Regarding Backpressure - United Bimmer Community

**Joey** · 04-24-2005, 09:11 PM

If backpressure is, as I understand it to be, essentially a clogged airmass in the exhaust stroke of a piston, and it causes the pistons on the opposite crank position to meet with resistance, how does that result in improved fuel economy?

I ask because it would be fairly easy to cut apart my muffler, catalytic converter, and intake box and modify them to be much, much less restrictive, but I know they're not that way from the factory for a reason.

**komodo** · 04-24-2005, 09:38 PM

Quote:

Originally Posted by Joey

If backpressure is, as I understand it to be, essentially a clogged airmass in the exhaust stroke of a piston, and it causes the pistons on the opposite crank position to meet with resistance, how does that result in improved fuel economy?

I ask because it would be fairly easy to cut apart my muffler, catalytic converter, and intake box and modify them to be much, much less restrictive, but I know they're not that way from the factory for a reason.

Less low end torque. Back pressure creates torque.

When I removed my cats, it hurt my speed off the line a bit, but made up for it greatly after I got above 40mph or so.

witeshark · 04-24-2005, 10:09 PM

Would you need to change the engine computer so it's adapted to the new back pressure?

**Addicted?** · 04-24-2005, 10:17 PM

i hear mods such as pullies can help you get some low end power back

David

**Joey** · 04-25-2005, 10:46 AM

You mean underdriving your accessory belts?
That makes better use of the power you're already producing (through the drivetrain at least), so I can see that. I wonder how pushing against a motor would generate more rotational force.

Edit: This explained a bit.

**the afro guy** · 04-25-2005, 11:50 AM

changing the pulleys in a naturally aspirated car will do bugger all.
an engine needs a certain ammount of back pressure to crate torque, too much back pressure and the engine will be struggling to put out much power at all and if there isn't enough its the same story.

one way to get near the optimal backpressure is to go for a full exhaust system - extractors and everything. or you can just make small improvements such as a cat back exhaust which is whati have. mine is a 2 1/4 inch cat back system, i think the engine feels a bit healthier for it, seems to be doing less work for hte same results.

back pressure isn't really something to worry about unless you a building a very high performing engine though.