engine rebuilt question

damian911 · 07-20-2006, 09:52 AM

engine built question???
i have a 83sc stock engine in need of a rebuilt....i was talking to a engine rebuilder with a pretty good reputation here in houston who told me basically i could go as far as using the je pistons 95mm with the 10:3:1 cr on my engine and still keep everything the close to the same namely the cis, w/ lil head work too, but that i would have to use 93 octane as gas. How accurate is this, can this be done?? this is not exactly what says in the dempsey book of engine rebuilding, haaa.....I basically want to increase the hp. (200-215) in my car overy stock as much as possible and keep the cis, exhaust and emissions are not a concern and changing the cams and pistons were expected expense of course....pls ur input would be appreciated......txks, damian,,,oh yea can the cylinders stay the same w/ higher compression ratios like in the 10's???

David · David's Garage

I've heard 9.8 is about as high as you'd want to go. If you reuse the cylinders, you'll probably have to have them recoated.

What part of Houston do live in?

damian911 · 07-21-2006, 09:44 AM

I heard 9.8 was the highest too, but i read it in a depseys book. I live in north houston... Question David, have u ever had your engine rebuilt and if so to what specs. and who did it??

Wayne 962 · Wayne 962's Garage

10.3 is close - you could have problems, but if you do, you can always alter the timing a bit to prevent detonation. On a 3.2 Carrera motor, you can't adjust the timing, so your options are limited.

You would have to run 93 or better (race gas or octane booster), or you'd have to run twin-plugs to *guarantee* that you wouldn't have detonation.

The trouble is, if you increase your compression too high, then you will have to retard your timing, which results in power loss, which counters the whole point of raising your compression to begin with.

For a street motor, I recommend keeing it under 10:1.

-Wayne

damian911 · 07-21-2006, 10:58 AM

thanks Wayne.....I live in houston, does heat or cool weather cause detonation problems to increase.?? Oh its very hot here, its now 100 deg...if i did go to 10.3 cr. would i get possibly more problems or less with a hot engine or neither???

I had an engine rebuilder here say i could go as far as 10.3 and have no problems with 95mm je pistons with 10.3, 964 cams and a little head work....and kinda scratchn my head now...damian

PPilot · 07-21-2006, 12:20 PM

I am running the higher comp. RoW 3.2 on 93 octane with no problems...its compression ratio I think is 10.1 ??

damian911 · 07-22-2006, 03:55 PM

thanks ppilot i think that is the comp/rat. that i was looking at anyways but on a 3.o. , nice to hear that ...oh what is ROW?

Henry Schmidt · Henry Schmidt's Garage

Cylinder pressure, ignition timing, fuel delivery and heat all effect the engines' tendency to detonate. Not to mention octane in ever decreasing amounts.
U.S. versions of the 80-83 911SC engine were prone to detonation with stock compression. 9.3:1. I don't know how many engines I've disassembled with broken pistons do to detonation.
Unless modifications are made to the fuel delivery system they will detonate over 9.5:1.
The problem seems to be a lean running condition at max torque (4600+- a few rpm). CIS injection will tend to go lean at cruise in most applications but when you throw a lambda control unit into the mix, lean is an understatement.

Lean running and high compression are a formula for disaster. The is why the W.O.W. (9.8:1, 210ph) engines have a special fuel distributor, warm up regulator, ignition distributor and no O2 sensor.