# Thread: compression ratio

1. ## compression ratio

What The Stock Compression Ratio on 2007 Rxp.

2. Per the Owners manual at sea-doo.com, Specifications Section, the compression ratio is 8.4:1. So at sea level, you should see 14.7 psi X 8.4, or 123.5 psi or better... Ron

3. I believe 8.5-1 pretty high for a boosted mtr

4. thank you

5. 8.4:1 is what the Owners Manual says... Ron

6. thats on the lower end. lowest ive seen on a FI engine is like 8:1, Ive seen some as high as 9.5:1. I'd prefer it to be a little higher personally..

Just since I have all the spare parts around, I was going to make a motor with the 155 pistons (10:1 I beleive?) with an oem RXP S/C . I figure with 5psi boost I could make a reliable 200+hp, and still be pretty safe as long as I keep the intake charge nice and cool. would turn up the boost more if those pistons weren't cast... I ran 10psi on my S2000 at 11:1 haha.

Originally Posted by jawz
I believe 8.5-1 pretty high for a boosted mtr

7. Originally Posted by King_Of_Fun
8.4:1 is what the Owners Manual says... Ron
OOPS! (believe)

8. I Was Thinking Of Going 9:5:1 On My C-kit

9. Raising the compression ratio from 8.4:1 to 9.5:1 would have the effect of increasing the efficiency of the motor by about 2.05%. So, for a 215 hp engine, that would raise the power to about 222 hp, or a 7 hp gain. But, the motor would be so much more sensitive to fuel octane and operating temperature. Plus, if you then went with higher boost, you would probably need race gas.

The formula for the efficiency of a gasoline piston 4 stroke motor (otto cycle) engine is given by the formula below. This is the maximum possible thermal efficiency obtainable of an ideal (imaginary) piston cycle. It is not the efficiency of the motor. Actual measured motor efficiency is typically about half of the calculated values due to internal friction, pumping losses, etc, etc.

r = compression ratio
k= ratio of specific heats (for air, it is 1.4)

So, for a stock compression ratio of 8.4, the maximum obtainable efficiency for that thermal cycle is 57.3%

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