I'm pretty sure he's talking about the exhaust.

 

Yeah, that post was pretty clear thought you were wondering about the rest of the thread

 

When thinking of this you must think of thermodynamics.

If the exhaust cools, whilst volume stays the same, energy has been lost. Energy is what is required to spin the turbo and compress the intake air. Alot of energy, around 25-50hp is typical depending on the setup.

More than anything this helps the spoolup times for a street car.

 

It is a common misconception that the exhaust turbine half of a turbo is driven purely by the kinetic energy of the exhaust smacking into it (like holding a kid's toy pinwheel behind your tailpipe) While the kinetic energy of the exhaust flow does contribute to the work performed by the turbo, the vast majority of the energy transferred comes from a different source.

Keep in mind the relationship between heat, volume, and pressure when we talk about gasses. High heat, high pressure, and low volume are all high energy states, low heat, low pressure, and large volumes are low energy states.

So our exhaust pulse exits the cylinder at high temperature and high pressure. It gets merged with other exhaust pulses, and enters the turbine inlet - a very small space. At this point, we have very high pressure and very high heat, so our gas has a very high energy level.

As it passes through the diffuser and into the turbine housing, it moves from a small space into a large one. Accordingly, it expands, cools, slows down, and dumps all that energy - into the turbine that we've so cleverly positioned in tho housing so that as the gas expands, it pushes against the turbine blades, causing it to rotate. Presto! We've just recovered some energy from the heat of the exhaust, that otherwise would have been lost.

This is a measurable effect: Stick an EGT upstream and downstream of the turbo, and you see a tremendous difference in temperature.

So, in real world terms, what does this tell us?

All else being equal, _The amount of work that can be done across an exhaust turbine is determined by the pressure differential at the inlet and outlet_ (in English, raise the turbo inlet pressure, lower the outlet pressure, or both, and you make more power) Pressure is heat, heat is pressure.

Raising the inlet pressure is possible, but tough. Lowering the outlet pressure is easy - just bolt on a bigger, free flowing exhaust. I've seen a couple of posts from people who added aftermarket exhausts, who report "my turbo spools up faster now" Well, that's because by lowering the outlet pressure, you increased the pressure differential, and now the exhaust gas can expand more, and do more work. That increased work pushes harder on your turbo, and it spools up faster. You should also see less boost drop at redline, because if an exhaust system is flow-limited, once you pass the flow limit of the system, any additional gasses you try and force through it only raise the outlet pressure. Higher outlet pressure, lower pressure differential, less work, less boost.

If you have an engine mounted turbo then you dont have to worry too much about the exhaust temp. leaving your exhaust pipe... However, if you have a RMT such as the one from STS then you should definitly worry about how much heat you are loosing from your pipes before it reaches the turbo. My advice would be to have your entire exhaust from headers to turbo coated by a company such as JETHOT or HTS. JETHOT charged me $350 for the headers and $15 a foot for the exhaust. This would definitly speed your exhaust's egress to the turbo.
...GOT BOOST...GOT SMILES

I just got my 87' Z back after 8 years of missing it....... I drifted to the Import sceen (240sx w/turbo'd SR20) The guy who bought my car in 98' thrashed it pretty bad but I will put it back together...and it will be boosted

 

ghost 7.62, that's only your first post and I like you already. . .

 

Agreed. That's a great explanation man.

 

Wow! You know your stuff. Welcome to the board (from another newer guy). That's a great description of what's really happening. I think a lot of guys miss the correlation between retained energy due to heat and focus on the hot-side kinetic energy only.

By the way . . . it's not chemistry, it's physics. Not so "basic" after all huh?

 

If you're refering to me then it is chemistry as well as physics. When me and the other guy were arguing Adiabatics, that was chemistry. And part of what this gentleman is talking about is abiatic efficiency, which is also part of chemistry.

But yes, on the whole the general efficiency of a front mount turbo versus that of a rear mount is a matter of physics And they are all very basic concepts.

 

Though I agree with you in that it is the exhaust gas pressure that ultimately spins the turbine wheel, the fact that energy is lost in the form of radiant heat is still a vital part of the equation.

Energy is energy, no matter how it manifests itself. And a cooler gas has less energy than a hot gas, manifested in the form of velocity and pressure.

I'm sure you know this, but the guys arguement is still valid. Atleast as far as his conclusion

 

Engineermike you are welcome, Ive been surfing around a few of the 3rd gen sites and this one seems to have the most open minded people.

Jerminator96 you are absolutly correct this is all about how chemistry and physics combine to form one or possibly multiple results.

Here is an abstract proposition for anyone who doesn't quite have the concept:

Have you ever used a whistling teapot, the concept is the same. If you will notice, as the steam shoots out of the small hole it is moving at a fast rate from the high pressure area to a lower pressure area...(PHYSICS). Now as the steam leaves this high pressure area it cools rapidly, as it cools it slows down. Now I didn't say it lost it's energy. Its energy is simply transferd into water droplets...(Chemistry)

This transfer of energy is the SOLE reason why heat retention is SOOOOOOO important in a RMT.

mile_high thanks dude I learned about turbos from trial and error on my old 240... blew up 3 KA24DE engines befor the SR20DET swap. If anyone wants to learn about turbos I whole hartedly suggest they ease over to some of the Import sites.

 

You dont see that **** on STS' website...

 

I built 3 turbo setups so far and most of my learnings came from turbomustangs.com. Funny how us turbo GM guys have to learn from "the enemy". . .

Mike

 

That is why they use coated pipes. Still laggy, but fun though.

 

Luna, yes it is true that STS uses coated pipes... but if anyone is putting together their own RMT then they should take note and have their exhaust coated.

 

engineermike, its always cheaper $$$$ to let the other guy figure out what will and won't work. Believe me I know