Backpressure

SWB · Post by **SWB** » Wed Sep 02, 2009 1:11 am

If you have the wrong valve sizes, cam profiles, or basic tuning, then back pressure will probably increase the power output of the engine. The error occurs when someone uses that as proof that "some engines", or even so much as "all engines" run better with some back pressure.

The next question is, how are we determining on a dyno whether or not a header or exhaust system is working better? If you installed a system with greater back pressure and that is what your improperly tuned or assembled engine combination wanted, and it picked up, is that the system/header to use?

I'm guessing the right way to measure it would be with the airflow data (cfm and bsac).

Piledriver · Post by **Piledriver** » Wed Sep 02, 2009 5:01 am

My guess is that the "backpressure is required" fallacy came from folks who put an open header on and/or a low/no restriction exhaust on and didn't know they needed to rejet...

Procision-Auto · Post by **Procision-Auto** » Wed Sep 02, 2009 6:52 am

Piledriver wrote:My guess is that the "backpressure is required" fallacy came from folks who put an open header on and/or a low/no restriction exhaust on and didn't know they needed to rejet...

That, and not understanding tuned lengths of exhaust for the RPM range
the engine is used imo.

chevy_power · Post by **chevy_power** » Wed Sep 02, 2009 4:47 pm

I tried to explain to them both that the burned valves was a direct result of not rejetting the carb to compensate for the MORE efficient exhaust system. they told me it was because of cold air coming up the header. What's sad is this teacher is instructing people that don't know much and they believe him. Then they will run around saying, "I learned that an engine needs backpressure in my class so i KNOW that it is true." There's no hope

dwilliams · Post by **dwilliams** » Wed Sep 02, 2009 10:52 pm

A lot of the "backpressure is necessary" stuff comes from the flathead Ford days. If you ran dump tubes (primitive headers) the exhaust was basically an open megaphone... and power would drop when the carburetion went lean.

You can check your own backpressure. Drill a 3/8" hole into the exhaust pipe just behind the collector, screw a 1/8 NPT oil pressure gauge adapter in, and use a foot or so of 1/8" copper line before going to rubber hose. Run the hose into the cockpit and use an ordinary fuel pressure gauge. Some restrictive OEM exhausts are over 10 PSI at WOT.

Some late model street cars have a convenient test port ahead of the catalyst.

SWB · Post by **SWB** » Thu Sep 03, 2009 12:02 am

chevy_power wrote:I tried to explain to them both that the burned valves was a direct result of not rejetting the carb to compensate for the MORE efficient exhaust system. they told me it was because of cold air coming up the header. What's sad is this teacher is instructing people that don't know much and they believe him. Then they will run around saying, "I learned that an engine needs backpressure in my class so i KNOW that it is true." There's no hope

Some of the WW2 fighter planes used exhaust stacks that were about 3" long. You might bring that up to him and see what he thinks. Those might have been water cooled engines, but even still, it kind of dispels the "air coming up the header" theory.

Maybe ask him if he's ever been to Reno, because all the unlimited planes run that as far as I know. Maybe they should turn their stacks around (facing into the airflow) to generate some back pressure? I wonder if that would help them go faster?

MadBill · Post by **MadBill** » Thu Sep 03, 2009 9:39 am

dwilliams wrote:...You can check your own backpressure. Drill a 3/8" hole into the exhaust pipe just behind the collector, screw a 1/8 NPT oil pressure gauge adapter in, and use a foot or so of 1/8" copper line before going to rubber hose. Run the hose into the cockpit and use an ordinary fuel pressure gauge. Some restrictive OEM exhausts are over 10 PSI at WOT.

Some late model street cars have a convenient test port ahead of the catalyst.

If you try this on a high compression engine with headers and a big cam, be sure to have a clamp or vice-grips to squeeze down the hose to act as a restrictor, thus damping out the extreme pressure pulses. Otherwise the gauge needle will slam from stop to stop until it destroys itself. (Don't ask me how I know this

)

Warpspeed · Post by **Warpspeed** » Fri Sep 04, 2009 3:53 am

I think we are talking at cross purposes here.

A "tuned" length exhaust system has to work at certain optimum pipe velocities (and optimum lengths) in order to generate the highest negative return suction wave during valve overlap.

Pipe tuning is a very powerful tuning technique, and YES you can make the primary pipes and collector too large a diameter to work properly.

An "untuned" system as used on about 100% of noise legal road cars, will have significant exhaust back pressure flat out. If it had no significant back pressure, it would just be far too noisy, and make the EPA man really unhappy.

So basically production cars have fuel curves and ignition curves to offer best power and best economy with the standard quiet factory exhaust system fitted.

So along comes "Joe Hot Rod", and replaces the quiet factory exhaust system with a short piece of four inch drainpipe. The engine runs like crap down low, but goes like a rocket at a million rpm.

So Joe tells all his friends that his engine needs back pressure, because back pressure makes low end torque. He is absolutely sure of this, because he has already proved it to himself.

What Joe has not done is RETUNE the engine, both fuel and timing to suit his open four inch drainpipe exhaust.

It would probably run even better with a set of properly tuned primary pipes and collector. But Joe is poor.
He just runs the factory cast iron manifold with a four inch pipe and he cannot afford a full dyno tune anyway, He just wants low cost bolt on power..

So it becomes a well known fact (and urban legend) that exhaust back pressure is necessary for low end torque. Plenty of guys have tried this and know it is the absolute truth. Reduce the back pressure, with no other changes, and the engine does not pull quite so well down low.

Aircraft are a different kettle of fish. Usually the engine is surrounded by a minimum size very aerodynamic cover, and there is just no room for full length tuned pipes that will run best at maybe 2,200 rpm. So short exhaust stubs are very often used.... Exhaust noise is not usually a problem with aircraft, none that I know of run a muffler.

SWB · Post by **SWB** » Sat Sep 05, 2009 12:59 am

Warpspeed wrote: Exhaust noise is not usually a problem with aircraft, none that I know of run a muffler.

Good post, but actually a lot of light airplanes do run a muffler for noise abatement. Otherwise I'd say you're 100% correct.

I was just saying that they will run better with a tuned system instead.

Warpspeed · Post by **Warpspeed** » Sat Sep 05, 2009 2:10 am

SWB wrote:
Warpspeed wrote: Exhaust noise is not usually a problem with aircraft, none that I know of run a muffler.
Good post, but actually a lot of light airplanes do run a muffler for noise abatement. Otherwise I'd say you're 100% correct.

I was just saying that they will run better with a tuned system instead.

I am sure aircraft would indeed run a lot better with a properly tuned full length exhaust. But there are often practical aspects of such an installation that might outweigh the power advantage
.
In something like a twelve cylinder piston engined WW2 fighter aircraft, the aerodynamic drag of a set of 24 pipe zoomies that need to be around six to seven feet long, might outweigh the extra power.

But it would probably look and sound great !