Targa Miata
August 6, 2008 - The Guru diff, freshly wrapped in a 4.10 ring and pinion.
It's a beefy thing, rated for 600 hp or so. With the number of diffs I've been juggling, I've started identifying them with a silver marker on the housing. Makes life a lot easier, and this way you don't accidentally install an open 4.30 diff in your rally car. To pick a random example.
entry 507 - tags: differential
August 6, 2008 - I love that blue.
The last time I installed the diff, I experimented with using Hylomar HPF instead of the usual RTV. Hylomar never really sets up, but stays tacky. This makes it really easy to separate later and to clean up. The old Land Rover guys use this a lot, and when I came across it in my toolbox I decided to give it a shot in something a little newer and hopefully more oil-tight. I'm pretty happy with the results, it's much cleaner to work with than RTV.
While writing this, I decided to check out the suggested uses of Hylomar from the manufacturer. And will you look at that, they recommend it for "differential covers". So there you go.
entry 508 - tags: differential
August 24, 2010 - I spent part of the day replacing the differential.
I'm going to try out a new unit that's a clutch-type instead of the Guru helical I have now. The car's tendency to spin an inside wheel on right turns should be abolished!
Or should I say, the car's old tendency. At the Open House, I didn't have any trouble with that. I always had a passenger which does alleviate the problem - but likely more important, I was running a different spring setup with a lower ride height than usual. I also didn't have the rear sway connected, but I know I've run without it before. Interesting. It could have simply been the passenger and the fact that I've been driving other cars and taking lines that alleviate wheelspin. Anyhow, it'll be interesting to see how this new diff turns out.
I've also pulled one of the front shocks off. Due to an assembly error on my part, it was leaking pretty badly at the track day and acting very poorly on right turns - maybe that was the secret! The good thing is that if an AFCO leaks, you can just refill it with the correct oil and it's as good as new. I have the oil, so I'll top it up and test the result on the track by Sept 4th at the latest.
entry 760 - tags: suspension, differential
September 5, 2010 - Track day test!
The good news? The new diff works beautifully. I could get on the power much earlier and get a solid drive off turns - left and right. It's a clutch-type diff. I had an idea this might be effective as the V8 cars from Flyin' Miata have shown an uncanny ability to hook up out of corners. On the first session, I managed a 1:03.796. That's nearly a full second off my personal best in the car. Now that is some serious progress!
It wasn't without a cost, however. I found the handling of the car was difficult. In particular, I had trouble with corner entry understeer. It didn't seem to be there for that first session, but almost seemed to get worse as the day went on. I tried to drive around it and I tried to tune around it with shock settings and a stiffer rear sway. Interestingly, the sway bar change didn't seem to have any effect. I also found I couldn't get enough rear bias into the brakes - even with the rears turned up all the way, I'd still lock up the fronts too easily.
I suspect what might have happened is that the old tires have finally given up. This is the same rubber I ran in the Targa almost exactly two years ago. They've seen 5 days at Laguna Seca with two drivers, probably a dozen track days on this track, some road use and of course the Targa itself. When the Seven is too low on traction, it gets a similar behavior. Oddly, the car doesn't feel slippery, it's just not hooking up the front under braking or turn-in. Am I going in the wrong direction? Hard to say. I might just have been trying too hard.
I have another track day on a big track next weekend, and I flat-spotted at least one of the fronts badly enough that it's done now. I have two more mounted tires and two unmounted ones in the garage. Both pairs from the Targa (or possibly pre-Targa testing) of course, but they haven't seen as many heat cycles. So I'll swap the flat spotted front tires out for the "new" unmounted ones and put the other pair on the rear, then take the existing rears with me as spares. I'm also going to double-check that front shock to make sure it's working as well as bleed the brake system again, and if things aren't any better then I might throw in some extra front camber if possible.
I ended the day a bit bummed out that I couldn't go faster, but I'm feeling a bit better now that I look at the times again. Another Miata at the event - a turbo car running some Nitto 225/45-15 tires on 15x9 rubber!- was putting down times right around the same as I, and his times fell off over the day like mine did.
Was the diff a success? Well, if I can sort out the handling, yes. Right now, the car wouldn't be happy on the Targa because it's too easy to wash out the front end, but if I can get that sorted then it's a real winner in terms of fast corner exit speeds.
entry 761 - tags: testing, differential
January 20, 2011 - The first of the V8 conversion parts is here.
This is the mounting system for the rear differential. Two tabs need to be welded to the rear subframe to locate the nose of the diff and that grey cross-bar bolts into the factory mounting spots. The big aluminum disc is an adapter that goes on the pinion flange of the differential.
entry 775 - tags: parts, differential
January 24, 2011 - Here's our differential.
It looks used because it is - it's the one originally used in Elvis. That car has a new rear end in it and this one landed in a box awaiting the Targa Miata. It's a Getrag rear originally used in the Cadillac CTS-V and has a 3.42 ratio and a Positraction clutch-type limited slip. It's a very effective package.
entry 776 - tags: differential
January 27, 2011 - Time to mount the Getrag differential in the rear subframe.
There's a cross bar that mounts the back of the diff, but we need to weld two tabs on for the front. The stock rubber bushing has been removed and replaced with a polyurethane part for better control of the diff. It's held in place with a set screw.
entry 781 - tags: differential