MikeGyver

We had a nice big discussion of this topic a while ago. Wall thickness doesn't so much effect thermal transfer, surface area does. Surface area is of course vastly greater on long tube headers.

A small A/R turbine housing directs airflow in a manner such that it hits the impeller more tangentially. Basically airflow intercepts the impeller at a more agressive angle. By appropriately sizing the turbine housing, any lost velocity (from heat) can be re-introduced.

97WS6SCharged

Depends on what you're comparing it to and what you're trying to do. It's a great system for the street and mild track use. I wouldn't want to use it for a race car though.

No, STS's turbos compensate for the lost exhaust gas velocity by using a smaller AR exhaust housing. You cannot reintroduce exhaust gas velocity lost through heat dissipation into the exhaust. If the same exhaust AR was used on a front mount system then it would come on very quickly and probably start glowing red hot on the top end as it would be a restriction.

MikeGyver

Yes, that's what I've been saying.

I have no idea what you're saying there lol

Again, thats exactly what I'm saying... re-read what I wrote, I think you read it backwards.

reamo04

I think he was just confused by the wording you used. Basically how you wrote it, I had to read it a couple times to figure out exactly what you were saying. At first it sounded like you said the smaller A/R brings velocity back. where really it just compensates and helps the turbo spool faster which after I read it a couple times is what you were trying to say.


FWIW, I would never run an STS on any car. if its street driven, its never going to be driven in the rain. If your worried about underhood temps, you can ceramic coat, and get a turbo blanket. Really, theres no reason NOT to go front mount.

MikeGyver

A smaller A/R DOES increase exhaust velocity across the turbine wheel... If you put a tighter nozzle on a garden hose it speeds up the water at the expense of increased pressure, same idea. By "appropriately sizing" the turbine housing you don't cause a restriction and limit the top end, nor do you loose your spool time.

That's funny, I can think of several off the top of my head.
1) its a BOLT ON system
2) It takes like 2 hours to change the spark plug on an LT1... I can't imagine working on it with even more crap shoehorned under the hood.
3) you dont have to modify the car.
4) stock underhood temps, a front mount wouldnt obviously be much hotter.
5) shorter tail pipe, Much less exhaust back pressure after the turbo
6) makes as much power as a front mount and you don't have the huge headache of cutting up and modifying the car etc. the kit goes on in a few hours...

Overlooking these very valid points is ignorant. You gotta give credit when it's due.
Bottom line, it's not easy to put a front mount in these cars for several reasons. I think your statement might apply if it was a PITA to put the turbo in the back instead of the front.

reamo04

ok, im not going to argue the A/R with you, because it seems it is pretty pointless.

1) there are BOLT ON turbo kits as well, no different than doing headers.
2) i can change all 8 plugs on my turbo kit just as fast as i could with longtubes
3) i didn't have to modify my car at all
4) my car runs same temp as before. it is literally 2 degrees warmer according to the temp sensors from my PCM
5) irrelevant. 3" pipe is 3" pipe, length is not going to much affect it. However, my exhaust exits right at the bellhousing
6) my big turbo makes more power than your small turbo ever will, and probably spools just as fast.... its been gone over and over plenty of times. Tell me, if rear-mounts are so great, why have I never seen one in the 8's?

MikeGyver

lol Then read it from TurboByGarrett.com...
"Using a smaller A/R will increase the exhaust gas velocity into the turbine wheel. This provides increased turbine power at lower engine speeds, resulting in a quicker boost rise. However, a small A/R also causes the flow to enter the wheel more tangentially, which reduces the ultimate flow capacity of the turbine wheel. This will tend to increase exhaust backpressure and hence reduce the engine's ability to "breathe" effectively at high RPM, adversely affecting peak engine power."


Really? radiator? Fans? A/C? Everything is all stock? Changing the plugs was just an example. If you put more stuff under hood of an already tight engine bay, it's not going to get any easier to work on.

Not so. All the ricers gain lots of power from using huge downpipes, you can't just write that point off. You'd be surprised how much power is freed up from having a short tail pipe. The turbo doesn't have to push the air out the tailpipe, which will greatly effect efficiency.
Do you have a muffler? or a cat?
You bring up another point, the STS kit is emissions and street legal. If you have to pass emissions, and can't, you can't drive the car...

Well of course a larger turbo can make more power
If you're going to accurately compare 2 things, you obviously need to hold everything else the same (like turbo size/motor).



Sigh... maybe cuz you haven't looked, since you're so against them? lol

...are you going to try to discredit this too somehow?
There ARE reasons to go with a rear mount over a front mount, unless you refuse to accept them, then I guess you only have 1 choice.

Rob94hawk

That vette is impressive. It kind of debunks the nay sayers against rear mounted turbo's. But I'm sure someone will knock it regardless.

boosted-lt1

The Vette is nice. Just want to drive the main point home...

Me:
Rear Mount Guy:
Rear Mount Guy:
This is exactly the point. There is no magic happening with a smaller turbo at the back of the car. A smaller turbo becomes increasingly necessary the farther from the pump you get because you lose more and more heat the farther you get - and pumping loss continues to go up. The fact that a rear mount setup requires a smaller turbine side (which we ALL agree on here) means that there is less exhaust energy to work with at the back of the car so you must decrease A/R at the least in order to accelerate and maintain impeller speed.

Rear Mount Guy:
Again, correct in-part and in agreement that A/R must go down to spool the thing. There is a penalty imposed by the reduction in capability we just had to make to get there. What happens if you put the turbo 25 feet from the engine, 100 feet, a mile? Efficiency of converting exhaust energy into impeller speed goes down.

Front mount guys aren't bashing the rear mount system or fellow boost enthusiasts that run them. They can work just fine. But there IS a reason that turbo X on a FM setup would be slower/less responsive and not provide the same output if put 10-12 feet further downstream.

-Scott.

MikeGyver

What's that all about..?

Just to make myself very clear, and I am NOT anti-front mount or anything (or S/C or Nitrous for that matter). I have alot of respect for you guys and what you've done with your cars and for the LT1 crowd. I'm not bashing front mounts and saying they suck, nor have I ever. I'm not being an instigator, I'm just defending rear mounts on many points that are erroneously or ignorantly founded.

The reason I say 'heat differential, not heat' is because if your exhaust temperature for example, is 1000degrees but so is your intake temp, there's no gain in air volume, which is what drives the ideal system. Since gaseous volume is proportional to temperature, it's the heat difference that matters, not simply heat, and rear mounts can achieve very low air intake temps.
Now the reason I say 'ideal system' is because in the real world you can have the turbine housing be a bit of a restriction and trade off less heat for more pressure to spin the impeller. The small loss in efficiency there doesn't make a huge difference (efficiency can be gained in other areas of the system like cold IAT's). Hardly anybody is pushing their engine to the brink, if they want to regain the small % of loss they can turn the boost up half a psi. These small cons can easily be outweighed by the pros of the system, refusing to acknowledge it and saying something like " 'X' is always better", is just silly.


Yup, I agree completely. (..as I said above, the tradeoff is increased backpressure.)

Again, I agree completely.

I think this is one of the most common misconceptions out there. I assume you're exaggerating in an effort to 'drive your point home'. I just went out and measured my exhaust pipe from engine to turbo, and it's right around 11 feet.
A typical front mounted F-body turbo pictured below is a solid 6 or 7 feet 'away' from the motor (again, I'm not badmouthing). In the case of rear mount's we're simply not dealing with an additional 10-12ft, more like 5 or 6ft. Because this isn't a huge distance, a rear mount with a ceramic coated exhaust pipe could very easily have even hotter exhaust than a front mount w/o ceramic coating. It's all just blown way out of proportion.

boosted-lt1

No offense. The original poster wasn't showing up in the quote box

evilundisguised

How is intake air temperature relevant to the turbine? You need a differential across the turbine, not the engine. As exhaust gas moves through the turbine, its pressure and temperature are reduced, and volume increases - that's the "differential" your turbo operates on.

reamo04

the smaller AR compensates for less flow. The A/R's smaller size is proportional to the loss of flow. You cannot re-introduce velocity. Simple physics. You can argue that all you want. It COMPENSATES, does not create velocity.

Yes, that vette is impressive. Its the only car ive seen running that fast with a rear mount though. IDK, the idea seems good, however it doesnt really seem that effective from what i've seen. I searched and searched for fast STS cars back when I first went turbo.

Yes, my first turbo kit was stock radiator, just thinner fans, factory position.

1996camaroSSclone

Aren't sts kits for vettes twin turbo anyway?

97WS6SCharged

That is a packaging constraint. Look at some of the other setups people have made that use headers which face the front of the car and place the turbo less than 3 feet from the exhaust ports of the heads. Hell, look at how a turbo buick is setup and see how much heat is lost between the head and the turbo.



I'll admit that Vette is impressive, but it should be fast considering how much time & money that guy has invested in it. How many people have better than $100k (probably closer to $200k) wrapped up in their F body?