Conventional vs. Rear mount Turbo...

Can someone give me a rundown on the performance differences b/w these 2 setups?

I've heard that lag is one big difference...which makes sense...but how much of a difference? Is it extremely noticeable?

I seem to remember Horsepower tv doing a rear mount on a C5 and they didn't seem to think much of the turbo lag downfall of the rear mount.

I don't have an FI set up yet, but I have done a LOT of research on it.

REAR MOUNT
THe way the rear mount turbos are designed is actually a great IDEA. They operate at cooler tempratures and they have little lag. THey also eliminated the need for cool down/warm up cycles and no deed for an intercooler. But, the EXH gases are less dense at the rear of the exh system which means (correct me if I'm wrong) You would need a smaller impeller housing to build boost easier. The gasses are even LESS dense when you add headers.

FRONT MOUNT
The exhaust gasses are more dense at the front, there for, the turbo builds boost easier. You would need a timer to eliminate dammage to the turbo. IMO, THe front mounts are more efficient. You loose under hood space but they make better power, and no lag other than spool time. Spool time relates to the size of the turbo. Manual transmissions need a smaller turbo to build that boost back up betwen shifts.

I seem to remember Horsepower tv doing a rear mount on a C5 and they didn't seem to think much of the turbo lag downfall of the rear mount.

You also have to remember that HPTV is a show that probably got paid by STS (or at least got free parts) to do an install on their show. If course they are not going to talk bad about it. :D

Hmm....good info...

What's a cool down/warm up cycle?

You need ot let the car idle for 3-5 minutes at start up and before you shut it off. Lets say you were just out driving, that turbo was boosting and you JUST shut it down. That impeller is spinning VERY fast (13k rpm, or higher, I dunno #'s on that).. You shut the engine off and also killed the oil supply to the turbo. That turbo is still spinning in there with no oil supply. Because of the high tempratures in that EXH side and in the turbo, you are also cooking that oil that is sitting in there. Heat soak, I think it's called.

THat's why most daily driven regular turbo cars smoke. -ie, Dodge Daytonas. That heat and not letting it cool down kills the seals in the turbo, then it's sucking oil into the intake. Turbo timers let the car idle for a set period of time after you remove the keys.

If you don't let it warm up it will not get sufficient lubrication, and we know what that does. ..:death:

ive always been scared of doing a rearmount, because i think if its a DD, and u get any moisture (perhaps get caught in some rain), that it would greatly shorten the life of the turbo

One annoyance with the STS is that it takes longer to heat up the exhaust system, so I have a hard time building boost when the car is cold. Takes about 5 minutes of driving before it really comes to life. I generally don't let the engine idle to cool down the turbo after driving it. I just drive it easy for a couple minutes before I get to my destination. It's been fine for 2 years as a summer daily driver. It's really nice to keep the AC for the summer. :D

A turbo at 13k rpm isn't doing much. They will spin 70-100k rpm in most cases. My blower spins at 65k rpm at max speed.

In most front mount systems, your turbo will have pressurized oil before your gauge shows pressure. :)

Cooking the oil in a turbo is called coking. Most people have enough common sense not to run their cars really hard and shut the motor off right after. Maybe if you're after that really choice parking spot or something.

A turbo at 13k rpm isn't doing much. They will spin 70-100k rpm in most cases. My blower spins at 65k rpm at max speed.


You know what I mean. I said I wasn't too familiar w/ impeller speeds. I'll remember it this time.

Cooking the oil in a turbo is called coking. Most people have enough common sense not to run their cars really hard and shut the motor off right after. Maybe if you're after that really choice parking spot or something.

or if your engine blows up... but you have a lot more serious problems then anyway. :)

I don't have an FI set up yet, but I have done a LOT of research on it.

REAR MOUNT
THe way the rear mount turbos are designed is actually a great IDEA. They operate at cooler tempratures and they have little lag. THey also eliminated the need for cool down/warm up cycles and no deed for an intercooler. But, the EXH gases are less dense at the rear of the exh system which means (correct me if I'm wrong) You would need a smaller impeller housing to build boost easier. The gasses are even LESS dense when you add headers.

FRONT MOUNT
The exhaust gasses are more dense at the front, there for, the turbo builds boost easier. You would need a timer to eliminate dammage to the turbo. IMO, THe front mounts are more efficient. You loose under hood space but they make better power, and no lag other than spool time. Spool time relates to the size of the turbo. Manual transmissions need a smaller turbo to build that boost back up betwen shifts.


If you want to use the word "density" then you have it backwards. Higher temps = less density... so the gases are more dense at the turbo on a rearmount setup = laggier.

If you want to use the word "density" then you have it backwards. Higher temps = less density... so the gases are more dense at the turbo on a rearmount setup = laggier.

Less Dense=Slower flow(which in turn equals lag) lol

ive heard you can fix that by wrapping all of the exhaust leading to the turbo to keep the heat in

Less Dense=Slower flow(which in turn equals lag) lol

cooler charge reaching the turbine housing = more dense charge

hi temp reaching turbine housing = less dense charge

Again, this was mentioned because the word density was thrown out incorrectly with temperature. Density = mass/volume (D=m/v) and as temp rises so does volume (PV=nRT). The increase in volume decreases the density. Using density wont properly explain why a turbo spools. I think engineermike explains it pretty well. Again, I am only mentioning density because Bayer had it backwards.

or if your engine blows up... but you have a lot more serious problems then anyway. :)

I had an old small block a long time ago that threw a rod cap through the oil pan and still kept running for a good 30-40 seconds. Sounded like all hell was breaking lose though. :eek:

I had an old small block a long time ago that threw a rod cap through the oil pan and still kept running for a good 30-40 seconds. Sounded like all hell was breaking lose though. :eek:
Well hell... A god oil filter and the turbo would be fine!

The worst blow-up I've ever had was when a cam snapped in a smallblock and dropped 3 lobes into the rotating assembly at about 5500rpm. When the crank smashed the cam chunk into the block, the block gave and it was the loudest, shortest ka-boom in the history of blown engines. The block came out in 6 pieces. :D

If you want to use the word "density" then you have it backwards. Higher temps = less density... so the gases are more dense at the turbo on a rearmount setup = laggier.

I prolly got the terminology backwards. I was doing some searching on LS1Tech and I found that in referance to headers and an STS set up.

This has been debated many, many times. Here's the long and short of the facts:

- Front mount makes more power, like for like. End of story. Dyno proven.
- The rear mount has to have a smaller turbine to get boost. The smaller turbine builds more exhaust backpressure and that hurts power.
- Front mount spools up quicker.
- Front mount has hotter underhood temperatures, which degrade wiring and hoses. However, lots of OEM turbo cars don't have problems in that area so it must not be that big of a deal.
- Turbo coking is pretty rare. 99% of turbo cars don't have coking problems and don't use turbo timers.
- Rear mount is more akin to a supercharger in terms of performance. They lose much of the thermodynamic advantage that turbo's have.
- Rear mount is much easier to install and cheaper than a front mount.

Mike

This has been debated many, many times. Here's the long and short of the facts:

- Front mount makes more power, like for like. End of story. Dyno proven.

Why?


- Rear mount is more akin to a supercharger in terms of performance. They lose much of the thermodynamic advantage that turbo's have.

Details?

Just want to pick your brain.

Why?

Details?

As we all know, a supercharger takes power directly off the crank. If it takes 75 hp to drive the s/c, then that's 75 hp less that goes to the flywheel.

We all have heard that a turbo has no parasitic drag. However, they do introduce pumping losses. Though, since the turbo captures energy from exhaust heat that would have otherwise been lost to the atmosphere, then the pumping losses are less than the parasitic losses used to drive a supercharger.

If a supercharger takes 75 hp to drive, then a properly sized turbo only takes 25, while a poorly selected turbo can take as much as 80. In the end, a typical turbo will make 25-50 hp more than an equivalently sized supercharger.

Enter STS. Alot of exhaust heat is lost between the motor and the turbo. This is exhaust heat that would have been used to spin a front-mount turbo. Since you dont have as much heat at the rear of the car, the wastegate has to pinch off to reach a desired boost level, forcing more exhaust gas through the smaller turbine housing required to get decent spool-up. This all equates to increase exhaust pressure and more pumping losses. These increased pumping losses rival the hp used to drive a supercharger off the crank.

It's all in the Thermodynamics. ..

Mike

Also worth mentioning is my car vs. a fella' named zombiess on ls1tech.

I'm running a T76GTS front-mount on a 388 LT1. With my 40 mm wastegate wide open, I'm making about 16 psi boost and pulling 146 in the quarter.

Zombie's running a T76 R-trim rear-mount on a 6.0 LS1. With his wastegate totally shut off, he's making 15 psi boost and pulling about 130 in the quarter.

Draw your own conclusions.

Mike

Also worth mentioning is my car vs. a fella' named zombiess on ls1tech.

I'm running a T76GTS front-mount on a 388 LT1. With my 40 mm wastegate wide open, I'm making about 16 psi boost and pulling 146 in the quarter.

Zombie's running a T76 R-trim rear-mount on a 6.0 LS1. With his wastegate totally shut off, he's making 15 psi boost and pulling about 130 in the quarter.

Draw your own conclusions.

Mike


AND THE WINNER IS.......!

Impressive. Why is rear mount cheaper? Is the turbo itself just cheaper?

I guess you wouldn't need special headers. That's where the turbo exhaust usually dumps isn't it?

Also worth mentioning is my car vs. a fella' named zombiess on ls1tech.

I'm running a T76GTS front-mount on a 388 LT1. With my 40 mm wastegate wide open, I'm making about 16 psi boost and pulling 146 in the quarter.

Zombie's running a T76 R-trim rear-mount on a 6.0 LS1. With his wastegate totally shut off, he's making 15 psi boost and pulling about 130 in the quarter.

Draw your own conclusions.

Mike


He also only dynoed ~520rwhp. Something isnt right with his combo.

Purevil (local guy to me) made 700rwhp with a 346ci M6 tranny rearmount T70 setup.

Either way, frontmount setups are consistently outperforming rearmount setups. A rearmount can be set up to run. There are some cars in the 9's (vettes/GTO's) with rearmounts... just need a good auto/stall/gear/2-step.

Impressive. Why is rear mount cheaper? Is the turbo itself just cheaper?

I guess you wouldn't need special headers. That's where the turbo exhaust usually dumps isn't it?


yeah alot more time and pain in the balls to make a front mount setup(to make power and keep options to make people happy) rear mount its just a buncha long pipes(for the most part) with very few bends.

This has been debated many, many times. Here's the long and short of the facts:

- Front mount makes more power, like for like. End of story. Dyno proven.
- The rear mount has to have a smaller turbine to get boost. The smaller turbine builds more exhaust backpressure and that hurts power.
- Front mount spools up quicker.
- Front mount has hotter underhood temperatures, which degrade wiring and hoses. However, lots of OEM turbo cars don't have problems in that area so it must not be that big of a deal.
- Turbo coking is pretty rare. 99% of turbo cars don't have coking problems and don't use turbo timers.
- Rear mount is more akin to a supercharger in terms of performance. They lose much of the thermodynamic advantage that turbo's have.
- Rear mount is much easier to install and cheaper than a front mount.

Mike

I couldn't agree more with Mike's reply to this post. :thumb:

This has been debated many, many times. Here's the long and short of the facts:

- Front mount makes more power, like for like. End of story. Dyno proven.
- The rear mount has to have a smaller turbine to get boost. The smaller turbine builds more exhaust backpressure and that hurts power.
- Front mount spools up quicker.
- Front mount has hotter underhood temperatures, which degrade wiring and hoses. However, lots of OEM turbo cars don't have problems in that area so it must not be that big of a deal.
- Turbo coking is pretty rare. 99% of turbo cars don't have coking problems and don't use turbo timers.
- Rear mount is more akin to a supercharger in terms of performance. They lose much of the thermodynamic advantage that turbo's have.
- Rear mount is much easier to install and cheaper than a front mount.

Mike

Great post.

It took some time really uncover the true results of the rear mount design. I have debated this on LS1Tech and summed up the results in this statement. STS trades performance and efficiency for simplicity. This is essentially the trade off.

It took some time really uncover the true results of the rear mount design. I have debated this on LS1Tech and summed up the results in this statement. STS trades performance and efficiency for simplicity. This is essentially the trade off.

One thing I don't like about STS is the oil pump needed to return oil to the pan. That's the only part that doesn't fit into "simplicity".

That's one thing I like about front mounts (as long as they are mounted high enough) -- Unless gravity goes awry, they will always drain right :D

Unless gravity goes awry

I hate when that happens. :lol: