sublimate

What viscosity oil does a turbo require? Can it use gear oil, or is it too thick?
And can gear oil withstand the high turbo temps?

If the answers are yes (which they probably aren't) you could make a sweet tranny/diff cooler that also provided the turbo with oil instead of running the lines all the way back to the engine.

Vilhuer

If gearbox oil works 5sp GTS has everything ready in theory. In practise gearbox oil pump would not work as it only pumps when car is in motion.

Chris Lockhart

From what some of the Chevy LS-1 guys are saying, the rear mounted turbo does not have as bad of turbo lag as everyone seems to think. There is a slight power loss below 3K rpm as someone mentioned, but for a street driven car, they felt that the drop was negligeable. The soft power application is good for driveline reliability, and they say the top end rush is absolutely awesome.

Several of you are questioning the reason for a rear mounted turbo vs a traditional underhood set-up. Several reasons: more room at the back, cheap and "relatively" easy to install, (and to remove if you want to sell the car, pass state inspections, etc....Can't exactly do that with a traditional set-up) at reasonable boost levels there is no need to intercool, as the charge temp drops due to ambient airflow over the exterior of the charge pipe. Probably more plusses that I can't recall, but you get the point.

If I had the time and funds to set out to build a turbo 928, I would do it the traditional method. But for a "bolt-on" set-up, this system is a pretty good compromise to get you 100+ additional hp. Smart IMHO.

James-man

Go for it Chris!

MarkRobinson

I'll be giving rides to anyone who wants on within a couple of weeks in my 928 turbo S. I welcome criticism at that time...though, I dont think I'll be hearing nearly as many "comments" as I'm seeing here regarding rear-mounted one-off installations. Dyno's anyone?? PM me for more info.

sublimate

Uh oh, Mark posted. Now this thread will be deleted for sure.

Fabio421

Damn you!!! The thread was going so well too.

James-man

I'll take a free ride if you come to North Carolina.

MarkRobinson

"King of deletion" Na, turbo's are fun....we all like fun.

Tony

Id place some screen mesh in there to prevent that?

either way guys this is an extremely inovative thing for our 928s.
Kudos to the PACNW and Mark who have both taken on there own projects with different approaches.
Cant wait to see some vid, sound bytes and some charts!

So...if i got one of these to feed air to my twin screw???

Jim bailey - 928 International

Tony you would have the advantages of BOTH !!

m21sniper

"So...if i got one of these to feed air to my twin screw???"

It would actually be the other way around. The twin screw would supplement the turbo via low end boost. A clutch to disengage the SC after a certain RPM so the Turbo can take over would be awesome.

Chazz

I don't see why the lag would be worse with the rear turbo than with a front mounted turbo with intercooler. Assume an IC with dimensions of 2ft. by 1ft. by 4in. That results in an external volume of 1037 cu. in. Using an IC design rule of thumb that internal volume is 33% of external volume results is 342 cu. in. inside the IC. Add on tanks either side if the IC 4in. by 4in. by 1ft. results in 311 cu. in. of tank volume. Figure about 5ft. of 2.5 in. pipe from the turbo to the IC and then to the intake results in about another 300 cu. in. The sum if these volumes is 953 cu. in. Divide that by the approx. 5in. cross section of a 2.5 in. pipe from the rear turbo to the intake and you have 190in. or 15.88ft. of pipe available for equivalent volume to the front mounted system. Judging by the pictures, the compressor outlet to inlet in the rear mounted system is more like 10ft., a 38% improvement, not to mention that the straight pipe will have a lot lower pressure drop that the manifold of an IC.

sublimate

There will be some additional lag because:
1. There's more exhaust volume that'll have to be pressurized before the turbine will spool up.
2. The exhaust will lose more energy (in the form of heat) before reaching the turbine, so the engine will have to get to a greater rpm before getting the same boost as a front mounted turbo (assuming the same model turbo).
3. The pipe from the compressor to the engine intake won't be as efficient at cooling as an intercooler (although it will cool some) so to get the same intake temps you'll have to add additional intercooler volume.

There are some methods to mitigate these:
1. Will be hard to get around - maybe by reducing the pipe diameter (which'll also reduce the surface area, helping #2), but that'll increase backpressure on the engine (not good).
2. Can be reduced some by using an insulating header wrap on the headers and exhaust pipe to the turbo.
3. Could be reduced some by making the intake pipe a more efficint heat exchanger (basically make it into a long intercooler), by increasing the surface area, adding cooling fins, etc.

And of course a different (smaller) turbo can be used to reduce lag - but that'll reduce top end power.

All in all I think a rear turbo could do well (which is why I'm working on one of my own), but the proof is in the pudding.

PorKen

Not to mention the many 90+ degree bends, which make the effective length even longer. The pressure might end up the same, but fewer bends will flow more.

1a. Dual (concentric?) pipes with valve? I still want to try this with my NA engine.

4? Sequential turbos?! Or a big ol' wastegate, like (Audi?) uses.

Plenty of room for an air/water intercooler too...