3.0 Twin Turbo Project - Engine Build / Parts advice?

[911TT33]

Ok, we are about to embark on a rebuild project for my friends 911SC street twin turbo.

Heres what we are contemplating:

Scrap the CIS and fit a 3.2 Carrera manifold for aftermarket EFI ECU
Aftermarket fasteners throughout
Twin plug the heads
New PhBr valve guides
New valve springs
New retainers

Option 1 - Slip in 97mm 930 7.0:1 pistons/cylinders (modify oem rods accordingly) for 3.12L capacity

Option 2 - Retain the stock 8.5:1 pistons/cylinders and just limit boost to 1.0bar.

Option 3 - Use 1.0mm or 1.5mm shims under the cylinders to reduce CR to 7.5:1

Question: which option do you guys think is best for a street turbo that may see occassional track events?

Which path would you take? Discuss?

[Raceboy]

I would leave the 8.5 CR as is and use max 1.0 bars of boost. That would make a VERY quick car and not even compareable to factory 930's with their on-off type of power curve.
You can do that because of EFI+twin-plug.
Shimming the cylinders is bad idea because you are losing squish area which is pretty important to prevent detonation and improve performance. If you would be stuck on 10+ CR and those pistons, then shimming would be reasonable but with 8.5 CR I wouldn't even consider it.

[psalt]

What year SC ? Alusil or Nikasil ? Factory turbo parts are a better choice, the pistons are forged, SC pistons are cast and some of these cars, when new, had a problem with broken piston lands even in N/A form. Also see the archives for failed SC turbo engines.

If you are even considering putting it on a track, split the case, new bearing, upgraded pump, send the rods to Henry and go with a new set of turbo pistons/cylinders and turbo exhaust valves. You will also have to have the heads milled flat. If you really want higher compression get the Mahle Motorsport version.

If you are using later SC heads, there is the mismatched port size to consider.

[Raceboy]

I would not take the history of failed turbocharged SC engines too seriously as pretty much none of them had controlled fuel and ignition with twin plugs. You cannot expect the engine to last without proper tune.

Regarding cast vs forged pistons. Well, let's analyze this from the prospective of single cylinder power. On standard SC (204 hp) one cylinder produces 34 hp which is ridicolous. This is in the same catgeroy of euro 924 (32 hp) and 924 has also cast pistons FYI.

Piston material treatment (=strength) is most relevant when one wants to up the revs considerably. On turbocharged STREET car it is a moot point. My turbocharged 924 (with cast pistons) has more than DOUBLE the horsepower of standard car (260 hp) and that makes 65hp per cylinder.

911 SC with that hp per cylinder is easy. Ring land failures are from detonation, either caused by bad fuel or bad CIS. It is common on other cars to with bad tune, not just SC.

I would leave the engine as is presuming that it is healthy or if you pull it apart anyway, just make a refresh and twin-plug it just like you are planning. And making sure that it is tuned well after, and you won't have any problems.

7.0 CR with modern fuel and spark management is a joke, seriously.

[911TT33]

Thanks all. His car is a 79 SC, which has the bigger ports and 8.5:1 C/R pistons, not the 9.3:1 of the 80+ 911SC.

As this is going to be 95% street with the odd track event at the local Porsche club, he wants it to be extremely zippy around the streets and responsive, so we are in fact leaning more towards the 8.5:1 and leaving the boost at 1.0bar.

He's ordering a pair of Garrett GT28RS turbo's with twin 1.0bar TiAL 46mm Wastegates.

Definitely want to make sure the head is twin plugged, and whatever necessary prep-works are required to the top end too (guides, retainers etc).

What are you recommendations for guides/retainers/springs for turbo application? It's not going to be a high-RPM monster, and he's sticking with the SC CAMs.

[Raceboy]

I would get the smallest GT28 variant you can get, with .63 or even .48 AR turbines.

[911TT33]

Originally Posted by Raceboy

I would get the smallest GT28 variant you can get, with .63 or even .48 AR turbines.

Yep, gt28rs ball bearing turbos with 0.63 A/R hot sides is the plan.

[Lukesportsman]

Has thoughts changed recently? I don't understand the "need" of twin plugging a mildly boosted low rpm engine. One of the biggest issues with any Hemi and especially a large bore hemi is flame front with humped high compression heads. It seems like a relatively modest $$ build if using CIS cams and stock rods. Why not spend the money on better pistons to keep/improve squish area and get ideal compression. Then throw in some 964 cams to make it responsive and zippy off boost on street. I like the 8-8.5:1 idea for street, but with proper electronics to monitor AFR save the coins on head work.

Im not saying it cant help, but if nothing else is pushing envelope, then I'm not sure the hassle and cost is going to improve it. There is a reason Porsche didn't do it on their 3.3 turbos. Its all in trade offs and a CIS pistoned/cammed 6k rpm engine.....I'm not sure.

Now you bump up the compression/cam/boost/rpm/timing/use on track.......twin plug starts to come into light.

Of everything debated, UNLESS heads need done, I would move twin plugging and necessary ignition upgrades down the list. Everyone may disagree and I'm running custom GT20's on mine so response was my target too.

[Raceboy]

I often wonder why people don't get tired of saying "if Porsche didn't do this or that, it's not needed" etc.

Every 2 valve engine that has spark plug anywhere than centered benefits from twin-plugging and especially turbocharged engines that are used on the streets (=with pump fuel). Why? Because twin-plugging moves the MBT closer to TDC and hence reduces the detonation possibility and gains more power both because you can reach MBT with less advance and because reduced engine loss of combustion counterworking to piston that moves up.
Porsche did not use it on it's turbocharged street engines because of ridiculously low CR, big AR turbine and crappy fuel management (CIS). Porsche's main goal (like evry other car manufacturers) is not to offer best product possible, but to offer a product that just-just performs and does it cheaply (both by production cost and low warranty case count). Also the power improvements you see over model years are not developed on the fly but are carefully calculated and implemented gradually because once you make an ultimate engine/car how do you proceed next? I know this because I work in the car industry as a quality specialist for years.

Regarding OP's engine, no engine component can withstand poor tune and twin-plugging costs next to nothing when you go EFI and direct ignition.
He doesn't need forged pistons, Carillo rods etc for street driven, efi'ed 911.

Things tend to go backwards in 911 crowd, they think about the efi as a last thing, but in reality when turbocharging is involved on 911, it should be first thing to think about. Get rid of CIS and non-programmable ignition, one gets much better power, control, driveability and reliablity.
This is not a folklore, I talk this from my own personal experience (and it is backed by many others in both air and watercooled world).

[Flieger]

What does MBT stand for?

[Raceboy]

Originally Posted by Flieger

What does MBT stand for?

Minimum Best Timing for maximum torque (at given rpm).