more hp...on smaller exhaust?

[zachm611]

I was talking to a friend of mine who is probably getting a golf gti. and i told him to open up the exhaust a little. and he said no, he has to make it smaller to get more power. but i thought opening the exhaust would allow the engine to breath better and the turbo would spool faster with a bigger exhaust? and now im confused. is he right or am i just crazy?

 

[HIDPLANET]

Ive always thought that N/A vehicles need a certain amount of back pressure to operate efficiently. But for forced induction vehicles, a 2.5" or larger exhaust should be used.

 

[SupraMario]

Ive always thought that N/A vehicles need a certain amount of back pressure to operate efficiently. But for forced induction vehicles, a 2.5" or larger exhaust should be used.

yea, NA = Back pressure
Turbo needs flow.

 

[phoenix6]

N/A for sure needs smaller... look at my car, prev owner sticks a 3" on there, no cat.. stil on the car.. torqueless pos.

 

[Jeff Lange]

Bigger is not always better, and it's not really because of backpressure either, though people like to think that. If you have a really large exhaust on an N/A you get much lower velocity, with a smaller exhaust the exhaust will exit the vehicle faster. But it depends where you want your power band as well.

Exhaust tuning makes a much larger difference on an N/A car versus a turbo where you really just want a high pressure differential across the turbo's hot side.

Same principle applies to Toyota's T-VIS system, and why the 4A-GE with the intake ports 2/3 the size of the earlier one makes 20 more horsepower. (Well, not all 20 of that is associated with the head, since it has higher compression pistons as well, but higher velocity is good.)

 

[JustAnotherVictim]

On our N/a's the 3 inch makes the power band kick in much higher than with a smaller exhaust.
At least in my experience. I wouldn't really feel any pull until about 4k with a 3" compared to stock.

 

[Keros]

This is my take on exhaust systems, please correct me if I'm wrong...

On a turbo motor, depending on horsepower (i.e. ammount of heat being produced), the size of the exhaust can be too big. Lower horsepower motors will not be able to push enough heat out out of the cylinders (think BTU/cubic ft of exhaust, not so much higher temperature) to keep the exhaust gasses flowing at high velocity... cold exhaust=slow exhaust. By too big, here, I mean like rediculously big, +3.5"... 3" turbo back exhaust is never a bad thing.

A wider pipe will cause the exhaust to drop heat faster as it moves, slowing down the overall exhaust pulse velocity. So, bigger pulses and more pulses will keep more heat moving further down the exhaust system. Think of it as having a 15ft long copper pipe and flowing a trickle of hot water though it... it'll be pretty cold by the end of it... but then on that same pipe, flow a shit-ton of hot water though it, and it'll be pretty darn hot at the other end.

I think that on a Golf GTI turbo 4cyl... if it came with a 3" exhaust package and he dropped it to 2.5-2.75... maybe he could get slightly more power just through upping the exhaust velocity. But I think the better way to get more power is to get more air and fuel in the cylinders so that he can cram more exhaust (thus hotter exhaust, as that there's more flow in the same space, keeping more heat moving) down the manifold and out the tail pipe.

 

[MDCmotorsports]

This is my take on exhaust systems, please correct me if I'm wrong...

On a turbo motor, depending on horsepower (i.e. ammount of heat being produced), the size of the exhaust can be too big....

Its only too big when it can't fit under the car. :biglaugh:

Pre turbo? Yes you are on the right track. Too large of header tubing can make a turbo car sluggish. Ive personally seen a WRX respond faster on stock ported cast manifolds than a Borla Header. The reason? The header's tubes were too large of a diameter to keep the velocity up.

After turbo? Bigger is better.

 

[foreverpsycotic]

is the GTI a vr6 or a 1.8t?

 

[PROJECT N00b]

so and ideal size for nd Na would be 2.5"?

 

[Junior]

Pre turbo? Yes you are on the right track. Too large of header tubing can make a turbo car sluggish. Ive personally seen a WRX respond faster on stock ported cast manifolds than a Borla Header. The reason? The header's tubes were too large of a diameter to keep the velocity up.

After turbo? Bigger is better.

gotta say that I'm somewhat surprised by this honestly. I was always under the impression that a turbo provides more than enough back pressure for any engines needs (infact enough that it's costing you big power by blocking itself). And that charge velocity shouldn't be that important 'cause you're force charging it from the intake side.

the charge velocity theory is very sound tho, and your experience certainly speaks to verify it.

 

[MDCmotorsports]

gotta say that I'm somewhat surprised by this honestly. I was always under the impression that a turbo provides more than enough back pressure for any engines needs (infact enough that it's costing you big power by blocking itself). And that charge velocity shouldn't be that important 'cause you're force charging it from the intake side.

the charge velocity theory is very sound tho, and your experience certainly speaks to verify it.

A turbo does provide enough back pressure for the engine & cams. Hence the "bigger is better" post turbo.

Dealing with velocity...

Not only my experience, but a host of other turbo gurus as well. Take a look at Corky Bell's work some time. He will tell you the same thing when it comes to keeping higher exhaust gas velocities.

:biglaugh:

 

[Poodles]

can't go too big on a turbo, on an N/A it's a different story.

there is no such thing as backpressure. It's velocity and scavenging. A good header will have smaller tubes that increase velocity and make a vacuum to to the point of scavenging the exhaust gasses from the cylinder. THis makes for lots of fresh air and fuel resulting in great lowend torque.

This is another level of tuning that's expensive, and is usually done on engine dynos when people want to really build a car for a certian powerband. There is a limit to how small you can go, as it becomes a restriction.

Having open pipes without a collector of any kind can actually have LESS torque and powerband than a well tuned header and full exhaust.

The exact opposite is true for anything after a turbo, the shorter and larger the better...

 

[IJ.]

Look at single cylinder 4 stroke bike motors for an education in exhaust tuning (I used to build a megaphone system with an inverted cone on the end that gave great results back when I was a kid)

There are some nice gains to be had for naturally aspirated motors in this area but most headers are an interference fit compromise.

 

[Junior]

to really give you boys fits. What's your thoughts on exhaust systems for supercharged engines?

 

[Poodles]

same as forced induction doesn't need scavenging (well, depends on the supercharger)

 

[MDCmotorsports]

same as forced induction doesn't need scavenging (well, depends on the supercharger)

Incorrect.

A supercharged engine should still use an exhaust header, collector & system.

Now, if we are talking top fuel - THATS a whole other story.

 

[Tanya]

Ran a 2.25" catless crushed bent on my black '85... LOVED it. Thought it was the perfect size. Gained 12whp from the header and exhaust.

Now I'm running 2.5" mandrel bent from the cat back, car seems to love it. I'm probably going to bet that it wouldn't love it so much if the cat was not there though... but we'll see if that's true eventually.

 

[annoyingrob]

Ok, reasons WHY you need a smaller exhaust on an NA, and bigger on a turbo:

Air has mass, and thus, momentum. It requires energy to get it moving. With a smaller diameter exhaust, the velocity is increased. Due to teh increased velocity, the exhaust doesn't have time to stop moving before the next exhaust pulse exits the manifold. The momentum of the moving exhaust will help pull the exhasut along. Basically, less energy is used trying to push the exhaust through the pipes.

The downside to this is that the smaller exhaust will ultimately be a restriction on the motor at higher RPM. When you put a larger diameter exhaust on an NA, the exhaust slows down too much, or even stops moving between exhaust pulses, requiring more energy to push the exhaust out. However, in the higher RPM, it works well. So basically, the larger the exhaust, the higher up the powerband is pushed.

Turbo cars follow the SAME principals pre-turbo. Not many people understand this, but exhasut gas velocity is VERY important in spooling a turbo. If you have two manifolds, one with 3/4" runners, and one with 1" runners, the 3"4" runners will always spool faster. Ever wonder how little rotaries can push such huge turbos? Exhaust gas velocity. Now, that's not saying that a 3/4" runner won't choke off the moter higher up, it's a trade-off. Spool for ultimate power, just like swaping the exhasut housings.

Now, the turbine in a turbo spins on one principal, Pressure differential. You want the LEAST amount of restriction after a turbo to create the largest pressure difference on the two sides of the turbine. This allows the exhasut gas velocity to remain at a maximum. Ever wonder why 7Ms always boost higher with DDPs? This is why, a greater pressure differential across the turbine.

So, if your friend thinks a larger exhasut on a TURBO car will lose him power, he's dead wrong. Unless the exhasut is so fucked up that it's creating a highly turbulent flow, he can't lose power.

 

[Junior]

Incorrect.

A supercharged engine should still use an exhaust header, collector & system.

Now, if we are talking top fuel - THATS a whole other story.

ya that's pretty much what I thought. Only your primary sizes are gonna need to be big enough to deal with the extra air-mass.