Rear Mount VNT turbo on S2

[ssschaib]

For starters, I don't care if you think rear mounting the turbo is stupid. It's what I'm going to do so don't waste yours or my time trying to tell me all the reason I shouldn't do it.

My build thus far:

89 Porsche S2 Cabriolet
megasquirt MS2 installed
Full sequential injection with 36 Lb Giant White Bosch Injectors
Full sequential ignition running LS truck coils
Pentometer TPS from Volvo 850
AFM eliminated, using GM IAT in intake pipe.

My plan (for winter 2017):

Rear mount a VNT turbo. Likely a GTB2260VK or GTB2566VK
Use a "S" intake manifold so throttle body will be on firewall side to cut down on plumbing.
Ceramic coat or exhaust wrap the entire exhaust system back to turbo. (Cat is long gone)
Tial Wastegate. (Probably overkill)
Planning to run 8 PSI.


My question/concerns:

I'm wanting to use a VNT turbo because it will eliminate any lag from being a rear mount. Also, since it's a rear mount, the EGT will be low enough as to not damage the turbo. My question is should I ceramic coat the exhaust, heat wrap it, or do nothing to it? Traditionally, turbos like to be hot which helps them spool. The VNT doesn't like being that hot and since it changes it's AR, do I even want to keep the incoming exhaust as hot as I can all the way to the back. Wouldn't it make sense to have the exhaust cool before entering the turbo since it will adjust the AR to spool fast regardless? And cooler exhaust should equal a cooler charge.

Do you think I can run more than 8 PSI? Alot of modern engines run high CR and still run 12-15 PSi of boost without forged internal. IE. the Ecoboost engines. Sense I have ignition control, could I not remove timing and add boost?

I've read that my block likely has the turbo oil feed on the balance shaft housing blocked with a plug. Is this plug just a threaded bolt that I can remove and screw a fitting in to? Does the oil return on the pan also have a removable threaded plug or do I need to tap threads?

Do I even need to run a wastegate? VNT's typically don't, but they also usually use an electronic actuator to control the amount of boost. I don't think the Megasquirt is capable of controlling this actuator so I was going to use a vacuum operated actuator. I was then going to use a wastegate as a stop-fail measure because I've read that the vacuum operated actuators tend to overboost. Anybody have any experience with this?

Thank you for any help/insight.

[v2rocket_aka944]

Ceramic coat
Yes to 8psi since you have timing control, the s2 head has a very good combustion chamber.
Oil feed plug is tapped in the upper balance shaft cover, oil drain is already tapped in the oil pan with a big threaded plug.
Can you find a vnt with built in WG? Even if not, i'd run one just in case you ever needed to " turn it off " to limp home without boost (safety measure)

Looking forward to seeing progress with this.

[flash968]

the rods won't likely handle that much boost. been there done that. bent quite a few at just over 6psi, and there was no detonation involved. you'll need to swap rods to 951 rods and resize them, or go carillo like i did.

[northern_aqs]

Are you going to rely on oil supplying the turbo by gravity to (and back from) the back of the car? My understanding is that people will usually run a remote, closed loop oil system local to the turbo. Getting the oil supply to the turbo won't be the issue, getting it back might be.

[v2rocket_aka944]

Originally Posted by flash968

the rods won't likely handle that much boost. been there done that. bent quite a few at just over 6psi, and there was no detonation involved. you'll need to swap rods to 951 rods and resize them, or go carillo like i did.

were the rods bent the early 968 "thin" ones?
944S2 has the same rods as 944 NA and late (89+) 944T.

[flash968]

it wouldn't matter. the difference between the early and late rods is not all that much, and it is not where the rods bent. the reinforcing they did on the rods was for stretching, not bending. they found that the early rods stretched at high rpms, so they beefed them up a bit at the place where they would stretch. unfortunately they did not beef up the center area, so they are both subject to high pressure bending.

[ssschaib]

I've got 2 spare 3.0L engines. I'll build one with forged internals on a stand. Will probably do CR to 9:1 or 9.5:1. As far as oiling, I'll be using a Turbowerx scavenge pump to pump oil back to the front. I'll be utilizing the existing ports on the engine that the turbo cars use. I flirted with the idea of a standalone dry sump system, but ultimately decided against it for cost and complexity. Not that the system would be complex, but it will add more failure points.

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[911tracker85]

check the 928 forum here and RL. I have seen several threads about rear turbos on 928s

[Rasta Monsta]

Originally Posted by ssschaib

It's what I'm going to do so don't waste yours or my time trying to tell me all the reason I shouldn't do it.

Ironic, to say the least.

[azbanks]

Can you give us more details on your MS build/install?

[ssschaib]

Sure, what would you like to know? Car is at the upholstery shop so I can't taste any pics but I'll answer any questions. I can upload my tune file if you'd like. Still messing around with the IAC control but other than that, is pretty much set.

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