Thread: 2 stroke hot rodding

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I am going to take my dads old 78 polaris 440 cobra snowmobile and run it in the antique snowmobile drag races this winter.
Any suggestions on how to get a bit more out of one of these engines and keep it looking stock (ie exhaust syst and carbs)
i am planning on new rings and taking 10 thou off the heads to bring the comp up a bit, dont know if there would be much more i could do other than try to port the intake and exhaust but dont know what to exactly do that way.
Any thoughts

I'd guess all the standard stuff.

Can you punch out the bores, put in bigger pistons? larger air cleaner? hotter spark? advance the ignition? hotter fuel?

Can you Port the Engine in this Class? I would see if I could Port the Intake, Exhaust, and Transfer Ports-along with the higher Compression ratio, it would certainly help-sometimes gas porting the Pistons will hold the Rings to the Cylinder better for a little more power too-

I was lead to believe that the reed valves on a 2 stroke is where the magic happens.

Before you start porting, speak to someone who is familiar with this particular style of engine....you can really dork up the timing by porting randomly.... just like 2 stroke dirt bikes, you can kill the power by a bad port job. Find a reputable snow machine engine builder that knows Polaris...they can tell you better than us that have zero Polaris experience.

mike in tucson

Two strokes are awful finiky animals. Many mods can actually kill power if not done right. Compression increases can screw up the scavenging, or lean it out and fry pistons. Like Robot sez, get hold of a 2 stroke wizard before you attack that sled.

how about a NOS kit for a 2 stroke? they make them.

I'v often wondered if we could use snowmobile 2 strokes in light cars like T buckets. a 800-1000 CC engine is approxamently 150-250 HP stock, most are liquid cooled, the engines can be tuned extensivly now because they use EFI, NOS, turbochargers, all common practice.. 400 HP consistant ( w/o spray ) is not out of the ordinary... they have adaptors and stuff to hook them to 4 wheeler transmissions, and some of them can be had in shaft drive. could easilly adapt a smaller shaft to a larger shaft/ yoke., and a lot of 4wheelers now have reverse.

Hey matt i dont know how long they would last, i know from personal experience big 2 stroke air cooled dirt bikes wont take the high rpm runs and sieze up.
Snowmobile engines i think are more durable.
But i am not going to dig too deep without finding someone that knows a little.
The old girl was the fastest 0 to 60 stock that i had ever seen in its day but i was 110 lbs lighter back then at least it wont spin as much

My friends have tuned them up and stuff, but I'v never gotten into a snowmobile engine. I know they should take the RPM's, cause they can run wide open across the frozen lakes w/o problems... and normally they can do 80-140 MPH in that situation.

I know that the Skidoo TNT 440 was supposto be a Hot machine in it's heyday, which was around that time.

Thus far all i have ever worked on are two strokes.

Opening up the exhuast is a good idea, DO NOT, i repeat DO NOT run straight pipe! she will run but like crud. not to mention fouling your plug, and all sorts of other nasties. i recomend looking up the "two stroke tuners handbook" i tried to attach it but its 7.7 megabites lol. I have not yet read it in its entirety but it looks really good.

tune your carb real well. i know this sounds like a no brainer but i got a 1978 Honda Express Moped to really scream yesterday after a simple adjustment. (opened up the jet needle one notch)

Cook your muffler on a bed of red coals 1/2 to full hour on each side to burn off any carbon build up.

definetly check with a 2 stroke tuner whom is familar with your engine. porting these bad girls really helps but if you mess up you will dearly pay for it .

you can gasket/port match without any troubles. leave the actual port in the block and crank to the pro.

depending on your comp ratio try some higher octane gasoline(or add a little race fuel to your tank). if its 6.2:1 like my two old ladies that will hurt performance instead .

anything you think will benefit, HOT RODDING isn't it great?!

Best of luck buddy!
2-stroke all the way,
-CJP

On the old Clinton go-cart engines, the round transfer ports were squared up and the dividers thinned as much as possible. IIRC, the intakes were squared on the bottom only, or the intake timing was changed too much. That's about the era that I gave up on two-strokes! I hate 'em! Any sled I ever took for a drive, I ended up walking home.
My brother had a 440 TNT, seemed like a good runner. We put the engine in a newer Free-air sled, aluminum instead of steel frame. Not as tough, but lots faster.

My family used to drag race snowmobiles in the late '80s, we had two Yamaha's, a '74 GPX 440 free air (mine - C stock) and a '76 SRX 340 liquid (my brothers - B stock). My son was the driver of both machines.

We could not do anything to the engine that didn't come stock in it including carbs, exhaust or clutch. I believe the overbore could not be over ten thousanths and still be legal, ours were totally stock. Some guys would grind off the blades on their water pumps; the rules stated if a engine came with a water pump, you had to use it; it didn't say anything about the blades though.

The "tricks' is what made them fast. Mine was set up by a professional racer, the biggest "trick" was the airbox, it came stock with one and you had to run it, and you couldn't drill it full of holes either. What he did was to put a stud hanging down from the hood, and when closed it would tip the back of the airbox so it would open up and draw air in. Made a BIG difference.

The SRX didn't have anything that wasn't stock. To make it fast we relied on "reading the plugs". We just had to have a good balance between the plugs and carb jets. We consistantly won if we were the only one with a SRX. It was fun Blowing them 440 Polaris's away with our little 340.

One big reason we won a lot is because we had a good driver, he was always first off the line and could steer it pretty good with the skies off the ice.

My son tied a world record with the SRX at a Speed Run" in Wisconsin one year and the temperature was 28 degrees below zero F.

We got out of racing them because of a rule change - our machines were too old.

The attached picture is the sign we had on the side our our truck. Our last name is Flynn, and the guy that desinged the sign played with that. I'm Fly'nn, ya we flew all right.

PS If you want to "Soup up" the Polaris, mill the heads and/or leave out the head gasket........ makes them Free Air machines FLY.