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story by paul van valkenburgh • photos as credited
Sometimes the simplest, most innocuous question requires a complex and demanding explanation. Or …
Great article!
How is the wheelspin thrust limit calculated?
You can also look at the data you can also look at the acceleration Gs. If the acceleration Gs drop off you need to adjust your gearing.
Gearing for race cars is pretty simple, but track dependent.
You want to be near the top of the rev range in top gear at the end of the fastest straight.
You want optimum power available exiting the "most important" corner.
You want second gear to be at the start of the powerband coming out of the slowest corner.
Sometimes the last two are at odds with each other. When that happens, I'd pick the option that forces fewer shifts.
Everything else is picking the middle ground between the must haves.
I like the "final drive ratio by the redline speed in fourth gear, divided that by the top track speed" thumbnail for picking a new FD. However...what if your engine's max horsepower is significantly below the engine's RPM capabilities (redline)? Let's assume that the gear ratios cannot be adjusted from stock.
Say, for example, due to regs restrictions your peak horsepower is at 7400 and falls off fairly quickly, but the engine is fully capable of spinning to 9,000 w/o damage (though it ain't doin' much up there)? If you'd typically shift to top gear at 8000 then do you consider that your "redline" for the purposes of this calculation? Or higher? If higher, how much?
Thought differently, should you shoot for the expected top speed to coincide with peak horsepower? It would seem that at that point, as the car is simultaneously approaches max drag and peak horsepower, that any fall off on the backside of the HP curve would result in statis.
Asking for a friend (of course).
This made me remember when Kenny Roberts was riding for Yamaha. They only had a 350cc bike while the rest just brought out 500's. They were at Daytona for Speed Week and the only way they could compete was to adjust the intake, exhaust and port timing (these were 2-strokes) to all peak in the same rpm range. They won the race, but if you were in the wrong gear trying to exit a corner, you had nothing.
My challenge car build has a 1 gear ratio of 9.11 and top speed of 59mph in 1st and 96 at top of 2nd . I figure it should stay in 1st whole autocross. 3000lb car 2jz boosted .
1st 3.794
2nd 2.324
3rd 1.624
4th 1.24
5th 1.0
6th .794
Rear dif 2.41 limited slip
I have not driven yet , don't know how much the tires will spin when boost hits.
GregAmy said:I like the "final drive ratio by the redline speed in fourth gear, divided that by the top track speed" thumbnail for picking a new FD. However...what if your engine's max horsepower is significantly below the engine's RPM capabilities (redline)? Let's assume that the gear ratios cannot be adjusted from stock.
Say, for example, due to regs restrictions your peak horsepower is at 7400 and falls off fairly quickly, but the engine is fully capable of spinning to 9,000 w/o damage (though it ain't doin' much up there)? If you'd typically shift to top gear at 8000 then do you consider that your "redline" for the purposes of this calculation? Or higher? If higher, how much?
Thought differently, should you shoot for the expected top speed to coincide with peak horsepower? It would seem that at that point, as the car is simultaneously approaches max drag and peak horsepower, that any fall off on the backside of the HP curve would result in statis.
Asking for a friend (of course).
Peak HP is peak acceleration for a given speed. Upshifting will have the engine make more torque, but with less gear multiplication there will be less torque at the wheels. "Horsepower" as a figure is just a shorthand way of figuring torque at the wheels.
I wouldn't want to make max acceleration at the END of the straight. The beauty of having a redline a lot higher than peak power is that you can increase your average horsepower to the ground by running past horsepower peak. (I can provide the math for this if you are interested)
Tl;dr: Rev the sucker
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