Does exhaust affect low end torque?

Gary Farmer · Apr 11, 2016

Gary Farmer said: ↑

Good post Keith! Thank you for your contribution.

Did some number crunching with the 1,2,3 punch combo a bit modified for max effort, to show everyone what the potential is.

This is within the Dyno program simulator, so take it for what you will. I've (and others) found it gives a pretty decent general idea.

I took the Crower level 2 cam, put it at 110 ICL (this will be 2* retard for those using a typical timing gear, since 4* advance is incorporated into the cam by default), or 2* advance using a wheel.

This location helps to raise the powerband slightly over the default 4* advance position, which seems to work best even with the 2 barrel, and puts it closer to the OEM cam's powerband.

I then used a static compression of 9 1/3 :1 (9.33:1) to put dynamic at almost 7 3/4 :1 (7.73:1) for optimal use with 93 octane pump gas, leaving a margin of error and safety based on elevation, atmospheric changes, fuel quality, etc. using the now 60* intake valve closing point (up from 58* at default location of 4* advance).

One main advantage of this is being able to use stock cast "10:1" pistons (the 10cc dish) without having to do much (if any) machine work to the engine, permitting the pistons to sit in the default location below deck. Cam can be positioned to suit the actual compression the build comes out to, to compensate if it comes out to be too high or low above target goal.

A 500 CFM 2 barrel is used (@3.00" vacuum) with the 'high torque' dual plane intake setting (which it is).

Small tube headers (1 5/8" for the Buick 350) with open exhaust is assumed for this measurement.

Heads are stock iron with stock valve size and mild port/bowl cleanup flowing slightly better than untouched heads.

VE is between 80-83% between 2000-4000 RPM.

Power figures will be flywheel.

Average torque output is 421 ft. lbs. between 2000-3500 RPM, peaking at 2500 RPM @428 ft. lbs.

Average hp output is 283 between 3500-5000 RPM, peaking at 4500 RPM @293 hp.

Power figures will be less without headers, but will exist closer to these numbers than an exhaust system without any scavenging effect.

This is what a large 2 barrel with good scavenging and a cam specialized (and installed specifically for) low-mid range power can do.

This is an effective and usable powerband between 2000-5000 RPM.

The stock cam won't perform quite as well as this, but it's not too far behind and trades hp for tq.

Gary
Click to expand...

For basis of comparison, I did the Federal Mogul CS647 cam using the same above formula, only with the same DCR for 71* IVC, making SCR 10.21:1.

I discovered that the discrepancies between the two cams (Crower level 2 installed 2* retarded vs default and the Federal Mogul cam installed at default 3.25* advanced @.050; 1.75* retard @.006) weren't as far apart as I had originally remembered.

It still swaps torque for hp, but not as much.

The results are 420 ft. lbs. @2400-2800 RPM peak, averaging 410 ft. lbs. between 2000-3500 RPM; with 297hp @4650 RPM peak, averaging 287 hp between 3750-5250 RPM.

It has a wider powerband vs the Crower level 2 cam, for a tradeoff of only -9.5 (average) ft. lbs. (but a wider torque band), and +4 hp.

Efforts put into increasing power can often result in a narrower powerband, which can ultimately result in decreased overall performance when the differences are small (with power numbers and similar powerbands). A wider poweband is much more desirable than a few more numbers, particularly on a street machine.

Add in the increased lifespan of the Federal Mogul cam (YES, this is TRUE, no matter how much you want to disbelieve it. Less strain on components = greater lifespan on those components), its ease of installation and less expensive (no special valve springs (or the machine work to install them) required, for one) vs the Crower cam, and you'll see which cam is better suited.

To reiterate, this is all done with a 500 CFM (@3.00" vacuum) wet flow 2 barrel carburetor.

I'm trying guys, honest, but the more I do the numbers, the more the finger points back to the CS647 cam.

If you want measurable differences, it appears it's back to the Crower level 3 cam, but then you're getting into the range where the 4 barrel (or fuel injection) becomes a greater gain and worth the move over the 2 barrel carb. Obviously, anything more than this will give greater gains, but then you're moving the powerband up into higher RPMs (and concentrating it within a tighter band) in order to make this power by moving more air using the iron Buick 350 heads--but these are limited unless you're a wizard with head porting.

Once the aluminum heads become available, things will change...

Just an FYI for anyone interested.

Gary

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Does exhaust affect low end torque?

Gary Farmer "The Paradigm Shifter"

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