Freestyle maximum compression for 91 and 93 octane gas

J

jetskiron

Jetskiron
Location
california
What is the maximum compression I can use for 91 and 93 octane gas, assuming I don't change the timing?

I have a 97 Yamaha Superjet with 701 motor and TNT pipe, etc. I use it for low end work, freestyle riding. I curently have 195 pounds of compression and run 91 octane gas. I'm using a WestCoast head with 650 domes to get the 195 pounds. I want a little more horsepower if possible and I'm considering putting on a different aftermarket head with no domes and just having it milled down to raise the compression to maximum. How difficult is it to change the timing if necessary?
 
S

Schmidty721

someone turf my rails
Location
WI
You are very close to what is considered safe for pump gas. Running that west coast head without a girdle kit is risking a crack in your cylinder. I would switch over to an ADA or similar head that has a full girdle kit to support that cylinder. You have to run "domes". Heads with interchangeable domes let you adjust the compression ratio to your liking Ie. 38cc, 35cc, 33cc.

As far as more power, your not going to see much by increasing compression from where you are at. Your best bet now is to have some port work done or consider a larger displacement setup. An ignition such as the ATP EPIC that allows you to adjust your timing curve will also help with the low end punch you are looking for. Just remember that it has to work as a whole package... engine, ignition, pump etc to get the end results you are looking for.
 
J

jetskiron

Jetskiron
Location
california
Thanks to all. I read the http://www.2strokeheads.com/tech2.htm link. I guess this is a lot more complex. It did force me to take a closer look at the two motors. The lower compresion motor cylinder is 1/2 inch taller and head (PSI Racing Genesis II Type 2SJ) is another 1/2 inch taller the the West Coast head. I think I will just leave my 97 SJ alone at the 195 psi. I have been running it that way 2 hours a week for the last 15 years in the summer and luckily no cracks.

To answer some other questions, the 97 has been ported, carbon reeds, fuel switch bypassed, msi ignition, lightened steel flywheel, skat 8/16 impeller (worked better than my solas concord 13/15 cut back), wet wolf pump cone, protec pump nozzle. Maybe just keep the other motor as a backup, and start looking for one of those Richter skiis with the DASA motor. I aint gettin any younger! Any bargains out there, let me know! :)
 
Nuklear6

Nuklear6

I Dream 10' Surf!
Location
Bordentown, NJ by week / Brigantine, NJ by weekend
Can anyone one tell us why cranking pressure goes down when exhaust port height and durations go up????????

oooooooooooooo me me me!..............

Two strokes only make compression from the exhaust port up (when it closes). Therefore the higher the exhaust port the less stroke time is effective to the compression of A/F.

Keep that in mind when you wanna ride and your cylinder has a minor flaw below the exhaust port..... im not saying the flaw will have no affect. Because it will have a minor effect on the crankcase pressure used to push the new a/f up the transfers (when the reeds close). But experience has proven they still run well when the flaw is below the exhaust port.
 
stefan

stefan

Location
denmark
Acording too my engine builder blue it also has something too doo with spuich/porting he ran 240 psi in hee's owne ski with 93 octane
 
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