heat and turbochargers

[hybrdthry911]

ive been having a discussion on another board about heat and how it affects the flow of a turbocharger.

someone stated that the exhaust gases will drive a turbo charger at the same exact rate on a manifold mounted turbo vs a rear exhaust mounted turbo.

my claim was that the exhaust gases will lose velocity due to transfer of heat through the metal in the exhaust system. and if exhaust gases are colder they dont move as fast. and with less velocity and the same mass, the exhaust gas now has less momentum to push the turbocharger. his argument to this was the the air would be more dense and have the same effect. and that the heat loss is pretty much insignificant.

what do u all think. please explain in detail please.

[19Cutlass94]

Honestly I dont think it makes a difference at all. If you have noticed, with a car thats already warmed up to operating temperature, the exhaust coming out of the tail pipe is still plently warm. This is why the mufflers and the whole exhaust system is hot, and could burn you ( providing your actually dumb enough to try this ) The temperature difference between the exhaust gases coming out of the manifolds is not enough of a difference to change anything.

The exhaust gases would come out of the manifold at the same velocity that it does out the tailpipe. There just isnt enough cooling going on to make a difference in terms of how dense the particles of air are. Your pushing hot exhaust gases through a turbo in which the warm air is already not very dense to begin with.

I just dont think theres enough cooling going on through the exhaust system to make a difference.

[jays05]

Heat = velocity. The hotter the exhaust gases are, the more exhaust volume you have, spinning the turbine wheel faster. Case in point, just about any turbo car will have better transient response after the turbo has been spooled up once or twice. On my STI I saw a noticable improvement in transient response just by wrapping the turbine housing, both on the stock turbo and a gt3582r. But on the other hand, after ripping through one gear I'm not sure if it actually makes a difference anymore.

[jays05]

Originally Posted by 19Cutlass94

The exhaust gases would come out of the manifold at the same velocity that it does out the tailpipe.

No, the exhaust gases lose velocity as they move through the exhaust system and cool down. What comes out of the tailpipe is cooler, denser, and moving a lot more slowly than what comes out of the manifold.

My egt gauge reads around 500c at idle, and by the time it comes out the tailpipe it's what, maybe 80c? 100c?

[Captain Moai]

It's a simple matter of energy. There may be the same MASS flow rate from two seperate points on an exhaust system, but the enthalpy of the system will be entirely different between them. There are just too many opportunities to lose energy in a larger system than a smaller one.
In a "small system", a gas particle just exiting the cylinder head is packing a lot of energy. It's moving faster. It hits the turbine blades mounted in close vicinity. It hits them fast enough and hard enough that it causes surface friction. A ton of molecules later, that friction translates into the exhaust manifold glowing red. There's a large amount of energy transfer as the particles transfer their high kinetic energy into the surface of the turbine section. The overall enthalpy of this system is high, becuase there's an ample supply of energy to do work. In fact, there's so much energy that a large amount of heat is formed as a byproduct.

The particle exiting the tailpipe is cooler. It doesn't have a lot of energy. The energy it has is partially from the other molecules backing up behind it and shoving it forward. [hence why cooler gases tend to be more dense as they pack together]. It strikes a rear mounted turbine with a lot less force. Not a whole lot of excess heat here and that's primarily becuase the gas is pretty well cooled down by the time it gets back there. Cool gas moves slower than hot gas. What happens when a slower moving substance strikes a turbine blade? The blade can only move as fast as the energy of the gas allows.

Now, seeing as turbochargers are essentially giant heat sinks as well as turbines and their response is derived from the energy [pressure] DROP across their surface, where would you mount yours in order to provide the maximum amount of energy transfer?

Life is a series of opposites. I think about it this way: Where can I mount the turbine such that it's the happiest and the compressor housing [being the opposite of the turbine], is the most miserable? AHA! As close to the cylinder as possible.
Want the compressor housing to be happiest? Mount it in an area as isolated from heat as possible - the tail end of the car. Of course your turbine section will have to suffer with loss of efficiency.

[Shawn96VR-4]

Rear exhaust mounted turbo is stupid anyhow. Period.

[intern8tion9l]

it just seems that if nothing else it would at least get all f'ed up sitting under the car

[skyphix]

A "proof of concept" is that a lot of the big inch NA guys run graduated exhausts, starting at 3.5" at the headers and working down to 2.5 or sometimes 2" at the tailpipes. This is big in those "cheater" classes where your car should look nearly stock. It isn't allowed in F.A.S.T. but I know of several Buick racers that have actually picked up as much as .1 and 1-2mph just from downsizing the tailpipe portion from 3.5 to 2.5. The rest of the exhaust is either 3.5 to the mufflers or if they have an X pipe, the Xpipe will be 3" and the exhaust stays that way to the mufflers.

I guess the idea is to keep the exhaust velocity up, and it seems to work for them.

[19Cutlass94]

Ive never heard of downgrading an exhaust on an N/A car... Although with carb engines you do need a certian amount of backpressure, but you get that through the exhaust size.

The old exhaust on my dads car was like that, and it killed the engine. It robbed so much power than when we put on a bigger exhaust it was like a new motor.

Do you have any articles or anything from that?

[skyphix]

I'll see if I can dig something up. IF you have 3.5" collectors on your headers and it goes to a 2" exhaust of course its going to suck.

What these folks do is run 3.5" (or 4") collectors, then maybe 24" of 3.5" pipe, a 3" x-pipe to 3" inlet 2.5" outlet mufflers and 2.5" tailpipes.


It is for the same reason that the 3" exhaust on my Buick is simply too big. There is not enough exhaust gas to fill the 3" pipe after it has cooled despite the fact that I have 3" collectors on the headers. The exhaust gasses cool down and slow down, causing an increase in backpressure instead of improving flow.

I will try to find the article that refers to this phenomenon as well as a few "Real world" examples of graduated exhausts.

[Redline Motorsports]

A hot pipe will flow at a much greater velocity then a cold pipe. I raced snowmobiles for years and we always used to put the sled on jack stands on the starting line and would crank the machine to redline a few times to get the "heat into the pipe".

The rear mount STS system takes into consideration the thermal loss that occurs from the exhaust traveling from the exhaust port to the turbine housing. In essence they "gear" the turbine wheel to scroll up faster with less velocity. We have installed several of thise kits with great luck. They are not a high end kit like the APS units we install but are effective. We recently did a new C6 with twins on a stock motor and made over 500 to the wheel on 7 psi. I also have a friend of mine that has a C5 with a 408 making over 760 RWHP and runs 9.30's on drag radials!

Personally I prefer front mounts!

Howard