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longer exhaust duration... why and when??

longer exhaust duration... why and when??

longer exhaust duration... why and when??

(OP)
Why do many engine builders insist on longer exhaust duration for racing cams?

what is the effect of power and torque doing this as apposed to cams of same duration?

The practise seems to be mostly used in older 2V V8 engines but with todays aftermarket heads and exhausts i think the port and exhaust design is not the reason.

So is this just habbit from when exhausts and head design was not so well refined? or is it something that is used to overcome other dimensional limitations or optimizations like bigger inlet valve size at the expense of exhaust valve size?

could it be anything to do with the use of carburettors instead of EFI?

In my experience I have always had better success with equal durations but I havnt played with any wedge pushrod or carburettored engines for a very long time

 

RE: longer exhaust duration... why and when??

Interesting timing for this post as we just finished the dyno tuning of our latest vintage 1600cc small valve DOHC Lotus twincam  engine.  First, let me say that I have had the benefit of knowing and dealing with several engineers, pioneers that I consider genius in the field of camshafts and what I've learned over the last half century plays a big part in what cam profiles I use.

The first set up was with a 283n/250x (at .050") set that made ~181hp/131tq...Excellent bottom and mid range power, less in the top end. 4000 to 7500 rpm--an easy profile to drive on track.

Second setup was 276/280 @ ~185/130 with excellent mid range power and very good top end. 4500-8000 rpm. Great cam/compromise for our vintage 1966 Lotus Cortina.

Third setup, 256/256 @ ~150/140 (an old Cosworth L-1 grind from a vintage F-2 twincam) with really good bottom end and mid range with a tolerable top end. Would have made an excellent street engine by today's standards.

Last and, what we are now using, 270/270 @ 197.2hp/8100rpm and 135.2lb/ft/6800rpm.  Comes on around 5300 and pulls great to 8400 where we usually shift (rev limiter is set at 8800). Excellent cam as long as you keep the revs up. Not the best for a novice, but we love it.

With a DOHC engine you can push the split and timing event numbers all over the place, but as I have found, you cannot re invent the wheel and, the old tried and true will usually be the best choice.  There is simply little new in the automotive field that has not been tried before by someone in the last century.  
There are just so many ways to 'skin a cat'.  If you like to play around with cams (as I do) then, "have at 'er"...

Rod


   

RE: longer exhaust duration... why and when??

(OP)
Thats similar to my experiences with my 6cyl bmw race engine were the best results are always come from cams with equal durations or even slightly larger intake for a nice broad power band

but as per my original post so many cam grinders insist on longer exhaust durations and im wondering what engine design conditions would require this

RE: longer exhaust duration... why and when??

Compensation for restrictive exhaust system, cat, silencer, small dia tubing, stock manifold?  

I'm using an off the shelf Kent SP310 in my 1330cc Mini and it has a strange split to compensate for the five port head that gives something like ~276/280 and ~280/280 (it's cataloged as 310/310 ?)that seems to work better than anything I've used before. I'm getting 117hp @ 7400rpm at the wheels.  I have not had this engine on a engine dyno but a good guess would be a gain over WHP of something like 16 to 20 %...Perhaps 135 to 140 hp.?

That's a couple things that I can think of, off hand.

Rod

RE: longer exhaust duration... why and when??

Just an uninformed observation here, but you see a lot of the old school V8 grinds, both factory and aftermarket, with split duration.  Somehow I seem to recall that opening up the exhaust duration buys you top end (maybe this is with a restrictive on-road exhaust system) without sacrificing bottom end too much, whereas increasing intake duration directly impacts bottom end.  Not to say that is the best recipe for ultimate peak power.

RE: longer exhaust duration... why and when??

I would also say na vs forced induction like turbo charging plays a role too.  

RE: longer exhaust duration... why and when??

(OP)
Im referring to na applications, i can see where turbo or supercharging would have quite different requirements and could well require different durations

any thoughts on carb v's efi?

RE: longer exhaust duration... why and when??

So long as the air flow with the carbs or EFI is the same, how will the exhaust know how the fuel was metered. Tuned length induction will influence the effectiveness of valve overlap, but that is equally true for a modern EFI tuned length manifold or a tunnel ram or Webbers or SUs or whatever on IR manifolds.

Regards
Pat
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RE: longer exhaust duration... why and when??

1D Navier-Stokes simulation comes in handy here.

- Steve
 

RE: longer exhaust duration... why and when??

(OP)
regarding the exaust not knowing what the intake is doing i was more thinking of the intake duration possibly needing to be reduced due to the charge having to navigate things like venturies, plenims and unequal length runners etc as apposed to fairly large open disign with longe equal runners typically used in EFI

RE: longer exhaust duration... why and when??

Have you ever compared a big set of Webber on IR manifolds or twin 4 barrel Holleys on a tunnel ram to a single point EFI system.

Sure, EFI is generally more modern in manifold design, but that is due to individual designs, not a feature of the system as such.

Regards
Pat
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RE: longer exhaust duration... why and when??

(OP)
yes thats what im getting at, sure there are carburettor set-ups that are more like efi but in those cases is an equal duration better??? but most situations where longer exhaust durations seem to be used is when a carburettor is feeding into a single plenim such as your typical american V8  

RE: longer exhaust duration... why and when??

I've heard that motorbike engines are nowdays using tuned exhaust lengths and geometries (similar to 2 stroke engines). The explanation I was given is that they're supposed to let some of the fresh charge 'escape' into exhaust and returning pulse rams it back into the cylinder, giving a slight increase in amount of charge. I'm in no way an expert, but it does sound plausible...

If something like that was employed, I would think longer exhaust duration might be of use.

RE: longer exhaust duration... why and when??

WolfHR, the exhaust ram-back effect you describe may be occurring in such applications, but only in one or possibly two narrow rpm bands.

RE: longer exhaust duration... why and when??

Yes, I was assuming they'd tune it near max power revs (or slightly below) since at lower rpms this reflected pulse would hit the valve too soon and eventualy ram in some exhaust gasses, whereas at higher revs there would be still slight 'positive' effect of earlier exhaust valve closing (pulse hitting the valve before it's closed, but having no time to ram in the charge), and after that point there would be no beneficial effect*.

* this *ass*umption/simplification was based solely on revs and excpecting relatively equalish exhaust gasses velocity (of which I have no clue)

RE: longer exhaust duration... why and when??

Referring 'inlet design' and 'exhaust tuning' v 'cam timing'...That's a total can of worms. Anti reversion, sonic tuning, tube size/length and a variety of camshaft timing applications have been done, by the forum members many years ago.  I remember contributing to the conversation/discussion but I don't know how to access such an old thread, perhaps ten years old. Perhaps Greg or Pat P. may know how to find it.

From memory it would seem the better prepared/designed race engines would not need a "crutch" in the form of "split timing" which is NOT to say I have not used such to good effect in the past.

These questions have always left me with that "itch that I cannot scratch" feeling. There is ample 'anecdotal evidence' that 'proves' just about every application I can think of. I doubt I will ever be totally convinced that I am "there". Maybe it's that "what if" that I just cannot shake.  Somehow I don't think that I am alone with that particular affliction...especially here in the forum.

Rod

RE: longer exhaust duration... why and when??

Rod

By split timing are you referring to different timing between inlet and exhaust, or are you talking different cam profiles on different cylinders to compensate for different port layouts from cylinder to cylinder or even with symmetrical port layout, differences in time between firing for adjacent cylinders because of limited possibilities in firing order on say a typical V8 where adjacent cylinders typically end up with some at 90 deg and others at 270 instead of all on the same bank being 180 apart.  

Regards
Pat
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RE: longer exhaust duration... why and when??

Hemi wrote "the exhaust ram-back effect you describe may be occurring in such applications, but only in one or possibly two narrow rpm bands. "

two smokers with long tapering diverging and converging cones mounted the "right" distance form the exhaust port can achieve a decently wide powerband even without variable exhaust port timing trickery. That suggests to me the "suction" wave and the inverted "plugging" pulse are both being managed pretty successfully.

This dyno chart of a 250 cc/ 15 cubic inch 70s motocrosser was made on one of the only dynos I believe.  The Schenck eddy current dyno that started at Webco, moved to Cycle magazine, and may now reside at Kerker.
http://czmadness.net/articles/CycleMay73-06.jpg

I'm guessing megaphones on 4 strokes might help similarly.

RE: longer exhaust duration... why and when??

Both, Pat.  In the Mini the Kent SP310 the 'SP' is Scatter Pattern where there is different profiles for each cylinder. Seems to work well enough for this application.  I've found the practice less than perfect in other cams, however.

"Split timing"...In the DOHC heads it's pretty easy to vary the profiles by changing either cam individually, thus some of the odd timing specs I quoted.  I did have a noted cam grinder do a split pattern on a single stick, to approximate the DOHC timing I was using on a TRD 2TC. That was in '97 and I got 'disinterested' in the project and the thing is still sitting on an engine stand in the corner of the shop...one of these days.

I can't address the V8 situation other than what I've read.  I'm not well versed in the variation of the V8 cam profiles from cylinder except that it seems to be rather commonly practiced by the big engine builders.

It really gets difficult when you take into account that simply advancing/retarding timing with two cams is time consuming but then add in varying the split...On single sticks I see the process involves varying the centers to achieve the same results as simply moving the individual cams on a DOHC setup.  Man that looks tough.

Now that I got through all that, you still need to make the In and Out match all the rest and one reason I heard a noted NASCAR engine builder claim that they had thousands of dyno hours on an engine design...and...that was over 30 years ago.  No telling what goes on today.

Add to all that---After all the years of doing it myself, we are using the cylinder head from a noted builder (that matches the very best flow of my old Brian Hart) and cams and engine assembly by one of the best Lotus engine specialists. I'm impressed by how much things have changed/stayed the same.  Gettin' old, Pat...Gettin' old.

Rod

RE: longer exhaust duration... why and when??

Tmoose, I'll read the article when I get a moment, but first, I have to ask, is there any data for that applicatoin that isolates the effect of the exhaust pipe tuning vs all the other VE tuning variables?  Probably nothing available publicly, anyway.  A wide powerband shows that they've done a nice job of combining port timing, ram tuning, in-cylinder flow, and whatever other control knobs they might have; it's not all down to the exhaust pipe design.

RE: longer exhaust duration... why and when??


Data?  
State of the art 1967 ?
http://www.bridgestonemotorcycle.com/documents/crankcase_volume6.pdf

Several SAE papers in the 60s/70s investigated intake port timing, manifold length, and other individual details although at relatively low rpm.  A few are posted here, plus some tech articles and the two stroke tuners guide by the late, great Gordon Jennings.
http://www.bridgestonemotorcycle.com/documents.htm
The subtleties of transfer port aiming alone can have a profound effect on power band width.

As soon as expansion chamber science advanced, concerns over crankcase compression upon "delivery ratio" became nearly moot.  A well done expansion chamber can take a deeper breath from a large volume than a small one.

RE: longer exhaust duration... why and when??

Tmoose, that's a lot to wade through...  can you direct us to a specific chart of data showing the improvement of "delivery ratio" over a wide powerband, via port tuning as an isolated independent variable?  I'm not doubting you, I just want to see some relevant data.

RE: longer exhaust duration... why and when??

Hemi, come to think of it it makes sense that it would have wider rev band than it's given credit for (my failing when considering it was that I was looking at it as some sort of 'wall' travelling through the pipe, whereas I now think it would be better described as a wave which causes constant change in pressure along the exhaust pipe)...

As for specific data, I think one will be hard pressed to find anything as complete as given in Blair's "Design and Simulation...". I don't know if I'm allowed to post an excerpt from it (I think with green brigade rampaging, any discussion of two-strokes is nowdays purely academic)- in the chapter 'Empirical Assistance' he analyzes wave reflecting in rather complex racing exhaust and gives plots of exhaust and cylinder pressure vs. crankshaft angle at several points within powerband (say 8-12+ k rpm). Unfortunately, there is no data for untuned exhaust to compare it with*. I do not want to break any rules, but if I'm told it's OK, I'm willing to post those few pages for purely "academic purposes"...

* and anyways, the data is as good/reliable as the guy interpreting it- and in this case you *don't* want to rely on my judgement

RE: longer exhaust duration... why and when??

crankcase compression ratio's inluence here here, with some intake and exhaust stub thrown in, at uselessly low rpm.
http://www.bridgestonemotorcycle.com/documents/crankcase_volume6.pdf

fig 8 and 9, page 4 of 17.  DON'T LOOK at any of the other pages or pictures.

==================
intake length with stub exhaust pages 92 thru 94
http://www.chuckbunnell.com/kart/jennings/Jennings12.pdf

highly doubtful the result would be the same with a real exhaust attached.
crankcase compression - pages printed 96 thu 98

I think on 2 strokes of that era adding greater transfer port timing (which must add to case volume, and thus reduce compression ratio) could be counted on for more and "better" power.  

 

RE: longer exhaust duration... why and when??

Tmoose, I appreciate your effort to clarify but I'm still stumped.  If I correctly found the figures you referred to, the independent variable (on the x-axis) is "crankcase clearance volume", so it is not obvious how exhaust pipe length tuning can be extracted from the data to infer its effect on the width of the power band.

RE: longer exhaust duration... why and when??

I thought that the object of the tuned exhaust was to plug the exhaust port just as the intake port so that all that mixture couldn't escape before the exhaust ports closed. That's the big advantage of the uniflow design.

RE: longer exhaust duration... why and when??

i would have though a larger exhaust duration compared to inlet would be a result of an underperforming exhaust port.

Some of the ealiest BMW 4 valve heads had exhaust ports that flowed extremely well and accordingly they used smaller exhaust durtion which is the opposite.

RE: longer exhaust duration... why and when??

Exhaust is always easy. Isn't that why the exhaust valve is always the smallest? I think the main problems are when more junk is added to the port flange back.
So is this discussion about street legal engines or racing applications?  

RE: longer exhaust duration... why and when??

The OP was about racing cams, to which I can speak with a small bit of experience. "Racing engines" suggest that everything related to camshaft profile/timing should be optimized to 'that' particular engine configuration, inlet/exhaust, etc.  For a 'street' engine I can see an almost unlimited amount of variables to which I am not qualified to address.

Rod

RE: longer exhaust duration... why and when??

The requirements for cam design optimisation for a V8 vs a straight six can be quite different.

There are many factors involved.

I also found that on the M20, M30 (nearly always about 260 period for intake and exhaust from the factory) and M52 straight sixes with FIXED cam phasing they tended to like equal durations.

However when you introduce variable cam phasing the optimised intake duration becomes shorter. A case in point is the old stock M52 with a 228 duration intake and 240 or so on the exhaust. When the engine uses dual independent variable cam phasing the optimimum exhaust duration shortens a little.

On old Mopar Big block wedge V8s from the factory the duration of the exhaust was always much longer than the intake. In this case it was more to do with the relative flow of the intake and exhaust tracts. The other thing on a V8 to bear in mind that folks here are losing sight of is that the cross plane crank V8 engine is uneven firing when you consider one bank and typically when you cam it for high performance you'll go towards more duration for both intake and exhaust and therefore more overlap which can cause interference from other cylinders on the exhaust side (anti-tuning regions). This is why careful attension is often paid to V8s and and the right optimised profiles for arrived at in terms of intake and exhaust durations.
Regarding the  above question in reference to carburetted applications I'll say that the carbd intake manifold has to overcome alot more compromises and there are significant penalties to be paid in terms of fuel flow distribution (cylinder to cylinder) and flow restriction compromise and tuned induction lengths. Higher overlap arrising from performance optimised cam profiles simply exascerbates these problems.

 

www.auto-scape.com

RE: longer exhaust duration... why and when??

Very insightful commentary, Marquis!

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