Exhaust Questions ANSWERED!

[SnowSurfLax]

Ok, after MUCH research on exhaust principle (without delving directly into physics gas laws, thermodynamics, etc.) and to answer my own questions about what I can do with my exhaust system, I’ve made up a quick exhaust guide. I’ve included the links at the end if you want to get a little more in-depth knowledge. My only caveat is that I’m no physicist and I’m only giving the most basic explanation I can, so I’m not 100% correct on any of this. Also, keep in mind that forced induction changes some of the rules of the game, so you’d have to adjust for any kind of FI application.

To begin with, since exhaust system building and tuning is essentially an art, my suggestion is find someone who has mastered this art and pay him or her to do your exhaust system. Also, exhaust systems do not “produce power.” They reduce the power lost by the motor attempting to expel exhaust gases out through the exhaust pipes.

Here are the 3 main principles you have to take into consideration in regards to an exhaust system:

1. Exhaust Pulses2. Restrictiveness3. Exhaust Heat & Gas Expansion

Exhaust pulses are just how frequently the piston is pushing the exhaust gases out of the chamber. A 4 cyl. engine at 4000 rpms creates ~8,400 pulses a minute. Ideally you want each pulse to line up right behind each other. This is why headers are tuned to be the same length with minimal bends. If the pulse off cylinder 4 crashes into pulse off cylinder 1 (assuming they don’t cancel each other out), they don’t flow out of the pipe correctly and that creates a small amount of restriction and turbulence. Multiply that by the number of times it occurs in a minute and you can see the power loss.

Restrictiveness is the simplest to answer. The catalytic converter, a muffler, bends in the pipe and anything else that gets in the way of the flow of the gases all slow down the movement in the exhaust pipes and cause the engine to work harder to expel those gases out through the pipes. The ideal is to reduce the restrictions in the exhaust pipe to the minimal amount possible, but not without first taking into account for…

Gas Expansion. Exhaust gases are HOT and under high pressure. Exhaust gases cool as they work their way down the pipes. The problem occurs when the gases enter a larger area allowing them to expand which allows them to cool faster than the gases behind them. Cooler gases are denser and harder to move. This is how you can lose low-end torque by attaching larger pipes to your system. You suddenly have the hot, high pressure gases enter into a larger area allowing them to cool and expand and become denser and heavier. The hot, high pressure gases right behind them run into that denser air and can’t push it down the pipe as fast as they were going. Then the gases behind them can’t move those gases as fast, increasing the pressure back up the exhaust all the way to the piston, which must push harder to push the stream of exhaust gases out through the pipes and draining more power out of the engine. You see the power loss disappear at higher RPMs due to the increased speed at which the gases flow and the decreased rate at which the gases cool as they move down through the exhaust pipes.

So, all that being said, there are general rules for the size of pipe you want for the size of displacement you have. The following rules I stole from one of the links:

For NA engines:100 – 150ci motor = 2” tubing150 – 220ci motor = 2.25” tubing220 – 350ci motor = 3” tubingOR:1639 – 2458cc motor = 5.08cm tubing2458 – 3605cc motor = 5.72cm tubing3605 – 5735cc motor = 7.62cm tubing

I’ve heard and read about the requirements of backpressure. For a 4 stroke engine, this technically shouldn’t be required, but due to valve timing (as valves are not perfectly timed for 99% of engines) there is usually a small period when both intake and exhaust valves are open and a small amount of backpressure would be required so that none of the air/fuel mixture is allowed to vent out through the exhaust valves. This would technically allow a varying degree of lean burn and hence power loss and increased emissions. So depending on the timing of your valves and the force with which the air/fuel mixture is entering the cylinder, you will want the optimal amount of back pressure to keep from venting it out. (On the contrary, too much back pressure can cause the exhaust gases to remain in the chamber causing lean burn and loss of power.) However, manufacturers design a tiny, specific amount of that into the engine in order to facilitate complete burning of the fuel and reduce noxious emissions.

So, in summary, a properly designed exhaust system improves the efficiency of an engine by reducing the power required by the engine to expel exhaust gases.

Hope I helped clear up a lot of peoples’ questions. I’ve come to the general conclusion that the best order of upgrades (in regards only to moving air in and out of the motor) for the VG33E engine goes something along the lines of Camshaft, Intake Upgrade and then Exhaust Upgrade. Not the easiest way to go, but given that they’re basically air pumps it makes the most sense to me. It does allow you to get the optimal flow, as the cam on a given displacement with a given head will set the maximum amounts possible for intake and exhaust, the intake will determine if the motor can pull all air it can handle, and then the exhaust just determines the efficiency of expelling the air that the cam allowed the motor to breathe through the intake. Head porting fits in there after the camshaft, but I’m trying to keep this simple.

All of you Nissan Techs please feel free to chime in for additional information.

Here are the links:http://www.physicsforums.com/s...54363h ... /h...h.htm