Compression drop when engine is hot?? - Snowmobile Forum: Your #1 Snowmobile Forum

Chevyho · 02-12-2009, 06:12 PM

I have a 2000 zrt600.... I checked compression with the engine cold wide open throttle and got 125 across the board. Check with engine warm wide open throttle got about 122 across the board and with engine really hot wide open throttle got just under 120 across the board.

My question is why is the compression dropping once the engine warms up? I have heard as the piston expands the ring end gap gets bigger and drops a little compression?? is this true? And do I have anything to worry about? I will be redoing the top end in the summer just want to get through this winter. Any help is appreciated.

Spun222400 · 02-12-2009, 06:26 PM

your running till you hit about 90psi, 120 and up your good to go

Chevyho · 02-12-2009, 06:35 PM

Oh good to know! I was just kinda worried about the compression dropping once it warms up, or is that fairly normal? Thanks for the reply

EFI rocks · 02-12-2009, 06:52 PM

i have a question about that
does it feel sorta like boggy

Chevyho · 02-12-2009, 06:54 PM

oh not at all, very snappy, and pulls just as hard all the way to 8000. Are you having issues? or just wondering? I am not experiencing any issues what so ever just check the compression and didnt know if that was normal.

EFI rocks · 02-12-2009, 06:59 PM

oh cause i just redid my topend and when its cold it works good but as it warms up
it starts to get a little dragy almost but not quite a bog

Chevyho · 02-12-2009, 07:05 PM

hmmm.... have you checked your compression? Is it overheating? When I rebuilt my old sled after my rebuild I musta had an air bubble in the line and she kept getting really hot and ran like shit. Just a thought. Can anyone else re assure me that my compression is normal?

Chevyho · 02-12-2009, 09:35 PM

anybody else have an opinion?

**BC_Dan** · 02-12-2009, 09:48 PM

When the engine is cold, the residual oil on the cylinders and rings is thick and creates a better seal. When the oil is hot, the oil is not making quite as good of a seal, so the compression is lower. The warm compression is the important number and will almost always be a little lower than the cold compression.

Spun222400 · 02-12-2009, 09:52 PM

^^^^^^that guys good

dmbeard · 02-12-2009, 10:04 PM

Yep BC_Dan is the man!!!!!!!!

Chevyho · 02-12-2009, 10:16 PM

perfect, that is everything I need to know. Thanks alot guys.

**kccats** · 02-13-2009, 07:04 AM

That is why you see almost all sleds are now liquid cooled! The old free air motors are now gone! They had to have such loose tolarences that when hot they would real loose compression!
120 is good and even with a new piston doupt you'll see much over 130!
Good time to do a little porting!!!