Advice needed with gearing / revs

[Zero3210]

Before you think this post is in the wrong section, please hear me out.

After months of reading and contemplation, I finally made the decision to upsize from stock 17" wheels and primacies to a set of 18 x 7.5 +45 Wedsport SA20R (weight ~9.8kgs / 21.6lbs via an intelligent measuring system; though on paper supposedly ~8.27kgs / 18.3lbs) with 225/40/18 Yokohamas V105s. The intention was to improve grip and aesthetics, without abnormal change to basic driving character -- as overall diameter only increased by ~2%.

Immediate changes I felt were a more planted driving feel with lots of grip around the corners (haven't been able to slide since #sadface), braked better as well, but what I also noticed was that the car accelerated slower (as expected) but also less rev-happy?

I did some 3rd and 4th gear pulls on back roads and recorded max speeds for each gear:

Before:
3rd max speed ~133 kmh
4th max speed ~174kmh
5th max speed ~207kmh
6th max speed ~218kmh

After:
3rd max speed ~142kmh
4th max speed ~204kmh (revs took significantly longer to hit 7400 rpm and oil temps rocketed in the process)
5th, 6th no longer revs past 5500 rpm and speed no longer increases -- in fact speed starts to decrease and throttle appears to be cut? (as in throttle feels maxed out at 50% pedal input which wasn't the case with stock setup)

So... thank you if you get this far reading but does the experts here think that should be as expected? And if that is the case, can peeps point me in the direction as to how I can improve the revs? The car really feels like its struggling at 4th gear, and 5th/6th gear becomes essentially redundant. Kinda makes me sad to see all the potential locked instead of unleashed.

As a bonus, I thought revs are proportional to engine output and thus should not be affected by change in tires / wheel size so if someone could shed some light to this I would be eternally grateful.

Please help a homie out, my head if exploding from all the self-reading

[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.

[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.

[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.

[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.

[Zero3210]

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.

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.

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.

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

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.

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?

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.

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.

------------------------------------------------------------------

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.

[scottman]

Just buy a 4.56 or 4.88 FD and you'll be back where you were in revs

[churchx]

Were the wheels & tires really only thing that was changed between the tests?
Reading something like "would resetting the ecu or a decent tune fix the gearing" leaves impression that there might be other things done in that time frame too.

[Zero3210]

Originally Posted by scottman

Just buy a 4.56 or 4.88 FD and you'll be back where you were in revs

You need to help me out a little here. What's FD

Originally Posted by churchx

Were the wheels & tires really only thing that was changed between the tests?
Reading something like "would resetting the ecu or a decent tune fix the gearing" leaves impression that there might be other things done in that time frame too.

And probably battery iirc. No, never tweaked ecu or got a custom tune before but have always kept that option open for the distant future

[Grady]

Originally Posted by Zero3210

Car is pretty much bone stock apart from a SARD rear wing.

I think we just found your top speed issue!

Best fix will be a Turbo!

[churchx]

@Zero3210: if you had disconnected battery (though i fail to see why would one for wheel change, and that's why it didn't cross my mind from reading OP) then most probable cause for throttle/acceleration imho are related to that, not gearing ratio or wheel weight/grip change, due ecu having cleared learnt accelerator pedal travel & fuel trims. Then i'd retest after driving a while, or google how to quicken that, eg. "dead pedal fix", "vishnu reset" and along the lines ..

[Zero3210]

Originally Posted by Grady

I think we just found your top speed issue!

Best fix will be a Turbo!

Well, my 'stock' value measurements were done with the SARD wing in place though. Killer if I may have unintentionally overgripped my rear due to it.

Hahaha ahh from a simple wheel change I'm already facing issues. Can't imagine FI at this stage

Originally Posted by churchx

@Zero3210: if you had disconnected battery (though i fail to see why would one for wheel change, and that's why it didn't cross my mind from reading OP) then most probable cause for throttle/acceleration imho are related to that, not gearing ratio or wheel weight/grip change, due ecu having cleared learnt accelerator pedal travel & fuel trims. Then i'd retest after driving a while, or google how to quicken that, eg. "dead pedal fix", "vishnu reset" and along the lines ..

Thank you. I'll try that mini ecu reset sometime next week and see if it stirs up things. I'll keep you guys posted

[churchx]

It's not reset that should help. It's things that should happen over longer period of driving after reset, to relearn pedal travel & fuel trims (or some procedures to make it happen in quicker fashion). Extra reset/battery disconnect imho won't help if only to reset progress of that process.

[Zero3210]

Originally Posted by churchx

It's not reset that should help. It's things that should happen over longer period of driving after reset, to relearn pedal travel & fuel trims (or some procedures to make it happen in quicker fashion). Extra reset/battery disconnect imho won't help if only to reset progress of that process.

Understood, thanks again. I'll take it nice and slow and steer towards ecu learning and giving it time. For those that has a similar issue hopefully this link will help.

http://www.ft86club.com/forums/showthread.php?t=68328

Never a day I do not learn something new