compression and gas mixture

[Superjet Geoff]

I was wondering if i need to change the gas/oil mixture if im changing to a head running about 180 psi. If so, what does it need to change to? I think i am running about 40:1 now.



Also, does anyone know offhand what the head bolts on a superjet are supposed to be torqued to?



thanks,

Geoff

 

[the only couch rider]

your going to need to rejet as well if you step up in compression

 

[tor*p*do]

you are confused on the gasil mix versus the air:fuel mix
you will leave the gasil mix the same at 40:1
you will need to re-jet to get the proper air:fuel mix

 

[Endo]

Uhmm....I don't think a boost in compression is going to require re jetting. Just be sure to run 93 octane or higher...possibly 90 but I don't have my chart in front of me.....
and try this for the bolt specs http://www.drmikesandy.com/xh2o/Yamaha SuperJet Service Manual.pdf

 

[Superjet Geoff]

Thanks guys.

I was under the impression that the raised compression from a head would not require a re-jet, but that i will definitely need to when i slap my pipe on.

And I always run 91 or 93, whatever highest octane is available, so that won't be a problem.

 

[Frosty]

i rejetted when putting on my FPP B-pipe... then when i changed from my stock head to my ADA 35cc dome head i did not rejet... that was, oh 3 years ago... running fine... still running the FPP jetting specs.

 

[Matt_E]

your going to need to rejet as well if you step up in compression

No you won't.
Moderate compression increases without changing fuel type DO NOT REQUIRE REJETTING.

 

[CarFanatic]

so If I put a ada head on my stock sj. Would I still run the stock 50:1 ratio? I dont need to retune? I planned on doing the head, then a b pipe.

 

[Matt_E]

You can still run 50:1, though lots of guys run 40:1.



For retuning:

Moderate compression increases without changing fuel type DO NOT REQUIRE REJETTING.

 

[CarFanatic]

can I run 40:1 in my stock ski? I assume it give me a bit more power?

 

[keefer]

Not any noticeable power difference but you will have more lubrication. Oil displaces fuel, you want to be able to run as little oil as possible but there is a point where the lack of lubrication will do more harm than the benefits of more fuel. Just run 40 or 50 to 1 and dont worry about it, there are better ways to make more HP.

 

[CarFanatic]

cool. I assumed 50:1 was more oil then 40:1, giving you more lubrication? am I wrong about this?

 

[Matt_E]

Yes, you are.

It's 50 parts gas, one part oil (2% oil total). So 40 parts gas, one part (2.5% oil) oil is more oil.

Make sense?

 

[Endo]

40:1 is more oil than 50:1. And 32:1 is more oil than 40:1. The first number is gas the second is oil. Hope that makes sense.

And to really throw you for a loop...the more oil, the less gas when compared to air. The easiest way to remember this is that there is a specific mixture level of AIR to FUEL that is effecient. Stoichiometry...which we will not go into (I have have bad flash backs if discussed).

Oil is just an additive to help the metal to metal contact in a 2 cycle. To much and it can affect the effeciency of the engine (air to fuel ratio). And to little it can hurt the bearings/metal to metal contact (lubricity)

 

[djyox]

I heard somewhere that when you increase compression, that you should change your gap? less of a gap on your plugs...


Does that sound correct to anyone?

 

[ski4]

I heard somewhere that when you increase compression, that you should change your gap? less of a gap on your plugs...


Does that sound correct to anyone?

not that i have heard

 

[eel]

It won't matter with what you're doing,djyox.

ski4, hst did some "objective journalism" in hells angels

 

[ski4]

It won't matter with what you're doing,djyox.

ski4, hst did some "objective journalism" in hells angels

indeed

 

[Soul Man]

Jetting should only change if you change the airflow: less restrictive pipe, air filter mods etc. Compression won't change airflow, so jetting shouldn't change (may need a minor tweak).

CarFanatic: I wouldn't run more than 50:1 in a stock ski. you'll just waste money. You would also have to re-jet to let more fuel in since you'll have less gas per any given volume of your gas/oil mixture.
Pretty sure the only reason to increase the oil amount is to help rings seal in high compression motors, and also with higher compression the bearings will see more stress so a heavier oil film helps.
You can run a stock Yamaha motor on 50:1 and get years and years of use before you would even have to replace the rings.

 

[CarFanatic]

ah! ITs all clear now! Thanks guys. RIght now I'm running 50:1. My motor is completely stock right now. I'm a newb to pre mix ratios. I was confused because I had it backwards. For some reason I was thinking the 50 was oil and thats why I thought it was more in 50:1 then 40:1. It makes much more sense to me now though. I Think when I put the head on I'll run 40:1. 40:1 would hopefully lubricate better and help the motor last longer.