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Join Date: Mar 2009
Posts: 158
I know this is an old thread but I have some input. I have a 993NA 11.2 CR running 6 psi of boost. It was built by prototech in Florida. The previous owner use to run 93 pump gas. I had protomotive retune it for 100 oct. The powerband is awesome!!! Compared to my previous 993tt with K24 hybrids (hence the screen name) at 600hp x 600tq this setup is much more fun!! While only about 450hp the fun factor is greater because of better throttle response and driveabilty.

The block is stock!!

I am tearing it apart this winter and upgrading the rods. While apart I am thinking of changing out the cast pistons. I would like to stay with the high CR low boost setup.

I read a lot about this on the web. Effective CR varies with cams and other volumetric efficiencies. I plan to use the apexi AVCR to vary boost in order to limit cylinder pressure. By decreasing boost at the motors torque peak (peak efficiency) and adding it when the efficiency falls you can flatten the torque curve out and not exceed the limits of the motor.

Peak cylinder pressure limits will be dictated by fuel, ambiant temp, oil contamination, and head design.
Old 11-14-2009, 09:27 AM
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Join Date: Nov 2003
Location: Chesterfield IN
Posts: 1,316
Thank you for posting this. If you check the archives you’ll find I started a thread on my project turbocharging the 3.6L N. A. with twin 996 turbos. I’m pleased to see my projections for engine characteristics and performance are proven out by your experience.

While this forum is mainly guys working with 930 Turbo based motors (most are highly modified) there are a few like me who just feel the need to apply turbos to less conventional applications.

Please start a thread and post pictures of the installation, manifolding, fuel system changes, oiling, etc.

Last edited by copbait73; 11-14-2009 at 12:04 PM..
Old 11-14-2009, 10:01 AM
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269

Just my two cents:

High compression w boost is an interesting combination. I suspect it is mostly helpful with drivability below 4000rpm. Above that boost becomes more effective and hight compression becomes a limiting factor.

Are you sure you need to replace the rods? For most applications, even to 600hp+ they seem to be fine. It is not typically the power stroke that is the problem. It is the centrifugal force that comes with higher than stock RPM. Thus, if you are planning on running over say 7K rpm do the rods.

I could be wrong but I believe that your combination is equal to about a 13/1 CR motor. Inter-cooler and turbo efficiency becomes very important and is probably where most effort should be placed. I suspect it is Protomotoves IC and choice of turbos for there efficiency is what allows them to somewhat bend the boost to compression rules of thumb.

Not sure that managing boost by RPM is going to add much in your case as you are running so close to the edge already. I do not know if it can take much more boost at lower rpm's. It might help to pull boost back a bit at TQ peak where combustion pressures are at there highest. Also, I suspect you might start to run into decreasing turbo efficiency as you approach higher rpm so trying to add boost will increase inlet temps and increase susceptibility to detonation.

I would think managing boost by rpm and load might be best used for filling in low rpm and pulling back boost to stay withing other limitations like stress limits of the drive train or decreasing efficiencies in other areas like a fuel limit, turbo limit, inter-cooler limit.

If me, I would put effort into keeping the heads sealed like adding fire rings and by all means, get forged pistons and consider having the heads and piston tops coated to reduce heat transfer.

Not an expert on this, just what I think.
Old 11-14-2009, 04:16 PM
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