What do you gain going from 11.5 to 12.5 CR on 3.8?

[JoeMag]

What sort of gain (torque, hp, power band, etc) would you see in a 3.8 going from CR of 11.5 to 12.5? 993 RSR cams, rsr enduro valves (51.5 / 43.5), rsr headers, plastic 964 intake (yea I know, no $ for itb's now).

approx how much would I need to bump up fuel (% wise)?

[mpeastend]

I have a 3.8 (3.74 actually) with 12.5 CR. While I don't know how much is gained from going up one full compression point, I do know what the fuel needs are. The engine builder told me that for track applications I need to put in 110 leaded. I use the benchmark/standard...Sunoco purple. I was told that, in a pinch, for driving on the street where you are not using full throttle, and it's not too hot out, 100 octane would be acceptable, but to be very careful doing it as this is only in an emergency. For track use you cannot get away without using leaded fuel as unleaded only goes up to 104. This will limit your compression choices if you need a catalytic converter. Hope this helps.

[JoeMag]

mpeastend -- who built the motor?

[Plavan]

Heat

[mpeastend]

My motor was built by Auto Associates of Canton Connecticut. My car is very similar to Matteo's IROC, the build is documented in this thread:

My '74 RS IROC project

There are quite a few pics of the motor, dyno sheets, etc. towards the end of the thread. There was a brief video posted by a friend while picking up the car. Turn up the volume at the 0:59 min mark to hear the PMO's, cams & Ben's 2-in, 2-out create a symphony...and that was through the trees on route 202, at the bottom of the hill that the shop sits on, a good distance away. Enjoy!

http://video.google.com/videoplay?docid=2875195681434852402

[smokintr6]

Joe Mag, What fuel are you running in the car at 11.5:1?

[engguy]

About 20hp or so if your lucky.

[mpeastend]

BTW, the higher compression gives you that nice crisp crack when you blip the throttle (assuming ITB's or carbs) & that immediate noise to no noise when you turn the car off. You also get that nice "funny" smell of the exhaust due to the higher octane fuel and, if your car is tuned right, that chalky gray/white residue, in the exhaust outlets. Me likey!

[bob barnes]

Joe,

A race engine is a system. One factor has little impact unless you take advantage of it! Your intake system is a limiting factor. Your Exhaust may be a limiting factor. Your cams are a limiting factor because there are better cams for the 3.8L race motor. Your fuel Mgt injection system may limit your HP.
There may be very little difference in HP because you are already restricted with the above pieces. However, 370HP is available at that 12.5CR if all the other items are in place.

Bob

[Eagledriver]

There are calculators on the net for this question. You just type in the information and it will give you the power increase of just the compression ratio change. Google Horsepower vs compression ratio.

-Andy