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Discussion Starter · #1 ·
Does anybody know the ungoverned theoretical top speed of the ISF? I am not referring to the ungoverned drag limited top speed but rather the redline limited top speed in 8th gear, if the ISF had enough power to reach it (or should I say at which point my ISF has enough power to reach it). My wild guess is it should be around 200 mph since according to the manufacturer's specifications it is only at 4400 RPMs in 8th gear at 170 mph. Does anybody know how to calculate it based on gear ratios? (Also, sorry if this is a repeat of some other post that I missed.)
 

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Discussion Starter · #7 · (Edited)
I read somewhere that Lexus claims the ISF is capable of 185+ without a governor. If that is true, than my guess is you would probably need at least another 100 horsepower to be able to push the ISF past the magic 200 mph mark...an accurately measured 200, not 200 mph on the speedometer. I am basing this on the fact that it takes a significant power increase to give a sportbike capable of 185 mph the ability to hit 200. Maybe the superior aerodynamics of a sports car over a sportbike make it a little easier...I don't know.
 

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Discussion Starter · #9 · (Edited)
259 Mph !!!

I just did a google search and found several Theoretical Top Speed Calculators (theoretical meaning they don't take the drag coefficient or tire resistance into account). Assuming that you could build the ISF motor to produce enough horsepower to achieve such a speed...based on a 6800 rpm redline, a 2.94:1 final drive ratio, a 0.69:1 8th gear ratio, and a 255/35R19 rear tire size...according to the calculator http://www.catherineandken.co.uk/sti/tyres.html the theoretical top speed of the Lexus ISF is 259 mph!!! There are several other calculators out there, and I will try them out later. As it looks now though, the only things limiting ISF owners with regards to top speed are the governor, power, (and eventually aerodynamics, suspension, and tires with safety in mind if you were really going to push it to an extreme level).
 

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Discussion Starter · #11 ·
each gear

Just for kicks...with a 6800 RPM redline, 255/35R19 rear tires, and a 2.94:1 final drive axle ratio, the theoretical redline-limited top speed in each gear is as follows:
1st - 4.60:1 - 38 mph
2nd - 2.72:1 - 65 mph
3rd - 1.86:1 - 96 mph
4th - 1.46:1 - 122 mph
5th - 1.23:1 - 145 mph
6th - 1.00:1 - 179 mph
7th - 0.82:1 - 218 mph
8th - 0.69:1 - 259 mph
rev - 2.18:1 - 82 mph

I love the fact that we have eight speeds. Eventually I might have to swap out the ring and pinion for something numerically higher to improved acceleration. Even if a 3.55:1 were used, the theoretical top speed in 8th would still be 214 mph.
 
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