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Help with comp ratios and turbocharger..

So im getting my 79 SC motor rebuilt and would like to have a low pressure turbocharger on it.

this would be a street motor with access to 110 octane for track days.

Ive been talking to JE...
what kind of compression ratio should i I use for 91 octane...?

can i make 8.0-1 and 10-psi work?

what could i run for track days?

my old school mechanic is telling me 7.5-1. 10psi for the street and 14psi for the track...

what else should i do why im inside? Im kinda on a budget..


i would use a draw through CIS setup(ala BAE turbo) for a year until i can go with Tony's mega-squirt EFI or similar...
I wont turn up the pressure until i can fit an intercooler on there.. that is a priority..

thanks for any help guys

-nick
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Old 06-16-2005, 05:21 PM
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I think you should either wait and get the proper turbo system matched to the engine or just build a nice N/A hot street engine
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Old 06-16-2005, 06:04 PM
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Thanks for the reply.

"proper turbo system matched to the engine"

what do you mean by "proper"? and "matched"? Do you mean sourcing a turbocharger with a map suitable for my application?

"just build a nice N/A hot street engine"

the $$ to power ratio of the NA stuff is too high for me.


should i just re-phrase the question without all the other bull?

what kind of PSI can i run safely with 7.5-1 and 8-1 with 91 octane and 110 respectively on a 3L CIS motor with a decent intercooler?

thanks for your help
-nick
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Last edited by air-cool-me; 06-16-2005 at 11:31 PM..
Old 06-16-2005, 11:26 PM
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Nick my compromise is using the stock heads stock compression 8.51 with a 7th injector, intercooler, and a MSD boost timing retard unit, 11lbs boost via a k27.
You will always want more boost and bigger, but for reliable street with plenty of kick this is what I am running.
The performance going from 5 or 6lbs your used to and 9 or 10 is significant try it you will like it. If you really want more, than you will need 930 heads that can handle the heat generated and forged pistons.
If your on a budget like I am it is best staying within the parameters of what the stock engine can handle use all the safty measures like fuel enrichment, intercooler and boost timing retard and cross your fingers when you stand on it. We cant afford engine failures and you have to draw the line somewhere. I would love to build a 500 to 600hp turbo 930 engine for my 914 someday but right now I am very happy with 300 to 400 out of the SC engine.
I have my engine apart right now and I am considering new pistons and cylinders I may even jump up to 3.2 displacement. I will let you know what I decide to do.
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Old 06-17-2005, 05:31 AM
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Quote:
Originally posted by air-cool-me


what kind of PSI can i run safely with 7.5-1 and 8-1 with 91 octane and 110 respectively on a 3L CIS motor with a decent intercooler?

thanks for your help

-nick
I think it depends on your CIS system. Some run leaner than others. I have read that a lot of people are running 8 to 1 at 1 bar pressure with 93 octane. But that is with EFI. I think most of them do turn down the boost to .8 bar most of the time.

Sorry not very helpful

Dean
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Old 06-17-2005, 05:32 AM
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well im not looking for a monster.. just i can afford the extra 900$ for any comp ratio i want..

im going to go for a better turbo.
an intercooler is a must for tack days down in AZ.

i figure the 8.0-1 i can take the same steps as you but include a small decrease in compression to give me some more wiggle room.


10 psi is all i want for the street use... nothing more.

i just want 300hp at the track and with 8.0 or 7.5 to 1 ,110 octane and 1 bar or less.

is this a reasonable goal?


chappy let me know what you do.

-nick
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Old 06-17-2005, 02:20 PM
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You would have about 300hp with 9lbs and a 7th injector with the stock compression and still have the low end drivability with stock 8.51. Sutter ran his that way for many many years without a problem, the k27 or garrett turbos are so much quicker to the punch than the Rayjay I think you would be surprised at what performance 9lbs will get you without an intercooler with 92 octane gas. I started adding all the other stuff like the intercooler and pushed the boost up to 11 not because I was disappointed with the performance at 9lbs just because I ran out of things to do to my car and I love tinkering on it. The 914 was damn fast running at 9lbs coupled with the short gear lsd tranny. I sometimes have to ask myself why am I trying to push it further. My answer is there aren't any 12 step turbo boost groups that I can join to help me with my addiction.
I say keep it simple throw a 7th injector for fuel enrichment and a intercooler, stay at 8 or stock compression and keep the boost under 11 your eyes will be as big as moons until you get used to it. Then start looking for a 930 engine.
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Old 06-17-2005, 07:26 PM
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There ya go! A setup that is matched well for the intent and on a budget. If you can mention 'budget' AND Porsche in the same paragraph!!
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Old 06-18-2005, 02:17 PM
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