Long post time:
We started with a front-end-wrecked '91 Miata street car and stuck a '95 XCR600 motor and clutches in it. That worked okay, but the motor had been built (big-bore and high-compression heads) and was a pain to tune and keep fed. We run between 800 and 1000 miles per race weekend, and high-octane fuel prices were killing us.
It was a surprisingly easy swap: we are using the stock Miata differential and driveshaft mated to a forward shaft made by welding the back end of a Miata transmission output shaft to the left end of an XCR jackshaft. The driven clutch is flipped on the jackshaft and the motor is in with the PTO end pointing aft. There are three bearings under the car to support the shafts: one big one that the snout of the Miata driveshaft slips into, a stock Miata transmission output bearing in the middle of the forward shaft, and the stock clutch-end jackshaft bearing at the front.
We weren't certain the car would work at all, but it is actually very good. It survived the first race despite being poorly tuned, so we went into the second race feeling pretty optimistic. We got the carbs and clutches tuned properly and the thing was super fast. Unfortunately, we broke the #2 rod two hours into the 14 hour race (probably by failing to properly tighten the sparkplugs, oops) and ran the rest of the weekend as a twin. We didn't know it at the time, but there was a fist-size hole through the crankcase. Again, oops. So that was the end of that motor.
We wanted to stay triple for packaging (and sound!) reasons, and wound up with a 2002 Mountain Max 700, bone stock. Much cutting and welding later it was in the car, and actually fits better than the Poo, which we had to sort of lean over to clear the hood. We stayed with the Polaris clutches, though, for various reasons. That turned out to be a mistake.
Our third race went pretty well, and we set the fastest race lap by almost two seconds. Up to this point we'd had zero belt wear or failure issues. The car was great, the engine was great, but our fuel tank (the stock Miata 10 gallon one) was limiting us to like an hour-and-a-quarter between stops, which killed us. I think we finished seventh or eighth out of maybe sixty cars, though we led early on.
So we did the obvious thing and installed a 22-gallon fuel cell. Which totally screwed up the handling. We didn't have a chance to test and the car ended up much more ***-heavy that we were expecting, but whatever. We raced it that way in March at Sears Point. A two-day race Saturday and Sunday, and another eight-hour race on Monday.
The car was very quick despite sitting on the rear bumpstops with a full fuel load, and we were leading when we broke a belt. That kicked off a series of issues that lasted all weekend long: multiple belt failures, cracked drive clutch that we couldn't remove without swapping engines, cam weights interfering with Yamaha clutch, radiator cap failures (one of which cost us the new motor which meant a top-end swap in the car), and finally a waterpump leak. Lots of wrenching, not enough driving. Ugh.
I will say that Yamaha builds a seriously stout motor. The thing ran for like ten minutes without any water in it before it tightened up enough or lost enough compression to lose power.
Anyway, we are rebuilding and re-gasketing our engines and think we know why we we were having belt problems. Sears Point has three really slow corners, and the previous track (Chuckwalla) has none. We were smoking the belt on the sheaves coming off those corners. We also had an alignment issue which we solved Sunday night. We've traded the P-85 driven for a TEAM TSS-04 and am setting it up for more belt pinch. Oh, and we're stepping up to a larger and more-modern belt. We were running XCR600 belts. We'll be spending some quality time on the chassis dyno to make sure we've got the clutching right before the next race in July.
Thanks for reading if you made it this far. Ask questions if you've got them.
Scott
