Full-circle cranks

[Simon Murphy]

Hiya, 

I was exchanging emails with an ex-karting buddy in the UK, and sent him some photos of the internals of a Haddock-tuned Mc91 that I picked up over here.

He expressed surprise that it doesn't have a full-circle crankshaft.  I'd never really thought about that before, but I don't recall ever seeing a Mac with one.  Is that right?  If so, I'm guessing there must be a good reason, but I can't think of one.

Can anyone enlighten us, please?

Simon.

[Tom Day]

Simon,
  That is a modification. Macs did not come that way from the factory. Maybe someone else can chime in here but I think they helped more for enduro racing. I have ran one and it did not seem to help running vintage sprint racing.

[steve miller]

Im putting one in my 140 over 13 ported mc94 for sportsman class ..ill keep ya all posted ...lol...

[Blaine Young]

That puppy should run even better than the MC9 that I modified in the mid sixties before I understood anything about them.   I thought (somewhat incorrectly) that if the goal is to get the intake gas out the exhausts as fast as possible it made sense to cut a passage directly from an intake port to the exhaust.  Surprisingly the engine would actually run but not near as well as your proposed MC94 sportsman.

Jim Akerman made circle cranks and published articles on building Macs and West Bends to his specs.  Those engines really ran (at least in endures).  There was a fellow that built a West Bend that always was in the top five in b-limited at Mid Ohio against a field of all B-bombs.

Smokey

[steve miller]

..Carl H. has a few mc94,s not sure if there 13 ported or a 140over but im pretty sure he,s got 3 on a fox kart .. maybe he will post a picture ..

[doug tenney]

Yup, Carl has a few Mc94's.  Stickers, that is.  Nothing brings a twinkle to Carl's eye like watching someone slowly circling one of his karts, scratching their head 'bout that sticker!


Regarding full-circle cranks: I asked the exact same question on the old forum, roughly four years ago.  It kicked off a pretty good thread, I thought.  I recall that Jeff Campbell explained that karters discovered that the benefits achievable with tuned pipes overshadowed what could be achieved by increasing crankcase compression ratio.

In his book, Gordon Jennings wrote that there was an optimal range for crankcase CR (not just, "more is better").

[Jenning's book focused primarily on piston ported engines.  He was skeptical of the performance potential of reed motors, because of the obvious restriction created by the reeds.  However, I think when you ditch the reeds, the importance of mastering the acoustic tuning aspects is magnified.]

Akerman's full-circle crank modification used metal hoops on each side of the crank pin to act as dams to contain epoxy filler.  Whenever I see a Mac crank that has undergone the modification, I'm struck by how thin the metal ring has to be in certain spots, in order to clear the crankcase.  Looks sporty to me, but what do I know?

[Tony Severino]

First you nee to machine .040 off the diameter of th crank throws to add the rings that hold the epoxy after you fill the crankshaft you re-machine the rings to the original diameter.

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Before epoxy.

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Pic 3

[doug tenney]

Tony, you must've posted that last pic to confuse me! I'm pretty sure the crank goes in from the side... lol

All kidding aside, it looks like you've probably done a few of these. I'm curious what you've found, and if you've gone so far as to measure the results (i.e., dyno hp, lap times, etc.).

I would have to believe that it would change the power curve in some way... and maybe the motor would prefer a different pipe configuration after going "full circle".

[carl haydt]

[steveohara]

Due to the outside diameter constraints of the McCulloch block the Mac engines are ba1ance challenged to begin with as evidenced by the need weld on additional weights for the larger bore engines. Modifying a Mac crank to be "full circle" adds weight in the wrong place and makes the ba1ance factor worse not better. A great deal of research has been done on the effect of crankcase volume and compression ratio and the conclusions are that the returns are diminishing and no gains are achieved after a ratio of around 1.5-1 has been achieved. The "stuffing" gained by modifying to a full circle crank with epoxy filler does little or nothing for performance of the engines, especially if the engine is fitted with an expansion chamber but it will cause more vibration due to the change in ba1ance factor. That being said, they do look cool and if you like messing with the stuff then why not. After all, vintage karting is not about breaking speed records or winning races, it is about having fun and building full circle cranks makes a great conversation subject.
I put the full circle crank in the same category as the swiss cheese holes people punch in the Mac blocks all the time... it was in a magazine back in the 60s so it must have been good! LOL
Steve O'Hara