Can You Limit Shift It?
 

When I was young, and had things with rev limiters, you could do really lovely shifts, by just putting pressure on the shifter, and just let the limiter make the shift. ;)

limiter make the shift? What?

hes talking about clutchless shifting i believe. put slight pressure on the shift lever, and when you take your foot off the accelerator it comes out nice and easy then you can slide it easily into the next gear when the trans and engine speed is matched. i dont do it in my personal vehicle, because why the hell would you, but in the heavy duty trucks i drive a work, clutch pedals are only for starting and stopping, their transmissions dont even have syncros, rev matching is the only important thing.

no, i do not clutchless shift the brz after i banged it off the limiter wtf
the posts you guys see come and go on this forum blows my mind

... No...

thanks, but i'd rather replace a clutch than the syncro's.

The limiter would have to have a tremendous amount of hysteresis programmed in.

It would still be way slower than a traditional, relaxed up-shift.

For all but the most skilled operator, the syncros might as well be made of cheese because they'd last about that long.

yep. the clutch is made to be replaced. the syncro's require full transmission replacement...

I can't, but I'm sure you can!

I suspect you could do it. But, as others have pointed out, it would be 1) slower and 2) unless you are a shifting god, REALLY damaging to the transmission.

It's slower for a couple of reasons. Let's say the rev limiter kicks in at 7500. If it's soft limiter it may cut fuel or spark to a cylinder or two. The engine will not simply fall on its face.

The idea behind what you want to do is to put pressure on the shifting mechanism and when the power cuts, the load on the transmission goes away and the pressure on the mechanism is enough to execute the shift as soon as the load is removed. This works quite well on constant mesh transmissions - think motorcycles. In fact, back when humfrz was a youngster it was the idea behind the first quick shifters on bikes. It does not work as well on synchromesh transmissions.

So, if you're expecting the rev limiter to cut the power so you can gently slip into the next gear it's not going to happen. The rev limiter (if it's a soft one) will usually result in the engine developing a case of the blind staggers which even in the event you are the shifting god we mentioned earlier, won't take enough load off the transmission to get the shift done. So you're sitting there with your face hanging out and synchronizers in the the transmission turning to slurry and the engine banging and popping away. Ell seven - loser.

The other reason it's slow is 7500 is way past the power/torque peak. My experience in my car is that the power begins to drop off as you approach 7000. After 7000 both of the stock engine's curves drop like a flume ride at six flags. You can push it farther, but that will slow you down. You're better off (in my opinion) shifting at about 6800-7000. That drops you down in to the mid 5000 range which is very close to the place where the engine's torque and horsepower curves intersect which is generally (again, in my opinion) where you want to be after an upshift.

The window for floating gears on this car is pretty short. There is no practical reason to do it so why bother :iono:

Shifting by banging the limiter is just....why?

The FRS/86/BRZ transmission is pretty forgiving. It's pretty easy to clutchless-shift on the car.


Can you limit-shift on this car? Never tried it. However, the rev-limiter on the FRS/86/BRZ is a hard fuel cutoff. It doesn't bounce, so with that considered I am doubtful it will limit shift.

I've heard of shapeshifters

No, where the curves cross means nothing as far as optimal shift points. Crossover rpm is exactly the same for any engine, and only depends on what units you are using. English units, torque in lb-ft and power in hp are "equal" numerically at 5252 rpm. But this is meaningless as the units are different. In metric (N-m, kW), crossover is at a different fixed rpm (apparently 9543rpm), again for any engine or motor.

Optimal shift point for max acceleration would be to shift far enough *beyond* peak power rpm (which is 7000rpm for stock FT86) that you end up at the same power level at the reduced rpm in the next gear up. For us, we can't rev high enough so optimal shift point is at 7400rpm redline.

7500 is way past the peak, but HP and torque cross at 5252rpm because of the formula used to calculate HP.

HP = Torque x RPM ÷ 5252

LOL. I drove truck for a long time and shifted them without using a clutch, as that's how its done. I used the rev limiter seldom but it worked quite well. I had motorcycles as well and some of them limit shifted really nicely, off the limiter.


The post is certainly tongue in cheek as syncromeshed transmissions don't like no clutch shifting much, but I have done it anyway with several cars. Its hard to make em' shift well.


Certainly the limiter will be beyond the top of any power range, but the point of the exercise is to come into the next gear at a good point to lay down power.

inb4 lol power

Yeah power. Its rather nice really, I have had way more, but this car just works so well once you use some RPM. I do it just to hear her sing. ;)

I'm puzzled how a rev limiter has anything to do with any kind of shifting.

Get a decent oil cooler. Don't cheap out.

Thanks for straightening me out on the intersection question. My former rule of thumb gleaned a long time back was to shift at the power peak and set the gearing up so you land on or near the torque peak.

I'm still a little leery of the argument in the second paragraph, I understand maximizing the area under the curve but my experience on track suggests otherwise. Do you have empirical support for that assertion.

Again, you'll want to shift *beyond* peak power rpm, and don't worry about where the peak torque rpm is. Doesn't matter.

Hopefully I can explain it such that empirical evidence isn't required:
What makes a car accelerate? Forward force applied at the rear tire contact patches. This force is a direct function of engine power, NOT engine torque which has to be multiplied by gear ratios to get rear wheel torque, and so depends on what transmission gear you are in.

Power is torque multiplied by rpm, and it is also force multiplied by velocity. 1 horsepower is 550 lb*ft/s. Forward thrust in pounds at a given speed is rwhp multiplied by 550 lb*ft/s divided by speed in ft/s.

Say you're at the track at 7000rpm in 3rd gear in a '13-16 FT86, making stockish 173 rwhp.
7000rpm, 1.541 3rd gear, 4.1 diff, 24.7" tire diameter => 119.4 ft/sec. Forward thrust at the ground is 173rwhp * 550 lb*ft/s per hp divided by 119.4ft/sec = 797 lb.

This is equivalent to running the *engine* torque through the gear ratios and dividing by tire diameter to get thrust at the rear wheels:
173rwhp at 7000rpm => torque = 173hp*5252/5700 = 130 lb-ft
130 lb-ft * 1.541 (3rd gear ratio) * 4.1 (diff ratio) = 820 lb-ft torque at rear wheels, divide by the tire radius or (24.7"/2) = 1.03 ft = 796 lb. Same result ignoring small roundoff error.

If you upshift NOW, at peak power at 7000rpm, you land in 4th gear at 7000*1.213/1.541 = 5510rpm. Power at 5510rpm is about 148rwhp, engine torque 141 lb-ft. So you're now operating with greater engine torque than you were at 7000 rpm, BUT torque at the engine is not what matters! Thrust or torque *at the wheels* is what matters...
Thrust force at contact patches is 148hp * 550 lb-ft/sec per hp divided by 119.4ft/s = 682 lb. of thrust, quite a bit less than in 3rd at 7000rpm...

Or calculating torque at wheels, 141 lb-ft * 1.213 * 4.1 = 701 lb-ft, significantly less than the 820 lb-ft in 3rd gear at 7000rpm.
701 lb-ft divided by tire radius of 1.03 ft = 681 lb. thrust (same as above calculation using rwhp and road speed)


Alternatively, winding it out past peak power rpm up to 7400rpm redline before upshifting to 4th:
At 7400rpm, power has fallen to 158rwhp, speed is now 126.2 ft/s
Thrust at contact patches = 158rwhp * 550 lb-ft/s per hp divided by 26.2 ft/s = 688 lb.

Or calculating from engine torque (158*5252/7400) = 112 lb-ft, way less torque at the engine, but again that's not what matters!
112 lb-ft * 1.541 * 4.1 = 708 lb-ft at the rear wheels
divided by 1.03 ft tire radius = 688 lb.

Upshifting to 4th at 7400rpm puts you at 5825rpm, at about 155rwhp (140 lb-ft). Note this is still a drop from the 158rwhp you were making in 3rd gear at 7400, so thrust still drops a bit, from 688 lb. to 676 lb.
Torque at the wheels is 140*1.213*4.1 = 696 lb-ft

TL/DR:
Between 81mph and 86mph (119.4 ft/s and 126.2 ft/s), you can either have 681 -> 676 lb. of thrust in 4th gear, or 797 -> 688 lb. thrust in 3rd.
Or in terms of actual rear-wheel torque, you can have 701 -> 696 lb-ft at the wheels in 4th gear, or 820 -> 708 lb-ft of torque at the wheels in 3rd.

More torque at the road wheels, more accelerative force if you keep it in 3rd all the way to redline...

Ideally you'd want to rev a bit higher than this even, such that you land at the same power level at lower rpm in the taller gear.

For best acceleration in this car, you'll want to rev to redline in every gear.

fwiw here's the rwhp dyno numbers I used in the above calcs:
https://media.ed.edmunds-media.com/s...011123_600.jpg

Exactly. For just about any naturally aspirated sports car you will want to shift at redline for maximum acceleration. Older cars and some pushrods are exception.

BRZ/FRS definitely needs to be taken to redline.