spurdy Posted December 25, 2007 Report Share Posted December 25, 2007 I was running a stock stroke ported motor with 17cc domes on VP red. I just added a new set of of 4 mil ported cylinders with custom 19cc domes. What sould I expect form a compression stand point. The compression on the stock motor with 17cc domes was about 175 psi. Now, with the 4 mil and 19cc custom domes the compression has jump to 205 psi. I spoke to the builder about this during the porting and we talked about compression and what fuel I was using. One question is how high on compression can I go with the VP red fuel. I think I understand why the dome size needs to change from the stock motor to the 4 mil. The 19cc dome is what is started with and when the 2mm is machine for clearance the total compressed volume changes effectly changing the 19cc dome to something smaller. Is this correct? Should I get new domes. I have posed the same question to the builder and am also waiting on his response. Thanks for the help. Quote Link to comment Share on other sites More sharing options...
wesw Posted December 25, 2007 Report Share Posted December 25, 2007 dont know what octane vp red is but 205 is a shit load of compression. check the squish and probably need some 20 or 21 domes sounds like where did you get the domes from Quote Link to comment Share on other sites More sharing options...
HotCarl Posted December 25, 2007 Report Share Posted December 25, 2007 what is your elevation? 175 psi with 17cc domes? really? either you are way up there or you had leak somewhere before and now you don't. Quote Link to comment Share on other sites More sharing options...
spurdy Posted December 25, 2007 Author Report Share Posted December 25, 2007 dont know what octane vp red is but 205 is a shit load of compression. check the squish and probably need some 20 or 21 domes sounds like where did you get the domes from About 900 ft. And now that you mention it I did have a leak which is what lead to the rebuild with the 4 mil crank. Leaking between the case halves in the front under the jugs. I got the domes from Kevin Herr (he also did the port work ). He has suggested going with some 21 + 2 domes. So, if I had a leak in the crank case, would that still lead to a false reading in the compression chamber? Maybe my compression tester isn't the greatest. It was the only one I could find when I needed it and both tests have been done with the same tester. VP red is 105 octane. Quote Link to comment Share on other sites More sharing options...
dajogejr Posted December 25, 2007 Report Share Posted December 25, 2007 I wouldn't trust 105 octane on 200 PSI plus... Crankcase seal won't effect cranking compression test. My old 4 mil had a bad crank seal and a BADLY leaking base gasket, still had 185 PSI, sealed it all up, same... Quote Link to comment Share on other sites More sharing options...
locogato11283 Posted December 25, 2007 Report Share Posted December 25, 2007 i wouldnt run 205 pounds period. thats just something bad waiting to happen. youd be a lot safer back down around 180. Quote Link to comment Share on other sites More sharing options...
spurdy Posted December 25, 2007 Author Report Share Posted December 25, 2007 I wouldn't trust 105 octane on 200 PSI plus... Crankcase seal won't effect cranking compression test. My old 4 mil had a bad crank seal and a BADLY leaking base gasket, still had 185 PSI, sealed it all up, same... I didn't think a leaky crankcase would effect compression. I am leaning towards a shitty compression tester. I picked this one up last minute last summer. Needed one on the fly and this was the best I could come up with. Tried removing the end from the tester (testing the short threaded end) and now my readings are all over the board. When using cut domes....how do you formulate what dome do you start with to get a comparable compression result after cutting the dome for a 4 mil stroke? What do you guys think I should have for compression with a 4 mil. long rod, 795 pistons, 19cc domes cut for the 4 mil., at 900 ft elevation (no spacer plate)? I will check the squish tomorrow. Wife is after me to help with xmas stuff tonight, LOL! Priorities! Locogato: What are you running? You"re just up the road from me...... Quote Link to comment Share on other sites More sharing options...
Wildcardracing Posted December 25, 2007 Report Share Posted December 25, 2007 I didn't think a leaky crankcase would effect compression. I am leaning towards a shitty compression tester. I picked this one up last minute last summer. Needed one on the fly and this was the best I could come up with. Tried removing the end from the tester (testing the short threaded end) and now my readings are all over the board. When using cut domes....how do you formulate what dome do you start with to get a comparable compression result after cutting the dome for a 4 mil stroke? What do you guys think I should have for compression with a 4 mil. long rod, 795 pistons, 19cc domes cut for the 4 mil., at 900 ft elevation (no spacer plate)? I will check the squish tomorrow. Wife is after me to help with xmas stuff tonight, LOL! Priorities! Locogato: What are you running? You"re just up the road from me...... I would do what your builder recommended and run some 21 +2 domes. 205 is a lot of compression. Quote Link to comment Share on other sites More sharing options...
wesw Posted December 25, 2007 Report Share Posted December 25, 2007 105 octane you need to be in teh 165-175 range imo Quote Link to comment Share on other sites More sharing options...
spurdy Posted December 25, 2007 Author Report Share Posted December 25, 2007 105 octane you need to be in teh 165-175 range imo 165-175 was the range Kevin and I were shooting for when we discussed building my motor. Now we need to figure out how we are going to get there. I would like to understand better how you decide what dome to start with to obtain the desired result. Or is it just a shot in the dark based on experience. Quote Link to comment Share on other sites More sharing options...
locogato11283 Posted December 25, 2007 Report Share Posted December 25, 2007 your builder should know what domes to run with his porting that will get certain compressions. in all my experiences 21cc domes will get you right around 170 psi, 20cc about 175 psi and 19cc about 180 psi.. but thats me on my setups. i can tell you one thing though, 205 is too much and its recipe for disaster. youre putting a lot of stress on your crank and pistons running that much compression. Quote Link to comment Share on other sites More sharing options...
spurdy Posted December 28, 2007 Author Report Share Posted December 28, 2007 (edited) OK. Here is what I've found out. I purchased a new compression tester. The right side is consistently 170-175 psi. The left side is consistently 190-195 psi. I removed the head to check for anything in the cylinder that could effect volume such as coolant or assembly lube and found nothing. I checked the domes. Both are 19 cc with a 13 degree angle and a recess of 0.097" (4 mil cut domes). The squish is 0.042" - 0.043". The motor has only been run about 2 -3 minutes running time before I found the high compression. So, in discussing this with the builder we decided to run the engine to operating temperature and recheck. I ran the engine for about 15 minutes and reached an operating temperature of about 150 F on the water return line. The surface temperature of the exhaust reached about 100-110 F with the right side trailing by about 10-15 degrees. I shut the engine off to check compression. I found the same compression results. I also found the right plug looking pretty good ( a little darker than chocolate brown) and the left side plug to be an ashen white. Could I have a plugged jet on the left side carb effecting the compression results? I have a couple of pictures. The one one the left is from the right side and the one on the right is from the left side. Sorry they are backwards! Thanks for any ideas. Shannon Edited December 28, 2007 by spurdy Quote Link to comment Share on other sites More sharing options...
Bobby Banshee Posted December 28, 2007 Report Share Posted December 28, 2007 OK. Here is what I've found out. I purchased a new compression tester. The right side is consistently 170-175 psi. The left side is consistently 190-195 psi. I removed the head to check for anything in the cylinder that could effect volume such as coolant or assembly lube and found nothing. I checked the domes. Both are 19 cc with a 13 degree angle and a recess of 0.097" (4 mil cut domes). The squish is 0.042" - 0.043". The motor has only been run about 2 -3 minutes running time before I found the high compression. So, in discussing this with the builder we decided to run the engine to operating temperature and recheck. I ran the engine for about 15 minutes and reached an operating temperature of about 150 F on the water return line. The surface temperature of the exhaust reached about 100-110 F with the right side trailing by about 10-15 degrees. I shut the engine off to check compression. I found the same compression results. I also found the right plug looking pretty good ( a little darker than chocolate brown) and the left side plug to be an ashen white. Could I have a plugged jet on the left side carb effecting the compression results? I have a couple of pictures. The one one the left is from the right side and the one on the right is from the left side. Sorry they are backwards! Thanks for any ideas. Shannon I've never heard of a jetting issue effect the compression. With your different cylinders being so far off, it looks like your rings haven't seated on the one side. Quote Link to comment Share on other sites More sharing options...
HotCarl Posted December 28, 2007 Report Share Posted December 28, 2007 You might want to pull the jugs back off, because it sounds like you have a ring proble or they are not evenly ported. If both exhaust ports are not identical you can get different comp readings. WHen compression testing is done it should be done with wot, so a pluged jet would be a none issue there, but could cause a difference in the plug color, but most likly is is because of the problem that is causing the comp difference. haha I said jugs. Quote Link to comment Share on other sites More sharing options...
spurdy Posted December 28, 2007 Author Report Share Posted December 28, 2007 You might want to pull the jugs back off, because it sounds like you have a ring proble or they are not evenly ported. If both exhaust ports are not identical you can get different comp readings. WHen compression testing is done it should be done with wot, so a pluged jet would be a none issue there, but could cause a difference in the plug color, but most likly is is because of the problem that is causing the comp difference.haha I said jugs. I know the compression test is to be done at WOT. Why is this necessary? Is it so that the maximum amount of air is available for compression? If so, I should be able to do a compression test with the carbs off the engine. This would be a great deal easier to test because I have to remove the gas tank and pull the slides out of the carbs to remove everything to remove the head. Especially if I am going to remove the jugs. If I had a ring problem, wouldn't this make lower compression instead of higher compression? With a 19cc dome cut for a 4 mil crank i should be around 170-175 psi according the the builder. Indicating that the 190-195 psi cylinder is the one giving me a problem. Thanks. Shannon Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.