Back pressure and torque

[John Hess]

> ----------
> From: 	Keven D. Coates[SMTP:keven at ti.com]
> Sent: 	Wednesday, August 27, 1997 6:17 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.
> 
Come on, use examples (i.e. a potato stuffed up the tailpipe would
DEFINITELY increase back pressure at higher RPMs and would NOT increase
exhaust velocity at ANY RPM).

> 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.
> 
Actually, it has more do with pressure waves traveling at speeds
governed by the diameter and length of the header than back pressure;
but, we will accept this explanation.

> 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
> 
	The whole intent of those of us who caution against
significantly changing the back pressure in an engine is that one really
needs to determine the usage (and the resulting requirements) before one
jerks any singular part of the system around.  Back pressure changes (on
a modern fuel injected engine) can have DRASTIC effects on what your O2
sensor THINKS it is seeing.  Indeed, low enough back pressures (on
unheated O2 sensors) can prevent the system from ever going closed loop!

	If one is simply measuring static back pressure, one is
obviously missing the point.  In an engine, what is sometimes referred
to as back pressure at high RPMs can actually be measured as a NEGATIVE
pressure at some specific lower RPMs (at the exhaust port).  The term is
commonly misused solely as a function of RPM.  Back pressure will
actually exert the same positive pressure at all speeds.  The tuning of
the port will determine if more pressure is placed on the source of the
back pressure, thus minimizing its results.