intake/exhaust=torque loss

[450whpGSX]

how come when you add an air intake or exhaust system you lose low end torque?

 

[Denji]

You do not. You have a turbo that gives your car plenty of back pressure.

 

[psychlow]

BACK PRESSURE IS ALWAYS BAD. IT DOES NOT MAKE POWER. At any RPMs.

Don't take offense to this, because I used to think the same thing about backpressure. The real reason why this happens is complicated, and that's where this misconception comes from.

The technical reason that adding intake/exhaust mods on an N/A car reduces low-end but improves top-end is air velocity and pressure waves.

Narrow runners on an intake manifold are good for air velocity in lower RPMs, which is good for getting more air into the cylinder at low engine speeds. The problem, at higher RPMs, is that these narrow passages become a restriction, so that when a lot of air is trying to get through, it becomes a bottleneck. So you put an intake manifold with wide runners on that allow more air flow at high RPMs to get more peak horsepower, but this reduces the velocity of the air at low RPMs.

Another way that intake manifolds are RPM-specific is in the length of the runners. When the intake valve is open, the engine draws air into the cylinders from vacuum created by the piston moving down the bore. When the intake valve slams shut, the pressure wave is deflected back up the intake manifold. You can tune the length of the runners so that the pressure wave bounces again and returns back to the intake valve at the same moment the intake valve reopens so that you're getting as much air as possible into the cylinder, but this only works over a certain RPM range. Factory intake manis lengths are tuned to do this during low to mid engine speeds. Aftermarket intake manifolds change the runner length to maximize peak horsepower production, but again this lowers power in the low/mid RPMs.

Exhaust changes are very similar. Because air has mass, it also has inertia. When air is moving through a tube of a given diameter at a given velocity, it'll can undergo what's called scavenging in a certain range. This means that the pressure waves of air exiting the exhaust pipe are pulling out exhaust gas from engine at the same time that the engine is actually pushing it out. One way that this occurs is that as gas travels through the exhaust, it cools. When air cools, it takes up less space, creating a vacuum. The reason it only has a maximum effect during a certain range is that you want an exhaust pulse exiting the exhaust pipe at the same the exhaust valve opens, because the exhaust pipe goes from high pressure to low pressure, again creating a vacuum.

To make it even more complex, your cams play a role in this by how much 'overlap' they have. If the intake valve is open at the same time that the exhaust is undergoing scavenging, it'll suck more air/fuel into the cylinder from the intake manifold.

Again, from the factory, the manufacturer wants to increase gas mileage and low-end torque, so the put an exhaust that works well for low engine speeds, but is usually not only too restrictive for making lots of power, but also won't scavenge well in high RPMs. By putting a larger diameter exhaust on, you're changing the scavenging period upward in the RPM band because you're actually slowing down the exhaust pulse. If you go too large, you won't get a good scavenge effect at all (but you'll still be able to eliminate the bottleneck of a small exhaust.)

Differences for turbo cars:

On a turbo car, these effects are reduced by the fact that there's a compressor wheel in the way of the intake and a turbine wheel in way of the exhaust. You don't have to worry about intake manifold pressure waves so much during boost, because the compressor wheel is pushing back against any pressure waves created by the intake valve slamming shut. It still plays a role, but it's not nearly as dramatic as on an N/A car. So you'll just want to have an intake manifold and cams that won't pose as much of a restriction under boost if you're going for maximum power. If you want streetability and better spool up time, you can keep the stock cams and intake. (HKS 264 cams seem to have very good low end and still cause a peak HP gain, but that just shows that the stock cams have room for improvement.)

For exhaust, scavenging is very hard with the turbine wheel in the way. You could theoretically move the turbine wheel faster (for better spoolup) if you used an exhaust tuned to do this at low RPMs, but most people report better spoolup with a larger exhaust because of the lack of a restriction. My theory on this is that by having the turbine wheel in the exhaust, it smooths out the exhaust from pulses to more of a constant flow (the reason turbo cars have better sounding exhausts on a 4-cyl than N/A cars) and also removes much of the heat from the exhaust. Without those two variables, you won't get a very good scavenging effect. So the only thing you'll really want to worry about is having an exhaust large enough to not cause a bottleneck for a exhaust gas (the larger the better.)

Cliff's notes:

N/A cars:
Small diameter intake mani/exhaust, stock cams = good velocity = good low/mid range torque, but a bottleneck at high RPMs.

Large diameter intake/exhaust, larger cams = less velocity = good high-end HP, but less low/mid power.

Turbo cars:
Small diameter intake mani, stock cams = good velocity = faster spoolup, better off-boost power, but a restriction under boost and at high RPMs

Large diameter intake mani, large cams = less velocity = slower spoolup, less off-boost power, but less restriction under boost and at high RPMs

Small diameter exhausts only pose a restriction. Go with the largest exhaust you can get for better power on and off boost and faster spoolup.

The same theories above also apply for head porting.