Rear-mounted turbo option

There was a thread about rear-mounted turbos. I can't believe nobody here mentioned this option that's so full of WIN that Charlie stopped using the word.

I don't know what to say.

It's a cool idea in an unconventional-for-the-sake-of-it way, but is there any advantage over a traditional front mounted setup? Unless the car has a REALLY tight engine bay I don't see it.

Jay wrote: It's a cool idea in an unconventional-for-the-sake-of-it way, but is there *any* advantage over a traditional front mounted setup? Unless the car has a REALLY tight engine bay I don't see it.

advantage: more piping so it's slower to boost.
that's all I got.
lol

Jay wrote: It's a cool idea in an unconventional-for-the-sake-of-it way, but is there *any* advantage over a traditional front mounted setup? Unless the car has a REALLY tight engine bay I don't see it.

like this?:

How can it work? Unless the turbo input flange and the output are separated inside that muffler.....

DrBoost wrote: How can it work? Unless the turbo input flange and the output are separated inside that muffler.....

guessing they built up a chamber for that output to go through? i don't know. maybe someone on acid who knows how to weld had a really bad idea. lol

Those muffler/turbo pics were taken from a stealth Mustang build. The rear seat was cut out and the turbos poked up through the floor. The whole point of that was so the car would appear to not be turbocharged.

All I can think of is the insane lag that setup would have. Keeping underhood temps down and space are definite bonuses though.

Jay wrote: It's a cool idea in an unconventional-for-the-sake-of-it way, but is there *any* advantage over a traditional front mounted setup? Unless the car has a REALLY tight engine bay I don't see it.

Massive advantage is that you can leave the factory manifolds/catalysts completely untouched. Very emissions friendly. As far as the car is concerned, you're just driving about 8000 feet below sea level sometimes.

I had the opportunity to do an engine R&R on a twin turbo F-body. The turbos were neatly tucked next to/behind the engine, and the car was not gutted in the least (still had working A/C and ABS), but between the two turbos, the two intercoolers, and the attendant air/oil/coolant plumbing, it was a kind of beautiful nightmare in packaging that also required a tubular subframe to pull off. The whole thing smelled of Very Expensive with all of the low-production one-off castings and careful attention to detail.

A rear mount turbo setup just needs ducting and a dry-sump style oil scavenge pump, which I must point out that this "traditional" system also required. The turbos were so low that they drained directly into a maybe 1-1.5l mini-sump in front of the transmission pan, and a scavenge pump (mounted next to the throttle body!) pulled the oil up out of that and dumped it down into the oil pan.

FWIW, I'm told that the car would run mid-low 10s at roughly 140mph, at 3800lb, on street tires. (No, I never drove it. I try not to drive cars with four-eight times my personal cars' power, lest I get Tim Allen Syndrome again) Not bad for a car that has all amenities and is fully OBD-II compliant. But a rear mount setup probably wouldn't cost half as much.

You don't necessarily have more lag in a rear turbo set up. You don't have to run an intercooler and so you eliminate all the cubic footage of air in the intercooler.

I sat down with Corky Bell once and he worked the math and proved to me that you actually had slightly LESS air to move in a rear turbo than a traditional front intercooler set up.

carguy123 wrote: You don't necessarily have more lag in a rear turbo set up. You don't have to run an intercooler and so you eliminate all the cubic footage of air in the intercooler. I sat down with Corky Bell once and he worked the math and proved to me that you actually had slightly LESS air to move in a rear turbo than a traditional front intercooler set up.

Now that's interesting.....

I always thought it was a neat idea... seen (online) a v8 s10 with the turbos mounted in the bed... a 4.3L with the turbo mounted in a cut out bed tool box could be neat...

that muffler setup I don't really understand... it's obviously been cut up and worked around but why goto all that work for something that could be done much more simply?...

In reply to donalson: Just to hide the turbos I guess.

corytate wrote: advantage: more piping so it's slower to boost. that's all I got. lol

This. Once the novelty wears off all you're left with is lots and lots of lag.

There's an opportunity for a one-car two-or-three-setup comparo magazine article.

Just saying ...

GameboyRMH wrote:

corytate wrote: advantage: more piping so it's slower to boost. that's all I got. lol

This. Once the novelty wears off all you're left with is lots and lots of lag.

I highly doubt that. The difference in lag would be pretty insignificant. You hear horror stories around the internet because somebody with a physics degree and calculator told us that going to a huge intercooler and bigger intercooler piping would give a lot more lag and blah blah blah.

I've NEVER seen more than a 200rpm difference between intercooler sizes or making huge jumps in intercooler piping size.

This would likely not be much different. All things being equal, i'd be HIGHLY surprised if you saw even 500rpms of additional lag.

If the lag bugs you, run a small turbo(s). Any car you'd be contemplating a rear turbo setup for likely already has a pretty decent sized engine. Lag shouldn't be a problem unless you're looking for eleventy billion horsepower with a huge turbo, in which case, it would lag if it sat in the engine bay anyways.

And it's not just about that path anyways. It'll be different on different motors with different turbos etc etc etc.

Stacy David has done a couple of shows in past yrs with rear mounted turbos ... one of them was a 'Vette

I don't see how you would have to move less air, and frankly I am not going to waste the time running the equations right now. I am at home, not at work.

But what I do know from experience is exhaust at the exit is cooler that exhaust at the engine, and that is bad for the efficiency of the turbo. The enthalpy is what the turbo runs on, not just a glorified 2 sided hair dryer. Enthalpy is the total energy in the system. So less heat, means less energy, back pressure from the rest of the exhaust system means less KE reaching the turbo. You also have the drag of the return piping, which I would guess is equal to the losses you would have in an engine mounted turbo back to the mufflers. so in the end you have a net loss of KE due to losses in piping.

I don't care about lag, that, from my understanding, has more to do with turbine profiles than piping.

Less energy in, less energy out. Unless you have figure out how to break the first law of thermo, these don't work as well as the upstream versions.

In the immortal words of Outkast "NEVER EVER, ever ever"

Oh yeah and for the record, this is a basic enthalpy question in almost every thermo book available to any engineering student. Any wants to question it, we can meet at the local book store, I will pile every thremo book I find on your head until you work the equation for yourself or your brain is turned into a greasy spot on the floor. Either way you will learn.

I wonder how the between-shifts lag is with a decently designed ducting setup.

Rally car "boost tube" designs have a simple large diameter volume just before the throttle plate, maybe 18-24" long. My theory, that nobody has yet to call me out on, is that the velocity in that chamber is low enough that it doesn't take much energy to get the airflow started again after the throttle is shut and then whacked back open. Before you can get exhaust gases to spool the turbo, you must first get air into the engine...

The design of the start of the chamber may also help to prevent any possible reversion back towards the turbo.

So if the ducting is relatively small diameter up to a certain point, then steps out to 3-4", you may be able to take advantage of inertial effects to reduce/eliminate boost drop-offs between shifts. That's where lag is, really.

In reply to Knurled:

I thought that is why they have the back fire systems to keep the turbo spooled?

I don't know too many beers, too much Japanese.

They don't really use those anymore. Not sure if it's noise restrictions, or having found better ways that don't involve playing Musical Turbos on a regular basis at services.