JKM

Is it required in ANY engines? be it diesel , LP , gasoline , etc , 2 or 4 stroke, ANY engine, I had a discussion about this today , and i was told that back pressure was not required in any situation, is it true? and if so , why all the babble about backpressure with snowmobiles and what not?

Forrest Nearing

backpressure is not required, but what happens is if your exhaust is too large, you run into reversion of the exhaust gas... which BECOMES backpressure...

the exhaust needs to be tuned and sized to allow the gas to travel as quickly and efficiently as possible... going smaller (from too large) and using different primary lengths, collector styles, crossover styles, etc. will actually help scavenge or "pull" the exhaust out of the cylinder... or a kidney pipe on a 2 stroke...

on a turbocharged engine, there is plenty of pressure between the valve and the turbine on the turbo... you don't want any more!!! and there's no reversion once the exhaust passes that turbine... and there's no PULSES after it passes the turbine... so no crossovers, no chambered mufflers, no small exhaust to try to scavenge... once it leavs the turbo, LET IT BREATHE!

Forrest

AlpineRAM

On a 4 stroke the backpressure would not be necessary if you had a cam without overlap- but since all "modern" engines do have valve overlap reducing the backpressure from stock levels or changing the resonance behaviour of the exhaust can create a situation where fresh gas goes traight in the exhaust. Not too bad on a diesel, but on a gasser the fuel is already in the air so it's not too nice- add a catalytic converter and it starts getting expensive.
On 2 stroke engines the main concern is resonance. Since the fresh charge is pushing the waste gasses out it's kind of difficult to avoid A keeping old stuff in and B pushing fresh stuff out.
On a turbodiesel the fresh air is used to cool the internals in some cases by pushing some fresh air in the exhaust on purpose.

HTH

AlpineRAM

JKM

So , is backpressure ever REQUIRED? I just always hear people talking about exhaust modifications that will eliminate the backpressure , hence making the engine run poorly.

Begle1

I never exactly understood back pressure. I think that on a 4-stroke it only everr makes a marginal difference anyways.

If you have valve-overlap, which most engines do (i.e. all), a wee bit of back pressure is needed to keep the fuel from coming out the exhaust.
On a Diesel, I don't think they ever inject with the exhaust valve open, do they? Still, that turbo has a LOT of backpressure, so you won;t kill anything.

Look at the P-51 Mustang and other old piston aircraft- they had 6 inch exhaust pipes and flew under grueling conditions for thousands of hours. I don't think it really matters any way.

It sure doesn't make that big of a difference for John Force...

SjLingenfelter

On a Naturally asperated car....if you dont have any back pressure and the motor is stock besides exhaust your valves can and will burn out. Now on the big block motors it a lil differnt but to make a engine last longer little back pressure is needed. Like said before cars with turbos on them dont need any more bends, chambers, etc because the exhaust goes from valve to turbine wheel.

Forrest Nearing

again... backpressure is not required nor is it desired...

but what people fail to realize is that too large an exhaust will cause reversion and or cause the exhaust gasses to stall in the pipe which CREATES backpressure!!!

backpressure is bad... ALWAYS... the less backpressure the better... but you can't just run without an exhaust system... OEM exhaust systems are designed for emissions and sound reduction... high performance exhaust systems are designed to cut backpressure and actually draw the exhaust out of the chamber (and intake charge in on overlap)

a properly designed race engine with proper intake wave pulse tuning and a proper scavenging exhaust will actually operate OVER 100% volumetric efficiency...

an engine is an air pump... backpressure is bad...

Forrest

signature600

So why did my friends 460 Ford burn 6 valves from running all weekend with no pipe after the collector?

I know there is probaly a reason besides no backpressure, but from what Forrest is saying, it can't be...

Could you try to explain a bit, Forrest?

Thanks,
Chris

Forrest Nearing

reversion...

never heard of burnt valves happening w/ an open longtube header... sounds like the headers might have been too big for the engine combo... or collector length too short...

Forrest

Forrest Nearing

if they were open shorties or manifolds, then burnt valves are pretty common... short primaries allow the cylinder to pull in fresh air through the exhaust valve on overlap.

signature600

Gotcha, so that's why my long tubes on the Chevelle with staight pipes is fine, huh?