the sled is a 11 pro 800 turbo. i just rebuilt the motor because it had 90psi on both cylinders. i put new wisecos and rings in and when i checked compression it was still 90. i measured the bore, ring gap, and everything was within spec. all new gaskets, cleaned up the exhaust valves, and put new reeds in. ive tried two different gauges and get the same reading. i deglazed the cylinders with a 320 grit hone brush. im lost as to where to look now. any ideas? do the rings need to seat?
why low compression?
- Thread starter k_newman66
- Start date
I'm not familar with the Pro motors, but I had an old Indy 500 that ran great and only read 85 PSI cranking compression.
Don't some turbo kits use different cylinder heads to drop C/R, then run more boost?
Port timing can have a huge effect on cranking compression, so you may get porting done, check cranking compression, see a drop, and think you were ripped off by your builder. Then you ride it and notice it rips, cranking compression doesn't equal dynamic compression.
Was the sled warmed up? I would think the rings do need to seat to get the best seal.
Oh, altitude makes a big difference on what you'll get for a cranking compression test. At 10,000' I wouldn't be surprised to see a 30% drop in the PSI readout of a cranking compression test versus the same motor and gauge at sea level.
How does it run?
Don't some turbo kits use different cylinder heads to drop C/R, then run more boost?
Port timing can have a huge effect on cranking compression, so you may get porting done, check cranking compression, see a drop, and think you were ripped off by your builder. Then you ride it and notice it rips, cranking compression doesn't equal dynamic compression.
Was the sled warmed up? I would think the rings do need to seat to get the best seal.
Oh, altitude makes a big difference on what you'll get for a cranking compression test. At 10,000' I wouldn't be surprised to see a 30% drop in the PSI readout of a cranking compression test versus the same motor and gauge at sea level.
How does it run?
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i havnt started the sled up, my fuel control box is getting worked on. it has the stock head and im at 3500ft
S
skippy
Well-known member
The rings definitely need to seat in order to get a true reading . Are you sure you don't have a low compression head on for the turbo, that could be giving you low numbers maybe... I don't know sh!t about turbos but don't they use like 11-0, 12-0 compression ratios in some applications?
H
High Velocity
Well-known member
I did a compression check on my '12 Pro before installing my Power Addiction head and I only saw 110 psi at 3000 ft elevation.
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skippy
Well-known member
This is the classic problem with a compression check.....its only a rough guideline for ring wear,
Everybody should buy a Leakdown tester and do a proper leakdown check to really see the condition of the motor before tearin it down. A motor can have 70 pounds of compression and rings can be perfect at 3% leakage. Altitude,temp,porting,how much oil on cylinder ect ect all change the compression reading. In a 4 stroke motor you can have 15-1 compression but with a huge cam it will still only pump 100 psi on a compression guage... just a fyi for everybody.
How do you properly perform a leak down check?
leak down tester
there is a gauge setup that uses a compressed air source with a regulator to fill the cyl with air. basically just watch for changes/drops in pressure on the gauge...there is a percentage of "leakdown" which is acceptable and that will change based on what type of motor you are working with....4s/2s auto/sled/lawn mower/etc. it will also allow you to check for leaks in oil seals on 2smokers wich is nice and check valve seating on 4s.
there is a gauge setup that uses a compressed air source with a regulator to fill the cyl with air. basically just watch for changes/drops in pressure on the gauge...there is a percentage of "leakdown" which is acceptable and that will change based on what type of motor you are working with....4s/2s auto/sled/lawn mower/etc. it will also allow you to check for leaks in oil seals on 2smokers wich is nice and check valve seating on 4s.
90 psi at higher alts is probably right, the static compression on the pros is pretty low due to port timing, (just as 2xm3 said on a 4 stroke, cam specs can mess with a 4 strokes cranking compression, same goes for a 2 stroke and port heights..) go run the sled, does it run good?
On my 2011 turbo pro @ 3000ft I had 102 psi pto 105 psi mag after 4 pulls WOT if that helps any. I would take it for a couple trips around the yard and then check it.
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Here is a good article on the 2 different methods.
http://www.wcengineering.com/articles/leakdown.html
Here is an in-depth article on the actual procedure.
Car Craft Leak Down
http://www.wcengineering.com/articles/leakdown.html
Here is an in-depth article on the actual procedure.
Car Craft Leak Down
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see bdoes post above..I like the testers made by b&b or moroso...GREAT tool
http://www.summitracing.com/search/...s/?keyword=leakdown+tester&kr=leakdown+tester
Warm up motor, pull plugs, set piston at the exact top of its stroke, hook guage up to air, zero it, hook to cylinder...read leakage done deal
pretty interesting also is the temp, on our blown alky motors in the boats stone cold they would leak about 35%, (aluminium block) at racing temp less than 5% if that.
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thanks for the input, ill have to look into the leakdown tester. the sled starts up on the first or second pull. im still doing the three warm ups/cool down, but i told the poo dealer here i had 90 psi and they were surprised it still ran. they should be around 120 is what he told me. just curious why it wouldnt be closer to 120 after a rebuild. and like a half inch from the top of the cylinders theres two holes on the exhaust side that go to the exhaust valves. what are these for? my other ski doo doesn't have these.
PROCEDURE
you said in an earlier post you got 90psi with two different gauges, explain the procedure you are using to check compression. 2 pulls? 5 pulls? wide open throttle or closed?
you said in an earlier post you got 90psi with two different gauges, explain the procedure you are using to check compression. 2 pulls? 5 pulls? wide open throttle or closed?
Ive tried it warm and cold. Wot and give it 7 or so pulls. I put a little oil in and it didnt go up.
ring seat
i certainly am not an expert but i would think the rings need to be seated to reveal proper compression but not sure on that.
i certainly am not an expert but i would think the rings need to be seated to reveal proper compression but not sure on that.
To sum this all up.
Go out and ride it.
Take everything your dealer says with a grain of salt. Sometimes they know what they're talking about, sometimes they have no idea and just make it up (just my experience, I know there are some better ones out there).
Oh and the holes in near the cylinder tops. Those lead into the exhaust valve bellows, so when cylinder pressure gets high enough (higher RPM) the exhaust gases that enter that little hole inflate the bellows against the exhaust valve spring, and in effect raise the guillotines (exhaust valves) up and out of the exhaust port.
On some motors there are two of those little holes in each cylinder. One for valve operation and one to bleed off compression so the sled is easier to start. I don't know if the Pro motors have the decompression holes. They aren't necessary though, and on the old bigblock 800's SLP recommends having the decomp holes plugged, they say they saw 150-200 RPM increase from plugging the decomp holes.
Go out and ride it.
Take everything your dealer says with a grain of salt. Sometimes they know what they're talking about, sometimes they have no idea and just make it up (just my experience, I know there are some better ones out there).
Oh and the holes in near the cylinder tops. Those lead into the exhaust valve bellows, so when cylinder pressure gets high enough (higher RPM) the exhaust gases that enter that little hole inflate the bellows against the exhaust valve spring, and in effect raise the guillotines (exhaust valves) up and out of the exhaust port.
On some motors there are two of those little holes in each cylinder. One for valve operation and one to bleed off compression so the sled is easier to start. I don't know if the Pro motors have the decompression holes. They aren't necessary though, and on the old bigblock 800's SLP recommends having the decomp holes plugged, they say they saw 150-200 RPM increase from plugging the decomp holes.
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compression
I did compression test on my 2012 pro in a warm shop, motor cold
125 PSI on each cylinder with wide open throttle and electric start
2200ft elevation and a snap-on gauge 450 miles on motor all stock
I did compression test on my 2012 pro in a warm shop, motor cold
125 PSI on each cylinder with wide open throttle and electric start
2200ft elevation and a snap-on gauge 450 miles on motor all stock
