Banshee0028 Posted June 16, 2008 Report Share Posted June 16, 2008 Recently did a rebuild with a 4mil, woods port, 20cc custom domes, single carb, etc....I ran 100\110 oct gas since the rebuild since when I first tested it the comp tester showed 200+ psi! Now its about 180 in each cyl, however I want to run 93 octane not race gas! Should I go up to 21cc domes to lower the compression closer to 160 or 150 (jetting is close to prefect maybe a little rich). I dont want to go higher to 22cc as thats pretty much stock. Not sure how at sea level these 20cc domes have so much compression. I know it also has to do with the porting, but its a fast woods ported 4mil. I tested it with 2 different testers and they are both within 3-4 lbs of each other! Could it be a problem with the special cut domes for the 4mil and coolhead? It runs great!! Quote Link to comment Share on other sites More sharing options...
okbeast Posted June 16, 2008 Report Share Posted June 16, 2008 That seems a tad high. Have you checked your squish clearance? Quote Link to comment Share on other sites More sharing options...
camatv Posted June 16, 2008 Report Share Posted June 16, 2008 exhaust port height has as much to do with compression as dome size. lower port = more compression.. i have 17cc domes that are only at 160psi.. and run pump gas in that motor.. Quote Link to comment Share on other sites More sharing options...
Banshee0028 Posted June 17, 2008 Author Report Share Posted June 17, 2008 (edited) That seems a tad high. Have you checked your squish clearance? When I put it together I think it was around .45-.48 exhaust port height has as much to do with compression as dome size. lower port = more compression.. i have 17cc domes that are only at 160psi..and run pump gas in that motor.. How does that affect performance? Would it be better to have that raised to lower the compression, or is that something that I cant change since its already ported? Edited June 17, 2008 by Banshee0028 Quote Link to comment Share on other sites More sharing options...
No Limit Powersports Posted June 17, 2008 Report Share Posted June 17, 2008 No that squish is fine a little loose but it will be fine for a trail rig. I would go up to a 22c to get the compression down...its not stock because you said it was a 4 mill stroker so it will take a larger combustion area to make the same compression because now the piston has more area to make compression. Also a big factor is timing whats your set at with 160lbs i wouldnt go any higer than maybe 3* to be safe. Quote Link to comment Share on other sites More sharing options...
Banshee0028 Posted June 17, 2008 Author Report Share Posted June 17, 2008 No that squish is fine a little loose but it will be fine for a trail rig. I would go up to a 22c to get the compression down...its not stock because you said it was a 4 mill stroker so it will take a larger combustion area to make the same compression because now the piston has more area to make compression. Also a big factor is timing whats your set at with 160lbs i wouldnt go any higer than maybe 3* to be safe. Right now its +4. I hear what you are saying about the compression with the 4mil...However with the special cut "+2" domes doesn't this take that into account, or is it different? Either way I agree, I think I need to get some 21 or 22cc domes .... Quote Link to comment Share on other sites More sharing options...
Snopczynski Posted June 18, 2008 Report Share Posted June 18, 2008 Recently did a rebuild with a 4mil, woods port, 20cc custom domes, single carb, etc....I ran 100\110 oct gas since the rebuild since when I first tested it the comp tester showed 200+ psi! Now its about 180 in each cyl, however I want to run 93 octane not race gas! Should I go up to 21cc domes to lower the compression closer to 160 or 150 (jetting is close to prefect maybe a little rich). I dont want to go higher to 22cc as thats pretty much stock. Not sure how at sea level these 20cc domes have so much compression. I know it also has to do with the porting, but its a fast woods ported 4mil. I tested it with 2 different testers and they are both within 3-4 lbs of each other! Could it be a problem with the special cut domes for the 4mil and coolhead? It runs great!! Too much for pump gas, but I would run race gas and that compression for a trail bike. Quote Link to comment Share on other sites More sharing options...
jbooker82 Posted June 18, 2008 Report Share Posted June 18, 2008 You need to shoot for 150 for pump gas. Your going to notice a drop in midragne power when you step it back down. Guess that is the nice thing about a cool head. Just throw your domes back in if you want to go back. Quote Link to comment Share on other sites More sharing options...
dajogejr Posted June 18, 2008 Report Share Posted June 18, 2008 Get a new tester. It originally showed over 200 PSI, now it's showing over 180 on 20cc domes? That's 20 PSI plus drop, and 180 is high for a stock stroke motor on 20cc domes. Get a new tester. Quote Link to comment Share on other sites More sharing options...
Snopczynski Posted June 18, 2008 Report Share Posted June 18, 2008 Get a new tester.It originally showed over 200 PSI, now it's showing over 180 on 20cc domes? That's 20 PSI plus drop, and 180 is high for a stock stroke motor on 20cc domes. Get a new tester. He already checked it with 2 different testers. I would personally run it, your gonna lose power everywhere if you drop the compression down. Quote Link to comment Share on other sites More sharing options...
Banshee0028 Posted June 19, 2008 Author Report Share Posted June 19, 2008 Get a new tester.It originally showed over 200 PSI, now it's showing over 180 on 20cc domes? That's 20 PSI plus drop, and 180 is high for a stock stroke motor on 20cc domes. Get a new tester. Hey Daj, yea i did use 2 different testers a Craftsman, and some other brand and now they are both within 2-3 lbs of each other. The 200 lbs was right after I rebuilt it 6 months or so ago, so I guess its broken in a little? Its also a 4mil. I might try 21cc domes, but still deciding...Gas prices are friggin crazy Quote Link to comment Share on other sites More sharing options...
Snopczynski Posted June 19, 2008 Report Share Posted June 19, 2008 Hey Daj, yea i did use 2 different testers a Craftsman, and some other brand and now they are both within 2-3 lbs of each other. The 200 lbs was right after I rebuilt it 6 months or so ago, so I guess its broken in a little? Its also a 4mil. I might try 21cc domes, but still deciding...Gas prices are friggin crazy so your runnin a spacer plate, and your domes are not cut? Quote Link to comment Share on other sites More sharing options...
Washburn Posted June 19, 2008 Report Share Posted June 19, 2008 just keep it that way and run 110 octane. If you go down on your compression you will greatly sacrifice low and mid pull and thats what you want with a trail bike. Quote Link to comment Share on other sites More sharing options...
Banshee0028 Posted June 19, 2008 Author Report Share Posted June 19, 2008 so your runnin a spacer plate, and your domes are not cut? Snop, no I have the special cut domes for the 4mil..No spacer plate crap... :cool: Quote Link to comment Share on other sites More sharing options...
Banshee0028 Posted June 19, 2008 Author Report Share Posted June 19, 2008 just keep it that way and run 110 octane. If you go down on your compression you will greatly sacrifice low and mid pull and thats what you want with a trail bike. yea I may do that for now..Plus it should "last longer" until I need a rebuild. Once it gets down to 150 or so I guess it would be safe to use pump gas then... :biggrin: Quote Link to comment Share on other sites More sharing options...
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