MAC .21 Turbo Crank

[MikeP]

I have an old MAC .21 inboard that I tried the turbo crank mod. on. I was very pleased with the way it ran. The second week out the crank pin that drives the drum rotor broke off.

Do you think this failure had anything to do with the grinding on the crank?

Any thoughts on MAC turbo cranks?

Mike

 

[Bob Kensill]

I think you mean that you "streamlined" the crank. When you "turbo" a crank,you are modifying the crank opening into the crankcase so it helps "pull" the fuel/air mixture into the engine. Maybe you could post a picture of your "mod" so we could get a better idea of what might have happened.

 

[izitbrokeyet?]

Any thoughts on MAC turbo cranks?

from what I've seen: that motor runs very well without any mods.

 

[gil sonsino]

Modifying cranks is critical with its balance.Everytime it is done it must be rebalanced carefully to avoid problems like this one so think about it next time!.

Gill

 

[MikeP]

Here is a pic of my mod.

Mike

 

[DaveMarles]

How can that possibly improve performance? It will certainly affect the balance. That looks like a definite 'go slower' mod.

Dave

 

[Grimracer]

Looking at the modification Turbo seems to be the right term..

Im not sure if it is or not but if the mod works like Mike was thinking then the velocity of the air/fuel would be increased/compressed as the counter weight/Fan Blade (very loose term) spins.

The mod and amount of marital removed does look quite radical.

Its fun to experiment.. seems to me that one thing that might happen with a mod like this is that you drastically change the volume in the crank case.. this is like running to large a carb.

grim

 

[gil sonsino]

"The mod and amount of marital removed does look quite radical"

Yes Mark

turbo cranks are designed to decrease the air drag produced inside engines but when this is done to cranks like this one the balance is lost and something wrong would happen. All my .21 crank mods are carefully verified in a dynamic electronic balancer used for kart engines and lot of work is needed due to their small size.lThe final result is the same produced by an unbalanced prop!

Gill