Snowmobile Fanatics banner

1 - 20 of 28 Posts

·
Registered
Joined
·
700 Posts
Discussion Starter #1
My sled has 165psi cold in both holes and Im looking to run 93 octane. I can put an early indy gasket which is thicker to lower it a little if i have too to keep it on 93.

Wondering who runs what pressures with what fuels.
 

·
Registered
Joined
·
15,722 Posts
150 with 93.
 

·
Registered
Joined
·
8,108 Posts
It is hard to equate cranking PSI with a given octane. If you had higher ports it will have less cranking PSI but you may or may not have more running compression with the pipe stuffing air back into the cylinder. Best to go with what your engine/head guy says.
 

·
Registered
Joined
·
700 Posts
Discussion Starter #4
I realize port hight and exhaust duration play into compression and burn paterns. Scavanging also has effect on it. I was just hoping someone with stock ports and a similar setup could shed some light for me. I just milled the head at my house and wound up with 165psi. Im thinking about just test driving it and looking for early signs of piston wear. Im using the newer indy gasket which is basicly no thicker than a few sheets of paper so i can always throw a cometec gasket on to lower the pressure or even the old fat early indy gaskets to lower it more yet.

Thanks
 

·
Registered
Joined
·
8,842 Posts
what sled is it on. if its a polaris triple that is way to high in my book. but like ripperd said it depends on the motor. most polaris triples run 120 ish for 87 octane. 140-150 is 93 octane.
on the newer twins its a little different.

heck run high test and watch the piston for pitting on the top of it.
 

·
Registered
Joined
·
8,108 Posts
Originally posted by Boss302man
[br]I realize port hight and exhaust duration play into compression and burn paterns. Scavanging also has effect on it. I was just hoping someone with stock ports and a similar setup could shed some light for me. I just milled the head at my house and wound up with 165psi. Im thinking about just test driving it and looking for early signs of piston wear. Im using the newer indy gasket which is basicly no thicker than a few sheets of paper so i can always throw a cometec gasket on to lower the pressure or even the old fat early indy gaskets to lower it more yet.
Thanks
If you know how to mill a head, just figure out the actual compression ratio that you now have.

Compression ratio = volume at BDC/volume at TDC

Then you can use that number and get a recommendation.
 

·
Registered
Joined
·
2,304 Posts
What is your piston squish?
This is a major factor in detination due to low octane. You are running fairly high compression so I would think your squish is tight.
Rule of thumb, 55 to 65 thousands you must run a high octane pump gas (like 93) if you are under 55 thou then you must run a race fuel
And also what Ripperd had stated above.
 

·
Registered
Joined
·
6,044 Posts
Measure your squish.
This will at least give you a general guideline.
55 thousanths and pump gas is OK.
50 thousanths and maybe some race fuel mix.
45 thousanths and about 50/50 race fuel.
Again, these are just general guidelines and definately not set in stone.

You basically want to run the lowest octane with out detonating or pinging.
165 does seem high for pump gas to me too, but I also know that there is more to it than just PSI.
Porting, pipes (scavenging), timing, they all have an effect.
 

·
Registered
Joined
·
700 Posts
Discussion Starter #12
alright. So by squish do you mean the amount of straight edge left on the edges of the clyinder head?
All our snow just melted off so i dont really have the option of takin it for a ride. I did run it for a while on a stand but the belt was off and i was just free revin it now and then. I doubt that would give and signs since there was no load.
 

·
Registered
Joined
·
12 Posts
I have zr 700 -00 and I'm thinking of increasing the compression in it. I'm also planning to polish the ports. The original squish gap is specified to 0,069 to 0,091 and the effective compression ratio is 6,0:1. How much to you think I can decrease the squish if I use 94 octane?
 

·
Registered
Joined
·
6,044 Posts
Originally posted by Jowen
[br]I have zr 700 -00 and I'm thinking of increasing the compression in it. I'm also planning to polish the ports. The original squish gap is specified to 0,069 to 0,091 and the effective compression ratio is 6,0:1. How much to you think I can decrease the squish if I use 94 octane?
Look at the guidlines above and cut metal at your own risk.
You can't put it back
 

·
Registered
Joined
·
510 Posts
squish band clearance does not directly corelate to octane requirement. its far more complicated than that. you can not say if i have x squish what octane do i need. keep in mind as compression goes up squish velocity decreases.

compression ratio is calculated (full stroke volume + top of stroke trapped volume ) divided by top of stroke trapped volume.

and do NOT polish your ports its a waste of time just clean up slag and leave fine tooth texture. exhaust port can be smoother to resist carbon buildup.
 

·
Registered
Joined
·
12 Posts
But I should be able to measure the gap and make it 0,069 safely considering the specs in the service manual.

What about the height of the exhaust port. Shouldn't you take that in account when calculating the effective compression? My "top of stroke volume" is 30 cc and my cylinder volume is 350 cc. So my compression should then be: (350+30)/30=12,67 which sound like a lot in comparison to 6,0:1.

I'm about to rebuild the top end with new pistons and boyesen reeds. So I might as well check if there's any excessive slag and bad fitting between ports and gaskets. And time is not a problem because I'm enjoying it =)
 

·
Registered
Joined
·
2,304 Posts
Originally posted by AHood
[br]squish band clearance does not directly corelate to octane requirement. its far more complicated than that. you can not say if i have x squish what octane do i need. keep in mind as compression goes up squish velocity decreases.

compression ratio is calculated (full stroke volume + top of stroke trapped volume ) divided by top of stroke trapped volume.

and do NOT polish your ports its a waste of time just clean up slag and leave fine tooth texture. exhaust port can be smoother to resist carbon buildup.
Yes, but I think thats why myself and Studentdriver said rule of thumb check your squish band. A person can give equations until the cows come home. We gave answer that will help determine octane needs, and no its not gospel, but if you think that is wrong then the engine builders I know and talk to must be blowing sunshine up my a$$, that is one of the first thing they check (squish)
What is the answer to your equation as far as octane requirement is?
 

·
Registered
Joined
·
6,044 Posts
That's why it's called a rule of thumb.
It's not set in stone, but it does make for a reletively safe starting place.
That would explain the parts where NFG and I both called it a general guideling and not set in stone. (duh)

Tuning and adjusting would be needed to get it just right.
That would be looked at as common sence.

As I put in my post above, the idea is to run as low of octane as possible with out pinging or detonating.
That would mean to start with it a little higher and slowly back down til it's just right. At least that is the SAFE way to do it.
 

·
Registered
Joined
·
700 Posts
Discussion Starter #19
so i check squish how?
 

·
Registered
Joined
·
12 Posts
Originally posted by Boss302man
[br]so i check squish how?
Put a piece of solder trough the plug hole and against the cylinder wall. Turn the engine a few times and measure the tip of the solder. Modelling clay could work too.

I was at the library today and found another rule of tumb in A. Graham Bell, Two stroke performance tuning:

Cylinder size [cc] Clearence [mm]
175-250 0,8-1,0
300-500 0,9-1,2

He also states that for every 0,6 to 1,0 points of increase in compression, octane (RON rating) should be increased with 5 to 6 numbers.
 
1 - 20 of 28 Posts
Top