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Sound suppression for Lysholm compressor with blow-thru throttles
2

Sound suppression for Lysholm compressor with blow-thru throttles

Sound suppression for Lysholm compressor with blow-thru throttles

(OP)
I am considering putting a twin-screw supercharger on an engine which has individual throttle bodies mounted directly to the cylinder head.  I plan to attempt to make this work in a blow-thru configuration, so I am not looking for people to dissuade me from that.  I have done enough research, and seen enough setups to be confident that it can be made to work, and work quite well.  

However I do have concern over the noise output from the compressor.  Blow thru setups are typically quite noisy.  I don't mind the sound under heavy acceleration, but I don't really want it to be screaming under normal cruise conditions.  The removal of the throttle plate in front of the compressor obviously has a negative impact on the noise transmitted when running with the bypass open.  

So, anyone have any good pointers to inlet noise suppression on a twin-screw Lysholm type compressor?  I have looked a little bit at resonators (tuned), but the tuned resonator might be limited to being effective in a relatively narrow rpm range.  

I would be interested in any good ideas, links, pointers, etc!!

Thanks,
Steve

RE: Sound suppression for Lysholm compressor with blow-thru throttles

Positive displacement supercharger + closed downstream valve = ? ? ?

RE: Sound suppression for Lysholm compressor with blow-thru throttles

(OP)
+ Large Bypass = no problem.

Its been done, hundreds of times, and even on a few "production" cars or aftermarket SC kits.

Do you have any answers for my question?

RE: Sound suppression for Lysholm compressor with blow-thru throttles

No problem until a bypass valve sticks, but you asked not to be dissuaded, so I will simply ask where you intend using these so I can be somewhere else.

I would expect the bypass valve to make a lot of noise on closed throttle at high rpm.

I would expect a throttle upstream of the blower to be quiet when closed at high rpm.

I will not help in nor encourage you to do something dangerous and stupid.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

RE: Sound suppression for Lysholm compressor with blow-thru throttles

(OP)
Secondary bypass/blowoff or pop-off valve can be used in case the primary bypass sticks.  I don't care how loud it is when I close the throttle at high rpm.  

I care how loud it is under normal cruise conditions.

I think i recall in past threads where your feelings against doing this is quite apparent.  I recall your concern about a bypass valve failure.  What other concerns do you have (if you can provide them concisely)?

It has been done on a production engine by Lancia, aftermarket supercharger setups on VWs and Miata's.  I don't recall reading about any deaths or global armageddon resulting from it, so I am curious.

The engine the setup is going on has ITBs from the factory.  I am quite reluctant to remove them.  I COULD just remove the throttle plates and connect the DBW actuator to a large single throttle body, but I am still not certain that would be the way to go.  If I do this, I have significant problems with engine management, since the timing of fuel delivery upon throttle tip-in is based on the rapid response of the ITB.  Even though a draw thru TS mounted on a compact manifold with a water-air charge cooler internal to the manifold has excellent throttle response, the lag, measured in ms, is probably enough to cause a brief rich condition and slight bog upon throttle application.

I can deal with hardware issues a lot easier than I can deal with something as fundamental as the timing of the injector firing in the software code.

Anyway, interested in your opinion, particularly if you have any ideas of how to quiet the compressor, or address the concerns that you have...

RE: Sound suppression for Lysholm compressor with blow-thru throttles

My two cents....

There will be 3 major sources of noise. You didn't describe your application much, but with individual throttle bodies I assume it is a motorcycle.
1. supercharger drive - Obviously a ribbed belt would be better than a toothed belt. An enclosed gear drive MIGHT be better but that depends on how stiff the enclosure is, the gear tooth profile, and backlash.
2. air intake - Provided you have the room you can treat intake noise with various types of silencers. (ex. helmholtz, 1/4 wave, expansion chanbers) If you don't have the room you are out of luck! Also make sure that your intake tract isn't vibrating and causing noise.
3. supercharger case (and other structures secured to it) - This will be tough to manage as absorbing/dampenging materials will act as insulation and might cause the SC to overheat. The best luck here might be a cover.

ISZ

RE: Sound suppression for Lysholm compressor with blow-thru throttles

(OP)
Application is BMW inline 6 cylinder.

Will use a ribbed belt for drive, as i have in past setups with non ITB engines of the same type.

The intake tract can be relatively long, and I can use non-metallics in it as well, probably in the inlet housing to the compressor as well, to help provide some acoustic damping.  I also think I will have room for a decent sized airbox, and possibly a resonator/expansion chamber.  Hopefully that will do it.

The compressor itself does not seem like it is a horrible source of noise thru the case in the current setups.

RE: Sound suppression for Lysholm compressor with blow-thru throttles

Stimpee,

Why do the blow through setups become more noisy than the suck through throttle types?  I assume that either a) the pressure in front of the throttle plate is greater causing more turbulence at the edge and therefore noise or b) the bypass valve has to be used as a means of part throttle boost contol which creates noise that is an inherent feature of the bypass?

I am not sure I completely understand why a blow through would necesarily be MORE noisy, can you expand?

Happy new year!

RE: Sound suppression for Lysholm compressor with blow-thru throttles

(OP)
During cruise with a draw thru system, the partially closed throttle plate must block a lot of the inlet noise of the compressor.  Couple that with the fact that the compressor is not doing much "work" due to low density of the inlet air, and the result is that you barely hear the compressor at idle and cruise conditions.

I have seen several blow-thru setups and they are much much louder at idle and cruise.  Presumably this is due to the lack of the throttle plate blocking the air (and noise), as well as perhaps due to the fact that the compressor is passing more "mass" thru it due to the inlet being at ambient pressure.  However I can't imagine that this is a huge contributor, since the outlet of the compressor in this case is also at ambient due to an open bypass...

Not sure, but empirical evidence shows that blow thru twin screw setups are much louder under normal cruise (partially or significantly closed throttle) conditions.  At WOT I would imagine that there is little difference.

RE: Sound suppression for Lysholm compressor with blow-thru throttles

OK, i see.  So basically the extra noise is due to the fact that the SC is un-throttled at intake meaning that it is passing ambient pressure air and doing alot of work, plus the fact the the throttle plate itself would act as suppressor.

The lack of air pasing through the SC will be quite a major contributor as if there is no air then the drive system of the SC will be running almost unloaded except for the system inertia.  The drive will not make alot of noise until it is under load (compressing air).

Surely any pulse or wave tuning cannot apply before the SC, i.e. Helmholtz, 1/4 wave etc as we are not dealing with any engine exited noise only the linear noise of the SC.  No pulse waves will pass the SC and back out causing noise.  Maybe try the open end of the system first as this is where you have the SC and BMW have probably made quite a good job of the other bit.

One way to cut down this noise is to increase the level of dampening or insulation in the inlet tract before the SC.  Maybe a box-inside-a-box chamber or muffler between the SC and air filter would be the easiest solution without sacrificing any response?  Also if the noise is eminating from the SC housing itself, then a cover or dampening mounts may help.

RE: Sound suppression for Lysholm compressor with blow-thru throttles

You can use conventional intake tuning (ie expansion chambers, quinkies, and helmholtz) to try and reduce the noise from the s/c. The main excitation will be at number of lobes*s/c rpm.



Cheers

Greg Locock

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Sound suppression for Lysholm compressor with blow-thru throttles

Greg's answer while absolutely correct to the extent that it goes, does not address the noise generated downstream of the blower and being emitted through the manifold and air duct walls. Heavy walls and curved surfaces will help reduce that noise as will wrapping the manifold and air duct with sound insulating materials. These might create a secondary problem by also insulating the heat in.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

RE: Sound suppression for Lysholm compressor with blow-thru throttles

"Surely any pulse or wave tuning cannot apply before the SC, i.e. Helmholtz, 1/4 wave etc as we are not dealing with any engine exited noise only the linear noise of the SC.  No pulse waves will pass the SC and back out causing noise."

Unless someone changed the laws of physics all "sound" or "noise" is made up of pressure waves in air. ISZ

RE: Sound suppression for Lysholm compressor with blow-thru throttles

Pulse waves are generated by the rotors as they take a gulp of air then close of then the next one takes another gulp.

Regards

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RE: Sound suppression for Lysholm compressor with blow-thru throttles

Yes, if it was a rootes type. This is a screw compressor.  Screw compressors have little to NO intake pulsing.  Sorry Pat, there is no exitiation frequency to speak of with these SC's.

Even the rootes type with its three twisted rotors does not have the same problem anymore.  As the lobes are twisted, only a very small section of the intake between the lobes is ever opened up at any one time.  A correspondingly small exhaust gap will be beginning to open on the other side.  This does not cause huge pressure fluctuations and therefore inlet tract pulses.  Also the timing of these small pulses have been timed to allow cancelling.  The more lobes, the less pulsing.  

What I am trying to say is that the flow is quite smooth from the lysholm SC.  The noise generated from the flow of air through the SC will rise in frequency as a function of engine speed and will also be too high to exite any large resonance in the inlet tract materials anyway.  This reduces the usefulness of the suggested methods of inlet tract tuning.

Assuming that the inlet tract is well constructed between the SC and throttles (being BMW it will be), and that the noise is from the SC itself, the part I would attack first would be the mouth of the SC to the airfilter as this is likely to be where the noise is coming from.  From some SAE papers I have read, the only way to STOP the noise that a twin-screw makes off-load is to stop the air passing through it.  It has a compressive element that slightly loads the SC at all times.  

I think that proper management of the bypass valve could almost cancel the noise if it is big enough.  Or what about a clutch system?  It depends on the application really.  I assume its a road car hence why you are concerned with noise, probably an ///M engine?  Why not just have the SC set to come in at say 50% throttle?

Other than that, as suggested myabe insulation on the SC body and some kind of broadband muffler on the intake will help.

Greg

RE: Sound suppression for Lysholm compressor with blow-thru throttles

(OP)
While screw compressors may not have the same "pulsing" as a roots, the internal compression in the unit (pressure ratio built-in of approx 1.4) results in a case where air is being compressed, and the "re-expands" when the bypass valve is open.  This expansion results in a "popping" sound as it is often characterised, and the screw compressor is typically regarded as being noisier than the newer style Eaton type roots blower with the 3 lobed twisted rotors.

I would still imagine that the number of lobes (male and female rotors have different numbers, 3 by 5 for the lysholm compressor, and 4 by 6 for the Opcon) times the operating rpm of the compressor rotors will provide the appropriate frequency range to avoid.  In my case I will want to mitigate noise in the 2000rpm-3000 ( or perhaps 3500) rpm range.  Outside of that range I am basically either lugging the engine, idling, or revving it out for power in which case I don't really care about noise under power.

The bypass will be left open probably up to 40%-60% throttle opening depending on how I tune it, with it closing progressively at the appropriate vacuum level to start generating boost.  

It is a road car, and is the S54 engine in the 2001-2006 E46 M3.

Insulation on the SC body will probably be effectively accomplished by the general packaging of my setup, and muffling or resonation chambers in the inlet piping, which will probably also be composite and high damping from a material perspective, are all on the table.  

Steve

RE: Sound suppression for Lysholm compressor with blow-thru throttles

Steve,

Interesting, I hadnt considered the expansion scenario.  Maybe then a tuned-pipe solution immediately at the SC exhaust to cover the transitionary period would work in this case?

Also, the correct placement of the intercoler will go some way to providing a plenum style voulme which will dampen the popping noise you speak of.  The closer the intercooler is placed to the SC output, the better the response.  

Greg



RE: Sound suppression for Lysholm compressor with blow-thru throttles

(OP)
On the previous setup I designed for the S52 engine (no individual throttle bodies), I had the compressor mounted directly on the plenum (cast aluminum), and a water-air charge cooler core mounted inside the plenum with some volume after the cores.

Compressor noise in that setup is barely noticable at idle and cruise.  It is certainly noticable at WOT, but not unbearable.

That setup was much simpler though, due to the standard draw thru setup.  I used a cast aluminum inlet housing to the compressor, steel inlet piping (3" or 3.5") and the throttle in front of the inlet about halfway down the piping.  There were a few silicone couplers, and then a hot film MAF with a cone type open filter CAI just past that.

Steve

RE: Sound suppression for Lysholm compressor with blow-thru throttles

The most powerful loudspeaker I ever worked with used a sliding plate to chop a stream of compressed air.

That chopping action is the direct cause of almost all intake noise in cars.

Cheers

Greg Locock

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Sound suppression for Lysholm compressor with blow-thru throttles

I initially said I thought this approach was stupid and dangerous and promised not to help. I should have kept my word. More than enough said

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

RE: Sound suppression for Lysholm compressor with blow-thru throttles

(OP)
Pat, with all due respect, you need to thicken your skin a bit and relax.

Also, saying this is a stupid idea is rather closed minded, and short sighted for an engineer.  Do a bit of research.  As I said, it can, and has been done, by automotive experts, engineers, and manufacturers who most would strongly agree "know what they are doing".

A lot of things we as engineers design and develop, have inherent risks and in many cases dire consequences if we are wrong.  That is why there are engineers.  To identify the risks, solve problems, develop solutions, and hence MITIGATE those risks.

However, thanks for calling me stupid.  I will try to apply that as motivation.  I love being told something can't be done, and then going ahead and doing it, and doing it well.  Proving prople wrong can be quite satisfying...

If there was a "rolleyes" smiley available on this forum, now is when I would choose to apply it.

RE: Sound suppression for Lysholm compressor with blow-thru throttles

Steve,

I was thinking that you could possibly tune out the noise in the transitionary period using a tuned length of pipe running up to the intercooler if it is close enough.  May I ask what is the exact model of SC that you are using?

Tmoose
The noise from this SC is produced at the exhaust when the gas expands again.  Not at the inlet, not due to it "taking gulps of air".  Without internal compression there would not be a pulsing at exhaust either.  And how do you know from those cases that you posted that the unacceptable noise is generated by the 'pulsing' and not the drive or re-expansion anyway?

Greg, I agree with the prnciple completely, but imagine the loudspeaker that chops the air never completely covers the hole.. that each blade leaves a gap either end.  It wouldnt be so loud.  That is what manufacturers have managed to do to make these SC's very quiet.  The problem with the twin screw is the re-expansion component generating noise as Stimpee has qualifed.  This is not the same as the 'stopping and starting of airstreams with inertia' problem.

Greg

RE: Sound suppression for Lysholm compressor with blow-thru throttles

Warmington (Automotive)
Yes there is an edit function in the preview post.
After you have submitted it you cannot take it back.
B.E.

RE: Sound suppression for Lysholm compressor with blow-thru throttles

(OP)
Greg,

I am not certain I understand what you are suggesting.  

I am using a Lysholm 2300AX compressor.

I am still a bit unsure whether I plan to use an air-air intercooler with the associated piping, or whether I will integrate an air-water core into the manifold plenum similar to my previous setup.  The air-air has some advantages, and in some ways might damp the likely brutal throttle response that this setup will probably have with the instant boost hit of the twinscrew combined with the ITBs.  However using the air-liquid setup will ultimately result in a more compact setup, and will also allow me to later integrate it into a twin-charged setup if I decide to really go nuts, where the twin-charged setup would have air-air cooling after the turbo, and air-liquid in the manifold after the compressor.

In the case of a twin-charged setup, I would remove the 2300AX and switch to the 1600AX for better low-end boost response, while the turbo would maintain the upper end while keeping the compressor revs reasonable.

RE: Sound suppression for Lysholm compressor with blow-thru throttles

I saw a couple of videos of the VW G60 engine fitted with the Lysholm SC.  Those things are REALLY loud.  How loud was the previous setup with the twinscrew SC post TB Steve?

I think you could tune out the noise in the transitionary period with careful sizing of the pipe to the intercooler for a certain frequency, whether using an expansion chamber or a tuned resonator, but it will be realively narrow spectrum.  Maybe with careful tuning of the bypass valve and this pipe size/length, you can get rid of most of the noise during the period where it is uncomfortable at cruising speeds, but its still going to be really noisy as soon as the SC is fully hooked up.  Best bet is just old fashioned sound deadening measures on the SC casing and inlet tract I guess.  Have a look at this little setup, not suggested but its one way of doing it with a not so happy ending. Straight lobe SC though.

http://autospeed.com/cms/A_2570/article.html

Out of interest regarding the "positive displacement SC before a closed valve is suicide/stupid/ignorant" comments, Nissan didnt think so:

http://i17.tinypic.com/6prihdv.jpg

The TB is attached to the inlet manifold AFTER the charging system. There are no other valves used other than the bypass pictured.

Greg

RE: Sound suppression for Lysholm compressor with blow-thru throttles

(OP)
The draw thru system i did previously is actually barely noticable at idle and normal highway cruise, etc.

It is only moderately loud at WOT, and not unpleasant at all.

Not sure what I will be willing to live with on this setup.  Interesting the link you posted.  However that is a very very quiet car to start with, and they chose probably the worst case supercharger on the planet (straight lobed roots) to create the noise.

I guess we will see how bad it is, and then figure out what I need to do.  The last ditch option will be to stake open the ITB's or remove the throttle plates, and put a big TB at the SC inlet.

However I will only be able to do this (I think) if I build the system so the SC discharges directly into an inlet plenum which then contains the air-water intercooler.  Otherwise the throttle response effects will probably be undesirable.

RE: Sound suppression for Lysholm compressor with blow-thru throttles

I have run roots blowers with a down stream throttle, but never a screw blower, at least not yet.

A blower air bypass system will work fine, the most successful home made bypasses I have used have been based on a modified external turbocharger wastegate,

Your concerns about light throttle rotor noise are very valid with this system. I can only make two suggestions. Use rubber hose (radiator hose) to route the air from the bypass back to the blower intake.  That will tend to damp any high frequency resonances. An fiberglass absorbtion type muffler might work, but I have never been able to find room for anything like that.

If all else fails, fit a second large throttle body immediately after the air filter. The idea here is not to throttle the air, as much as reduce the air intake opening size, and keep the noise in.  This sounds rather suspect I know, but it does in fact work rather well.

Arrange it so at idle or light throttle there is minimal pressure drop across this additional throttle body, but it only needs to be open just a fraction.  Make sure the linkage opens this faster than the main throttle, so it is never restrictive.  You will find that it can give a very worthwhile silencing effect without any real disadvantage.

   

RE: Sound suppression for Lysholm compressor with blow-thru throttles

(OP)
Warpspeed,

My system has a drive-by-wire throttle setup where there is an existing throttle control motor, with a linkage to the ITBs.  I could possibly slave off this linkage somehow to open an aux throttle, however I am not certain that the existing drive system has adequate "horsepower" to open a big throttle body.  

I think if I was going to go to the trouble of doing that, it might make sense to just dispense with the ITBs altogether?  I am not sure what the advantage would be to keeping them at that point, other than slightly improved throttle response perhaps.  In this case, with the free revving nature of the engine, coupled with the instant boost hit of the twin screw, I don't think I would really lose much in responsiveness though.  

In my case, due to the layout, and the "bling" factor of having the ITB's I would probably keep them in place and either fix them in an open position, or likely just remove the throttle plates altogether.

Anyone have any thoughts or insight to share?  This would ultimately meet my needs in that I would get the power delivery I desire, keep the ITBs in place for "looks" and simplicity, get the improvements in noise abatement and safety of using a draw thru system.

The only drawback would be in the fundamental view of "giving up" on the attempt to do something a bit more challenging than the norm!

RE: Sound suppression for Lysholm compressor with blow-thru throttles

Maybe you could slave another throttle actuator from the same control signal? (assuming the ECU can handle the extra load).  

You could use a very lightweight plastic moulded 'throttlebody' at the airfilter controlled by a second throttle actuator, it would only have to be slightly bigger than the total area of all the individual bodies and open at the same rate in order that it prevents any restriction.

In the back of my mind I can picture something just like this fitted to a VW... Can't seem to remember what that was, probaly some resonator that opened at a certain rpm.  It was definately cable controlled though.

I for one would be very interested to see how the system performed in the blow-through setup before you ditched the ideas and comformed!    

RE: Sound suppression for Lysholm compressor with blow-thru throttles

You could attach a conventional cable to the throttle pedal and use it in parallel to the fly by wire system. If there is different degree open vs pedal position issue between the two systems, make sure the suck through throttle opens fastest under all circumstances.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

RE: Sound suppression for Lysholm compressor with blow-thru throttles

The way I did it, I ran a second throttle cable direct from the accelerator pedal as Pat suggests.  

My car has a manual gearbox, but there was provision for a kick down cable if an automatic transmission was fitted. This made fitting the second throttle cable dead easy, as everything was already there at the pedal.

This is reasonably simple to set up, usually a cable operated throttle body has a "snail" cam. The secret is to use the same snail cam on both throttles to give identical opening angles, but make the first throttle body sufficiently large in diameter never to be restrictive.

Wedge this first throttle body wide open, and set the warmed up engine idle speed.  Then allow the first spring loaded throttle body to close against it's idle stop. Because of the extra size, the  first throttle body can then be adjusted so there is no change in idle speed, or no significant airflow restriction.

Opening and closing this first throttle with your fingers at idle, will quickly demonstrate the significant induction noise reduction this system offers at small throttle.

RE: Sound suppression for Lysholm compressor with blow-thru throttles

(OP)
Interesting ideas.  I think adding a cable to the throttle would be problematic, but I am not certain.  The DBW throttle assembly is quite complex, and quite expensive on the BMW, and quite likely also quite quirky!  I think I would be better off slaving either an electronic throttle off the existing command signal, or mechanically slaving it off of the ITB actuator.  

I can say that my biggest concern over running in blow thru (once the appropriate bypass redundancies are in place per Pat's warnings), is the noise.

If I could abate the noise via resonators, airbox, etc, it would be simple.  HOwever I fear that it will not be adequate.

Not sure if I should just design to accomodate a pre-compressor throttle "just in case".  Or whether I should design it into the system from the get-go, and then simply remove it if I determine it is not required.

Decisions, decisions...

In any case, I thank those of you who have been sticking with this (especially Pat, who is obviously compelled to chime in, even though he doesn't want to!!)  I appreciate having some qualified minds to help me think thru the ideas.

Thanks!
Steve

RE: Sound suppression for Lysholm compressor with blow-thru throttles

I have no idea what a screw blower sounds like with an open air bypass, but a roots blower at idle sounds like a big angry bumble bee droning away.  At cruising speed it rises to a fairly intrusive hum, but the strength dies away with further throttle opening (as the bypass progressively closes).

Full throttle with the bypass fully closed, there is no problem, until you change gear in a manual, then it really roars.

Either music to your ears, or a bloody awful racket, depending on your age, social class, and general attitude to the gentle art of motoring.

RE: Sound suppression for Lysholm compressor with blow-thru throttles

(OP)
In no case will my system be put together with an open air bypass.  The bypass will be a true bypass, and plumbed back in.

If I do a blow-thru setup, I will have an open-air pop-off or blow-off valve, as a fail safe, and also it will likely activate during brief throttle closures such as shifting similar to a turbo, but that I can deal with.

I just will not be able to deal with a constant SCREAM from the blower under normal cruise conditions, etc.  

RE: Sound suppression for Lysholm compressor with blow-thru throttles

Even completely plumbed back, the blower intake will still effectively have an acoustic path back out through the air filter.  Pipework is great for transmitting sound.  

At the very least, some sort of rudimentary muffling will be required. My own problems with this, led to the second throttle body idea, which does provide one fairly effective solution.

The only other idea I have come up with to solve this noise problem, might be to vent all the unused air down the exhaust pipe (after the cat converter). That should be practical if a MAP sensor is used. Not possible with a mass airflow meter though. I have not yet attempted this myself, but may possibly try it one day on a future project.   

RE: Sound suppression for Lysholm compressor with blow-thru throttles

(OP)
On my current setup, there is a path around the throttle from the compressor inlet housing to right after the MAF and air filter, for idle air bypass.  Granted that is a smallish 1" diameter rubber hose, but it doesn't seem to transmit any noise.

Let me get this straight though, if I read what you are saying, you have done or attempted a blow-thru setup in the past, and ended up adding a throttle on the inlet just due to the excessive noise from the blower (in this case a roots)?


Steve

RE: Sound suppression for Lysholm compressor with blow-thru throttles

You say "compressor inlet" that suggests a turbo ?

Roots and screw compressors have a pulsing output which creates an entirely different, and more severe noise problem. Bypassing some relatively smooth flowing air around a centrifugal compressor can be made virtually noise free.

Yes Steve, I have run roots blowers with a down stream throttle, and a modulated air bypass. It worked exceptionally well from every aspect except the noise problem from the bypass air.  The second throttle was a very crude but effective solution.

RE: Sound suppression for Lysholm compressor with blow-thru throttles

(OP)
When I say compressor inlet, I am still talking screw compressor, not turbo or centrifugal.  Since the Lysholm unit is a true compressor, I didn';t consider that an issue!

It is looking like I should plan to include the auxiliary draw thru throttle, since it is likely that I will find continuous noise to be "problematic".

RE: Sound suppression for Lysholm compressor with blow-thru throttles

Hi,
Some people here have arguments against the idea of the blow-through and by that pointed out some of the potential difficulties/dangers of that system. Thanks for that because that's the way I was planning to go a I still am actually but now more conscious of these issues.
As Stimpee told in the beginning of this thread, some production cars are setup like that. The supercharger I was planning to use comes from a Mercedes M271 engine which is used in a number of different forms. It is an Eaton M45 equipped with an ECU controlled throttle bypass valve and installed in that famous blow-through together with an electronic throttle. It has 3 noise dampers, first between air filter and charger, and two others -one of them called "broadband"- between charger and intercooler.
I have a MB booklet here presenting the engine and its ancillaries containing nice pics and tech specs like the different torque and power curves, charge pressures, pulley ratios but hey it's copyrighted material. I won't drop it on the internet but I could e-mail scans to those interested in this matter for purpose of technical study…
I have the first damper after the charger too, could take some pics.
Regards,

   Jean


RE: Sound suppression for Lysholm compressor with blow-thru throttles

Perhaps you could describe the size and general construction of these acoustic dampers. No doubt MB have put quite a bit of time and effort into developing this system.

Some pictures would be great !

Blow through has many advantages, and if you are planning to run a decent sized front mounted air/air intercooler, there is really no alternative to fitting a down stream throttle.  

RE: Sound suppression for Lysholm compressor with blow-thru throttles

Warmington wrote - "Yes, if it was a rootes type. This is a screw compressor.  Screw compressors have little to NO intake pulsing.  Sorry Pat, there is no exitiation frequency to speak of with these SC's


jcd06 wrote that the Mercedes M271 "has 3 noise dampers, first between air filter and charger......"

So, the Mercedes device is designed for broadband type flow noise, rather than anything related to noise at frequencies related to SC rotor lobe/blade pass?

RE: Sound suppression for Lysholm compressor with blow-thru throttles

Even a roots style blower has almost no pulsing at the rotor inlet. But both types can pulse significantly at the discharge.

There is rarely an induction noise problem if the throttle is placed upstream of the supercharger.  But a downstream throttle always requires some type of an air bypass system.

It is when this bypass is open at light throttle and engine idle that pulsing supercharger discharge air can find it's way back around to the main air intake.

RE: Sound suppression for Lysholm compressor with blow-thru throttles

So here are the pics finally.

http://users.skynet.be/fb924332/Folder1/P1260053a.jpg
The complete thing, this is all I have...

http://users.skynet.be/fb924332/Folder1/P1260054a.jpg
That's the first damper after the charger.
The plastic insert is held in place with those metal clips.
Not a big deal huh.
The damper upstream of the charger looks more or less the same type as I can see on the
cut-away drawings in the boolet. The second damper downstream of the charger I would rather describe
as half a meter of perforated tube surrounded by a plain one. It's not clear on the drawings if the space between
both tubes contains somekind of a damping material but it's definitely an absorption system.
In the text they call it a broadband damper. No idea what gives it that property,
perhaps different perforation diameter or distance between the holes.

http://users.skynet.be/fb924332/Folder1/P1260056a.jpg
This one is a view without that bypass manifold.
The bypass valve is a DC motor opening the butterfly against a spring.
It has potmeters for positioning feedback; I think it can be called a "servo" then.
Each rotor has three lobes and is covered with a black, rather rough coating.
Not at all the smooth feeling Teflon coating like I saw on another Eatons.
Perhaps someone can tell me more about this kind of coating cos I never saw it before.

In my application noise will not be an issue unless it would be louder than the exhaust.

RE: Sound suppression for Lysholm compressor with blow-thru throttles

Really interesting Jcd06.

The servo driven air bypass idea is o/k for the vehicle manufacturer, because they can fully map it's operation in the ECU.  But for a project car, getting it to work properly would be quite a challenge. An entirely pneumatic system using an adapted turbo wastegate wins hands down for simplicity, and it will work perfectly well, onece a suitably light wastegate spring has been fitted.

Your best bet might be to just install everything on the engine and first get it drivable and properly tuned. That should keep you fully occupied for quite a while.

The induction noise problem is difficult to anticipate. The space available, and pipework layout will then dictate what is possible, or practical, along with some experimentation.

Fortunately changes and additions to the atmospheric side of the induction system should be possible without too much difficulty once the car is drivable.

All the noise (and any vibration) will come either directly from the mechanical blower drive system, or out through the open air filter. Minimal noise should radiate from pipework or intercooler surfaces.

RE: Sound suppression for Lysholm compressor with blow-thru throttles

(OP)
Warpspeed,

The problem with your "approach" of get it running and then deal with it, is depending on the severity of the "problem", and dealing with the problem, may completely dictate how I ultimately decide to lay out the system!

If I can get the blow-thru setup to work, and work well and safely, as well as manage the noise, then I would be inclined to use an air-air intercooler setup on the car.  I would do this for both simplicity (less failure points vs water pump, etc), and also since it would (in my opinion anyway) slightly damp the throttle response a bit.  The car's throttle response is already pretty amazing, and bordering on abrupt.  I fear with a twin screw on top of that, that it would get to be difficult to drive.

However if I KNEW that I would end up punting, and either adding a pre-compressor throttle, or even further, end up removing the ITBs or blocking the original throttles open, then I would probably build an integrated air-water cooled intake manifold like I have in my previous system developed for the earlier BMW 6 cylinder models.  The main problem with this approach is that the timing of the delivery of fuel to the cylinders could be hard to deal with, since there could be some lag in the airflow relative to the stock setup, and it could result in a stumble upon throttle application.  

Those 2 approaches would be very different from a fundamental standpoint, so it is hard for me to figure out how to proceed without knowing the answers!

RE: Sound suppression for Lysholm compressor with blow-thru throttles

It is a tough decision that only you can make, and the intended purpose of the car may influence your thinking.

Individual throttles are going to give by far the best throttle response, especially on closing throttle. That is something few people realize.  But throttle lag can severely impact gear changing if you have a manual gearbox. With an auto is hardly matters how bad throttle response is.

At least ninety percent of the planning, engineering, and fabrication will be involved in mounting and driving the supercharger, and moving other engine accessories out of the way.

The induction pipework is simple and fairly readily changed in comparison.  In fact, until the supercharger is bolted into its final position, it may be rather difficult to plan the exact details of the pipework.

I would get that part of the project out of the way first, there will be enough problems to keep you awake at night just getting that far. Then tackle the pipework as stage two.

RE: Sound suppression for Lysholm compressor with blow-thru throttles

(OP)
Again, the issue arises, since the fundamental position and the orientation of the compressor will be different (or at least in my mind, I would intend it to be different) depending on the approach.  If I do an air-water setup, the charge cooler is located internally in the intake plenum, and the compressor attaches directly to the plenum with the discharge to the side similar to my current setup on the older BMWs.

If I go air-air, I would very likely (assuming it will fit) orient the compressor differently, in a manner that is more conducive to piping/ductwork for a front mount intercooler.  

If I end up doing the air-air setup, and then later decide on the draw-thru throttle, then throttle response will likely suck.

If I go to the air-water setup with the integrated manifold, that is probably a more straightforward design approach for me, but I fear that the throttle response will be too abrupt.  

The good thing with the air-water integrated manifold is, that if I desire to put together a twin-charged setup down the road, it would be "easy" to make a turbo feeding a front mount intercooler and then feed that into the compressor with the second stage of charge cooling after the compressor in the manifold.  

Ugh...

RE: Sound suppression for Lysholm compressor with blow-thru throttles

I feel your pain stimpee, been there done that.

Throttle response with an air/air cooler and the throttle on the blower intake will not just be bad, it will be completely undrivable.

With perhaps eight to twelve litres of total contained air volume after the throttle, close the throttle and it can take several seconds for the engine speed to drop back to steady idle rpm from flat out full boost. I tried this once and was quite shocked by how bad it actually turned out. Forget that idea.

For best performance and drivability use a front mounted air/air intercooler with the individual throttles. It will be a dream to drive, and have no intercooler or supercharger heat soak problems in slow traffic. Especially if you bypass the air back to the blower intake from after the intercooler. But the small throttle transmitted noise problem will require some acoustic ingenuity.

Next best would be throttle on the blower intake and an air/water cooler with absolute minimal dead pipe volume. No nasty noises, but it will not be so responsive. The air water intercooler system will be a real pain to develop, and it will badly heat soak in the engine compartment, and be nowhere nearly as effective at intercooling in typical stop go street driving as you expect.

Either of these systems requires an effective air bypass around the blower.

Twincharging does not really change anything, the air from the turbo just goes in where the original air filter was, regardless of what else there is. Twincharging is a fantastic idea, but keep that for stage three.  Solve all your supercharging problems first, just leave sufficient space for the turbo for later.

If I was doing this myself, I would keep the individual throttles, fit the largest front mount air/air cooler that would fit, and fit a second slave throttle body to the blower intake.  I know that ain't particularly easy in your case, but it is only one problem to solve.  

Doing it any other way is going to have heat soak and response disadvantages, as well as more potential problems.

RE: Sound suppression for Lysholm compressor with blow-thru throttles

(OP)
It does appear to be obvious that the design of the bypass setup will be critical.  I also agree that a throttle in front of the blower and then an air-air intercooler setup (with the requisite cooler and piping volume) would never work without keeping the ITBs in place and functional.

I don't necessarily agree that a water-air setup will be as bad as you state, particularly since I have done it before, and achieved very excellent results.  However I will concede that in slow stop-n-go it will get hot, and takes a minute or so of driving at normal (30mph+) speeds to cool the liquid back down.

So in your suggestion, is the purpose of the slave throttle body at the blower intake basically for noise (and adding the secondary benefit of another fail-safe if something went wrong with the primary ITBs)?  I am not sure how difficult it will be to do that, but given the location I am thinking for the compressor, getting access to the existing mechanical throttle linkage may not be that difficult.  The big question is whether the existing linkage and drive setup for the throttle can handle the load of an additional large throttle body.  I would imagine that I can get away with a pretty light spring on the throttle body though.

I am partial to the simplicity (functionally) of the air-air setup.  Hopefully the piping won't be too much of a bear...

Thanks for your continued feedback!

Steve

RE: Sound suppression for Lysholm compressor with blow-thru throttles

The problems I had with my own water air setup was that everything in the engine bay heats up to perhaps 85C (185F) including the intercooler core.  That rapidly heats the water reservoir, even if that is located in a cool remote location.  Even a very large front mounted water/air heat exchanger will take a couple of minutes to cool down all this stored water. My own experience was that this was just not up to the job in mid summer heat, and heavy traffic conditions. All the extra complexity did not justify the rather disappointing results.

With an air/air cooler mounted way out front, well away from engine heat, all the engine bay pipework and induction system will still heat soak, that is unavoidable.  But when you plant your foot, the rush of cold air through the hot pipework has a surprisingly low actual temperature increase.

This is where a sudden rush of really cold air is most welcome, with a quick stab of acceleration. If it takes ten seconds for the induction temperature to fall after full heat soak, it is pretty useless.

If the intercooler is properly ducted, the radiator air will first pass through the intercooler, and the radiator fan will cool the intercooler core even when fully stationary and idling in traffic.  In my experience air/air works vastly better for a city car than air/water/air.

The idea of having an extra throttle body up stream, is to reduce the open flow area of the air inlet, to reduce the escape of noise. This this will be far more effective than you might imagine. There should not be any mechanical loads on this extra throttle beyond the return spring.

I believe it is now a mandatory design rule in most countries that dual throttle return springs must be fitted. These are usually one inside the other fitted directly to the throttle shaft.  Where two throttles are to be fitted, one spring can be removed from each, resulting in about the same throttle pedal effort as original.

This is still a fail safe system, because both throttles are connected in series flow, and only one need close.  Strictly speaking there are still two throttle return springs, so it should still comply. But regardless, it is still a safe modification to do.

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