Hey everybody, I used to post regularily years ago here and have been keeping up with posts but now I have a serious question,I am in Queen's University and I am a member of the schools Formula SAE team (engsoc.queensu.ca/formulacar).

I want to run a polaris 750 engine out of the FST snowmobile with a CVT but competition rules state that our maximum displacement must be 610cc. My original idea was to debore the 750 with custom sleeves/pistons so that each cylinder would have a bore of 76.7mm with the stock stroke. Now this is quite a decrease in bore size (stock is 85mm) which I think i may run into problems with combustion chamber sleeve matching, With the cylinder head being vastly wider than the sleeve we would run into all sorts of inefficiencies (compression, flame propogation, and thermodynamic). I know polaris make a kit for the predator to decrease the displacement but I believe it comes with a new head with matched combustion chamber.

Now my question, is there a way i could effectively match the combustion chamber and smaller sleeve without a custom head or reshaped CC? Or would a 4.15mm decrease in radius cause a problem with combustion especially if we are turbocharged? To me this difference is huge but maybe its not as bad as I think?

Any help you can give me on this would be great!! Thanks guys.

 

The only way to do it right would be to rechamber a new head or maybe have that one shaved down quite a bit then rechambered. Without redoing the head compression will be affected, and squish band width/clearance wont be right. As you may already know, head design has everything to do with power, heat, and detontation of a 2-stroke. It wouldnt be as much of a problem as you would think to either build or rechamber the stock unit.

If you go that route dont forget to do a port map layout or degree the port timing to match it back up for proper peformance. But also keep in mind that a smaller bore may not require the same amount of exhaust port timing or width to achieve the best power as the much larger bored unit.

 

Hey, thanks for the reply, I really appreciate your input. I forgot to mention that the 750FST is the four stroke turbo snowmobile and not the 750 cleanfire two stroke. Bearing that in mind, port timing and width arent an issue, im just worrying that with the head and sleeve not matched I would end up a huge compression loss (which would be favorable for turbo) but ultimately create problematic behavior that would make tuning the TEC3 a nightmare. Am I totally off base by worrying about this?

If youd like to check out last years car, go to Queen's Formula SAE.

 

You'll get better performance from a similary tuned 600 than a reconfigured 750. Maybe look into AC, I think they have a turbo 600 four stroke.

 

Arctic cats engine is a 660 which puts me in the same boat, i want the 750 because I know polaris engines and CVt's much better than I know arctic cats. Loss of performance isnt a huge issue as competition rules state I must pull through a 20mm restrictor in the throttle body. I will be running TEC3 to map the engine and achieve near stock power levels (with resitriction). Any other thoughts on this problem anybody?

 

im not much of a quad person, but maybe try investing in a Polaris Fusion 600 H.O.? i know there powerfull as hell and win alot of races

 

Well modedwarrior if I knew a little more of what you are trying to do I could give you a little more/better advice.
What does the "event" involve. Speed? Fuel efficiency? Both? Is there going to be a weight penalty for cubes? Can you use any power adder? Any thing else?

I can tell you this about a wedge head. You don't want to lower the compression by destroke it with a shorter rod. A hemi, ya, you can do this with no efficiency penalty or problems. With a wedge, you will wreck the quench and have a tunning nightmare.

I don't know how much $$$$'s you are going to have for this project but you can also sleave the cyl. head. This will cost some $$$'s though.

 

hey guys, the engine must be four stroke so 600 fusion is out of the question

Doctor turbo: The event entails acceleration, autocross, endurance and fuel economy.

The problem im thinking is that by deboring I am going to run into a negative squish band width which may cause a swirl effect and causes the edges to heat up too much causing knocking. I don't know enough about this stuff being only in second year but by fourth year I have no doubt ill posses the knowledge to figure this out, I just want to get a head start on it

 

Now I remember, I talked to you last year. How did you do in your last race?
modedwarrior the problem with a dead space is aaaaaaaa well...........a dead space. Bad for power, bad for emissions, bad for mileage. That is why Chevy went to a open chamber head design. There is no dead space. Now all of these problems are small % wise but if you are in a race, you want all the power you can get.
The best design for what you are trying to do is a flat top hemi. Good flame travel and low compression for your turbo.
If you have a weight penalty for cubes I would make a small engine and really put the boost to it.

The best advice I could give you is REALLY read over the rules and take advantage of their oversights. That is how you will win races. Your going to have to have a realistic budget and stick to it. With sleaves and custom pistons your talking big $$$$'s.

 

Hey, It wouldnt have been me you were talking to but I know there are other teams that have members on this board. You were talking about flame travel now are there any papers i can read regarding this? are they calculated via CFD methods or pure testing methods. Sleeves and pistons aren't so bad provided we can recieve sponsorship for them