How much compression should my 800 have?

[ForeverRMK!]

It's a 09 800 rmk with 400 miles since the update was done... My warrenty runs out today so i pulled it out of the shed last night and gave it the torture test.. Ran good except if u held it at 7500 rpm it would bogg then i would have to slack off and hit it again then it would go.. Checked compression and it had 100 psi on the pto and 105 on the mag side. Phoned the dealer and he said that "should" be good,,said polaris doesn't give a spec for compression... Does that make sence to anyone?? Said to bring it in and they'll reflash it again for the bogg.. Machine is all stock....

 

[yamaha1]

i have an 800 assault and the sled has about 120 on each side, and i didn't find that new flash any better. the best way to make it run better is to get a power commander 5 and the auto tune really works good. p.s slp has an air fuel map for the auto tune

 

[Polarblu]

140-148 is where they run new. Anything lower than 120 needs attention, below 90 and they dont start so good. I have 2 in my shop that have 115-118 and they run fine but i advised the owner to do the top ends. Also check your power valves i found both of these to have broken springs and full of water.

 

[rockydaug]

depends on gauge but at 2000 ft, you see a lot of 120-125 psi. the general rule is to be equal, but 100 sounds low, remember try another gauge.

 

[overkill131313]

I have always used the 100PSI is rebuild......I like to see 120 and better! was compression test done hot or cold? throttle wide open?

 

[OnlyPolaris]

yes that is pretty low. should be up around 125. make sure when doing the test you hold the throttle wide open and give it 5 quick pulls.

 

[MrMorgan]

I just went through this. My gauge was buggered so I had a shop do it. They said 90 psi. I had another shop do it and they got 138 psi. Bad test? Bad gauge? I don't know, but I have never run much lower than 120. Apparently, aside from lower power, harder starts, etc., if there is too much play, the slap can cause the piston skirt to break. Anyone else confirm this?

 

[overkill131313]

I just went through this. My gauge was buggered so I had a shop do it. They said 90 psi. I had another shop do it and they got 138 psi. Bad test? Bad gauge? I don't know, but I have never run much lower than 120. Apparently, aside from lower power, harder starts, etc., if there is too much play, the slap can cause the piston skirt to break. Anyone else confirm this?

it is your rings that give the compression.....well cylinder out of round and taper and ring end gaps all play a factor.......scratches in cylinder but if the piston is that sloppy in there....I think you have bigger isues than just compression lol! just my 2 cents!

 

[ForeverRMK!]

Yes the engine was hot but didn't hold the throttle open when i did it and the gauge is a few years old.. I always thought they should have atleast 120 psi also but when i talked to two service managers they said 100 psi "should" be good.. That's what didn't make sence to me, they had the same response like they're trained to say that or something.. I'll do some more tests with my fingers crossed and see i guess because it does start and idle good.. The only reason i checked the compression was because it was going off of warranty.. Thanks for all the responses guys!!

 

[overkill131313]

go down to Crapy Tire and get a can of compression.......it cheap........ like 10-12 bucks but the install kit is a little more. I would say just add 20 pounds but if your going to run some race fuel you could put in 70 psi..... just watch the gauge on the install hose!

 

[ElectroTech]

100 PSI with 11.8 compression,fairly aggressive factory porting and decompression holes sounds okay to me, my sled here at 2000 with cut head for 50/50, heavy porting and decomp plugged had 125 ish with new pistons on perfect cylinder. Thats a gauge that reads bang on on many different motors. Best bet is to get a gauge and stick with it, take a measurment before you even ride and then monitor it through the sleds life. Set 15-20 percent change over time as your rebuild interval. Other things to look for are unexplained drops in RPM and speed. Compression on an EFI motor has very little effect on starting, unlike a carbed motor where the engines ability to pump is the determining factor in starting, if its super low on compression its simply not going to pull fuel into the cylinders, where an efi sled that has fixed rpm/throttle position based fuel/spark delivery could fire with the engine not turning at all if you had a way of triggering the system.