Too much Compression?

[dericsdad]

Just finished setting up a new air cooled 53mm cylinder, everything looks perfect on paper... 190 deg. exhaust dur. ( matches our pipe), piston goes perfectly flush with the bottom of the exhaust port, and have .034" squish. Checked compression after a couple warm-ups, and it is at just over 195 psi, will probably be a good 200 after the rings seat! We usually run 180-185. This year i am going to run straight 110 race gas rather than blending it. Is this compression too high? I haven't calculated the compression ratio yet, but I am sure it's up there. Thanks, Dan.

[wvspeedfreak]

Just curious how this held up.I just put a 52mm together with .032 squish and it was at 195# of compression.From what I have read,this is a borderline too high compression reading.

Originally posted by dericsdad
Just finished setting up a new air cooled 53mm cylinder, everything looks perfect on paper... 190 deg. exhaust dur. ( matches our pipe), piston goes perfectly flush with the bottom of the exhaust port, and have .034" squish. Checked compression after a couple warm-ups, and it is at just over 195 psi, will probably be a good 200 after the rings seat! We usually run 180-185. This year i am going to run straight 110 race gas rather than blending it. Is this compression too high? I haven't calculated the compression ratio yet, but I am sure it's up there. Thanks, Dan. we run a 54mm with 190 exhaust and it runs over 200 lbs of compression. i thought it was high but it sure does run good. our last motor ran over 200 it ran well for a year and a half races only and i took it off at 185 it is still a good cylinder. we could go faster with lower compression. we run vp c12 straight with klotz r-50 50 to 1 ratio on short tracks and 45 to one on 1/2 mile track. 8 laps of never letting off. thats 4 miles of can your motor hold up?lol

[hotquads1]

that compression is a little higher than I like, sounds like a stock head that got milled and the combustion chamber was not reconfigured for the correct compression and squish. reshaping the combustion chamber will result in better squish .30 or less and still maintain reasonable compression of 155-180, This is my preference , I am sure other builders have opinions that may differ,
marc

Marc you are completely right factory head been milled down . you think a new high compression head will give us anymore.just wondering always mill instead of buying new.let me know your opion marc, gary anyone.

[BradLoomis]

I have built motors over 250 PSI and all it does is rob RPM and premature wristpin/bearing wear. I snapped a connecting rod at 290, it was smoothed and polished.

[hotquads1]

Originally posted by tgcheeseman
Marc you are completely right factory head been milled down . you think a new high compression head will give us anymore.just wondering always mill instead of buying new.let me know your opion marc, gary anyone.
.
Milling the head is fine, but the combustion chamber should be reshaped to provide ample area , we normally start with a target squish and machine chamber accordingly. Without having a measure (cc') of original volume it will be difficult to tell you how much material will need to be removed. You will have to enlarge the combustion chamber portion of the head and retry untill you get the result you wish. Does the head have a machined squish band ? what head are you working with ?
marc

[LT80]

I modded a stock head. Is OK but a aftermarket head would be my choice.
I'm with Brad. High comp+ lower rpm's.
I like 150-160 pounds comp. for a 12k+ rev.

Originally posted by hotquads1
.
Milling the head is fine, but the combustion chamber should be reshaped to provide ample area , we normally start with a target squish and machine chamber accordingly. Without having a measure (cc') of original volume it will be difficult to tell you how much material will need to be removed. You will have to enlarge the combustion chamber portion of the head and retry untill you get the result you wish. Does the head have a machined squish band ? what head are you working with ?
marc the head has been milled and reformed to a hemi head .probally the same as many others its a 54 mm head that came with the cylinder kit with the big k on the side.it had the combustion ring in it but now like i said is a hemi head but i wonder if i use a aftermarket head will it drop my compression thats what i want about like lt80 said 160 pounds and i dont want to reshape the rings. i am actually not sure what my squish is.my port timing is set at 121 and 190. thats all i am saying lol. lt80 how are you ?havent talked to you in a while?

[dericsdad]

Just to update you all. Did some changes to our port timing per Justin's recommendations.... and also lowered the squish to around .027". With the new Exhaust and transfer durations sert, my compression guage says it has about 198 psi, with the koso head. Justin said he thinks with that head and squish it isn't that high, more like about 185 psi. I haven't checked it with a different tester yet.

It ran VERY strong all year. We managed nearly every holeshot once we fixed the carb(worn out pilot jet), and Deric could then hold it wide open without it flooding out. He even got a holeshot on a Cobra 70.

It pulls really hard and even up to about 12k., and even works well in the tight supercross style indoor track here just by swapping in a larger rear sprocket.

I may try running VP U2 race gas next year, if it is in the budget, just to see if it likes it.

Only thing I found is that after 6-8 races the piston rings start to curl over, probably from all of the heat/compression.

May also experiment with a Wiseco piston, maybe the rings will hold up better. (they are very heavy though)

Dan