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jpnovak 09-11-2008 09:17 PM

Help a Newbie design for boost
Yeah Yeah. I swore it wouldn't happen. Some of you may recognize my name from the tech board. Yeah, from rust repair to paint and body to any mechanical project I have been there, done that. I am always intrigued by a challenge and I think I found my next project.

Last spring I took a ride in a 930 that had all the usual mods that CIS could still handle. Somewhere around 450HP or so was enough to really keep my mind going back to boost. The hit of the turbo was unforgettable. I admit, things happen quickly at that rate of acceleration.

So, here we are several months later and I am building a new engine. Its a NA 3.2SS for my track car. It is the usual high compression, big cam, EFI ITB motor. Near 300 at the crank is about right. My thoughts really turned to the dark side when I CC'd a JE 95mm PC set on the shelf to find 8.7:1. This is too low for my projects but they may have another application.

I have a set of twin plug heads on the shelf. I have a 12 plug dist, OK I have the parts to build one. I have EFI capabilities. I even have the ability to fab some ITBs. Seems like I could just bolt on a turbo and run some boost. I know its not that simple but here is a more detailed plan.

95 x 70.4 = 2994cc.
twin plug
single turbo
964 cams or maybe something hotter.
I would also fab an intercooler that was hidden under the decklid.
Combined with 8-10psi of boost I think I would be pretty happy.

here is where my knowledge falls apart. All you boost-a-holics can help me out. Let's start with exhaust. Can I just get a $400 OBX header set to mount a turbo to the car? I guess I need a wastegate attached to this header. I think there might even be a mount. Does the zork tube come straight off the wastegate or is that an uber short "muffler"?

From the turbo there is a pressurized outlet pipe that enters the engine bay and connects to the intercooler. Somewhere there is a BOV in the plumbing. I was thinking that I would need a cut and bent intercooler to wrap around the fan and clear a stock decklid. I would be willing to go with a ducktail that might give more clearance. I assume that the intercooler should cover the decklid opening to ensure maximum airflow draw by the fan. Can you just take a core, cut it in half and weld plates, intake plenums and outlet plenums to the intercooler. I have seen intercooler cores that are temptingly cheap. What flow numbers do I need? How do I determine pressure drop?

The intercooler outlet would likely go to a 3.6 plenum that sits on top of some ITBs. I have some motorcycle ITBs that would be just the ticket. I even have the intake adapters already cut that press-fit to the ITBs. The heads are the 1978 variety, twin-plugged.

How hot of a cam can you run? Does a narrow-lobe center, high overlap cam work as well with a turbo? I am wondering about high rpm HP abilities due to lower dynamic compression. The 964 cam is pretty mild in my book that doesn't have boost. I guess that is pretty aggressive under boost.

Is there a general rule of thumb regarding cylinder pressure? I can easily figure a dynamic compression ratio. That number combined with the amount of intake pressure gives you a cylinder pressure under compression. how much can a 911 motor handle? Let's say this was a 20k mile motor which for me is about 6 -7 years. I guess my question is can I run 8.7:1 under 10 psi boost with EFI controlling AFR and timing?

Fuel delivery: I am currently running a MSII setup. This did wonders to wake up a tired motor even compared to carbs that were on the otherwise stock 3.0. The programming aspects are easy. It shouldn't be a problem selecting the right size injectors. Now What about fuel pumps? What fuel delivery rate? What pressure? rising rate regulator to allow for the wide range of flow? Low Z injectors are a must in this case. I assume a very short duty-cycle at idle will be needed to keep within the 85% duty-cycle at WOT redline.

Bottom end: Can a stock 3.0 bottom end handle this much HP? ARe there any crank oiling mods? These are needed for high rpm but I think this motor would be done by 7K. Are the stock rods strong enough? I assume good rod hardware is needed (ARP, Raceware, etc). High-zoot head studs are a must as well.

I know this a long, late (for me) night rambling about boost. Help me sleep at night by telling me this will never work. :D

mb911 09-12-2008 05:07 AM

obx would be fine then you could run any stock 930 turbo set up. You can use a gt30 and it will bolt up to the obx and use any muffler designed for a 930. the zork tube mounts right to the turbo should you choose a wastegate dump mounts right to the waste gate and just vents to atmosphere. the mssII should have plenty of control for what you need for this application. You can run 3 bar fual pressure and that will work fine for all injector sizes but will have to change injectors out as they peak duty cycle at different HP setting but you should know that. oh and you can run the stock fuel pump with no issues. give me a call I could help you figure most of it out

hobieboy 09-12-2008 05:58 AM

My 2 cents...
The 1st thing I would research is your static compression ratio. Factory 3.3 uses 7.0:1 compression and runs 14psi of boost. 934/935 runs 6.5:1 compression for 1.4bar boost (& the engine lasts 24 hours). I think the general rule of thumb is boost = 1.5 times static compression. So running 8.7:1 might be high IMHO, depending if you are using it on track or street (where you won't even get 10 sec boost at a time).

I don't have experience with OBX. But, the WG does come off the header. WG can vent via straight pipe to atmosphere. You'll also want/need a BOV (between air filter & butterfly somewhere).

I am also using MS-1 extra so the control part should be fine :cool: I'm also using Carrera FPR and stock 930 fuel pumps. With 550cc injectors, I'll get to 85% duty cycle at max boost & RPM on track.

Good luck & have fun with the project...

rsrfan 09-12-2008 09:41 AM

check out the link above for some good ideas on intercoolers for your project.


beepbeep 09-12-2008 11:27 AM

ITB's aren't terribly effective on turbocharged car and 8.7:1 C/R is a tad too high for high power outputs, but otherwise it's a sound idea. Stay clear of raising rate FPR, use ordinary 3 bar FPR. Injectors shouldn't be a problem. 400-500cc should be OK. Get a good turbo to begin with. Stay clear of KKK "shed-modified" (sorry..."flow-prepped") junk. Garrett GT35R should be fine. Intercooler isn't so complicated...basically "the bigger the better". If you are technically inclined, you can also use water/air heat exchanger and thus get rid of intercooler packaging problems (and gain new with cooler in front). I don't see the reason for using 12-plug distributor either. Get yourself twin EDIS-6 coilpacks and your good to go. Sell 12-plug distributor to some MFI-lover.

964 cams should be just enough for medium output turbo engine.

BOV is a goes between plenum (after throttle) and turbo intake (or into air, for that sweet ricer sound). Wastegate needs to be connected before the turbo. Discharge into air is OK. Also, there are turbochargers with integrated wastegate.

Good luck!

NathanUK 09-13-2008 01:11 AM

Jamie, seems you are pretty spot on with your ideas. You should read this whole 930 section of the forum, you will find all the answers to your questions. It would be great to see you post as you progress with the project.

patkeefe 09-13-2008 08:38 PM

So, welcome to the dark side. All things considered, this boost thing is pretty much lots of fun.You can do it pretty easily as long as you don't get too greedy for HP. It seems the stock bottom end holds up well; I have ARP bolts, and 993 head studs. Large port SC heads and the 8.5 CIS pistons. I am running 8 PSI boost on CIS. Ben's is an EFI unit, I believe, 3.2. Jbrinkley has the same setup, and his dyno sheets were quite impressive. I would imagine any EFI is easier to deal with that the CIS system.

jsveb 09-14-2008 11:35 AM

Sounds like great project.
Will be watching...


jpnovak 09-16-2008 07:32 PM

Thanks for all the advice. A few questions arise.

What compression ratio allows for big boost? 7, 7.5, 8:1? It would not be hard to fly some material off the top of the pistons. Actually, it would be pretty easy. I think the edges of the JE's are a bit sharp at the valve pockets anyway.

How much cam is too much cam? Where does boost make up for the fall in torque due to duration and lift limitations? I see builds with SC or 964 cams. From a NA perspective, these are really mild. There are grinds that build just as much low-end torque and a ton more high rpm torque. I would think that combined with boost you could really build some big HP numbers.

I will keep reading this sub-forum. There is some good info here.

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