5.6 cu in Modified Mc 91?

Scott Grundfor · Post by **Scott Grundfor** » Mon Mar 07, 2011 9:14 pm

I have a highly modified mc 91 that has been bored and sleeved so that the bore is now 2.250 and has been de stroked to 1.36. The intake port area has been stuffed with devcon and milled flat. The transfer passages go through the devcon. The flywheel fins have been shaved and it has the 91 Carb (with out the venturi) and std 91 manifold and reeds. It looks very well done. The crank has been welded and counterbalanced. Now it's 5.6 cu in. Whats the point?

Scott Grundfor

Dick Teal · Post by **Dick Teal** » Mon Mar 07, 2011 9:56 pm

Scott,

The shorter stroke will slow the piston speed down so the ports are open longer. This usually means that the engine will have more peak horsepower at a higher rpm. That's assuming that the compression has been adjusted to compensate for the different stroke. The negative effect will be less torque.

My best guess is that someone was messing around with an enduro engine where the rpm is high for longer periods of time.

I'm sure someone has more experience than I do and may have a better explination.

Dick Teal

Scott Grundfor · Post by **Scott Grundfor** » Mon Mar 07, 2011 11:03 pm

Thanks Dick.

Seems kind of wierd that you would decrease cu in for more power.

steveohara · Post by **steveohara** » Tue Mar 08, 2011 2:10 am

Scott,
I doubt the engine was built that way to get more power. More likely, someone just had a pile of parts laying around and built a running motor out of them. I have built a few like that myself for folks that did not have complete engines with matching parts.
The reduction in stroke and displacement is far more detrimental to performance than any offsetting gains one might hope for from changes in port time/area, piston speeds, rod angle, balance change etc.
Regards,
Steve O'Hara

Scott Grundfor · Post by **Scott Grundfor** » Wed Mar 09, 2011 1:16 am

Thanks Steve,

It looks like the motor has never been run. I'm going to put it together and see what happens. Perhaps I'll stick it on my dead axle Bug and terrorize the A400, WB510 and PP47 guys?

steveohara · Post by **steveohara** » Wed Mar 09, 2011 3:47 am

Perfect use for the motor. One that I built like that is run in the Sportsman class out here and the owner has a lot of fun with it. One suggestion... even though a motor like that will rev way up and keep pulling hard I suggest you limit the motor to 13,000 rpm otherwise it will pitch that heavy piston right out the top eventually.... end of fun!
Steve O'Hara

ted johnson · Post by **ted johnson** » Wed Mar 09, 2011 8:15 am

Scott, did they do the old Jim Akkerman trick of pushing plastic tubing coated with mold release thru the ports and transfer passages, then pouring in the Devcon around the tubing? Akkerman did that with his 820's, and won at least one Nationals that way. Seems it would be a heck of a lot easier to do West Bends with the open transfers! I wonder about the difference in port velocity, due to having the same passage diameter the whole way VS. having the open passage with the port providing a venturi effect.... Still, no argueing with the fact that Akkerman's Bends were fast.

Scott Grundfor · Post by **Scott Grundfor** » Thu Mar 10, 2011 8:47 pm

Yes, I was wondering how you would get perfect transfer holes through the plastic filler. A tube with mold release would explain. I tried to attach a few photos but it says " Sorry, the board attachment quota has been reached." Que Pasa?

5.6 cu in Modified Mc 91?

5.6 cu in Modified Mc 91?

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