Exhaust backpressure

[JIMTRON]

Why do people think this is beneficial? Backpressure doesn't make torque

[Preen59]

Why do people think this is beneficial? Backpressure doesn't make torque

Backpressure aids in scavenging. Are you familiar with pressure waves?

Basically if you have no backpressure, the charge that escapes out the exhaust as the valve closes is lost. The pressure wave bounces back from the collector and 'shoves' the small amount of charge lost out the exhaust port back into the cylinder before the exhaust valve closes. This does 2 things:

1. It ensures (well helps to ensure) the maximum amount of charge is burnt, thus producing a higher BMEP, so more torque.

2. It insures a better more even cylinder fill, which, well refer to No. 1.

Yes, too much backpressure is a bad thing, because as the RPM increases, the exhaust has to flow more volume, and if it can't get out of the cylinder, the charge doesn't get into the cylinder.

Everything is a balance. Backpressure is (i'm pretty sure) part of "everything".

:)

[Jarred]

Backpressure isn't all of what it's cracked up to be. but it's all relative.

gas speed is more important, as is maintaining a constant flow.

It's all relative to speed and pressure and temperature, and a whole lot of thermodynamics is involved.

[Preen59]

Backpressure isn't all of what it's cracked up to be. but it's all relative.

gas speed is more important, as is maintaining a constant flow.

It's all relative to speed and pressure and temperature, and a whole lot of thermodynamics is involved.

It's just as important as everything else. In reference to NA engines, anyway. Forced induction engines are much, much less reliant on backpressure because they scavenge better.

[gldgti]

It's just as important as everything else. In reference to NA engines, anyway. Forced induction engines are much, much less reliant on backpressure because they scavenge better.

there are VERY FEW people on the forum who could put it so eloquently and correclty as you have preeny.

i vote STICKY and LOCK!

[Preen59]

there are VERY FEW people on the forum who could put it so eloquently and correclty as you have preeny.

i vote STICKY and LOCK!

Haha thanks man. I've tried not to flame anyone in doing so. :)

It's all about the learning. ;)

[JIMTRON]

Backpressure aids in scavenging. Are you familiar with pressure waves?

Basically if you have no backpressure, the charge that escapes out the exhaust as the valve closes is lost. The pressure wave bounces back from the collector and 'shoves' the small amount of charge lost out the exhaust port back into the cylinder before the exhaust valve closes.


I am familiar with pressure waves, exhaust pulses have low pressures following the initial high pressure wave. It is the low pressure at the end of the pulse combined with the velocity of the exhaust that scavenges the burnt gases from the cylinder. Also, as the opening of both intake and exhaust valves overlap, the low pressure created in the cylinder assists in drawing in the intake charge.
It is not backpressure you want, but exhaust velocity. Unfortunately, to get a higher velocity, you need a smaller diameter exhaust, which gives higher backpressure. This I am pretty sure is where the balance comes into play.:)

[Guy_H]

I suppose it really depends on exactly what you are trying to achieve - design the exhaust from the exhaust valves out, but do you want a torque low motor, or a high end high flow (dragster straight out of the heads job!). When I was at VW racing a while ago, they had several different configuration headers (for the 2.0 16V race engine) depending on where the car was going to run (type of course) so they could tune the powerband to suit!

[twin eng, twin turbs]

Great technical knowledge coming out in this topic. :biggrin:

Another part of the back pressure equation in the interest of scavenging is the design of extractors, a good set of 4 into 1 tuned length extractors will have a collector in the firing order of 1,3,4,2 rotating the exhaust gases as each runner draws the gas from the next one out. The gases spiral down the exhaust as they merge together.

This is barely scraping the surface of the scientific research involved but a really good racing exhaust is designed with the combination of camshaft profile, intake manifold length and diameter, bore and stroke etc. Its all relevent...
I had a book with the formulas required to design a tuned length set of extractors and the diameter of them and the system aswell. Manufactured correctly you can move your power and torque up and down the rev range to a degree.