REAR MOUNT TURBOS- PERFECTLY GOOD IF DONE RIGHT. IF...!

The old Compressor Racing E55 AMG Turbo. Probably the UKs best known rear mount turbo car- A car I helped build and develop with great success.

Rear mount turbos are pretty new to the tuning scene as 'tuning' goes, having only becoming well known in the last decade or so, and even now are relatively unheard of in the UK. Therefore almost every time they're spotted they're always treated with amazement, both in good and bad ways...
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The problem is, people's opinions of them tend to be polar opposites of each other.

MOST people, due to it being totally against everything they know, instantly say it will be laggy as fuck, won't spool, will be shit, and so on and so forth.
They're wrong if it's done right.

On the other hand, you got people, again have no actual experience of it, but love the idea and have seen a few on the internet, who tell you rear mounts are amazing, the best way to turbo, as good as a front mount, and so on.
They're also wrong.

A rear mount turbo setup from World War Two! This is a P47 Thunderbolt engine, turbo, and intercooler setup! Engine at the front, wastegates at the front too, turbo far left, near the tail.

Basically, rear mounts are neither shit or amazing, though they're perfectly possible to be shit if done incorrectly, just like a front mount turbo can be. What they are is simply another option if a front mount isn't viable for some reason, and done right can work fantastically well, often far better than most front mount setups, which are often sub-optimal, even when a 'pro' has designed them...

If ALL ELSE WAS EQUAL, a front mounted turbocharger IS BETTER, no iff's or but's, it is. But things never are equal with turbocharging, and there's a multitude of reasons a rear mount setup is useful in some applications, and if done right, they can perform as good or better than a typical front mount setup.

The E55 AMG rear mount I helped build for Compressor Racing was a perfect example of the above. Just like a whole lot of cars, there's next to no room in the bay for a turbo or two, so even IF it's possible, you'll likely be spending a few grand on custom manifolds, even before anything else. And then you've still got to find room for the intercooler, and then deal with the heat management issues for all of that too.
The only other option for the E55 is a supercharger conversion, that costs at least £7000 plus fitting, and makes about 500bhp at 9psi boost, about the limit of the stock (albeit modified) ECU, as we found ourselves.
The E55 rear mount turbo we built was on full boost by 3000rpm (despite a turbo capable of 1000bhp!), and the only time we put it on the dyno it made 465lb/ft at just 4.5psi and due to boost control issues we were only at 2psi by peak power but it still made 440bhp- About 90bhp more than standard.
In the end we sorted out the fuel and boost control issues and got it safely at 9psi. It was never dyno'd at this level, but it WAS timed with Racelogic timing equipment, the proper pro stuff magazines and companies use, not some crappy app or a stopwatch, and the results, despite horrendous wheelspin, were pretty impressive to say the least...
60-100mph time was 4.4seconds at best (backed up by a number of 4.5s runs).
To put that in to perspective, the later, best part of 500bhp, factory supercharged version of the E55 AMG does 60-100mph in 5.5seconds, over a second slower, and even a supercar like the Ferrari F360 is 4.6sec!
And it even made a pretty fun drift car, despite being a huge, heavy, rusty old boat, as these pics show...

And how much did the full rear mount conversion on the E55 cost to do, inc all the non-turbo related bits, such as the uprated fueling parts and so on? Under £2500. 
You'd not even get a pair of custom manifolds made for a front mounted turbo setup on the same car made for that, and even if you did, will be spool a 1000bhp capable turbo much sooner than 3000rpm? Would you even want it to? Probably not for both accounts, and it'd be highly unlikely to make much more power at the same boost either.

While it's MOSTLY pure lack of room that makes a rear mount particularly tempting, it's also been done, even with very high end race cars, for weight distribution reasons, and the simple fact that, done right, it isn't a big disadvantage...

So does this mean we should ALL have rear mount turbo cars? FUCK NO! If you've the room to easily do a front mount turbo setup that's not a complete fucking bodge or cost an absolute fortune, you may as well just do that. As said already, if all things are equal, it will spool faster and make more power than a rear mount. It might not spool loads faster and make loads more power, but still, really no point for a rear mount unless it makes things a lot easier for whatever reason.

BUT as it often does make life easier, I'm sure most are you are thinking "Ok, but say if I do want to go rear mount, how the FUCK do I do it so it's not the laggy mess the naysayers say? How DO I make it as close to as good as a front mount as possible?"
Well, I better tell you what I know and have experienced...

MANIFOLDS- Use standard cast log ones, or short runner tubular ones at very most. For fastest spool you want to minimise exhaust length and maximise heat retention, and regardless of turbo position, long runner exhaust manifolds have very little effect on peak power on turbo cars anyhow.

PRE-TURBO EXHAUST DIAMETER- This is one so many people get massively wrong, and hugely over-do. Treat all your pre-turbo exhaust like you would your manifold on a front mount system, so with a T3 flange and even 600bhp+ a single 2.5in pipe is more than enough, and a single 2in or 2.25in is plenty for lesser power cars. Even with big V8s running over 140mph in the quarter mile, a single 2.5in pipe leading to a T4 flange turbo is well proven. Overly big exhaust diameter will drastically slow spool, and is the biggest killer of performance on rear mount setups.

PRE-TURBO EXHAUST LENGTH- To minimise heat loss, shorter is better, so if you can actually have a mid-mount turbo rather than rear, even better. Just behind the front seats? Instead of passenger seat? It's all been done...

PRE-TURBO CATS- I've not experienced this one first hand, but while basic theory says they'd be bad for power and spool, they're fitted to a great number of very powerful but emissions legal rear mount cars. It's possible the reason they're not the spool killer you might imagine is because post-cat exhaust temp is incredibly high (it's part of their job), which will help maximise exhaust energy and heat at the turbine. But I've never tested this so that's speculation...

BOOST PIPE LENGTH- As discussed in THIS feature, boost pipe length isn't actually a big deal at all. Don't make it longer than it needs to be, but of all the things to fuck up spool on a rear mount, this is a minor one.
One thing you might want to take in to account, though on smaller engine applications (or just using bigger boost pipes) it's likely not an issue, is the instant off-boost punch at very low rpm or just as you open the throttle. On small capacity cars there's not a lot unless there's boost anyhow, but on the AMG it had a huge V8, and once it was rear mount turbo, sucking through inlet pipes 10 times as long but about half the diameter of standard, it lost a little of this punch. You'd not notice unless you drove it back to back, and even then I only really noticed as I wanted maximum punch for drifting, but it was there.
I had a solution though, I fitted this flapper valve taken from a Volvo boat engine, allowing the engine an alternative way to breath in N/A mode...

INTERCOOLING- Yes, compressor outlet temp is lower in a rear mount. Yes, the long boost piping allows for more heat loss. But regardless, the inlet temps aren't that much lower, so if you've got room at all, you'd be mad not to fit an intercooler or chargecooler.

POST TURBO EXHAUST- As explained in THIS feature, bigger is better, and with rear mounts needing all the help they can get, as big, and as short, a post turbine exhaust, the better. Even with a 2.5in pre-turbo exhaust, noticeably faster spool has been achieved by going from a 3in to 4in post-turbo exhaust. As a rule of thumb, your exhaust diameter should be AT LEAST 1.5 times your turbine wheel exducer diameter for best spool...

HEAT RETENTION- This is a big part of spool for any turbo, and a HUGE issue on rear mounts. Basically, the hotter the exhaust gas is by the time it gets to the turbo, the better your spool is. So heat wrap ALL of the exhaust, front to rear, double layered if you can, and of course, put a turbine blanket around the turbo too. The difference between heat wrapped and not is very noticeable.

TURBO POSITION- Under the car is most common, and nothing wrong with that at all, as long as ground clearance isn't an issue. On the Merc we went boot mounted, which personally I prefer for packaging reasons. When we did so we got no fucking end of people saying how can it breath in there, we need to cut holes to let air in, etc etc etc.
Nah, not really.
Boots are FAR from air tight, thousands of small holes and gaps which overall make up to far bigger than a typical 3-4in inlet of an airbox, and the boot itself works as a giant airbox.
No matter how many times we said this, people still said it, so in the end we drilled some holes in the boot to see if it made any difference at all.
Nope, not a fucking sausage, exactly as expected.

​WASTEGATE POSITION- The "Normal" rear mount position is right by the turbine inlet, but IMO they've decided that with zero testing and just their guess. IMO it's wrong. For me, the closer to the engine the better. The WW2 planes and the top drag cars have the wastegates just after the manifold collector. And I'd be more inclined to listen to war winning aircraft and cars that run 190mph in the quarter than some random other person who's probably not tested either way.

OIL FEED AND RETURN- While you certainly can have a stand-alone oil system for a rear mount turbo, it seemed like added complication for little benefit to me, so on the ones I've had dealings with, we've always just fed the engine oil the traditional way- An oil feed pipe direct from the engine.
Oil return is a bit different though. You NEED an electric scavenge pump so suck and pump the oil back to the engine, it won't go on it's own, you'll just end up with a smokey fucked mess. One bonus is, you don't have to return to the sump like on a conventional gravity oil return, anywhere will do- On the E55 we returned to one of the cam covers! 

TURBO SPEC- This is the biggie. Compressor side, well, it should be the same as you'd choose front mounted for the power you want. Turbine, well, smaller improves spool, but also increases backpressure which kills power, and personally I'd not be going much smaller than I would front mounted, maybe not any, and just concentrate on optimising the setup as per above. Just be conservative on turbine size, sensible, as you can't get away with overkill as easy as front, but the exhaust gas still needs to come out, and having a tiny turbine to try and get good spool is why so many rear mounts make shit power for their boost level.
Of course, there's OTHER options to allow a massive turbo without the risk of killing spool.
On the E55 AMG we did for Compressor Racing we used a variable geometry turbine turbo, a Holset HX55V VGT unit in fact, a flippin' huge 1000bhp capable beast. VGT turbos, while not the magic solution to all our woes, have the ability to spool like a small turbine, and flow like a large turbine, so are very useful for rear mounts.
The issue is big VGT turbos are rare as hell in the UK in good condition, and the poxy little ones used on diesel cars are useless, so it's not an easy solution.
Another idea is twin rear mount turbos, compound style, one small for spool, one big for power. I'm currently building a rear mount setup like this now.
Finally, and one I've never seen tested, but I'd like to try one day, is the effectiveness of twin scroll. I don't think it will be a magic solution, but for cars with a very easily done twin scroll setup, mostly inline sixes and twin rotors, I'd be inclined to run two small pipes to the turbo, one to each scroll...

AND THAT'S THE BASICS FOR YOU...
There's a whole lot of bullshit, incorrect 'facts', guesses, and downright lies about rear mounts, but hopefully what I've told you will set a few things straight, and to be honest is plenty enough, if you choose to follow my advice, for a pretty well performing rear mount setup.
BUT DON'T FORGET! Don't do it if there's no good reason to...

OH! Forgot to mention something that's actually a BIG plus point, as childish as it may be... IT SOUNDS FUCKING MENTAL. All you can hear is turbo spooling up and down, super loud chatter and spool and everything else. Sounds like your car has a jet engine rather than a normal engine, which is amusing. Click HERE to see a short vid of the AMG, inside and out, to see what I mean...

​And of course, it looks cool too...