I've read it multiple places and was just having a discussion with a car buddy, and he's saying that "you can't open up your exhaust too much or you lose backpressure" to which I informed him that backpressure doesn't help an engine (only when it creates perfect pulses in the headers to create vaccuums to help pull the air through).

Can someone please explain how backpressure helps torque? As far as I know...backpressure is a BAD thing unless it helps with the tuning in the headers. Any comments or explainations would be greatly appreciative.

 

with less backpressure you will lose a little torque, but you gain more at the top end, so its kinda a even deal, just depends on where you want it

 

How specifically does backpressure cause a lack of torque? Or does it have nothing to do with the backpressure itself? I'm confused on this topic, gimme some insite!

 

by adding a cat back system and headers you have dramatically reduced back pressure. This is good, however, your torque has been reduced at the lower RPM ranges and has increased at higher RPM ranges. In other words your low end torque band has moved up. This can be bad if you are a drag racer beacuse low end torque is what wins short races but if you are a high speed demon you have taped into something sweet. What I have just said really only comes into play when you take your gears into consideration, a pair of 3.08, 3.23, 3.55 or 3.73 will increase low end torque delivery offseting the decraesed back pressure. A car with gears of 2.73 nature will for sure feel the negative effects of the decreased back pressure. All that said I believe that a pair of headers and a cat back will increase you rear wheel hp by a solid 15. You have probably gained a good 10 pounds of torque. Also, keep in mind that your exhaust is only as good as your weakest link. Your weakness is the catalityc covertor. Many companies offer high flow cats and in your case it would be a great thing to do. You can expect another 5 to 7 hp if you install this.

 

I understand what you're saying about losing torque and gaining high-end HP (all associated with backpressure). I'm talking about the physics of it. HOW does LESS freely flowing air (backpressure) HELP an engine's torque? I don't see how this is possible. Specific details to how this works would be appreciated.

 

So would removing the cat for an i-pipe increase RWHP by 5-10 by reducing the backpressure?

 

KevinAce, this is something I've wondered about for awhile too, I'd really like to have someone with some real knowledge explain exactly WHY this happens, not just talk about its effects.

 

Yeah I always hear certain things but I never really believe something unless I see proven facts to back it up or just flat out physics to back it up. I see no way that increased backpressure can help a car's torque...

 

With all these motorheads in here, nobody is able to give me a legit answer?

 

I think Chasman has done a good job breaking it down into layman's terms. You're asking for some awfully indepth, truly technical and very involved information.

There's a few qualified folks on this board that could probably drop the physical formulas for you...but don't expect them right off the bat. Give it some time...someone like Injuneer will likely come around spell it out for ya.

 

I'll make a guess. With more back pressure the engine has to work harder to get the exhaust out. So if its working harder to get it out it must be producing more torque? With a free flowing exhaust the motor doesnt have to work as hard to push out the exhaust losing some of that low end torque?

Remember, im guessing!

 

think about this. when try to unbolt something when it is loose you just do it. but if it is stuck then you might hurt your hand on another object when it breaks loose. you aren't stronger at that point but for a short time there is a lot of force. hence low end torque with less high end power

 

I'll take a stab at it,

Adding a catback and headers allows for a better flowing exhaust. At high rpms the combination of higher flow and high exhaust velocity helps scavenge or evacuate exhaust gases from the cylinders which improves power.

Hang with me here, making this up as I go...

Now looking back at the lower rpms, the better flowing exhuast drops the pressure, allowing for more exhuast to flow, but the larger diameters of the headers and catback, greately reduce the exhaust velocity. This combination of flow and lower velocity does not scavenge the exhaust gases from the cylinders as well as the stock system would. More residual exhuast gases remain in the cylinders, which decreases the power, resulting in slightly lower torque.

Great question!! but my answer is very general, there are many more factors in exhaust system design (wave tuning for example) which effect overall torque and horsepower... Maybe your question will spark some interesting conversation...

 

This otta be good. I don't believe back pressure is good at all. A good exhaust system will scavenge the spent exhaust ensuring optimum room for a new charge.

Good scavenging requires no restrictions plus velocity. Header length and configuration will determine the optimum rpm for the best scavenging.

If exhaust ports are too large it may hhurt scavenging because of a decrease in velocity. But to say that back pressure is good for torque seems too simplistic. So no you gurus can set me straight.

 

That lack of back pressure potentially hurts low end torque is because of the affect it had on the overlap period in the cam cycle.

If you have no back pressure, once the exhaust valve opens, the exhaust escapes into the manifold/header tube rather quickly and it's almost all gone (so to speak) by the time the piston get up close to the point where the intake valve begins to open. Because most of the exhaust has escaped already, there is not alot of velocity left in the exhaust flow as it's leaving to help draw in the intake charge. So what happens is the intake only begins to be drawn in when the piston starts moving down, rather than being pulled in by the exiting exhaust. So you lose some intake charge, which results in a lack of power. Once the rpms start increasing, then the piston is moving fast enough to where the exhaust can't escape fast enough so the intake charge is drawn in accordingly.

If you have back pressure, the exhaust cannot escape as quickly so that thier is still some exit velocity there when the intake opens which draws in the intake charge. But the problem here is that as the rpms increase, the exhaust cannot overcome the backpressure quickly enough so it sort of compresses in the cylinder which when the intake valve opens the exhaust moves into the intake runner and then back into the cylinder on the down stroke so you get much less charge, so your power drops.

Hope that helped...it's kind of a quick explanation because I have a meeting to go to.

-Dave C. '97 Z28
1997 Camaro Z28 w/ 454 cid Big Block
1995 Corvette LT-1 6-spd (all stock )

PS: I just read Mobleman's response... he said about the same thing I guess.... I should have read down further first.