Thread: Too much crankcase pressure, novice engine builder

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How much gas do you have in the tank right now??????????If you could lower the level to near empty you could just add 5 gallons of a higher octane to see if it helps-------BUTTTTTTTTTTthose higher octane fuels do need to have jetting increased--------
do you have any jet numbers and cfm ratings of your carb and manifold???????? what do you have for exhaust? How /where do you use the exhaust guage?????? any leakage in gaskets or collectors?????

Originally Posted by jerry clayton

How much gas do you have in the tank right now??????????If you could lower the level to near empty you could just add 5 gallons of a higher octane to see if it helps-------BUTTTTTTTTTTthose higher octane fuels do need to have jetting increased--------

Ya' know Jerry, with due respect I disagree. If NegativeZero is wanting to run 91octane then adding 93 to the tank (or a bunch of octane booster) and then rejetting the carb to get the AFR right isn't really going to help, other than telling him that it's detonation. To run on 91 he's going to have to back out the jet changes, and solve the problem with timing/compression/repairs to rings or something else. Instead of "contaminating" the tank with different fuel I would back out some timing.

I believe that with aluminum heads and a 10.0 compression ratio running 91 octane should be good. I also believe that the compression leakdown test that you first suggested is the best next step to learn more about what's going on in each cylinder.

I hope that you don't get your knickers in a knot about someone disagreeing with you..............................

Originally Posted by rspears

Ya' know Jerry, with due respect I disagree. If NegativeZero is wanting to run 91octane then adding 93 to the tank (or a bunch of octane booster) and then rejetting the carb to get the AFR right isn't really going to help, other than telling him that it's detonation. To run on 91 he's going to have to back out the jet changes, and solve the problem with timing/compression/repairs to rings or something else. Instead of "contaminating" the tank with different fuel I would back out some timing.

I believe that with aluminum heads and a 10.0 compression ratio running 91 octane should be good. I also believe that the compression leakdown test that you first suggested is the best next step to learn more about what's going on in each cylinder.

I hope that you don't get your knickers in a knot about someone disagreeing with you..............................

I tend to agree. I don't understand how running higher octane gas relieves the crankcase pressure problem. If an engine is not allowed to breathe, IMHO, it's bound to blow up just like what happened to the 57 Ford I had.

It's a few gallons of fuel, I have a syphon though. It's not hard to empty. I have larger jets for my carb which is a 750 cfm 1411 Edelbrock. The manifold is a dual plane high rise also from Edelbrock. I don't know the flow number off the top of my head. Exhaust is a 2.5" diameter, H-pipe, with 1 5/8" full length headers. The AFR gauge is places about 6" behind one of the headers. No known leaks from the exhaust.

As is stands, since I have people here telling me this is likely a tuning issue, I'm going to go ahead and take my car to a tuning shop. I will get in touch with them before hand about all of this and give them the update and see what they would like me to do and what fuel they'd want me to have in the tank. I do plan on doing a leakdown test first however. When I get the equipment and do it, I'll be sure to post results here.

I'm interested to see if you think this is the right call, and I also appreciate all the info.