2005 XC 700, stock compression was 125#, it's now 115# with the SLP cheater head (0-9000ft domes). They say the stock head is agressive in terms of compression, how can that be at 125#? Said with thier head the sled will start easier and be more reliable, but I don't care how hard the thing pulls over, it certainly isn't hard with 125#. I've seen some SKI-DOO sleds at 140# stock. SLP claims their head provides more HP due to a more efficient chamber design, which I can understand, but I feel that I'm leaving alot on the table with the reduced C.R. Seems like if I switched to the 9000+ft domes, C.R. would probably back closer to stock or a touch higher. I ride in lower MI, which is around 2000-3000ft. I added a SLP single, cheater head, and reeds at the same time and the sled runs strong, I guess I should do a back to back on the head to see if I gained anything. Anyone ever play around with the high elevation domes when running at low elevation? Or does anyone know what the cranking compression is with the high elevation domes?
Low Compression with SLP Cheater Head
- Thread starter jjense15
- Start date
Can try the higher compression domes, watch your DET though and run clean fuel with with it. Bought a billet head for an '01 800 xcsp, 150lb compression stock, about 140 afterwards and was an overall better running engine with more pull in it. Ran 2g heavier over stock. You should be able to pull 66's with no problems.
And yes, the SLP domes are more effecient and run lower cranking compression to save the engine if you run lower grade fuel by accident or have no choice.
And yes, the SLP domes are more effecient and run lower cranking compression to save the engine if you run lower grade fuel by accident or have no choice.
Compression readings on your sled when cold really doesn't pertain to anything with the actual running compression of a engine while it is running. 2-Stroke vs. 4-Stroke comp #s are a totally different game.
All this test is good for is to determine the condition of the rings, when taking a reading when new, just broken-in over time to see if the rings are leaking.
Many things contribute to the actualy "running compression" of the engine while it is running.
The biggest one is the exhaust. The fuel/air gets pulled all the way thru the case/cylinders/ and into the pipe while the intake & exhaust ports are open. As the soundwave bounces off the covergant (the tapered part of the pipe) the piston comes high enough to shut off the intake ports, but leaving open the exhaust ports. This soundwave will start compressing all the fuel/air into the cylinder until the exhaust port closes. This is why pipe design is so critical to power.
This is what actually makes your real "compression".
The shape / size of your head/combustion chamber really is most effective AFTER the spark plug fires. The dome/squish band shape of the head controlls how the "flame" travels to the outside diameter of the cylinder.
Though the cold compression numbers dropped by 10#, it can actually be more aggresive of a design (more angle / less squish) and more prone to detination.
In a ported engine, you will actually LOOSE compression, because the exhaust and intake ports are ground taller, resulting in less distance for the piston to compress the air/fuel. If you do a cold comp test on a modded engine, it will usually be signigantly lower than a stock one. That's why ported engines rely more on a good pipe to run correctly.
Irregardless of what your gage says, SLP knows what they are doing. IMO, they are one of the best 2-stroke builders in the country for consumer based/bought parts. What they sell works.
All this test is good for is to determine the condition of the rings, when taking a reading when new, just broken-in over time to see if the rings are leaking.
Many things contribute to the actualy "running compression" of the engine while it is running.
The biggest one is the exhaust. The fuel/air gets pulled all the way thru the case/cylinders/ and into the pipe while the intake & exhaust ports are open. As the soundwave bounces off the covergant (the tapered part of the pipe) the piston comes high enough to shut off the intake ports, but leaving open the exhaust ports. This soundwave will start compressing all the fuel/air into the cylinder until the exhaust port closes. This is why pipe design is so critical to power.
This is what actually makes your real "compression".
The shape / size of your head/combustion chamber really is most effective AFTER the spark plug fires. The dome/squish band shape of the head controlls how the "flame" travels to the outside diameter of the cylinder.
Though the cold compression numbers dropped by 10#, it can actually be more aggresive of a design (more angle / less squish) and more prone to detination.
In a ported engine, you will actually LOOSE compression, because the exhaust and intake ports are ground taller, resulting in less distance for the piston to compress the air/fuel. If you do a cold comp test on a modded engine, it will usually be signigantly lower than a stock one. That's why ported engines rely more on a good pipe to run correctly.
Irregardless of what your gage says, SLP knows what they are doing. IMO, they are one of the best 2-stroke builders in the country for consumer based/bought parts. What they sell works.
