turbowrenchhead Posted August 19, 2013 Report Posted August 19, 2013 I have 20cc domes now and I'm getting 130psi on my tester. If I go to 19cc's domes what can I expect for an increase? When you increase compression does it make your mixture leaner because of more pull on the carbs? Quote
Zillaguy Posted August 19, 2013 Report Posted August 19, 2013 Its not that it makes it leaner, higher compression creates more heat... So, in turn you need more/ higher octane fuel to keep cylinder combustions temps down, so you don't detonate.. I run a 4mil on 19cc domes 150psi with +7 timing on pump fuel, and it RIPS hard with zero detonation... I'm using premium pump gas with Klotz R50 @ 32:1pre-mix Quote
turbowrenchhead Posted August 19, 2013 Author Report Posted August 19, 2013 Do you think I will have to go to a larger jet with just a compression change? Quote
turbowrenchhead Posted August 19, 2013 Author Report Posted August 19, 2013 My elevation is around 800'. Don't know what my porting numbers are. I enlarged the intake and exhaust. I opened up the exhaust on the side. Didn't mess with the top or bottom of the port and didn't mess with the transfer ports either. I went with a paper base gasket and my compression shot up to 170psi. Too much for pump gas. I would think 150-155psi would be the max on 93 octane. I ordered a set of OEM base gaskets and now waiting on them. I would like to get higher compression then what I have now. Quote
turbowrenchhead Posted August 19, 2013 Author Report Posted August 19, 2013 Now meaning the normal 130psi I have when running OEM base gaskets. Quote
turbowrenchhead Posted August 19, 2013 Author Report Posted August 19, 2013 I'm going to order a set of 19cc's and try those. The bike made 58HP on the dyno with 130psi. Quote
dajogejr Posted August 19, 2013 Report Posted August 19, 2013 Compression RATIO, not cranking compression, determines the need for octane. And Ratio can only be measured by knowing the inside of your engine, it can't be measured on a compression gauge. Quote
sprinklerman Posted August 19, 2013 Report Posted August 19, 2013 Compression RATIO, not cranking compression, determines the need for octane. And Ratio can only be measured by knowing the inside of your engine, it can't be measured on a compression gauge. ^^^ THIS Educate yourself on what this means, how to determine it, and go from there. If you are confident enough to port your engine,this should ( actually should have already been done) be easy for you,and is necessary for a happy engine that performs well. Cranking compression just wont tell you what you need to know. Quote
turbowrenchhead Posted August 19, 2013 Author Report Posted August 19, 2013 I replaced the paper base gaskets with OEM today. Now I have 160psi on both cylinders. It must be the pistons that are causing the rise in cranking compression. I bought the same brand name but they were a bit different in construction. Now I'm thinking I need 21cc domes to bring the compression down under 155psi. Quote
turbowrenchhead Posted August 19, 2013 Author Report Posted August 19, 2013 How much does a 1cc change in dome size change your psi rating? Quote
Calderon Posted August 19, 2013 Report Posted August 19, 2013 When I had the 20 domes my compression was at 150 and I went to the 19 domes and compression went to 160 Quote
Dixie normus Posted August 19, 2013 Report Posted August 19, 2013 10 psi per cc is what i usually go by Quote
turbowrenchhead Posted August 20, 2013 Author Report Posted August 20, 2013 Thanks for the input everyone! Quote
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