TeamRealtreeHD Posted June 21, 2009 Report Share Posted June 21, 2009 A buddy of mine finally got his bike up and running after it seized 3 years ago. He rebuilt it with the following: -Vito's Forged pistons, gaskets, bearings, etc. (their top end kit basically) He has a Pro Design Cool Head on the thing with 17cc domes No timing K&N filter in the airbox with the lid off, running stock carbs FMF Fatties with Power Core 2 Silencers 2 weeks ago was rebuild time, he has 8-9 gallon burnt through it as we speak We are at 1300ft ASL I did a compression test tonight and got 192 on the right, and 193 on the left. Now without any timing, doesn't that seem high??? It sure does to me for some reason. My bike last year, with stock HJR ported cylinders, running 17cc domes and +4 timing was right around 185. Those Vito's pistons can't be the difference, atleast not that much. Or could it? I would like to see what you guys think. I am stumped on that one lol Quote Link to comment Share on other sites More sharing options...
rubberneck Posted June 21, 2009 Report Share Posted June 21, 2009 couple of things. first off, 7psi could easily be mechanical error in the gauge, 2nd, you rbike was ported, so your kicking compression will be lower due to higher port hiegth. 3rd timing has no effect on compression, however it does play a large role in octane requirements. that bike needs to be on race fuel Quote Link to comment Share on other sites More sharing options...
TeamRealtreeHD Posted June 21, 2009 Author Report Share Posted June 21, 2009 He's running AV Gas I just thought that was really high, another buddy ran 18's with no timing with the same setup and had 155 per cylinder on the fresh build Quote Link to comment Share on other sites More sharing options...
278 Posted June 21, 2009 Report Share Posted June 21, 2009 Dropping a dome size to 18's will definantly drop the compression, but I'd say alot of it may have to do with the guage, different guages can range like 10 sometimes even 15 PSI. That and every bike will be different Quote Link to comment Share on other sites More sharing options...
rubberneck Posted June 21, 2009 Report Share Posted June 21, 2009 He's running AV Gas I just thought that was really high, another buddy ran 18's with no timing with the same setup and had 155 per cylinder on the fresh build No way, that guys gauge must be reading wrong 18cc domes on a fresh build should be way higher than 155 psi. I think your readings of 185 and your buddies of 192 with 17 domes are probably spot on. 17 is a pretty small dome. Quote Link to comment Share on other sites More sharing options...
bansheeseat$$ Posted June 21, 2009 Report Share Posted June 21, 2009 Another thing to consider is bore size.If his bore size is bigger then yours that will add more compression on the same cc dome. Quote Link to comment Share on other sites More sharing options...
shanYE west Posted June 21, 2009 Report Share Posted June 21, 2009 No way, that guys gauge must be reading wrong 18cc domes on a fresh build should be way higher than 155 psi. I think your readings of 185 and your buddies of 192 with 17 domes are probably spot on. 17 is a pretty small dome. I agree. :beer: Quote Link to comment Share on other sites More sharing options...
TeamRealtreeHD Posted June 21, 2009 Author Report Share Posted June 21, 2009 No way, that guys gauge must be reading wrong 18cc domes on a fresh build should be way higher than 155 psi. I think your readings of 185 and your buddies of 192 with 17 domes are probably spot on. 17 is a pretty small dome. Kevin told me my build last year would be 185-190, and he got really close for sure. My buddy's yesterday just seemed really high though. Both were tested with my guage. The other bike with the 18cc domes was tested on another guage, however. Quote Link to comment Share on other sites More sharing options...
stroker_2 Posted June 21, 2009 Report Share Posted June 21, 2009 Measure up the actual combustion chamber volume with the pistons at tdc and the heads bolted down. Also measure your exhaust port opening timings from tdc. Now calculate your "trapped" compression ratio. http://www.torqsoft.net/compression-ratio.html Quote Link to comment Share on other sites More sharing options...
FireHead Posted June 21, 2009 Report Share Posted June 21, 2009 Measure up the actual combustion chamber volume with the pistons at tdc and the heads bolted down. Also measure your exhaust port opening timings from tdc. Now calculate your "trapped" compression ratio. http://www.torqsoft.net/compression-ratio.html That website is trying to help you calculate static vs. dynamic compression. It's overly simplified in this case, but I suppose it will give you some sort of reference number. :geek: I would beware of a couple of the other things offered on that website as they are not so great. Quote Link to comment Share on other sites More sharing options...
mopar1rules Posted June 21, 2009 Report Share Posted June 21, 2009 That website is trying to help you calculate static vs. dynamic compression. It's overly simplified in this case, but I suppose it will give you some sort of reference number. :geek: I would beware of a couple of the other things offered on that website as they are not so great. like their software for measuring squish velocity?? i think its sucks. not even anywhere close to what my TSR spits out for me. Quote Link to comment Share on other sites More sharing options...
stroker_2 Posted June 21, 2009 Report Share Posted June 21, 2009 I was possibily a bit to brief in my previous post. What I wish to point out is that when there is any doubt about the compression ratio of an engine it is best to actually measure the trapped compression ratio. This is the compression ratio calculated using the trapped swept volume of a cylinder. i.e the volume of the cylinder from the point of exhaust port closure to the tdc position. As an example, if the exhaust port on a standard stroke(54mm) banshee opened at 27mm from tdc, the calculated trapped compression ratio would be exactly half of the static (also known as geometric compression ratio). In a two-stroke the cranking compression is directly related to the trapped compression ratio! If you raise the TCR, the cranking compression ratio will also rise. The static compression ratio has no relevance to the cranking compression. Factors that can cause the cranking pressure to appear different to what was expected include. 1/ Gauge accuracy 2/ altitude 3/ warm or cold engine. 4/ squish clearance 5/ dome shape of piston 6/ cylinder bore 7/ exhaust port timing 8/ worn rings 9/ worn bore Trapped compression ratio is sometimes known as dynamic compression ratio. However the name recommmended by the SAE is trapped compression ratio. Does not matter to much what it is called as long as you realize that they are the same thing. Below is a small sample table for the calculated trapped compression ratio. CCV is the combustion chamber volume in CC. Exhaust Port Opens from tdc CCV 26mm 27mm 28mm 13 7.43 7.68 7.93 14 6.97 7.2 7.43 15 6.58 6.79 7.01 Quote Link to comment Share on other sites More sharing options...
stroker_2 Posted June 21, 2009 Report Share Posted June 21, 2009 mopar, could you do me a favor and calculate the msv using tsr squish programme if I give you required dimensions. It would be great if you could put a screen grab of the output in a posting. stroker Quote Link to comment Share on other sites More sharing options...
mopar1rules Posted June 22, 2009 Report Share Posted June 22, 2009 i can run the figures for you yes. i need: rod length on centers-- rpm that peak hp is produced-- bore-- stroke-- exhaust port opening in deg or mm-- trapped comp ratio or full stroke comp ratio-- squish area ratio-- squish clearance-- Quote Link to comment Share on other sites More sharing options...
stroker_2 Posted June 22, 2009 Report Share Posted June 22, 2009 bore - 70 mm stroke - 70 mm rod - 125 mm exh opens at 104 deg atdc eng speed - 5000 revs/min trapped compression ratio 7.0 squish area ratio 0.5 (i.e 50%) squish clearance - 1.2 mm If you could throw a screen dump of output in your post would be much appreciated. Quote Link to comment Share on other sites More sharing options...
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