mahhddgtp

I've heard things bassackwards from people about exhaust.. I'm asking here because the FI guys know engines THE BEST out of any other section on the site.. Yes, I did a search.. I'm just not sure if I was using the correct words

Okay.... Can someone explain why you lose gas mileage when people run with exhaust with lower backpressure?

And is it ever bad to have no backpressure, and why is it harmful? Does it matter on the valve springs or size of the motor necessarily?

I've always wanted to have this cleared up... I understand how backpressure can actually help people spool up and stuff when it comes to turbos...

Once again, I did a search.

andereck

Gas mileage can be affected because a well performing exhaust makes room for more air/fuel mixture. More fuel used is of course less mileage. Basically the engine acts like its larger.

Backpressure is different than velocity, though both could be affected at the same time. If your system provides minimal backpressure while maintaining high velocity in the primary tubes then you have the potential to move more air and fuel. Velocity creates the low pressure area in the upstream exhaust ports which starts the intake cycle during overlap when there is basically no piston speed.

Decreasing backpressure via a major increase in area hurts velocity and could result in a less than optimal cylinder fill.

Massive tailpipes can hurt performance by presenting too much area to the atmosphere which results in more force pushing back on the exiting exhaust.

BIG BAD BLACKSS

iTrader: (4)

You mean the 14" ricer tips might actually HURT performance!

mahhddgtp

Sounds good.. Can anyone add anything?

What do you mean by less than optimal fill? Run leaner?

BIG BAD BLACKSS

iTrader: (4)

The cylinder cant fill with as much of a fresh air/fuel mix cause theres still exaust gasses in teh chamber that havnt been evacuated.

Basically better velocity helps suck the spent gasses out of the cylinder allowing for a better fill of fresh air fuel. As you know you cant burn exaust fumes, so the more fresh air and fuel that can get in the more power you will make.

mahhddgtp

So what would the differences be between dumping the exhaust at the:

Headers
Y-pipe
After Cats
Exhaust

any rule of thumb? And diameter of the pipe go? 2.5", 3", 4"?

I have an idea for each... But I'm not going to think about it to avoid confusion and so I can remember it easier.

lynx853

iTrader: (8)

you also have to remember, and it is true... when you have exhaust, your on the gas more... cause lets face it, who dosn't like the badass sound of high rpm's with longtubes!

BIG BAD BLACKSS

iTrader: (4)

I was told by someone (never confirmed it) that a 3" pipe will outflow 2, 2-1/2" While we are discussing exaust anyone wanna confirm this or tell me my buddy was on drugs. He was a ford guy, well lincoln to be specific.