longer exhaust duration... why and when??

[e21jps]

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

 

[Tmoose]

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.

 

[Lou Scannon]

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.

 

[WolfHR]

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

 

[Tmoose]

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.

 

[Lou Scannon]

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.

 

[L4189]

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.

 

[inline6]

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.

 

[dicer]

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?

 

[evelrod]

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

 

[Marquis]

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.

http://www.auto-scape.com

 

[Lou Scannon]

Very insightful commentary, Marquis!