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Thread: Cams?

  1. #1


    I have owned a 71 911T for the last five years, It is used for autocross and someday maybe club racing. My question is is it possible to replace the stock cams with cams from a E? will this make much of a performance difference (I have the stock twin pipe sport muffler)? Will I have enough valve clearance? Does the distributor need to have the advance curve changed? (Bosch dist.) Also the mechanic that rebuilt this engine in the early 90's said he thinks he remembers putting "S" pistons in, it's not that I don't trust him it's just that he did not seem real sure. Is there a way to tell what I have in this car without tearing it apeart? As a shade tree machanic I am greteful to have a forum like this- thank you Justin

  2. #2


    One way to tell is to remove the engine fan assy and see what type of barrels you have. On a "T", the cylinders are cast iron. On Es and Ss, the barrels are aluminum. For more piston details, you'll have to remove the heads. There's still a chance (slim) you might have T pistons inside aluminum barrels. If you have S pistons, your compression pressure should be higher due to the higher compression ratio (compared to a T engine).

    To determine the type of cam you have, you can put a dial indicator on the rocker arm, temporarily adjust the clearance to "0", then measure the lift as you slowly rotate the crank. Compare with specs.

    You also need to know how much valve-to-piston clearance you have. The experts says about 0.040" minimum or so (don't quote me though). With the cylinder at TDC and the dial indicator setup, you can run the rocker arm adjuster inwards until the valve stops against the piston. I haven't done it this way so I don't know if there's enough travel on the adjuster to go this far. At any rate, take it easy and don't force the issue. Since the T had the lowest compression ratio between the three models, you should be fairly safe in the valve clearance dept. I would check the clearance at any rate.

    I'd recommend the E cams. Using a longer duration S cam will result in poorer low end response compared to higher revs. The T's already-low compression ratio doesn't help. If you're autocrossing you know low end torque/response is more important than top end power.

    I also recommend recurving the distributor using the early S advance rates as a guide as well as rejetting the carbs.


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