exhaust backpressure

[blu3kgt]

ok i have a question... on my sohc all i did was cut the stock piping at the cat and then went 3" inch and then y pipe into 2.5 pipping with no mufflers... i love the way my car sounds, i thought it would be loud because of no mufflers but it sounds nice and deep.. but performance wise i did lose a hair of torque but i did get a lil bit of high end torque.. so i was wondering what role does backpressure play???wut does it do and why is it needed?? i wanted to gut my pre cats and put a high flow cat and keep no mufflers, would that make me gain even more high end torque and lose more torque.... or will i have to put mufflers on at this point because such low back pressure... the loudness is not a concern..

[AutostradaVR4]

this is actually something ive been looking into recently. from what i've learned, it's basically caused by some of the air/fuel mixture getting pulled strait into the exhaust, meaning less in the cylinder for combustion, meaning less cylinder pressure, and less torque.

basically, you should choose an exhaust based on the other components of the car, mostly the intake setup and camshaft/valve timing. Just like 700cc injectors wouldnt be effecient on a stock SOHC, neither would a 4" exhaust.

this is just what ive been reading. google "backpressure and torque" and some good things should come up.

[blu3kgt]

well wut you have stated does make a lot of sense but how would you explain gaining more high end torque like i did. before with stock exhaust the sohc had a lot of torque but it would stop pulling hard around 4500 rpms, now without the mufflers and resonators,i lost a little bit of low end torque but gained high end torque,it pulls hard all the way to 5000rpm now...

[2old]

I think most people oversimplify what is happening in there engines.

Back-pressure is bad. No if ands or buts...

The problems is that the pressure in a exhaust is not static, comes out in pulses in which it's frequency varies based on the engine rpm.

So if you imagine the pressure pulses as a wave (which it is... a compression wave that is) the high point of the wave is were the highest pressure is the the low point is the lowest pressure. The average between the two is the average pressure in the exhaust (shocking... it almost makes sense).

Now if you were to make the ultimate exhaust... you would want to design it so that you match the low portion of the exhaust wave to be present when the exhaust valve opens... This would help create suction out of the cylinder (this is called scavenging).

How you do this is you tune the exhaust so that the harmonic of the exhaust tubing match the harmonics of the exhaust (actually i think it's .75 of the harmonic but you should really look that up).

Unfortunately that if you do this you actually only tune the exhaust for that RPM... Hence why in racing you sometimes see "exotic" variable-length headers (this works on the intake side as well).

Now when RPM gets high, fluid-dynamics start to move away from laminar flow to turbulent flow. In which case the harmonics of the exhaust not longer dominate (although they still do for n/a intake). In which case that enemies to you is the resistance to flow length of restriction and the mass of the exhaust.

What I mean by this is the most horsepower your engine would ever generate is if there was no exhaust and it dumped to free air (at high RPM anyways). Therefore the ultimate exhaust would have lost no heat to the environment no back-pressure.

Therefore your objective is to get the exhaust out our your piping as quick as possible with the least amount of back-pressure.

Now we through in the concept of pipe diameter... The smaller the pipe, the faster the exhaust will flow and the larger you "low point" in your pressure wave, the easier it is to "scavenge" low at low rpm. The bigger the pipe the lower the back pressure the better the engine breathes at higher rpm (of course with diminishing returns).

Now believe it or not, this is the simplified version... There are actually even more factors that you have to take into account (like temperature density and viscosity changes as the exhaust cools in the tubing) and this affects why you will eventually see horsepower decrease if you go too large in exhaust diameter... But you should just go into engineering if you wanted to know that.

[AutostradaVR4]

gaddamn you know too much. what do you do for a living??? lol

so a theoretically "perfect" exhaust would essentially be a strait pipe that increases in diameter as the engine RPMs increase? in a way that its always harmonic with the engine?

and all this still goes right out the window when you bring a turbocharger into the picture right?

[Igovert500]

Once you put a turbo in the mix, it already creates more than enough backpressure itself...so ideally you would like no exhaust or hte biggest free-est flowing exhaust, after the turbo, possible

[2old]

Well in theory the best N/A exhaust would be a straight pipe that decreases in length as the RPMs increase and increases diameter to avoid turbulent flow (the objective is actually to create the maximum exhaust speed which creates a larger scavenging effect while still minimizing back-pressure).

You can pretty much assume that after the turbo the exhaust flow is turbulent and does not pulse so your only objective is to get rid of the exhaust and therefore Igovert is correct.

In front of the turbo the average pressure is much higher (and hence density). So much so that any scavenging effect is masked by the increase volumetric efficiency (IE: you get much more air in the cylinder from the increase in air density then you could by using the partial vacuum from scavenging)