backpressure

[3.4orNothing]

hi people,

i hear that when you change your cat back you will waste more gas because of the lost backpressure, i have straight pipe no cat and flowmaster and i think i miss a little low end torque and fuel loss

why do we rely on back pressure and the funny cars and dragster run open headers and they dont need backpressure just open pipe??

[BLLDOGG]

back pressure is a myth. you will notice a CHANGE is torque cause it will move up your power band. no loss just moved. the loss in fuel is your imagination or i heard a few days back in the exhaust section that headers make you run leaner. so maybe all exhaust work does it too

[HBHRacing]

yup. I agree with Blldoggs statment for the most part. usualy the only time you need worry about backpressure is when you have too much IE a colged cat

[3.4orNothing]

Originally posted by BLLDOGG:
back pressure is a myth. you will notice a CHANGE is torque cause it will move up your power band. no loss just moved. the loss in fuel is your imagination or i heard a few days back in the exhaust section that headers make you run leaner. so maybe all exhaust work does it too

thanx for your answer
you are right about the moved torque range
because i have a good top end but i miss a little low end since i changed the cat back
and with all my bolts on i did only a 17.77 e.t. at the track i have moroso air intake, msd coils and wires,accel plugs ,no cat and flow master..... was that bad e.t. because of the moved torque range??? should I put a catalyst to gain more low end???

[camaro_speedemon]

Adding a catback will increase your fuel economy, provided you stay off the gas.

A free-er flowing exhaust moves your torque higher in the RPMS. Low end torque does jack crap for racing. Think about it, when you're racing the only time your in low RPMs is when you launch, from then on out, you're staying high up in the revs. You the most power in the RPM range where you spend the most time.

When you're racing you'll never be below 3k RPM after launch, so why would you want a few more tq at 2k RPM?

[ReodDai]

I have dynos to prove with a cutout you gain horsepower and torque lol. However the peaks are shifted up a couple 100 rpm.

[OneQuickV6]

Originally posted by camaro_speedemon:
Low end torque does jack crap for racing. Think about it, when you're racing the only time your in low RPMs is when you launch, from then on out, you're staying high up in the revs. You the most power in the RPM range where you spend the most time.

Some of this is true, but not all! Low end torque is very important!! In a 1/4 mile race, you're going to win or lose it in the first half of the track. Your driving skill, suspension setup, and low end power make for a great 1/8th mile time. The second half of the race is just waiting for the line.

The 60' time and 1/8th are very very very important, please do not spread anymore information like this.

[HBHRacing]

^ agreed

[TXSIX]

Right, but how much difference does an exhaust system make when it comes down to 60' times?

[ReodDai]

Normally exhaust decreases your 60' ft if the car is stock before hand lol.

[HBHRacing]

there is a LOT that goes into exhaust theory. but really it can be broken down into a fundemental issue. the motor is like a big air pump. if your trying to pump out more air then you can pull in you lose power. Thats why a large exhaust on a stock low displacment car loses power. you can get back some of it by adding more air to the motor.

think of it this way.

look at a deseil.. VERY large displcment and a ton of air exiting the motor. and for this reason that have very large 4 inch+ exhaust.

now take something like a small honda. low displacment and very little air exiting the motor and they have a tiny exhaust system

[BLLDOGG]

big exhaust doesnt pump air out, it releases it. so...

Originally posted by HBHRacing:
the motor is like a big air pump. if your trying to pump out more air then you can pull in you lose power.

is false. you dont lose power

[HBHRacing]

I'm puting it in lamens (sp) terms. like I said. there is a lot to it. I could go on and on and on.. but it would not get us anywhere

[mjparme]

big exhaust doesnt pump air out, it releases it. so...

However, the piston is pumping out the air on the exhaust stroke so the engine is a big air pump. Having a bigger exhaust decreases the work the engine has to do to pump the used gases out (backpressure).

is false. you dont lose power

Everything HBHRacing said sounds correct to me. Care to expand on your very vague statement that his post was false? Like maybe providing supporting facts.

[OneQuickV6]

If I'm noy mistaken, the piston isn't forcing the exhaust out of the cylinder, it is actually retreating from TDC as soon as the exhaust valve opens. So really, the burnt mixture leaves because of the difference in pressures. It wants to go from where there is high pressure to where it's lower. This is not a detailed scientific explanation, rather just clarifying the way the motor works mechanically.