lol yeah okay.. it sounds dumb but its for a physics assignment, would a bigger diameter exhaust have a difference on the exhaust gas temperature? if so, minor or major?

it would make a difference due to the amount of pressure. Higher pressure will heat up faster and stay hot.

I concur...less room for gas to disperse and cool.

Physics answer: Yes.

But not so mucn because the gas is expanding, it already does that a LOT when it leaves the cylinder. The biggest difference in temp could probably be attributed to the increased surface area for heat transfer due to the larger tube diameter. If you're really doing this for a physics class, its a pretty complicated problem to solve numerically. You have to know a lot more about the operating conditions of the engine to correctly solve it. You must define a control volume (where in the exhaust you're talking about) and have conditions for the gas flow through it. The engine must be in steady state operation (i.e. not speeding up or slowing down, not changing load, etc.). What level of class is this for?

Real world answer: Nope.

It won't make any difference unless your previous exhaust was waaaay too small and causing a lot of restriction on a turbo car. The pressure difference would not be great enough in any other situation to change the temperature a measureable amount.

The temp of exhaust will be a function of pressure and velocity. You'd have to find out the heat capacity of the exhaust gases as a function of pressure and velocity. Using the fixed value of heat capacity you can calculate velocity. Its really an advanced physics formulation. Probably used by 4th year engineers.

so assuming the flow is isentropic (reversible and no heat addition), steady, quasi 1-D, and all of the same species (or not massivly different), we can model this as flow moving through an increasing area duct (from smaller area of the engine into larger area of exhaust piping)

Assume you know the incoming gas velocity, density and areas (v1, rho1, A1, A2), use a simple area velocity relation of conservation of mass flow: rho1*v1*A1=rho2*v2*A2

Now that you have the gas velocity in A2. Now you need pressure. You can get this from bernoulli's eqn: 1/2 rho*v^2+P=const

now that you have the pressure, its simply P=rho*R*T to get temp.

Do this for A2 for the two differnet size exhaust pipe sizes you are looking at and compare the temps.

**edit: i am ignoring an viscous (friction) effects and any heat loss that may occur and defining the control volume as the flow that is leaving the engine and entering the exhaust pipes, nothing upstream or downstream of that.

This post has been edited by orvillescelica: Apr 28, 2004 - 1:17 AM

lol wow i wasnt expecting that good an answer/s. i'm still just year 11 >> i'm getting new exhaust fitted on monday (2.5" cat back, 5.5" straight through), compared to the stock 1.75". I'll measure it then, finish my assignment and post the results on here (just in case anyone wants to know) >>

EDIT: oh btw, i'm just measuring the exhaust gases as soon as they come out of the muffler, holding the thermometer about 2cm away fropm the tip, right in the middle of it.

This post has been edited by ShadowFX: Apr 28, 2004 - 6:37 AM

the only real way to measure exhaust temps is with a EGT gauge.

on an N/A motor, exhaust temps is somehting you shouldnt have to worry about.

lol yeah its only for a small physics assignment but =P