Super Jet PSI vs. Octane rating chart??

EXPERIMENTAL

EXPERIMENTAL

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Jefferson City, MO
I know ADA has recommendations for what octane rating to use for different size domes, example: 35cc domes = 91+.
Is there a chart that specifically lays out at what PSI rating you should pump up to another octane rating? I did do a search for this on google, but it mostly just turned up octane ratings for different sized domes, but with ranging altitude affecting PSI ratings.
 
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Jefferson City, MO
I guess to be more specific, what is the maximum allowable compression (PSI) that you can safely run 93 octane? considering 33cc domes for my engine (to get a little more power) but don't want to have to buy higher (turbo blue) fuel to mix with my 93. Any input is appreciated!

engine is currently running 175 psi on 35cc domes, i believe the porting has something to do with the reason I am not hitting 185 psi (like protec says i should on their website).
 
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CARBONX2

CARBONX2

Location
South Africa
PSI off a guage is not that accurate for octane rating. You need to work out the full swept-volume compression as if it were a 4-stroke. I think 93 would be safe up to 13:1
 
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would that compression ratio change if it were a two stroke?

I have a stock bore, stock stroke, ported 701

if total displacement =701 then each piston = (701/2)= 350.5 cc

current total displacement= 350.5cc (cylinder)+35cc (dome)=385.5

new total displacement = 350.5cc (cylinder)+33cc (dome)=383.5

current domes =35cc: 35/385.5 = roughly 1:11.014

new domes = 33cc: 33/383.5 = roughly 1:11.621

is my math right? or is it more complicated than that?
 
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LegDragger64

LegDragger64

X
In addition to compression ratio , you have to consider ignition timing as well as rpms ski will be run at . Are you going to hold it wide open accross the lake , or will you be putting around jumping waves . Compression guage readings are good for monitoring engine wear .Check it when its new and write it down .
 
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So basically there is no real way of knowing what octane I should use? Should i just try 33cc domes and hope for the best? What should i look for if it is unable to run on 93 octane? could this cause significant damages?
 
kevbo

kevbo

Location
dfw
EXPERIMENTAL said:
I know ADA has recommendations for what octane rating to use for different size domes, example: 35cc domes = 91+.
Is there a chart that specifically lays out at what PSI rating you should pump up to another octane rating? I did do a search for this on google, but it mostly just turned up octane ratings for different sized domes, but with ranging altitude affecting PSI ratings.
Click to expand...

Lower compression will allow you to run hotter mixtures and/or earlier timing for a given octane. Higher compression needs cooler mixtures and/or later timing. Its a balancing act.
 
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finally, something that constitutes an answer, If you are curious about this question, the link above answers a lot of questions and also explains a lot a things that occur with engine PSI and porting. Thank you Snivilous for supplying that link
 
Snivilous

Snivilous

Location
SW UT
snirunner24 said:
So it has more to do with porting (temp)?
Click to expand...

Honestly I just read it fast once, so don't know all the details, but from what it sounds like yes. Such as, you might run well at 87 octane at 180psi, but if you mess with the porting and timing and change your domes for only 150psi, you probably want to increase octane even though your compression is lower. Like I said, I've only read it once and fast, so there's a lot more to it than that, but the main thing the guy stressed was that compression doesn't indicate what your octane should be.

Though I didn't see any rule of thumb or calculations for how to know what octane to run. Sounds like you just go from what the manufacturer suggests from stock, then modify it based on a lot of stuff.
 
V

vrizi

so does anybody actually know what fuel to use,.... I don't have a clue what I will need to run, I am hoping I can get away with 93, but how do you know. I was watching this thread hoping to get answers for my own ski.
 
Snivilous

Snivilous

Location
SW UT
Assuming the ski runs 87 stock, if you increase the compression only, then increase the octane (run 93 until around 180-190psi seems to be the rule of thumb). If you are advancing your timing, or doing anything with porting, but not increasing the head compression, then increase the octane also (even if head pressure decreases, increase octane). If you do both, then I assume you really start cranking it up.

That's just what I would go by, it seems to all come down to how hot the fuel is before it explodes to dictate what octane to run...
 
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