HR STU Build / intro thread
05-23-2017, 11:01 AM
#61
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There's not a 285/35 or I probably would run it. There is a 285/30 and the reason I don't run it right now is because of gearing. It's about 3 mph less top speed in 2nd vs the 275/35 tire. At Dixie I was on my stock rev limit at 68 mph, the corvette hit 72 mph in the same section (both using SoloStorm). I feel I need the extra mph for National events.
275/35ZR18 = 275*0.35 + 18/2*25.4 =324.9 mm radius (12.8")
285/30ZR18 = 285*0.3 + 18/2*25.4 = 314.1 mm radius (12.4") (96.68%)
285/35ZR19 = 285*0.35 + 19/2*25.4 = 341.05 mm radius (13.4") (105.99%)
If they made a 285/30 in 19" that would be almost identical in outer diameter to your 275/35 18, but they don't. Would the 285/35ZR19 be too much at 6% taller? (Would give you about the same second gear top speed as the Corvette, although you'd sacrifice in torque obviously.) Or is it just not worth the weight going to a 19" wheel?
Also, do all the aftermarket LSDs for these cars keep the stock gearing? Or would that be another way to tweak it?
05-23-2017, 11:16 AM
#62
I don't think the 19" would be worth it. I wouldn't want to go much taller and I'm not sure there are any good, light weight 19x10 or 19x11 wheels for auto-x. The 18x10.5 RPF1 are light/strong for the size and price.
The OS Giken I purchased used the stock ring gear. You could change that for better acceleration but then you couldn't run in STU. In the ST* classes you have to run the stock gearing. If you bump to an SP class you could use the gear ratios out of an automatic in a manual if that is helpful.
The OS Giken I purchased used the stock ring gear. You could change that for better acceleration but then you couldn't run in STU. In the ST* classes you have to run the stock gearing. If you bump to an SP class you could use the gear ratios out of an automatic in a manual if that is helpful.
05-23-2017, 11:35 AM
#63
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Ahh, gotcha. And I didn't realize that about gearing changes - just figured since you can replace the diff you could put whatever in you wanted - but makes sense.
05-24-2017, 12:09 AM
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Speaking of tires, one more question. I understand you want to go with the widest tire that will fit in the front (which I guess is ~285), but if the tire were available, would it make sense to go wider (staggered) in the rear? Probably not worth the cost to you since you already have the wheels, but for someone starting fresh would it potentially make sense to put a 305 30 19 (similar outer diameter to your 275 35 18) in the rear?
Edit: Ahh except 285 limit in STU, of course. Never mind...
Edit: Ahh except 285 limit in STU, of course. Never mind...
