Originally Posted by
aarcobra
A couple of observations on DIffs: The Motonica gear diff is a "spur gear" unit, another company had a "worm" type for off-road, called a "Gleason". As Garen said all gear diffs are quite similar.
The ratios Farmer is talking about are the "ring & pinion" ratios and are quite different from what the modern pans run, 48/20 or 2.4:1 for Moto belt drive pulleys vs. 3.3 to 4+:1 for the off-road units as mentioned. Instead of around 47spur/18 pinion (to get a 6.2:1 FDR) you need a pinion in the low to mid 20's for the Kyo/Los 3.3 and low 30's for the Assoc gearing. (Check my math!!!)
I my experience the issue with the silicone oil filled diffs is getting it out and refilling it. One trick is to mix in low viscosity shock oil to dilute the super heavy diff fluid, and as Garen says be sure the fluid is all cleaned out of any pin holes, motor cleaner sprayed in with a "straw" works pretty well. The shimming may need to be done for both the diff gears in the housing and the ring and pinion on the outside. Those big metal gears can be expensive so be sure to pay attention. Hopefully the pan car application will not be as demanding as the off-road application...
Good luck to all!
Right now I'm spitballing but here's the general idea. Tom's car, when it's running right, is nearly as quick down the backstraight as anything else on the track, and there's only a three tooth difference in spurs. Remember, we still intend to use gears, albeit single speed, but now we're only pushing a 5.25 pound car and we're doing it with a .21 vs. a .28, the .21 getting higher rpm. So my crude math skills plus general observation is that the gearing should neither dog nor wind out, plus we have gear options as well (and yes, I'm considering the difference between how it reacts as a 4wd vs. how it should a 2wd.)
If I have the thing built before February, I'll take it with me to the Winternats and run a few laps during practice. I know they aren't running pan down there but no reason I can't get a lap or two in, for testing purposes, and to make sure I'm on top of things before racing season starts up here.
By the way, I'm still trying to talk a guy on Ebay out of his MRP diff so I can complete a third GP180. I'd like to be able to sell at least one on Ebay, just so I know what the market worth of the thing is, but I haven't forgotten that you and Lon both want one, and we'll find a way to make that happen....