Backpressure and torque

[James Boughton]

For an excellent reference on this subject try "The
Scientific Design of Exhaust and Intake Systems."
All of my belongings are in boxes from moving so
I don't remember the author, but it is worth reading.

Jim Boughton
boughton at bignet.net

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From: 	Keven D. Coates[SMTP:keven at ti.com]
Sent: 	Wednesday, August 27, 1997 7:17 PM
To: 	Bronco Group; Tom Cloud; diy_efi at coulomb.eng.ohio-state.edu; EEC-EFIList
Subject: 	Backpressure and torque

Tom wrote,

>I keep hearing that the lack of exhaust back pressure
>might be the cause of my lack of low-end torque (which
>I can get back by advancing the timing, BTW) but then
>I've read that relieving exhaust back pressure **never**
>hurts low-end torque -- only that the engine needs to be
>set up for it .... that removing exhaust back pressure
>**always** increases torque and hp at **all** rpm's --
>and I've seen dyno curves to support that claim.

I can't help but involve myself needlessly in a purely theoretical claim, so
here it goes:

I would disagree with the "always" and "all" words, with the clarification that
what causes backpressure at higher rpms will sometimes mean higher exhaust
velocity at lower rpms.

When I had the stock '68 manifolds on my mustang (with a big cam, performer
manifold, etc.) it had more backpressure for sure, but there is more.  It had
small exhaust manifold ports, and so had very high exhaust velocity.  This
caused it to idle much better, and very low rpm torque was slightly better.
When I switched to 1 5/8" headers, the idle and very low rpm suffered.

Why?  Small primary runners, such as a a stock exhaust manifold, will create
back pressure at higher rpms, but at lower rpms will create negative pressure
waves from the high velocity of the air pulses going into a small runner.  So,
many stock manifolds will actually reduce backpressure at lower rpms
(comparitive to a larger tube system), and therefore cause more torque at those
rpms when compared with bigger primary headers.

Back pressure can also cause more torque at lower rpms by slowing the fuel/air
mixture from crossing right from the intake valve to the exhaust valve during
valve overlap.  This causes the same affect as closing the exhaust valve
earlier.  It will increase gas mileage (at that rpm) and torque by filling the
cylinder more completely and not wasting as much fuel/air.  

Ferrari decided that an adjustable backpressure system was preferable over the
increased complexity another system in their 355 (the one after the testarossa,
whatever number that was).  They found that they could increase low rpm torque
through adjustable backpressure.

Unfortunately the other affect of backpressure, whether it is caused by small
primaries or other restriction is killing all higher rpm operation.  My engine
would barely rev to 5500 rpms with the '68 manifolds and the single 2" exhaust.
After I replaced it with headers and dual 2.5" pipes, it would happily rev to
6500 rpm, with much more high rpm power.

The same is true of intake manifolds to an even greater extent.  The longer
high velocity runners of the stock mustang manifold will almost always will low
rpm torque battles with any bigger tube/shorter tube manifolds because of the
high velocity ram effect of the intake pulses.  But, it is too restrictive at
higher rpms to make the horsepower of some of the other intakes.

It's all a trade off, as usual.  Well, that's it for my theoretical soap box.  
Hope you all enjoyed it!

Best Regards,
Keven Coates
'68 stang
Texas Instruments