So what HP increases do you see going to a 11:1 or 12: 1 or 13:1 or the biggie 14:1??
well I know the bottom end is gonna increase, what about the top end?? is that much compression hard on the crank bearings??

 

A friend went from the woods 11.5 to the 14-1 had it dynod with that being the only change, he picked up 4 hp. That was on gas.

 

I once read that you get a 3% HP gain for each point you go up in compression, don't know if that's fact its just something I read somewhere, but in the case Crazy is mentioning the math holds up.

it is not much concern on the crank, she can take it take it well for a long time. the concern is the added combustion heat that would cause preignition and top end destruction if the right fuel is not used with the right piston. as long as the right fuel is readily available to a guy, I say go as high as you can on compression. in my case, the 110 octane required for the 14:1 piston is too inconvenient to get for me. I stuck with 11.5:1 and run pump premium 93 octane available everywhere. once you cross the 12:1 mark, you gotta start mixing race fuels.

 

Yeah we have a place down here called Downs oil, 110 to 114 oct no problem!

 

Horsepower increases through compression is on a curve.
That is, has compression is increased, the power lost through the power required to compress the charge is thus reducing the final power output.
Thus horsepower per point of compression, starts to be reduced.
Final horsepower increase per point is:
8 to 11 compression ratio - 3% per point of compression
Example: 8 to 9 = 3% 9 to 10 = 3%
11 to 12.5 compression ratio - 2.5% per point
12.5 to 14 compression ratio - 2% per point

 

Well what the heck is all that means?

 

say stock engine with 9.7 compression makes 40 hp

10.7:1 comp is up 1 point, so 40 X 3% for each point =1.2 HP

Now a 10.7:1 engine makes 41.2 HP

Beyone 11:1 compression, you only gain 2.5% instead of 3%, and same applies after 12.5:1, only increasing 2%. Make anymore sense?

I initally thought it was 3%per point of compression as in 0.1, but that would mean going from 9.7 to 10.7 would see an increase of 12 HP! lol I think not.

 

Out of curiosity, what is stock compression???

 

I think it is 9:5 or 9:7. Someone will cime in with the correct answer.

 

Section 11 TECHNICAL DATA
Subsection 02 (ENGINE AND VEHICLE)
VMR2001_029_11_02A.FM 11-02-1
ENGINE AND VEHICLE
VEHICLE MODEL 7449
ENGINE
Engine Type BOMBARDIER-ROTAX 654,4 Stroke, Double Overhead Camshaft,
Bush Chain Drive, Liquid Cooled
Starting System Electric (start in any gear with clutch applied or on neutral)
Number of Cylinder(s) 1
Number of Valves 4 Bucket Tappet-Operated Valves
Decompressor Type Automatic
Bore Standard mm (in) 100 (3.94)
Stroke mm (in) 83 (3.27)
Displacement cm 3 (in 3 ) 652 (39.7)
Compression Ratio 9.7: 1
Calibration Engine Speed 100 RPM 13 + 1
Intake Valve Opening 17° BTDC
Intake Valve Closing 45° ABDC
Exhaust Valve Opening 47° BBDC
Exhaust Valve Closing 15° ATDC
Valve Clearance When Engine Cold
(maximum 35°C/95°F) Exhaust/Intake mm (in) 0.1 to 0.15 (0.004 to 0.006)
Valve Length Intake mm (in) 90.91 and 90.76 (3.579 and 3.573)
Exhaust mm (in) 90.65 and 90.05 (3.568 and 3.545)
Plate Diameter Intake mm (in) 36 (1.42)
Exhaust mm (in) 31 (1.22)
Shaft Diameter Intake mm (in) 5.95 (0.24)
Exhaust mm (in) 5.935 (0.23)
Valve Seat Angle Intake mm (in) 45°
Exhaust mm (in) 30°
Piston Ring End Gap
New Minimum mm (in) 0.2 (0.008)
New Maximum mm (in) 0.4 (0.016)
Wear Limit mm (in) 1 (0.04)
Ring/Piston Groove Clearance
New Minimum mm (in) 0.03 (0.0012)
New Maximum mm (in) 0.065 (0.0026)
Wear Limit mm (in) 0.15 (0.006)
Piston/Cylinder Clearance Wear Limit mm (in) 0.09 (0.004)
Cylinder
New Minimum mm (in) 100 (3.9371)
New Maximum mm (in) 100.012 (3.9376)
Wear Limit mm (in) 100.03 (3.9382)
Crankshaft Balance Factor % 47 ± 3
Crankshaft Deflection mm (in) 0.1 (0.004)
Connecting Rod Big End Axial Play Wear Limit mm (in) 0.8 (0.794)
Connecting Rod Small End I.D. Wear Limit mm (in) 22.04 (0.868)
Connecting Rod/Piston Pin Clearance Wear Limit mm (in) 0.06 (0.002)
Camshaft
(intake and exhaust)
Bearing Point Ø mm (in) 21.95 (0.864)
Cam Height mm (in) 39.7 (1.56)
Timing Chain
(gap from sealing face to piston of chain tensioner) mm (in) 9.5 (0.374)
Lubrication Dry Sump Lubrication by Oil Pump Replaceable Oil Filter
Oil Filter Main-Flow Type
Air Filter Type Cleanable Synthetic Panelette/Foam
Exhaust System Type Bombardier, Stainless Header Pipe Painted Steel Muffler
Spark Arrester USFS Approved