Quote:
Originally Posted by EndlessAzure
None of this sounds like it would be caused by wheels/tires.
If the overall diameter difference is only ~2%, you shouldn't be having speedo differences like that. Are you in manual mode? Are you pegging the redline or holding at a consistent RPM when you mark your max speed? Did you put back on the stock wheels/tires to confirm that it was indeed due to them?
How hot is the ambient air temperature there? Car could be throttling due to overheat or high intake air temp or other factors taking away from power. You shouldn't be really seeing much acceleration after 190 km/h.
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My thoughts exactly.
My 86 is an auto but transmission behaving the same regardless of whether I placed it in auto or shift mode. On stock wheels, the auto shifts automatically @7400 rpm when flooring which is the same spot I mark my max speed. There are slight variations of +/-5kmh depending on road conditions, etc but I tried to only take values at flat roads during normal summer heat ~26 - 35 deg celsius (~80 - 95 F for you imperialists).
What I do notice though with the new wheels is that at 3rd gear it holds max rpm for about 3 - 5 seconds longer before it shifts which is unusual as in the past the shift is pretty much instantaneous when it hits max rpm. I had to release the accelerator slightly to encourage the upshift.
I am aware I'm not doing any favours to the poor FA20 by wringing it out like a crazy fool but I do also like to experiment and collect real world data. It's all physics and mechanics at the end of it which I take a mild interest in.
Quote:
Originally Posted by Grady
Your math does not add up. Please recheck your numbers.
According to your speeds your tires increased by 10% or so.
Was the last check of speed on worn out tires?
A better quality tire in the stock size may get the grip you want without the degradation in performance.
Put stock tires back on and retest. If the tire size is causing your issue I would go back to stock.
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I simply inserted the values on
https://tiresize.com/calculator/ and cross-verified the calculations manually which reads out to ~1.81% increase in overall diameter from the top of my head. Yes, the old primacies were fairly worn down to ~2mm left past the tread wear indicator.
Yeah, which is also why I am not particulary complaining with the performance deficit as I exchanged it with slightly better aesthetics and "fitment".
Forgive me if this is a noob question but would resetting the ecu or a decent tune fix the gearing? I'm most concerned about the gearing / revs becoming less rev-happy which gives a 'struggling' impression rather than actual top speed -- though it would be nice to be able to go faster
Quote:
Originally Posted by wparsons
How are you measuring speed? If you're using the speedometer in the car, or even the wheel speeds from data logging, the speed vs rpm won't change, even if you have a hugely different overall diameter tire on all four corners. Only a change in the final drive (or transmission internal ratios) would change the speed vs rpm (and as a result, the max speed indicated in any given gear).
What else has been changed on the car? Do you still have the original wheels/tires? I'd do a better test back to back with the original tires if you can.
I think you're completely over thinking this, and looking at the wrong cause.
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That was what I understood but does not seem to be the case. Speed measurement already answered as above. I also run a dash OBD2 reader which shows intake temps, manifold pressure, etc if that's of any benefit.
Car is pretty much bone stock apart from a SARD rear wing. Amazing kit btw -- did not expect the downforce and much better high speed stability. I did also change my battery recently from OEM to a 55D23R AMARON BATT which the agent recommended -- though I would not think that should affect anything?
Quote:
Originally Posted by NLSP
The larger tire diameter has effectively made your gearing longer, that's why it's less "rev-happy". I noticed this too when I swapped wheels/tires.
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Hmm yeah. But was hoping someone could point me in the direction of just at least utilising 5th or 6th gear as its pretty much there only for fuel economy now without speed increase.
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My stock wheels/tires are no longer road legal, and replacing the rubber would seem like a defeat at this stage. I do plan on future minor mods which may up horsepower by ~5 - 20 but I doubt that would fix my gearing?
Thanks guys I'll appreciate any other input -- if any.