turbowrenchhead Posted May 19, 2014 Report Share Posted May 19, 2014 I thought I would do a little experimenting on this last pair of cylinders since I got them cheap. I raised the exhaust port up 2mm. I did a cranking compression test with it on the bench and cranked it over with a large ratchet and socket off the crank gear nut. I also tried with a cordless and that was a bit faster, regardless I got the same reading on the both cylinders, 60psi. So is this normal when you raise the exhaust ports? Quote Link to comment Share on other sites More sharing options...
turbowrenchhead Posted May 19, 2014 Author Report Share Posted May 19, 2014 I have a .045 squish and 20cc domes. The last pair of cylinders were ported the same way except I didn't raise the exhaust port. Those made 150psi with the 20 domes. Quote Link to comment Share on other sites More sharing options...
turbowrenchhead Posted May 19, 2014 Author Report Share Posted May 19, 2014 Typo: Its suppose to be 21cc domes not 20's Quote Link to comment Share on other sites More sharing options...
turbowrenchhead Posted May 19, 2014 Author Report Share Posted May 19, 2014 Yes, it will lower cranking compression, but 60psi is incorrect. Thats what I'm thinking. How can it be so low? Quote Link to comment Share on other sites More sharing options...
turbowrenchhead Posted May 19, 2014 Author Report Share Posted May 19, 2014 For shits and giggles I tried blocking off the exhaust and seeing if the compression would go up. Now I get 90psi with a blocked exhaust. So that shoots down the theory that the exhaust porting is effecting the compression. Quote Link to comment Share on other sites More sharing options...
sheerider11 Posted May 19, 2014 Report Share Posted May 19, 2014 For shits and giggles I tried blocking off the exhaust and seeing if the compression would go up. Now I get 90psi with a blocked exhaust. So that shoots down the theory that the exhaust porting is effecting the compression.What?The compression only starts when all ports are closed. So well break it down to simple math. You have a sleeve that's 3 inches long. A hole (port) drilled 1 inch from the top. The piston moving up will push air out that hole till it's closed off. You now have one inch of volume to compress. Move that hole up 1/2 inch. Do that same test over. You now have 1/2inch of the volume to compress. Which would get you lower number since there is less room of closed off space. I'm trying to make it as simple as possible. If you can't understand that maybe some one can explain it better. Quote Link to comment Share on other sites More sharing options...
turbowrenchhead Posted May 19, 2014 Author Report Share Posted May 19, 2014 I understand how it works. However I'm kinda boggled how only 2mm would effect cranking compression that much Quote Link to comment Share on other sites More sharing options...
turbowrenchhead Posted May 19, 2014 Author Report Share Posted May 19, 2014 how did you come to that conclusion? Because the exhaust had no where to go? It is trapped in the head port? It probably doesn't work that way, who knows. I'm just thinking out loud. I'm really boggled why the compression is so low Quote Link to comment Share on other sites More sharing options...
turbowrenchhead Posted May 19, 2014 Author Report Share Posted May 19, 2014 Ok, so I pulled the head back off and the cylinders. I checked the ring gap. They are between .003 & .004. Piston/cylinder wall clearance is .003. I compared the old cylinder and new cylinder deck height. Old ones are at 102.37, new ones at 102.36. I had the new pair decked .004 because there was a nick in one of them. I measured how much I raised the exhaust port and I raised it 1.6mm over the old pair. So I'm really at a loss why compression is so low. Is it just because of trying to crank it over on the bench? Does the kick start actuate the piston's that much faster that you get higher cranking compression? Quote Link to comment Share on other sites More sharing options...
trickedcarbine Posted May 19, 2014 Report Share Posted May 19, 2014 Always check your compression gauge. If they are even breathed on wrong, they will show goofy readings Quote Link to comment Share on other sites More sharing options...
turbowrenchhead Posted May 19, 2014 Author Report Share Posted May 19, 2014 I've had my craftsman gauge for a long time and its been really good for several years. I've compared reading to snap on's and mac's and it was accurate. It's not the gauge. The next thing I'm going to try and putting the old cylinders and piston back on and see what I get for cranking compression on the bench using the same method I did today. Quote Link to comment Share on other sites More sharing options...
lowidenfast Posted May 19, 2014 Report Share Posted May 19, 2014 Always check your compression gauge. If they are even breathed on wrong, they will show goofy readings I can't comment on your original question but I have recently had problems with grossly inaccurate gauges myself. Maybe try another gauge before you go to a lot of trouble on this Quote Link to comment Share on other sites More sharing options...
BANSHEE HP Posted May 19, 2014 Report Share Posted May 19, 2014 Ffs Quote Link to comment Share on other sites More sharing options...
turbowrenchhead Posted May 19, 2014 Author Report Share Posted May 19, 2014 I tested my gauge on another machine. It read 120psi on a stock warrior. Seems about right Quote Link to comment Share on other sites More sharing options...
trickedcarbine Posted May 19, 2014 Report Share Posted May 19, 2014 Yeah, so maybe you are ok and just not getting enough crank the way you are trying. I've honestly never heard of using a ratchet to get compression. Did you try with an impact yet? Quote Link to comment Share on other sites More sharing options...
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