TonyT3hSleeper

Hey Folks, Im doing some upgrades to my Stang, after getting most of the other kinks out of it other than cosmeic. When I got the car it had Ford Shorty Headders on it mounted to a custom strait pipped exhaust, just 2 pipes (one from each headder) all the way to the tips in the back, there are no cats nor mufflers on the car would i get a higher horsepower yeild from having an H or X or just leave the thing as is?

Mods to car other than the exhaust

-Advanced timing 12 deg
-Smog pump removal

ArmchairDeity

As I understand it, you'd get more HP from an off-road H or X pipe. That said, I'm not an expert on it... I just know what I've been told. The crossover connection between the two sides of the engine sets up a sympathetic shock wave in the exhaust so that each time an exhaust valve opens the exhaust already leaving the pipes helps actually pull the spent air/fuel out of the cylinder. This doesn't happen with straightpipes. On the other hand, this probably only has much effect on very highly tuned engines, like when you're trying to get 290HP out of a 4-cylinder NA Subaru engine.

Still, as I understand it, you'd get more HP out of a catless x or h pipe than you would out of straightpipes. Someone else will have to confirm this for sure.

AdderMk2

you will get some power from putting mufflers on it too.. straight pipes are NOT helping your car at all

WhiteWindsor

Not to mention straight pipes suck for sound. You will sound like every other GMC or Ford truck

ArmchairDeity

I never thought of that, but yeah... without the proper backpressure, your car won't run at peak efficiency, which means losing some HP. A full exhaust system from headers to h or x pipes to muffler and tail pipe is probably going to find you a good bit of missing HP.

This URL has a good explanation of the idea, the the way the rolling shockwave helps void the cylinders and the concerns behind how you set up your exhaust. It's about Porsche 944, but the concept applies to about anything with an engine:

http://warnertechnology.com/Cars/backpressure.shtml

83GT5.0

Whats the point in having a decent amount of backpressure? Isn't the main point to get the exhaust gasses out as fast and efficiently as possible?

ArmchairDeity

Yeah, 83GT5.0, but what "fast and efficient" actually mean is what's at question... if you read the link above, it makes a bit more sense. It's not "backpressure" per-se so much as it is the appropriate flow velocity for your engine at a given RPM range. The key point that's covered in that link is the presence of a low-pressure pocket just outside the exhaust port on the engine created by the previous exaust moving thru the rest of the exhaust system. When the exhaust valve opens again, that low pressure cell effectively sucks the spent fuel and air out of the cylinder, clearing it for the next cycle that much faster.

You don't get that barometric effect with dual straight pipes...

83GT5.0

So then this low pressure is somewhat like a vacuum that helps suck out exhaust instead of only being pushed out?

AdderMk2

in a way, yes. Its caled the "scavenging effect".

as each exhaust pulse moves down the piping, it creates a slight vaccum behind it, pulling the following pulse along. That is also why equal length headers give an engine more power than unequal length.

golden_eye

Lots of good info in this thread. Lots of people are out to eliminate backpressure but don't realize that some is needed to help the engine run efficiently. If backpressure was totally bad then running open heads with no exhaust manifolds would be the best, but it obviously isn't.

I remember reading one of the import tuner mags a while ago (I was on break at work and there was nothing else to read)and lost even more respect for them when I saw the letters to the editor section. Some kid was asking about exhaust on his Honda Civic or Integra or something and was asking what size piping to go with. The editor responded, totally serious, that since eliminating as much backpressure as possiblewas important he should use a 3" system from the header back. WTF [&:], I wouldn't run anything over 2.5" on an N/A V-8, let alone an N/A or lightly turbocharged 4-banger. Silly ricers [>:].