Does anyone know how much the 760 crank is stroked? I know the stock crank is 83mm. I'm thinking about doing myself a 7mm stroker crank and maybe a 102.36mm bore on the stock jug. This would give me 740cc at a fairly reasonable price and still be able to have a nikasil cylinder. I'm also thinking of going to a smaller crank pin and use a roller bearing setup instead of the stock type babbit bearing.


MrHorsepower

 

i thought i heard 4-5mm but never really looked close. i was wondering if the 703 had a smaller stroke than the 760. which if it was 2-3mm with the 105 cylinder would give you in the range you thought.

 

Dang: This didn't paste well. This was a spreadsheet. Hopefully you can figure it out.

Bore Bore Stroke Mult. Displacemnt Kit Supplier
10 10 8.3 0.787 653.2 Stock Rotax
10.2 10.2 8.3 0.787 679.6 2mm larger Bore Pami, TVI
10.2 10.2 8.6 0.787 704.2 2mm Bore + 3mm Stroke Pami
10.55 10.55 8.7 0.787 762.1 5.5mm Bore + 4mm Stroke Pami
10.5 10.5 8.3 0.787 720.2 5mm larger Bore TVI
10.236 10.236 9 0.787 742.1 2.36mm Bore + 7mm Stroke ???
10.85 10.85 9 0.787 833.8 Pami 830?? Not sure. Pami




I think these are correct.

BoreXBoreXstrokeXmultiplier=displacement

That 740 has a lot of stroke. Thing would be a tractor - TONs of torque.

 

Where can one buy a stroker pin for a stock crank?? and how expensive is it?

I would think a big stroke,and a stock piston would be a GREAT set-up.




BIgger.

 

I would rather try the biggest bore piston and jug I coule with little or no stroke!

 

To me , When you go BIG bore,and stock stroke, you increase wieght,and increase the amount of friction on the cylinder(thus bigger piston), slowing the revs down.BUT increasing the amount of volume ,which still gives you more HP.

But when stroking,and still remaining stock bore, you add no wieght,or additional friction,AND will not have Head gasket problems when going high on compression. you will also increase the Volume thus increasing HP,and torque. you revs will be quicker, but you wont be able to rev as high. that should not be a problem as long as your making a much broader power band by stroking.



JMO[img]i/expressions/face-icon-small-smile.gif[/img]






Bigger.

 

is it not better to increase the bore not stroke? Then you dont lose the ability to rev.. But you would not gain in torque?

 

From your spread sheet it looks like the 760 is +4mm and the 830 is +7mm.
So what I'm thinking about doing would be an 830 crank with a little bore or maybe just stock bore.
I think with stock bore it would be 707cc. With this much stroke the engine would still be way under square at 100-102mm bore by 90mm stroke.

I'm going to check into crank pin diameters from all the different makes and find one that would work. The reason you normally cant stroke this far is because the rod will not clear the counterbalance. Going to a smaller rod journal is what the prostock boys have been doing for years. I dont see why I cant do it. I've already talked to my crank shop and they see no problem with it at this point.


MrHorsepower

 

When I say "it wouldnt rev as high" I didnt mean that it would be a dead stick and only rev to like 8000 rpm's. I have seen 760 PAMI rev 10,000 easily. but with a stroked motor, your HP/torque will start sooner,and that means you will be "going" sooner, also able to gear higher(faster acceleration) to take advantage of the extra low end torque.



In strait line drags torque is not AS impotant as it is in the trails,adn in hill shooting, but anytime you can increase power that is GOOD, but you have to make sure to set-up your quad to use the type of power you have.





BIgger.

 

looks like there may be more options soon[img]i/expressions/face-icon-small-smile.gif[/img]