Today I gave Mickey a call about what weight synthetic oil I should run and some other things. We BS for alittle about PW machine then he said he had some bad news!

Somehow he miscalculated the cc's on the 650(633) big bore kit instead of a 750 it really is a 715. Oh well! I still am trying to figure it out? 715-633=82 & 785-697=88 I wish I new more about bore and strokes. Maybe Mickey will be able to explain better? He noticed a big difference between a 715(750) and a 785(PW's) and did the math over to see his mistake. Somehow he also misfigured a 765cc also sorry Sgt. Rock! Not everyone is perfect especially with a busy man that has torque and HP on the brain!!! It was correct that he told me the truth also, I respect that!!! I would have never known if he didn't tell by the power gain it gave to my machine! That is what counts.

The next time I need to rebuild (hopefully not for awhile knock on wood) I will be adding a crank from V-force or Prairie to get those extra couple of cc's. Mickey said it will be well worth it! I priced one for around $200 I believe.

Mickey I want to see the difference, maybe we can line them up some weekday! Also can you stroke a v-force crank some more, maybe above 800cc? That new V-force your are getting could use some more, don't you think[img]i/expressions/face-icon-small-wink.gif[/img] A good time to R&D something like that, those weber carbs could handle some more cc's!

650 big bore brothers sorry to bare the news if you didn't already hear!

 

Yeah that is bad news. But there is good news: I bet it won't cause a loss of power!

(Bore x Bore x 3.14159)/4 is the piston area. (PI is longer than that, but 3.14 is all you really need)
Multiply the piston area by the stoke and you get displacement.
Multiply that by the total number of cylinders in the engine (2 in this case).

Do all this math in centimeters, not millimeters, so you will wind up with cubic centimeters.

If you do all the math in millimters, your result will be in cubic millimeters. Divide that by 1000 to get back to cubic centimeters.

650 bore = 80 stroke=63
700 bore = 82 stroke=66

8.2x8.2x3.14159/4=52.8 square centimeters per piston.
52.8 x 6.6 = 348.5 cubic centimeters per cylinder
348.5 x 2 = 697 cc (pretty close to 700cc)

Anyone know the bore and stroke of Mickey's Kits?

 

Stroke doesn't change, just between 650 and 700. Bore is 87 or 88mm I believe.

 

Ok now I am confused?

8.7x8.7x3.14/4=59.4
59.4x6.3=374.2
374.2x2=748.4

8.8x8.8x3.14/4=60.8
60.8x6.3=383
383x2=766

8.7x8.7x3.14/4=59.4
59.4x6.6=392
392x2=784

8.8x8.8x3.14/4=60.8
60.8x6.6=401.3
401.3x2=802.6

Where does the 715 fit in? Please someone help!

 

Piston shape on top is also a factor whether it is concave, flat top, or domed!!

 

This just goes to further prove that a big bore kit is not required but only cams, carbs, clutching and whatever else he does besides the big bore kit. The only way to sell these kits to people, and to add a little more power, is to increase displacement. I have seen this on all motors, GT Thunder has tested this out on the Z400.

 

Hmmmm, if Mickey made a mistake on calculating the cc size of the "750" big bore kit, I now have to wonder if that 2002 Prairie 800 featured in Dirtwheels a few years back (owner was Mike Shelley I think?) was actually only approx 750cc??? Do you guys remember that quad?

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

 

That is not true. It doesn't matter whay the shape of the piston top is for calculating displacement. It is simply the diameter of the bore squared time PI divided by 4, multiplied by the stroke. Absolutely nothing to do with domes, dishes, valve cutouts etc. Search on google, you will find this formula everywhere, no mention of piston shape.

However, the piston shape does come into play when calculating compression ratio. In fact, you sometimes have to measure the volume of cutouts in the piston using water and a syringe.

Wildwill, do you think that mickey was right about the cc in the first place?

 

Bore would have to be 85 to = 714cc

 

First of all it's hard to admit that I made a mistake. I am glad that Wild Will took it so well and didn't trash me for trying to be honest. But here is the good news I AM NEVER WRONG,[img]i/expressions/face-icon-small-happy.gif[/img], the other day when Sgt. Rock said he was going to make a seat cover that had 765 on it I wanted to know exactly what size it was going to be. When I sold him the kit I told him it would be approximately 15cc's bigger with the new 88mm pistons that made the 785 into an 800. When I went to figure the bore and stroke, I opened up my trusty Dirt Wheels magazine from October 2001 to check the stroke and on page 84 I found the specs to be an 80mm piston with a 60mm stroke when in fact it's an 80mm piston with a 63mm stroke. That bothered me all night so I looked in 4-Wheel ATV Action October 2001 on page 31 and found the right specs. So all my figuring was correct in the first place and Sgt. Rock will have a 766cc machine. It just goes to show you don't believe everything you read in the magazines and to double-check yourself.

This is how we figure bore and stroke and cc's:

Bore divided by 2 (or 80 divided by 2 = 40) x the bore x the stroke (in this case 63mm) x 3.1416 = 316.67 x two cylinders gives you a 633.34 for the stock engine. 40 x 40 x 63 x 3.1416

650 Prairie, 750 Kit, 87mm piston divided by 2 = 43.5 x 43.5 x 63 x 3.1416 x 2 = 749.03

650 Prairie, 765 Kit, 88mm piston divided by 2 = 44 x 44 x 63 x 3.1416 x 2 = 766.34

700 Prairie/V-Force, 785 Kit, 87mm piston divided by 2 = 43.5 x 43.5 x 66 x 3.1416 x 2 = 784.69

700 Prairie/V-Force, 800 Kit, 88mm piston divided by 2 = 44 x 44 x 66 x 3.1416 x 2 = 802.84

Sorry for the confusion. I hope this clears things up.