Myth or Reality, low backpressure=low torque

[doradox]

Being one of the "back pressure guys" in the poll, I'll give my thoughts...

1) As has been stated above, I think it's more an issue of semantics, not interpretation or analysis of technical data.

2) If "zero" back pressure is ideal, why have exhaust manifolds and exhaust systems at all? In fact, why don't all race cars have no exhaust systems?

My understanding is that you can "tune" the exhaust system using a combination of variables to maintain flow to help "scavage" the exhaust gases to achieve optimal performance within a specific power band. Like most things, one size does not fit all.

3) Then there is the more "aesthetic" nature of the sound produced and personal preferences. For example, I want a more restrained exhaust note, and don't want my car to sound like a high school kid's car... That will affect the design and effieciency of my exhaust system.

4) Lastly, there's the local noise law issues...Moving violations anyone?

From what I've seen, I believe there is as almost as much art as science to exhaust system design. Having performed fluid flow analysis in the past, I found that sometimes there are almost as many "assumptions" as known or predictive variables in the calculations.

Just my 2 cents...

To answer #2, you need an exhaust for several of the reasons you listed. With no manifold at all you can't tune the exhaust for maximum power. It's loud. It causes the failure of exhaust valves. The exhaust gasses go places you don't want them to.

The pressure is an unwanted consequence of designing a system that solves all the other issues. Every design solution is a compromise. It has to fit in the space given to the design engineer, not cost too much, not be too loud or heavy, tune for power at xxxx rpm, keep the cat converter hot, not be too close to combustibles, be easy to manufacture and easy/quick to install, and on and on. So, even though I create "backpressure" in my exhaust, It's something that comes from the other things I want. It's not something I purposely design in.

Steve

[John Coffey]

2) If "zero" back pressure is ideal, why have exhaust manifolds and exhaust systems at all? In fact, why don't all race cars have no exhaust systems?

Safety mostly. An open exhaust port would pretty much ignite anything in the engine compartment with a foot of the port. And if the rules allow it, most race cars run without an exhaust system, just the headers. Look at Top fuel, Pro Stock, etc.

[m4xwellmurd3r]

well also because afaik exhaust systems also help pull heat away from the head and valves. without them they burn up

[ozconnection]

I just checked up on this thread to see how it was going....and it doesn't seem to have gone anywhere for a while.

I was thinking about some sort of simple and practical 'test' that would prove this discussion one way or another.

Grab a potato. Cut it down so it fits snugly in the tailpipe and cut a smallish hole through it so that some of the exhaust gas escapes when the engine is running. Simple.

Of course, when the engine is running now, we have effectively increased 'exhaust system backpressure'.

Take the car for a drive and see how she runs.

Let us know how you get on!!

[Force Fed]

I think it's more a question of balancing the flow through the engine, than it is about pressures. Too much flow on one side or the other of the cylinder can cause problems or inefficiency.

[ozconnection]

Yes, that might hold some truth with forced induction.

[grantf]

i do not know if this pertains, but my dodge van was running really rough with little power until i found a hole in the exhaust manifold, for a quick fix I plugged it up with a bolt and some washers and now it runs great. I was thinking this might be a back pressure issue.

[Force Fed]

Chevy engines are the same, they run bad when they get an exhaust leak.

[ozconnection]

i do not know if this pertains, but my dodge van was running really rough with little power until i found a hole in the exhaust manifold, for a quick fix I plugged it up with a bolt and some washers and now it runs great. I was thinking this might be a back pressure issue.

If that were true, the car should've run better not worse. I believe its more to do with the proper pulsing inside the exhaust system which was restored by plugging the hole.

[grantf]

hu??? I do not understand, It dos run better! Pulsing???

[ozconnection]

hu??? I do not understand, It dos run better! Pulsing???

Look, I'm no real expert here and a lot of this stuff belongs in the physics lab in some university. What I do know is that there are powerful pressure waves travel up and down the exhaust system while the engine is running.

A hole somewhere in the exhaust system (where it shouldn't be) then dampens this 'good' pulsing. Also, a hole creates turbulent flow in the exhaust pipe or muffler. This turbulent flow will reduce the exhaust flow out of the engine, decreasing its performance.