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

 

[evelrod]

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

 

[e21jps]

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

 

[evelrod]

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

 

[Lou Scannon]

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.

 

[dicer]

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

 

[e21jps]

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?

 

[patprimmer]

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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[SomptingGuy]

1D Navier-Stokes simulation comes in handy here.

- Steve

 

[e21jps]

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

 

[patprimmer]

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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[e21jps]

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

 

[WolfHR]

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.

 

[Lou Scannon]

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

 

[WolfHR]

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)

 

[evelrod]

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

 

[patprimmer]

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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[Tmoose]

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.

 

[evelrod]

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

 

[Lou Scannon]

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.