I have seen a lot of threads where the sts kit is slammed because the exhaust loses a lot of heat before getting to the turbo. I have heard people say heat doesn't matter cause you can't turn a turbo with a blow torch and stuff like that. My point is not to say the sts kit is awesome or if it's crap, it's just to give insight as to how exhaust temp affects the turbo output. Feel free to make corrections.

Let's say a certain engine/turbo/exhaust combo has an exhaust total pressure of 45 psia before the turbo and ~15 psia after. That gives a pressure ratio of 3:1. Now assuming the turbine has an efficiency of 90% (polytropic), That gives a total temperature ratio 1.256.

Now this is the important part. We know the ratio of exhaust temp before and after the turbo, but that could mean 1500 (Rankine) before and 1194 after, or 1000 before and 796 after, or any other combination with the same ratio. The power output of the turbine is proportional to the change in total temperature, which is 306 for the first case but only 204 for the second case (the equation is massflow*specific heat*change in total temperature). Say the massflow is 50 lbm/min and the specific heat is .29 BTU/lbm *R. That means in the first case the turbine would be driving the impeller with 104 hp and in the second case 69 hp. You can see since the exhaust temp in the second case was 2/3 the first, the power to the impeller was also only 2/3. More power to the impeller of course means more boost & massflow.

My background is in jet engines, not turbos, so i realize some of my example numbers may not be realistic. Also, I know i can't just say the exhaust temp is higher in one case and everything else stays the same, but this is just a simple example to show a concept. Hopefully this will clear some things up for some people.

Cliffnotes: The power produced by the turbine is proportional to the change in total temperature across the turbine. And hotter exhaust means the change in temperature across the turbine is greater.

 

I'm interested in your thoughts on this. (just learning myself...)

Can you support the above statement. Because it seems that the exit temperature could be high too, then the delta isn't necessarily greater. (maybe it was in the part about efficiency and temperature ratio and it just didn't sink in yet).

Is the efficiency based on a given temperature? How does the efficiency change when the "before" temp is significantly different that the rated temp?

 

Take into account the basic laws of gasses and thermo dynamics. The hotter a gas is the faster it will travel; so in this case with the STS turbo by the time the gas reaches the impeller at the rear of the car the gas is moving a lot slower thus reducing the potential speed of the wheel and boost.

Thats why Exhaust temps are vital to Turbo cars.

 

It is just a fact of life "heat and air" move a turbo.

 

Is it the speed of the gas that spins the wheel?
Or is it the pressure differential that dictates the wheel speed?
Or is it the temperature differential as P Mack was explaining?



And 2FAST4U, you just basically told me, "It's true, because, well, because it's true..."
I'm too hardheaded for that to work on me.

 

PV=nRT

 

The beauty of thermodynamics is it relates all these things to each other. If you want to get down to the basics, what turns the wheel is the fact that pressure on one side of each blade is higher than the pressure on the other side of each blade. The speed of the gas spins that spins the wheel determines the difference in pressure on the sides of the turbine blade, much like the speed of air over a wing is needed to determine it's lift. The problem here is you have to know the shape of the turbine blades and it gets really complicated. The pressure differential does dictate wheel speed, but not by itself. The reason why temperature is so much easier to work with, is you don't need to know what the shape of the turbine blades are. It makes for one simple calculation that takes into account pressure, efficiency, blade shape, blade speed, etc.

 

The exit temperature is a certain fraction of the inlet temperature. So when the inlet temperature is higher, the fraction that is the difference between the two is a bigger number. The equation is:

total pressure ratio= total temperature ratio^(gamma/(gamma-1)*polytropic efficiency)

gamma is the ratio of specific heat at constant pressure to specific heat at constant volume. For exhaust i'm using 1.3.

polytropic efficiency is ratio of actual turbine work to ideal turbine work for a differential pressure change. I'm assuming 90% for this.

Turbine efficiency is normally given as a function of pressure ratio, mass flow, and turbine speed. I'd have to say the effect of temp would be negligable on efficiency.

 

Not sure how this will fit into this discussion but I thought I'd throw it in to help support or buck a theory.

On my STS setup I will spool up MUCH faster when the engine and the turbo itself are hot and have been running for a bit. If I were to fire up the truck and immediately hit WOT the turbo will take forever to spool and in some cases won't even brake stall up to the desired psi durring a race.

 

anybody who has taken fluid dynamics, or even physics should know this.

 

Ok, that helps. Now that I went back and re-read the original post, the talk about ratios makes sense to me. So very simplistically, 2/3 the temp = 2/3 driving hp, = approximately 2/3 the max boost that could be produced.

So if a given turbo could produce 20psi in a front mount, if the temp dropped by a 1/3, it would now be capable of only 13-14psi. (I suppose it's more directly related to cfm than psi though. But I'm sure those are closely related too, if you can define enough variables...)



Yeah, somehow I missed any exposure to fluid dynamics, that I can recall... Therefore all the questions. I do vaguely remember that a theory can't be self-supporting though. And to each new mind, even a well-proven concept is the same as a theory, until it is understood and incorporated.

 

I've always assumed that a hotter gas has more potential energy to move the turbo wheel to a specific rpm sooner than the same turbo spun with a gas with less potential energy, i.e., cooler gas... a rear mount can compensate for this using a smaller exhaust housing which allows for an equal spool up time, but now you've bottlenecked the exhaust pipe an even more restrictive exhaust housing thus limiting your overall maximum hp output.

 

 

my favorite thing about theory is sometimes no matter how hard you try to prove something works or doesn't can be totally off.

I got a t76 in place of my muffler on a stock motor and have full boost by 3700 rpms :-p .81 a/r

 

that's also a ball bearing turbo... if that turbo was up front I bet you would see full boost by 3k or less.

 

Not from what i've seen of others running t76's. Just gotta size the turbine housing appropriately. Just so happens this turbo came off my 3.0 liter import cause I felt it would be more suited to my 5.7 liter project. The 67 p trim will be a much better match for a 3.0 liter.

BTW, my friend also has a 67 turbo on is 370 9.0:1 motor, he hits full boost (9psi) by 2500! He could definately spool my 76 under 3k.

If the turbo hits full boost before 4k in 4th gear it means you need a bigger turbo

 

P. Mack, you can argue with STS owners till your blue in the face. Won't make a difference...lol. I have several STS customers that are now getting around to making it to the tracks, some are needless to say changing their setups .

Jose

 

Has anyone ran a pre-turbo EGT and post turbo EGT meter at the same time? That might be a intresting #

 

JZ, I'm not trying to argue with anybody. All I'm trying to do is spread some fluid dynamics knowledge so people can back up their arguments with real physics instead of "blowtorch" analogies because i know sometimes this stuff is counterintuitive.

 

Ah yes, the famous STS cheap shots. Classy...