# What size exhaust is the right size?

There seems to be a lot of confusion and misinformation about exhaust in performance circles. There are a lot of myths and people implying that it is a black art. One of the most common half-truths is that an exhaust system needs backpressure to work correctly.

Wait, why is back pressure bad? The engine need flesh air to burn fuel. The more air you have, the more completely you can burn the fuel. The more fuel you can burn per cycle, the more power you make. It is that simple.

Back pressure impedes the flow of exhaust out of the cylinder. If exhaust gasses are left in the cylinder there is less room for the fresh air that we need to burn fuel. Thus, with back pressure you can “choke” the motor.

But why is the backpressure idea a half-truth? This is because velocity has to be considered, too. If you can keep velocity up, you can actually pull more exhaust out of the cylinder and more air in through the scavenging effect. So there is a bit of truth in the idea that you don’t want too big of an exhaust, but the reason isn’t for backpressure, it is because you want to keep velocity up.

Tube size is what limits flow as in CFM or a volume over time. Velocity is just speed apart from volume. Take a garden hose for example. If you use a nozzle on a garden hose to choke the flow, you will increase velocity but decrease flow.

As I mentioned before, you do need to have enough velocity to ensure that exhaust gasses are not pulled back into the combustion chamber in the brief moment when the exhaust valve hasn't completely closed, the intake valve is beginning to open, and the intake stroke is beginning. But velocity alone isn't enough. You have be able to flow enough volume to ensure that you are completely able to empty the exhaust gasses from the chamber.

You can calculate the CFM (flow) required to support a motor of given horsepower. A commonly used estimate is 2.2 CFM per flywheel horsepower. Another common estimate needed is the CFM per sq inch of tubing which is 115 CFM per Area in cubic inches.

In the calculations I'll be doing below, I'll be assuming that we are using standard 16 gauge galvanized steel. This has a typical thickness of .065". This is important, because the size of the tubing is outer diameter and we need to do our calculations on inner diameter to be the most accurate.

Thus you can calculate the max HP supported by a 2" diameter exhaust tube through the following calculations:

Account of thinckness of the tube wall (r - .065)

1 - .065 = .935"

Find Area (r^2 * Pi)

.935^2 * 3.14 = 2.75 ci

Find Max CFM (Area * 115)

2.75 * 115 = 316.25

Calculate Max HP given Flow Rate (Max CFM / 2.2)

316.25 / 2.2 = 143.75 HP

So here's the estimates:

note: I used Excel to do the calculations here. That's why the rounding is different than in my example above.

Tube OD Tube Radius Tube ID Radius Tube Area Max CFM Max HP

2.00 1.00 0.94 2.75 315.84 143.56

2.25 1.13 1.06 3.53 405.94 184.52

2.50 1.25 1.19 4.41 507.32 230.60

3.00 1.50 1.44 6.47 743.96 338.17

3.50 1.75 1.69 8.92 1025.76 466.26

4.00 2.00 1.94 11.76 1352.73 614.88

So here is an example: Let us assume we have a motor making 400 flywheel horsepower and we'd like a single pipe exhaust.

A 3" single exhaust is good for about 340 crank horsepower. Our motor is 400 hp at the flywheel, we need an exhaust size larger than 3".

Now I'm not saying that 4" would work fine, either. 4" tubing supports much more power than we are planning to make. While the flow would be adequate, the velocity would be too low because the exhaust pressure would be diminished.

Thus, the rule of thumb with the calculations is to use the smallest tube diameter that supports the horsepower you are planning on making.