Snowmobile World banner

1 - 19 of 19 Posts

·
Registered
Joined
·
51 Posts
Discussion Starter #1
If a car engine can run on unleaded fuel with 10/1 compression,whats the maximum compression corrected for a two stroke ? Also what does this convert to in psi? thanks
 

·
Registered
Joined
·
51 Posts
Discussion Starter #3
mr670 thanks for the reply.why will cars and boats run with 165-175 psi on 87 octane while a sled will only run 145 psi on 92 octane?
 

·
Registered
Joined
·
3,910 Posts
Posibly because there timing is retarded or just not advanced and they dont run 8000rpm.
 

·
Registered
Joined
·
4,316 Posts
I thought you could run a max of 150psi with 87...then you have to switch to 92...
 

·
Registered
Joined
·
51 Posts
Discussion Starter #6
some good info guys .machz1 if the timing on cars advances as the rpm increases wouldn't theses new sleds with timming curves be exactly the same?therefore run the same high comp.
 

·
Registered
Joined
·
858 Posts
I run 155-160 psi, advanced timing on 91 and never had any problems.
 

·
Registered
Joined
·
4,316 Posts
The only reason I ask is that because my mxz is at 145 on 87 octane...and stock skidoos are usually around 140-150psi designed to run on 87. My understanding was (and I got it from SnowTech) that 150psi was the ceiling for 87 octane
 

·
Registered
Joined
·
530 Posts
I would think that you can't run as much compression in a sled cause of how high they rev.  I mean they are pretty much running over 5-6K all the time.  At least while your driving them.
 

·
Registered
Joined
·
300 Posts
It's all about tuning.  For example an Acura Integra Type R runs fine on 92 and it boosts 11.0:1 CR.  People turbocharge thses engines and keep the stock timing or close too.  It's all about getting the right amount of fuel at the right time.  It's true you can retard the timing to "play it safe" but the engine will suffer on the low end.  So all in all, you can't really say what the max psi can be on pump gas.
 

·
Registered
Joined
·
1,736 Posts
Looks like a bit of a misconception on compression with a few.

Octane ratings in fuel have more to do with their resistance to burning.  Higher compression ratios or turbos and superchargers will require higher octane fuel due to their higher internal pressures.  Higher octane helps prevent detonation.  I personally would have thought that snowmobile engines would have lower compression than a valved engine due to the way they breathe.  From what I've seen posted in here, that doesn't seem to be the case.  It is a misconception to use high octane fuel in an engine that does not require it.  Running too high octane is typically just a means of draining the wallet.  It can cause hard starting, poor idle, and loss of power in some cases.

When I rented the Yamaha on my Yellowstone trip.  The rentors stated that they wanted premium fuel to be used.  Well, Premium fuel in the park was running 2.04 per gallon, vs. 1.84 for regular unleaded.  I put the regular fuel in, and noticed no difference in performance at all.  

150 psi *roughly* equates to 8.5:1.  The fuel octane will have absolutely no bearing on compression ratio.
 

·
Registered
Joined
·
51 Posts
Discussion Starter #12
is there anybody running 10/1 compression in there street car and burning 92 octane?Is there anybody running 10/1 corrected compression ratio in there sled burning 92 octane?  All posts are appreciated.
 

·
Registered
Joined
·
139 Posts
the octane rating needed is not just with compression. Wolfman is correct that higher octane just has a higher resistance to detonate and it burns slower, and that there is no need to run higher octane then needed. running too high of octane can also be bad because it can cause carbon build up causeing higher compression then when you try to run lower octane again it will detonate.  A engine detonates for several reasons, compression is one.  timing is also one reason and engine design. cars have higher compression but there timing is not advanced as much.  my sleds timing is 27 degrees BTDC my car is 10 degreees BTDC.  Polaris's octane switch only changes timing how could it change compression, and that makes the difference between 87 and 92 octane.
 

·
Registered
Joined
·
361 Posts
I find this subject a little confusing also. My sled, a '97 Mach 1, is running at about 12.2:1 , has 148lbs. cranking compression ,has a squish clearance of 0.057", and runs fine on 92 octane fuel. Now, for this summer, I'm installing a 12.1:1 piston in my 4-stroke '85 Honda 200X atc, which will now require 102 octane fuel as per instructions. It now has a stock 9.25:1 piston, has 165lbs. cranking compression and runs fine on 90-92 octane fuel. I don't know if it's the squish clearances in 2 strokes, their rpm's, the timing curves ,or the supercharging effect of the pipes while in the powerband, as compared to the 4-strokes which just blow out all exhaust and have less unburnt gasses lingering ?
limskii is confused !  
 
 

·
Registered
Joined
·
476 Posts
Well, a few years ago i built a 400 chevy for my brothers Monte Carlo and it has 10.59 to 1 comp. ratio and he run's it on 92 oct. with steel heads. Squish is at .039" though.

   Auggie

PS: Remember that both two and four stroke engines have two comp. ratios. Mech. comp. ratio and effective comp. ratio. Most eng. builders really don't pay much atten. to mech. comp. ratios anymore.
 

·
Registered
Joined
·
1,736 Posts
OK, think of compression ratio as this:

8.5:1 - Maximum cylinder volume (BDC) will be 8.5 times minimum cylinder volume (TDC).  Your actual compression PSI will vary by both design (engine breathing) and age/condition (blowby losses.)

Now, I'm not sure what the snowmobile manufacturers advertise for compression ratio's on their engines.  However, if due to the port design of a two stroke, the amount of air/fuel mixture that is limited by the amount of time that it can enter the cylinder, then this, by my deduction, should result in lower overall cylinder PSI at TDC.  

Example:

A four stroke engine can intake its A/F mixture throughout its intire intake stroke.  A two stroke engine can only intake A/F mixture for a brief period while "near" BDC.  This would indicate to me that the 4 stroke engine should by design, show higher pressures despite a lower overall compression ratio.

Now, I do understand that this is not an exact science.  I've seen large differences in compression ratio despite the different auto manufacturers showing identical compression ratios.  ex:  My departed Rabbit turbodiesel spec'd 22.5:1.  The shop manual stated 500 PSI new spec, and engine teardown at 400 PSI.  My Mercedes 240D shows the same compression ratio, and only shows 360 PSI new spec, and 310 PSI teardown.  <shrug>
 

·
Registered
Joined
·
51 Posts
Discussion Starter #17
limski,4 strokes have valves which can close while the piston is at BDC therefore compressing the entire cylinder of air/fuel. A 2 stroke has no valves therefore cannot start compressing the mixture until the piston closes off the exhaust port on it's way to TDC. The bike with 12/1 compression is a true 12/1. The sled is12/1 uncorrected and approximently 8/1 corrected comp ratio_Octane numbers only pertain to corrected compression ratio,therefore their seems to be room to increase the comp and watch them go like....... As far as timming , its nice to know sleds advance as much as 27 degrees,but keep in mind cars are only at 10 degrees at idle and 35* atfull advance.  Thanks
 

·
Registered
Joined
·
3,910 Posts
Wolfman.  This is why you should use 91 octane even if its recommended for 87.  In a snowmobile your burining oil.  This causes a lot of carbon build up on the cylinder walls and on the spark plug.  THis doesnt make any diffrence when putting along the trail doing 60km but when you open it up on a lake these carbon deposits sometimes heat up to the point where they can ignite the fuel.  This is pre ignition or as me and my buddies like to call it A SILVER WOLF.  We call it this because the piston is still on its way up and the fuel has already ignited and the piston liquifies  and re hardens on the spark plug.  The plug has silver deposits which are melted aluminum.  

Higher octain fuels will prevent this because of there ability to resist pre ignition from a hot spot or carbon build up.

I have seen it many times.  People only running 87 on lakes wide open normally come of the lake by tow.  

My camaro has super high compresion and i need to run 91 or higher.   Even 91 is a little low 104 would be better.  If i use 87 or 89 my car wont shut off.  It will desiel.  Basicly just run by it self with no ignition just igniting itself because of the high compression and heat.  Also possible because its a carb.  

Also all stock skidoo sleds come with 150psi compression.  Except the fans and maybe the 440 racer.  

Rocketman your 670 came from factory with 150 and was rated for 87.  My Mach Z came from factory with 150 but minimum 91.  This is because of the timing.
 
1 - 19 of 19 Posts
Top