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
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??
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??
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??
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??
RE: longer exhaust duration... why and when??
RE: longer exhaust duration... why and when??
any thoughts on carb v's efi?
RE: longer exhaust duration... why and when??
Regards
Pat
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RE: longer exhaust duration... why and when??
- Steve
RE: longer exhaust duration... why and when??
RE: longer exhaust duration... why and when??
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??
RE: longer exhaust duration... why and when??
If something like that was employed, I would think longer exhaust duration might be of use.
RE: longer exhaust duration... why and when??
RE: longer exhaust duration... why and when??
* 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??
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??
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??
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??
"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??
RE: longer exhaust duration... why and when??
Data?
State of the art 1967 ?
http://w
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??
RE: longer exhaust duration... why and when??
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??
http://w
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:/
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??
RE: longer exhaust duration... why and when??
RE: longer exhaust duration... why and when??
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??
So is this discussion about street legal engines or racing applications?
RE: longer exhaust duration... why and when??
Rod
RE: longer exhaust duration... why and when??
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??