Why rear-mounted turbos on a front-engine car?
 

I've noticed some front-engine cars with an aftermarket turbo conversion feature rear-mounted turbos. What is the exact purpose of mounting the turbo in the rear of the car, far away from the engine? Wouldn't the boost pressure also gradually decrease as it has to travel a long distance to get to the engine compared to a turbo that's mounted in the engine bay, where the piping is much shorter? My theory is the engine bays of some cars have no room left for any turbos. I've seen this rear turbo setup on some Corvettes and 350Z's, but in some cases the turbo(s) are actually mounted in the engine bay, which kinda rules out my theory of a completely full engine bay.

This strange turbo setup doesn't really make sense to me, so my question is how is it in any way practical to mount the turbo(s) in the rear of a front-engine car?

Easier to find a place for it in the rear...

Just don't plumb it with PVC!

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Conventional wisdom is that the closer to the motor, where the most heat is coming out the exhaust, the more efficient the turbo set up will be

BUT....I've seen the rear mounted turbos on Corvettes and a few knowledgeable people that drove ./ ridden in them were amazed how well the cars performed. The exhaust gases do cool off some as they travel down the tubes to the back, but it looks like that energy lost is no big deal when it comes to getting horsepower.
I would be curious to see how a back to back comparison would be on a road course. You might think the spool up would have a fair amount of lag. then again the turbos are rather small on these installs. Wonder where the cavitation points are

Here's some propaganda from STS. Much of what they say makes some sense, but I'll bet the main reason is that the engine bay is packed.

STSTurbo.com: Why Rear-mount Turbo

STS makes a very good system and they put the turbos back there because that is where there is room.
It is actually more complicated to put the turbos in back because they have to have an oil pump in the rear to pump the oil back to the forward sump.

Why not use a separate rear-mounted oil tank, electric pump, and cooler?

Hell why not mount the entire engine in the rear....oh, wait....yeah.

I can't speak for them, but simple and compact is usually better. And cheaper. They had to sell these systems, and its easier to sell a $5k system than a $6k system.

The sds system i saw used the space for the stock mufflers for the turbos. not a lot of extra room for things like a tank and cooler.

It was simpler to use a small sump pump to put the oil back to the front where it is filtered and cooled.

Owner liability probably. Owners wouldn't change a rear oil system.

I think the turbo and intercooler should all live in a foot high lump in the middle of the hood.

Only on Hondas and Mustangs though.

I actually don't understand how this could work. Given the absurdly long run from the turbo back to the intake manifold, isn't the lag huge?

I have no idea. I DO know that on my Corvette, the engine bay was far from full; there was tons of room in there. Same with my Z--there would be no issue finding room for a turbo.

lag
 

lag is TURBO wheel spinning up NOT FLOW
small wheel spins up faster then a bigger one

vetts with 6-7 L get lots of flow
rear turbo also removes heat from confined space under the hood
not the best for max efficiency
but plenty for a street car with 400-500 hp

btw a small turbo like volvo used gains mid range power
not so much at peak for better fat tork drive-ability

Here they explain why they did it that way:
STSTurbo.com: Why Rear-mount Turbo

It is not the ideal place to mount them fundamentally. Lets say it is "25%" worse than putting them where they belong near the exhaust manifold and the throttle body.

Turbos are super fantastic and even 75% of super fantastic is pretty great (highly quantitative methods involved here).

They are generally used with big engined cars that dont struggle with low rpm torque anyways. A turbo vette is fine "spooling" at 4 krpm, it can still fry its rear tires from idle to 3999 rpm anyways. After first gear you can keep it in its powerband.