×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Fuel stand-off - intake reversion - what are the causes?
2

Fuel stand-off - intake reversion - what are the causes?

Fuel stand-off - intake reversion - what are the causes?

(OP)
A while back, I posted some questions related to a v-twin I am working on.

Here is another puzzling one.  The engine is a v-twin, dual overhead cam, 4 valve configuration - designed from a clean sheet of paper - not really a copy of anything.

The motor has severe intake reversion that comes on at around 4-5k rpm.  At the same time, the engine will not make peak power past low 5k rpm.  
Things tried have been:

1. Cam timing and duration changes - we have been all over the map with possible changes and combinations and the engine does not really respond positively or negatively to cam changes.
2. Intake manifold configs: dual IR runners, one carb to divide to either cylinder, 48mm and 56mm throat carbs.  All had reversion.

Is it safe to say that the engine will not be able to realize its power potential unless the reversion goes away?    Also, what other factors could contribute to reversion besides cams and intake systems?  Really look forward to some experienced opinions on this topic.

Thanks

RE: Fuel stand-off - intake reversion - what are the causes?

Intake runner length.

Intake runner cross sectional area.

Intake plenum volume.

Design and shape of bell mouth to runner.

Throttle plate position and size.

Intake valve closing point.

Valve springs and cam closing ramps and their effect on valve bounce.

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: Fuel stand-off - intake reversion - what are the causes?

What RPM is the engine designed for? What's the volume of each cylinder? What type of carb are you using - automotive fixed-venturi, constant-velocity with the slide actuated by venturi vacuum (standard motorcycle carbs), or flat-slide-type carbs in which the slide and needle also act as the throttle?

My experience, which is motorcycle engines with a separate carb / throttle body per cylinder, is that you will *always* have some degree of flow reversion, particularly at low revs. If the cam timing is long enough for the engine to make decent power at high revs then it will be long enough so that at lower revs it fills the cylinder first and then pushes some of it back out. If you run the engine without the airbox you can often see a mist of fuel being pushed back out.

If your problem develops at high revs and full load, it sounds like there is some sort of resonance developing between the intake runner and the cylinder (acting like a Helmholtz resonator). I can only see that situation happening if your intake ports or intake runners are WAY WAY too big and/or too short. You need to keep the velocity up so that the column of air stuffs the cylinder full but doesn't let any of it back out when the engine is in its powerband. I've never seen a production-design engine have that problem, but if you've gone out of bounds, maybe there's a first time.

The bike engines are just designed to deal with whatever reversion happens at lower revs. Constant-vacuum carbs have the slides close down at lower revs, which partially reflects the reversion wave; the carb is jetted to deal with whatever reversion is left. Modern fuel-injection systems use a drive-by-wire throttle that stays mostly closed at low revs, even when the rider asks for full throttle. Flat-slide carbs don't deal with the reversion; they have terrible bottom-end response ...

RE: Fuel stand-off - intake reversion - what are the causes?

(OP)
Some of the comments included so far, we have already experimented with.
I'll explain.

The design is a 4.25 bore, 4.00 stroke, 6.8" rod,  45 deg twin.  We would expect to see the best power in the 6500 to 7000 rpm range.  It seems that the reversion is not a problem "visually" at under 3500 rpm.  Once over 4000 rpm at full throttle it comes in and get's worse and worse as the rpm climbs and never goes away.
On the dyno, the mist of fuel/air coming out the carb mouth is around a 1.5 ft column.

In terms of determining if the intake runner cross section is too large, we have gone from as low as 1.8 id to as large as 2.25 dia just after the carb body.  In fact, the reversion seemed worst on the smaller runner.

Since the motor fits a Harley style frame, the intake runner length is fairly short in order to be packaged.  But, for tests, we have tried adding a spacer up to 3" long behind the carb with no better results.

The carbs tried have been 48mm mikuni flatslide and S&SD butterfly style with 2.25" throat.  Both had reversion at high rpm.

The intake cams have been tried as low as 226 degrees @.050 duration and 97 degree centerline (early closing point).  This was not better than larger cams with later closing - makes no sense so far.

Brian, it sounds like your experience is that the reversion goes away once the revs come up.  That is opposite of what we see.

Is it possible that the bore and stroke will not work well with the head flows we are working with on the 4-valve head design? We have very good low lift flow numbers as expected with the 4 valve design. Does the motor need to be more over square?

Pat, I remember you replied to my "optimum port velocity" questions a few years ago.  You mentioned that the intake cam should guide the valves to meet the piston and follow it down the bore and close as soon as can be done to achieve best power - can you explain further?

I remember you also stating that the intake runner volume should be slightly larger than a comparable 2 valve. Could you give recommendations for sizing based on this 114 cuin engine?



What keeps the motor from carrying out on top end?

Thanks for the help






RE: Fuel stand-off - intake reversion - what are the causes?

One thing to look for is incomplete combustion. If the flame
speed is too slow it will continue to burn during the camshaft
overlap period causing excess pressure in the intake tract.

RE: Fuel stand-off - intake reversion - what are the causes?

(OP)
Automotivebreath,

This isn't something I've even considered.  How do you go about finding if this is happening?  What test can be done?

RE: Fuel stand-off - intake reversion - what are the causes?

Okay, that's useful info to compare to my experience.

What's happening on the exhaust side, and how much cam overlap do you have? If you are getting a pressure wave coming back the exhaust system during the valve overlap period, it's going to spit flow backwards even before the intake stroke starts. If the exhaust system is too restrictive to let the flow out, there could be pressure left over at the end of the exhaust stroke. This gets *worse* if the valves have high flow at low lift.

Back-of-notepad calculations suggest that your exhaust system should have primary header pipes somewhere around 40" long and 2.5" diameter. On a 4-banger you would want to join with the pipe from another cylinder at that point and carry on with about the same diameter for some distance after that.

The short intake runners are unfavorable, but with that engine configuration I agree that there's not much that can be done.

Are you drawing air from a good-sized airbox? Most bike applications just contain the reversion flow and have the engine breathe it in again.

RE: Fuel stand-off - intake reversion - what are the causes?

One other thing with the incomplete combustion situation. The engines that I deal with range from 56mm to 76mm bore or thereabouts; 76mm is about 3", but the rev range is about double yours. It should be *possible* to get a good burn if the combustion chamber is right. If it's a 4-valve head, one would assume you've put the sparkplug dead centre, and have a squish-band around the perimeter of the chamber everywhere that there isn't a valve taking up the space, and hopefully you've used a low included valve angle and either flat-top or dished-centre pistons ... or not?

RE: Fuel stand-off - intake reversion - what are the causes?

Short of pulling the heads for inspection several indicators
can be used. Excessive EGT, intake inspection, spark plug
inspection and exhaust inspection. (dry/white deposits = good
combustion heat; wet/black deposits = insufficient combustion
heat)

Here’s a good ink to reading plugs:

http://www.empirenet.com/pkelley2/sparkplugreading.html

Another thing to consider is exhaust system efficiency;
properly designed it should pull on  the intake tract at RPM.

RE: Fuel stand-off - intake reversion - what are the causes?

agree, once saw an engine behave just like that. turned out to be restrictive exhaust. hard to imagine that happening in this case though. you probably already know...it going way rich when reversion gets that severe.

RE: Fuel stand-off - intake reversion - what are the causes?

(OP)
The air/fuel ratio is in the 12.5-13.0 area for best power.  At this setting, the plugs look white/slightly tan and dry.

The head/chamber configuration is modern - plug in center, shallow valve angle and squish band around where we could get it.  Spend lots of time on this.

The motor seems to make best power with only 26-28 degree timing.

Brian,

I have tried cam timing all over the place including large and small overlaps - it doesn't really seems to matter.  This the confusing part.  I believe that there could be something in the way of being able to see the results of cam changes.

The current header pipe is 1.9 id and around 38" long through flow thru mufflers.  This is the largest header pipe we used to date.  The outlet of the port is 1.800.

On the intake runner, would it make sense to try to build an automotive style plenum "box" after the carb to increase volume understanding that there will be a sudden drop in velocity as air passes into the box area.  There really isn't room to make the runner longer, but it could be larger in diameter before the heads.
This could contain/diffuse the reversion, but doesn't it make more sense to try to cure it in the first place?




RE: Fuel stand-off - intake reversion - what are the causes?

One thought is to see if you have a lazy port so...check velocity in both ports.
The other is (this may have been mentioned) that a reversion wave is timed to the RPM, if the positive wave hits the chamber during overlap it could spew out the intake....
Dont try to fix it with an air box as it will be miserable to tune with a sometimes wet reversion wave...

Cheers

I don't know anything but the people that do.

RE: Fuel stand-off - intake reversion - what are the causes?

Is there an edit?...I meant a positive exhaust wave into the chamber and spewing out the intake..


Cheers

I don't know anything but the people that do.

RE: Fuel stand-off - intake reversion - what are the causes?

I no nothing about v twins, but I am familiar with quad valve 4,6,&8 cyl. The fog ball is not that unusual for 1 throat/cyl carb setups, but if it is excessive, the only thing you said that may cause some issues at higher RPM is the tight lobe center (97deg) for the duration cam you mention. I am guessing that the cam is around 260 deg. actual which is about normal for a cam that is 226@.050". On engines I am familiar with, 112 LC is closer to typical for a well mannered engine and is a long way from 97deg LC. You didn't quantify how far you had spread the lobe centers in testing, but I would be more inclined to start testing in the 110 to 114 lc area if the fuel reversion is a problem. Watch your piston to valve clearance when moving the L.C.s around this much.

RE: Fuel stand-off - intake reversion - what are the causes?

He still has not qualified intake runner length or whether it's one or two carbies.

The stand off should tend to suck into the engine as revs and airspeed increases.

I agree 112 lobe centres on a 4 valve head. They scavenge a lot better than 2 valves and to much overlap increases the chance of exhaust pulses bouncing back into the inlet, although you should be able to drive through these events as they should only occur over a narrow rpm range.

It really sounds to me like cam timing is way out, like it lost a lobe type out.

I guess it's to basic to ask, have you checked the split overlap is at about TDC.

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: Fuel stand-off - intake reversion - what are the causes?

(OP)
I really appreciate the feedback - might just make some sense of this after all.

1. Runner lengths - intake port - 5.5 inch average between the two valves.  one runner is slightly longer than another.

intake manifold - 5 inch average - one cyl in further than another because of cyl offset.

total avg runner length - 10.5 inches.

2. It is a single carb, single throat 57mm, butterfly style. The carb feeds both cylinders through a common manifold which divides to each cylinder.

3. I have not checked the split overlap, but I can offer this information:

Recent cams and settings:

1. intake 226@.050 set at 97 deg lobe center
   exhaust 234@.050 set at 112 lobe center

lobe center separation = 97+112/2 = 104.5

2. intake 234@.050 set at 100 lobe center
   exhaust 226@.050 set at 106 lobe center

lobe center separation = 100+106 = 103

3. intake 234@.050 set at 100 lobe center
   exhaust 234@.050 set at 112 lobe center

lobe center separation = 100+112/2 = 106

All of the above do not come very close to the 112 lcs described above. Could this be an issue? Do the above figures point toward a tendancy toward reversion at higher rpm's?

If yes, then would a setting the like the following be more in line

intake 226@.050 set at 108 lobe center
exhaust 234@.050 set at 116 lobe center

lobe center separation 112.

RE: Fuel stand-off - intake reversion - what are the causes?

I guess it is twin cam so you can dial in each.

I would go back to basics and set it up at split overlap with 112 Deg between centres.

Fine tune from there.

Durations sound in the ball park to me.

Inlet tract is a lot to short.

Can you fabricate something up for dyno that makes both runners the same length, with a connecting plenum. Make the plenum quite large and put the carby in the middle, possibly facing away from the runner inlets so the airflow sweeps through a 120 deg or more curve. This might keep the reversion on the plenum and make a bit of room for longer stacks.

Are the valves definitely holding their seats without leaking. Is the head distorting under load and unseating the valves.

This is all grasping at straws, but your problem really does not add up.

Can you test with long, big dia open exhaust pipes.

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: Fuel stand-off - intake reversion - what are the causes?

I've seen this on F1 engines, and they seem to do okay with it.

RE: Fuel stand-off - intake reversion - what are the causes?

Are you sure about your real in cylinder A:F ratios.

Maybe it has good mixture with stand off leaning it out, but as you draw the stand off in, it is going rich and miss firing.

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: Fuel stand-off - intake reversion - what are the causes?

(OP)
Pat,

Very much appreciate your suggestions.  

The a/f ratios are being read by an oxygen sensor on the front headpipe.

The next step for me is to try the 112 centerline intake and exhaust.  

On the plenum idea, I think a "box" can be made up to bolt onto the existing intake manifold flange.  90 degrees to that inlet, the carb can feed through into the box. This should point the carb throat toward the front of the bike into the airstream - a good thing.

Have any ideas on what volume the box should be in relation to the engine size?  Something like around 50 cu in should be possible to package.  Wouldn't there be any concern about the sudden drop in velocity of the air once it first hits the plenum behind the carb?

RE: Fuel stand-off - intake reversion - what are the causes?

I would play with the lobe centers,and or exhaust  or ? you could build a big plenum and reduce the standoff but if the reversion and standoff are not causing  A/F mixture problems then its not a problem its a symptom. and you would not be too concerned with it if power and revs were meeting expectations. would you?

RE: Fuel stand-off - intake reversion - what are the causes?

(OP)
In my mind, the reversion is a problem because at the rpm it is noticed (visually) coincides with where the power seems to drop off.  As the rpm's come up, the reversion gets worse and never goes away. It makes me think that the rpm potential of the motor will not be achieved until the reversion is under control.

I would agree that if the motor was performing as expected, then the reversion would not be much of a concern.

RE: Fuel stand-off - intake reversion - what are the causes?

I guess what I'm suggesting is that you have a problem, that's causing power to drop fast and early and causing reversion/standoff, but that the reversion is secondary (a symptom and a clue) and that if reversion is not causing fuel metering problems (you say it is not) then i don't think its a problem in itself. as always i could be wrong.

RE: Fuel stand-off - intake reversion - what are the causes?

IMO reversion at RPM is a major restriction on archiving
optimal VE and power potential. At speed the time to fill
the cylinder is very short, when the intake valve opens
the first and only movement should be into the cylinder.
To accomplish this the pressure differential between the
intake and cylinder must be favorable. With V8 engines we
achieve this with the exhaust header, V twin... I don't know.

To me I see three options, find a way to reduce cylinder
pressure during overlap, raise the intake pressure or open
the intake valve later when conditions are more favorable.
 

RE: Fuel stand-off - intake reversion - what are the causes?

all true, no argument with any of the above. just trying to get causes and effects in order.

RE: Fuel stand-off - intake reversion - what are the causes?

(OP)
I should be able to try the following tommorrow.

Going on the idea of opening up the centerlines, I'll try with the cams at 226@050 intake at 112 cl and 234@050 exh at 112.

If this shows improvement (reversion and power), then probably go even further with lowering the overlap.  What would be preferable from there - later intake or earlier exh?

I have read in other places that lobe separation angles of up to 118 work for 4-valves - sound correct?

RE: Fuel stand-off - intake reversion - what are the causes?

FoMoCoMoFo, no offense intended, we agree the solution is to
look past the reversion and find the cause. With 2 valve heads
 and 4 into one headers the results can be amazing. The
picture below is from Larry Meaux, an awesome example of
completely eliminating reversion.

v114, I can't answer your questions on ideal valve timing
events. I'm very interested in the results of widening the
separation. Please keep us informed of progress.

RE: Fuel stand-off - intake reversion - what are the causes?

none taken.

RE: Fuel stand-off - intake reversion - what are the causes?

Does the oxygen sensor read both cylinders or just one.

One cylinder might be very lean and the other very rich due to charge robbing between the cylinders as I on a V twin, I would think the intake of one opens before the other closes. Twin carbies would be a lot nicer.

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: Fuel stand-off - intake reversion - what are the causes?

(OP)
Did the 112 and 112 intake /exhaust cl test yesterday.  As usual, this made almost no difference in the peak power or rpm where it made it.  The reversion did seem to be a bit less as compared to other cam settings in the past.  Still, though, during a pull on dyno, your hand will get wet with gas a few inches in front of the carb mouth. Can't believe this is optimized.

Pat, the o2 sensor reads only one cylinder.

Getting back to the plenum idea.  In terms of geting the engine to carry higher in the rpm range, say to 7K, does the plenum have a chance to be able to help this.  The idea would be that the single 57mm carb may not flow enough to supply the demand at 7k - maybe the pleunum would help?

If a plenum would be a good thing to try, how to size it?  Is there a rule to go by in terms of plenum volume vs. engine displacement?



RE: Fuel stand-off - intake reversion - what are the causes?

The plenum will make the carby seem bigger.

Some fuel stand off is normal on a high performance engine.

You might have one very rich and one very lean cylinder.

The airflow might be OK, but you problem is ignition system. Are you sure it is good. Are you sure there is no valve bounce.

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: Fuel stand-off - intake reversion - what are the causes?

(OP)
With the currrent set up, we will swap the o2 sensor between cylinders to check for lean and rich conditions.

Good to hear confirmation that the plenum will make carb seem bigger.

More than one ignition system has been looked at.  Also, we looked at the timing marks during a pull to 6K and it seemed like the ignition stayed fairly steady within around 2 degrees of setpoint.

Valve bounce is a possibility - We are not sure that there is no valve bounce.  How can this be identified without spinning the valvetrain on a test fixture with accelerometers.



Can the plenum be made too big?





RE: Fuel stand-off - intake reversion - what are the causes?

You won't have room to make it to big. Most are to small due to packaging restrictions. From memory, from quite a few years ago, at least one cylinder volume is minimum, engine size is probably getting well into diminishing return area.

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: Fuel stand-off - intake reversion - what are the causes?

v114,
Are you experiencing any detonation?

RE: Fuel stand-off - intake reversion - what are the causes?

(OP)
No detonation except when we radically change the ignition curve (very high advance rate).  Normally, we run around around 26-28 degrees total advance with no problems.

The plenum size I have scoped out will mount in place of the existing carb on the end of the manifold and will be made to be adjustable in volume with the smallest volume starting equal to one cylinder.

The carb will mount forward facing and be about 85 degrees to the inlet of the manifold with a generous radius to the entrance of the manifold to smooth the flow.

Think this may help rpm? Nothing else has so far.

RE: Fuel stand-off - intake reversion - what are the causes?

I keep coming back to combustion problems creating high cylinder
pressure during overlap but everything you describe relates
to good combustion efficiency. Have you giving much thought
to what phase each cylinder is in during the overlap period
of opposite cylinder?

Here's a link to information you might find interesting about
software for sizing engine air flow components.


http://www.maxracesoftware.com/pipemax36xp2.htm

RE: Fuel stand-off - intake reversion - what are the causes?

automotivebreath

That is why I suggested twin carbies at one stage. It would eliminate that as a potential problem.

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: Fuel stand-off - intake reversion - what are the causes?

Yes Pat two carbs would eliminate that possibility, it would
then be like tuning a single cylinder engine.

I read this tonight on harley tech talk:

If you've ever watched a bike on a dyno with the air cleaner
off, especially one with a healthy cam, there is a nice big
cloud of fuel/air hovering over the carb inlet. This is caused
by the air pulses from reversion, and also at high rpm by
the air getting accelerated, then hitting a brick wall
(intake valve) when the intake valve closes, and bounces back.
 

http://groups.msn.com/harleytechtalk/tc88fathead.msnw?action=get_message&mview=0&ID_Message=1152025&LastModified=4675628912205815110

RE: Fuel stand-off - intake reversion - what are the causes?

V114, is there any chance that you have lost control of your valve timing?  Valve float or cam follower pump up?

RE: Fuel stand-off - intake reversion - what are the causes?

(OP)
Did the run at intake and exhaust cams set to 112 centerline and basically nothing changed form other settings. Hp, torque, and rpm peak we all within about 5% of other cam settings.

It sure seems that there is something far overshadowing the normal tuning options here.  No matter what tuning options are tried, the power does not go up or down much at all.

We are not 100% that the cam drive train is keeping the timing fixed - we are in the process of installing cam position sensors and a scope to be able to see what is going on up at speed.  This will hopefully rule out problems in the valvetrain.

Thanks all for the feedback - will keep you posted.

RE: Fuel stand-off - intake reversion - what are the causes?

An adjustable strobe light is a very useful tool for veiwing valve train motion in slow motion

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: Fuel stand-off - intake reversion - what are the causes?

(OP)
Couple of things tried, again with no different power results:

1. Replaced the spring tensioned cam chain shoes with solid adjusters.  These were tightened after the engine came to temperature and there is very little play or room for chain to deflect. Thying to rule out cam timing changes as rpm goes up.

2. Still have not checked valvetrain with cam position sensors.

3. Completely new ignition system module and coil.

The next plan is this:

We now have new 213 degrees @.050 cams and .380 lift.  This is a big step down in duration as compared to previous.  We will be installing and testing early next week.

Two questions to throw out there.

1. If reversion is really coming from cam selection, does it make sense that 213 duration should show a significant difference?

2. We have not tried the plenum idea yet as listed earlier in the post.  I have been reading what I can on intake runner length sizing and we have established here that the current intake runner length is way short in this case (10.5 inches).  Is it a decent possibility that the rpm limiting we are experiencing is due to the short runner in combination with no plenum?






RE: Fuel stand-off - intake reversion - what are the causes?

I recently read a very good Paper on the subject of intake
bellmouth shape and it's impact on cylinder filling and
spit back by Prof. G. P. Blair.   


http://www.profblairandassociates.com/pdfs/RET_Bellmouth_Sept.pdf





Two more papers on intake sizing:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930090861_1993090861.pdf

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930084998_1993084998.pdf

RE: Fuel stand-off - intake reversion - what are the causes?

"1. If reversion is really coming from cam selection, does
it make sense that 213 duration should show a significant difference?"

I believe that something other than camshaft selection is the
primary cause of the reversion. If it were the cause I think
you would have determined it with the tests you have
performed. My current thinking is your problems are related to
tuning of the intake and exhaust runners as well as bell
month shape described by Blair.  

The new camshaft design will shift the RPM range and require
different runner sizes. This should be considered if the new
design shows benefit.  

RE: Fuel stand-off - intake reversion - what are the causes?

(OP)
Automotivebreath,

The current thinking on the smaller camshaft selection is to be able to detect whether or not we do actually have the wrong sized ports. We believe that if 213 duration works in increasing power and reduced reversion, then that may point in the direction of port sizes being too large.  

The theory being that the flow velocity (and resulting momentum) is not high enough to keep the flow moving toward the port.  This could actually be the whole problem.
It is puzzling though, because I thought that larger ports favored higher rpm.  We see good throttle response and low end performance with this setup.

RE: Fuel stand-off - intake reversion - what are the causes?

(OP)
Some more tests have been done.

1. Completely new style ignition system - revoved electronic module and replaced with a more conventional inductive pickup with weights for advance.
Result - no change, so ignition is not an issue.

2. Looked at A/F between front and back cylinders and saw that they were consistent

3. To test to see if higher port velocity would have an effect, we added material to the exit of the manifold where it meets the head to reduce the area by about 25%.  

Result - no change

4. We still have the question of valve bounce or something similar that could explain the reversion and how it gets worse as rpm climbs.

We are using seat and nose pressures of 65 lbs and 200 lbs respectively. The spring is a Manley 22105.  Total weight is 131g except the weight of the spring.

5. We still have not built a plenum.    

RE: Fuel stand-off - intake reversion - what are the causes?

what does the engine sound like when runing

RE: Fuel stand-off - intake reversion - what are the causes?

Does this engine have a hydraulic lash adjuster system? Sorry but I can't find it mentioned anywhere. Lifter pump up could explain it, especially with it getting worse with more rpm.

RE: Fuel stand-off - intake reversion - what are the causes?

How do the valve seats look?  
What does the cam manufacturer recommend for springs?

Manley says this is an Acura spring, suitable for up to 0.535 inch lift, but with a seat pressure of 82 @ 1.350".
And, if I read it right, they will tolerate up to .765" lift before binding.
http://www.manleyperf.com/sc/mk/honda/b-kits.html

These guys say Crower valves for that motor are a dainty 45 grams.  Stock intake head size is 33 mm.  http://www.tunerschoice.com/xq/aspx/dept_id.196/display_id.108/qx/Product.htm

I can't imagine your valves are anywhere near that small with a 4.25 inch bore (almost 1 inch larger than the Acura).

It looks like you may be running 20 lbs less seat pressure with a much heavier valve, although max revs are obviously much lower.
I'd be looking into the possibility of shimming those valve springs to get a seat pressure nearer 90 lbs, which is what CompCams springs
provide for a FORD  4.6L 4valve motor have.
 http://www.compperformancegroupstores.com/store/merchant.mvc?Screen=PROD&Store_Code=CC&Product_Code=26123-32&Category_Code=Ford

RE: Fuel stand-off - intake reversion - what are the causes?

(OP)
Engine sounds fairly responsive, not the best it's ever been, but not terrible.  On the dyno, you can tell that the engine is not pulling hard on top where is should and seems to labor

No hydraulic pucks - shim under style buckets

The current cam grinder has no recommendation for us on the springs - go figure.  I guess it's like that for some of the bike cam grinders.  This is why the question still exists on whether we are using enough spring to control the system.

We are headed toward increasing the spring to rule this theory out also.  

All of our cams have had no recommendation for springs and all cams have had fairly agressive ramps which could explain some things.  Maybe the valve bounce is occuring with all cams we are using and thus the reason why we are not seeing predictable changes.

RE: Fuel stand-off - intake reversion - what are the causes?

I still think it is most likely runner length and single carby and consequential interaction between pressure pulses from each cylinder. Twin carbs on IR manifolds will prove that point.

Second most likely is valve bounce having an influence on pulse in intake.

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: Fuel stand-off - intake reversion - what are the causes?

(OP)
In the past, with an earlier version of this motor design, we did rig that one with dual carbs and ir manifold.  We did that test to understand whether the reversion was due to interaction between front and back cylinders or just confined to each cylinder.  The reversion and lack of high rpm power was still there with the ir setup.  We then machined out the area between the runners to make the front and back communicate and then the rpm came up and the reversion looked much better.  The peak went from 5200 to about 5800 rpm - better, but still not what we wanted.  That motor also had 65 lbs on the valve seats. Could that have been bouncing valves?  Don't know
We have not really pursued this much more recently because it really does not package well for the rider but is seems to have been about the only thing so far to get the rpm to carry at all.

RE: Fuel stand-off - intake reversion - what are the causes?

I know a lot of reversion can be controlled by stepped headers...Have you looked into that setup?

All I know about reversions is from Jim Feuling and Jerry Branch
and if you Google them there might be some info

http://www.ssheaders.com/header.htm

Cheers

I don't know anything but the people that do.

RE: Fuel stand-off - intake reversion - what are the causes?

may be stating the obvious but do you have clearance on both
exhaust valves?. What kind of base circle radius are you using, do you know the lift per degree on profile, what have you set your seat pressure to and your over the nose rate, also what is your valve mass weight.

RE: Fuel stand-off - intake reversion - what are the causes?

Did v114 finally cure his problem or did he jump off the balcony?

RE: Fuel stand-off - intake reversion - what are the causes?

(OP)
The balcony looked very good, but we didn't jump yet.

The more testing we do, the more the issue points toward the problem with reversion and lack of power and rpm potential being in the cylinder head itself.

It doesn't respond well to cams, pipes, type of carburetion, ignition timing, valve spring rate changes. All the things that would normally have an effect in a normal working engine.

We are looking very hard again now at the cylinder head/port design.



RE: Fuel stand-off - intake reversion - what are the causes?

(OP)
As we continue to test, we have observed a common situation:

The brake specific value usually falls in the range of between .42-.45 under a full throttle run up to the rpm where the hp peaks. After peak hp, the brake specific number rises substantially for the rest of the run and climbs to approx .7 or .8 by the end of the run.  We are usually seeing rpm peaks in the low 5k range (not good) and we run to about 6200 rpm during a pull.  By the end of the pull, the brake specific number is around 0.8.
What does this say about the engine condition?  I understand this to not be a normal situation, but not quite sure on this one.  

Earlier in this thread, we described how the engine will not peak power beyond the low 5k rpm range.  Does this situation (low rpm peak and poor brake specific) point to anything specific to look at in correcting the problem?

Look forward to your comments here

Thanks

RE: Fuel stand-off - intake reversion - what are the causes?

Is a portion of the fuel you see leaving the carb in the wrong direction not coming back ?                       

RE: Fuel stand-off - intake reversion - what are the causes?

(OP)
I'm sure some is lost

RE: Fuel stand-off - intake reversion - what are the causes?

Have I missed something or have you NEVER removed the, any, exhaust stack/pipe/retrictions to see if the problem might lie there..??

RE: Fuel stand-off - intake reversion - what are the causes?

I believe they use 1.90" diameter head pipes almost a meter long... no mention of Pat's suggestion to run a pair of 2.25" pipes 40" long, though.

V114 - IMO you've stacked the deck against yourself with your novel engine design.

What did you discover in the combustion chamber design?
Have you considered a dry-flow intake tract?

Si Vis Pacem Para Bellum

RE: Fuel stand-off - intake reversion - what are the causes?

V114
 Have you made any progress on the reversion or did you shelf this project?

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources