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dean1484 GRM+ Memberand SuperDork
2/19/12 2:44 p.m.

Anyone know the changes in sizing you need when you move a turbo to the rear of the car? Installing it up front close to the manifold has been pretty well documented but I can not find anything about mounting it in the rear.

I would imaging that the exhaust side needs to be a bit smaller to spool up faster due to the cooler exhaust temps.

My 924s has a huge muffler and virtually no room up front for the addition of a turbo. It is something that has always interested me. It looks so much easier to do than the conventional turbo. Yes you need to deal with the oiling and in some cases water cooling but that is just some engineering and again there is allot more room at the back of the car for the addition of things like this.

Anyway back to my original question. How do you size for a rear mounted turbo?

SVreX SuperDork
2/19/12 2:55 p.m.

Hmm... your question interests me, and I don't have the answer.

But I'm more intrigued by your presumed answers. I don't think it would help to reduce the exhaust sizing- any reduction would be a reduction in flow. I'm not following the cooler exhaust temperature idea.

An increase in lag would be inherent in the design simply because of the longer piping. I'm not sure temperature would have an effect.

AutoXR HalfDork
2/19/12 3:21 p.m.

Turbo's that say "Holset" on them work the best

MadScientistMatt SuperDork
2/19/12 3:30 p.m.

I'd agree with Dean - you have the same compressor, smaller turbine. The energy comes from both heat and pressure, and you've lost energy from the front to the back of the car.

pigeon SuperDork
2/19/12 5:12 p.m.

Not a direct answer, but is there really no room up front for a 951 turbo setup to drop in? I thought that the 924 and 944 were the same between the inner fenders.

Knurled Dork
2/19/12 6:45 p.m.

I have never seen a 931, but the 951 sits the turbo on the intake side of the engine, because there was no room on the exhaust side.

noddaz GRM+ Memberand Reader
2/19/12 6:45 p.m.

Good question.. I have wondered things about rear mounted turbos also.
But there is room in a 924 for a turbo up front. Porsche did it from the factory... But enough of off topic... Back to the OPs question...

Vigo SuperDork
2/19/12 7:47 p.m.

The turbine turns mechanical energy in the air (basically kinetic energy) into mechanical energy in the turbine shaft.

Heat and pressure are just means to an end. The turbo doesnt necessarily work off of heat or pressure (although a LOT of people think this). The pump that's putting mechanical energy into the air (i.e. the engine) for the turbo needs heat/pressure, though. Heat/Pressure because there's an important relationship there..

Anyway, who needs an essay. Same compressor, smaller turbine. How much smaller? There's not much to go on. I learned ballpark turbine sizing just by observing what other people did (mostly on the internet) and how well it worked. There arent enough rear mounted setups documented on the internet to get that kind of benefit from just observing a vast set of examples.

I know how the smallest t3 spools bolted right up to a 2.5L.. pretty quickly. So if you wanted to downsize the turbine from there you'd be going down to the gt25/28 turbines. So if i were to make a broad generalization... you have a decent-flowing 2.5L and you want decent response and to make 300-400 hp.. I would look at something like a gt2871. Basically a t25 size turbine, with a t04b size compressor. In garrett terms, anyway. If you wanted even better response and to make like 320hp or less, maybe a gt2860 or even 2560.

dean1484 GRM+ Memberand SuperDork
2/19/12 8:01 p.m.

Yes you can replace the intake. engine mount and the exhaust system and the oil pan and add in oil lines and the cooling pumps and all the other stuff that was added to create a 951 and ti will fit under the hood. But what is the fun in that. Just go by a 951. It will be cheaper.

I have looked at several options with respect to forced induction in a 944 and the easiest and best bang for the $$$$ is either supercharging (an M62 is the perfect size for these cars) or to go with a turbo but not installing it like a 951. Basically I have narrowed ti to two choices for the turbo.

  1. Install it in the rear of the car (replacing the muffler)

  2. Install it similarly to how the Callaway Turbo was done.

Both have there problems. Both have upsides.

SVreX SuperDork
2/19/12 9:08 p.m.

Once you gain some space by moving to the rear, how about a pair of sequential turbos? Should help address the lag.

icaneat50eggs New Reader
2/19/12 11:29 p.m.


all being the same, losing temp = losing pressure.

JohnyHachi6 Reader
2/20/12 2:52 a.m.

^ yeah, from a thermodynamics standpoint (which is really what you want to consider with any turbomachinery), it's the exhaust stream enthaply that is being converted to mechanical energy in the turbine. The enthalpy is related to both temperature and pressure, the more you have, the better...

If you were to insulate your exhaust system all the way back to the turbo, there would be a lot more enthalpy available in the exhaust stream which would help mitigate lag to some extent. There are still some dynamic effects of actually moving that hot, high-pressure gas from the engine to the turbo when you step on it, and you won't get rid of these without decreasing the length of the exhaust system.

Also, I don't mean to be a shiny happy person, but in reply to Vigo, the turbine in a turbocharger really isn't converting kinetic energy to shaft work. There's very little change in kinetic energy upstream and downstream of a turbine. It's really the thermal energy and pressure (these combined are basically the stream enthalpy) that are being converted to shaft work in the turbine. There's a lot of confusion on this subject and I think it's related to relatively poor definitions of the various types of energy found in the exhaust stream.

Raze SuperDork
2/20/12 6:34 a.m.


Didn't we have this very same conversation almost exactly 2 years ago?


SVreX SuperDork
2/20/12 6:43 a.m.
icaneat50eggs wrote: pv=nrt all being the same, losing temp = losing pressure.

I agree. Not sure where I was going on the temp statement. Brain fart.

dean1484 GRM+ Memberand SuperDork
2/20/12 7:37 a.m.
Raze wrote: Dean, Didn't we have this very same conversation almost exactly 2 years ago? http://grassrootsmotorsports.com/forum/grm/sizing-a-turbo-for-rear-mounting/19240/page1/

Yes I did and I never for a good answer. I figured that someone may have an answer now that is a year later. I looked for the thread but with the site search function being what it is and google is a hit or miss proposition. It did find my M62 thread that I dug up this week.

Raze SuperDork
2/20/12 9:45 a.m.

Everything I researched at the time led me to the conclusion you need a smaller turbine and/or smaller turbine housing. That's one of the reasons Holsets come up in discussion is that for many of the turbos including the HY/HX 35s, 40s, 45s, HE351s, etc. there are multiple configurations for each. The problem with all of this is first figuring out which turbo you want based on a compressor map overlaying your engine flow numbers based on rpm range and desired boost level. This should size your turbo. Then you'll have to look into either hybridization of your given compressor with a slightly smaller turbine, or find a turbo with identically sized turbines with various sized turbine housing and go for a smaller one.

If you're using a smaller sized turbo, say a T3, below 15psi there are a couple options readily off the shelf that I have experience with and can relate hopefully for an example: Merkurs used a .63 A/R turbine for the Automatics, .48 turbine for the manuals (autos were limited to 10psi, manuals 15psi) with a .60 A/R compressor. Early SVOs used .63 as well. These turbos spooled slower (to protect the auto trans). The TurboCoupe used a smaller IHI turbo. In other words find a turbo with an appropriate compressor, get the A/R of the turbine, see if there's a smaller variant, or a smaller housing that could be fit.

There is one more possibility I will point out and leave up to you to investigate. Holset started running variable geometry turbos a while back that would electrically actuate a disc in the turbine section to change the effective area of the turbine housing. This led to less turbo lag and better operating range boost. They had issues with these and many can be found for a song on eBay. There was a DSM fella who junked the electric actuator and ran a mechanical setup. In your case you could use a system like this to 'tune' in a desired A/R. I'm just saying...

1988RedT2 SuperDork
2/20/12 10:04 a.m.

I wouldn't compromise. Keep it as close to the engine as possible. What mean no room? Punch hole in hood!

carguy123 SuperDork
2/20/12 10:08 a.m.

And since you don't need an intercooler who says there'll be lag?

Xceler8x SuperDork
2/20/12 1:43 p.m.
carguy123 wrote: And since you don't need an intercooler who says there'll be lag?

Are you saying an intercooler increases turbo spool times?

dean1484 GRM+ Memberand SuperDork
2/20/12 5:24 p.m.
Xceler8x wrote:
carguy123 wrote: And since you don't need an intercooler who says there'll be lag?
Are you saying an intercooler increases turbo spool times?

I could see it increasing the time it takes the boost to get in to the motor and this combined with the cooling of the charge (causing the volume to be reduced) could also add a little bit of lag.

I will post photos in a bit I was laying things out this afternoon in the driveway. Nothing spectacular but I really like the fitment and the alignment of things.

dean1484 GRM+ Memberand SuperDork
2/20/12 5:27 p.m.

You know.. .. . It just occurred to me that I should do a rear mounted turbo that then feeds a into a supercharger. . . . .

Raze SuperDork
2/20/12 6:58 p.m.
dean1484 wrote: You know.. .. . It just occurred to me that I should do a rear mounted turbo that then feeds a into a supercharger. . . . .

or another turbo

Vigo SuperDork
2/20/12 11:02 p.m.
There's very little change in kinetic energy upstream and downstream of a turbine.

Im not sure how you can say that with a straight face.

Oh, and one reason why Holsets might come up often in rear-mount applications is because they tend to have gas-v8-size compressors on too-small-for-gas-v8 turbines, which sort of goes with the same the whole theme that's been echoing in here about same compressor as usual, but smaller turbine (housing). At the end of the day the holset turbine has to sized right to spool off-idle on a diesel that, while it may have similar displacement to a gas motor from the same vehicle, has E36 M3ty VE and only revs to 3000 rpm.

JohnyHachi6 Reader
2/21/12 1:14 a.m.
Vigo wrote: Im not sure how you can say that with a straight face.

I don't know what's so hard to understand about it and it's certainly the case. I was wondering more about it today and started to talk to one of the Professors I work with at Tech about it. He's a specialist in turbomachinery who has been working in the field since 1979 and I asked him specifically about this - he agreed that there is very little change in kinetic energy across the turbine.

If you pick up an intro thermo textbook, I'm sure you can find an example or look at some of the relevant equations.

dean1484 GRM+ Memberand SuperDork
2/21/12 10:34 a.m.

Some photos of what I am thinking.

If anyone is wondering it is a K04 turbo form a Mazda 2.2L IT is designed for off the line boost in the Mazda so was thinking it would work reasonably well in a rear mount application with the 2.5L that is in my car. The compressor map for this unit looks to be about perfect for what I want. I am probably going to have to dial the boost back a bit. Possibly swapping the wast gate diaphragm for one set at a lower boost. I will have to see what is out there that I can work with that is interchangeable.

All I need is to get the adaptor mount plates and start welding. I am thinking of putting both the coolant and the oiling systems in the rear of the car with there own coolers and electric pumps.

I also was wondering if anyone had a source for oval tubing. At very least I would like to use it to run back form the turbo to the front of the car.

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