Optimal Shift Points: 240sx

[MrFox]

Recent discussion in the General Forum led me to think about rear wheel thrust and its relation to optimal gear shifting points. Using dyno data found on the internet (from import tuner no less... :rolleyes ) I generated the following analysis. Rolling radius is 1 foot.



What ya all think? Valid?

[nomuken]

could you explain how you generated this graph?

[MrFox]

We applied the method to the S2000 in the thread mentioned above:

[quote=" MrFox

Lets use the S2000's F20C as example.

240hp@8300rpm153ftb@7500rpm

So does the S2000 accelerate better at 7500rpm (peak torque), or at 8300rpm (peak power)?

At 7500rpm, the engine generates 153ftlbs of torque.First gear ratio is 3.133. Final drive is 4.10. Overall ratio: 3.133 x 4.10 = 12.84Thus, rear wheel torque: 153 ftlb x 12.84 = 1965 ftlb

Factoring in the leverage provided by the radius of the wheel will give us the amount of force that actually accelerates the vehicle.

The wheel/tire of the S2000: 225/50R16Rolling radius: (16/2) + (0.5*225/25.4) = 12.43 in = 1.036 ft

Thus, rear wheel thrust: 1965ftlb / 1.036ft = 1896lb

Repeating the exercise for 8300rpm gives us 1882lb[/quote]The shift point is the RPM at which the force curves of each gear intersect.

[Eswift]

cartest software provided nearly the same result!

[IvoryJ30t]

in a perfect world, you would shift at your horsepower peak, and land on your torque peak. thats optimal.

[Eswift]

in a perfect world, CVTs would be efficient and reliable enough to consider, too!

[IvoryJ30t]

too bad nothings perfect...

[MrFox]

in a perfect world, you would shift at your horsepower peak, and land on your torque peak. thats optimal.

Your perfect gearbox will be optimum.... for accelerating.

But real cars have to climb hills w/o lugging, cruise with decent MPG, generate pretty 0-60 times for the car magazines and bench racers, not have to row through 21 gears to get on the freeway, ect...

Engineering = Compromise

[IvoryJ30t]

exactly, everything is a compromise you cant have performance with fuel economy, ect ect, but when you ask what the optimal shift points are, im sure you want to accelerate, not "what gear should i grab when im on my way to the store"

[MrFox]

ect ect, but when you ask what the optimal shift points are, im sure you want to accelerate, not "what gear should i grab when im on my way to the store"

My point for this post was to show that the common old wives tale of "shift at peak power" or "land at peak torque" for maximum acceleration isn't true for the 240sx, or for that matter 99% of real world street car gear boxes - All the gears are spaced too far apart.

[IvoryJ30t]

well, the shift at hp, land at tq would be applicable if your building a gearbox to personal specs, if not please let me know...

as ive never driven a 240, my input on that would be on par with a three dollar bill.

[MrFox]

well, the shift at hp, land at tq would be applicable if your building a gearbox to personal specs, if not please let me know...

If you are building a gearbox to personal specs, the chief design criterion would be to fit the available gears within the projected target speed range of the vehicle. The engine would then be tuned to produce a torque curve that best matches the gears selected.

What you said is entirely correct, but the logic is analogus to growing longer legs in order to fit them in the pants you just bought.

[s13sr20chris]

dang those are some great analogys.

edit: oops, analogies

[GELLIS2586]

personaly built transmission + engine tuned to match transmission = race car or nut in your pants ride of your life which ever suits you best.

[Q45tech]

The only thing the graph doesn't consider is the tire changes in diameter as the speed increases......say above 80 mph.

Be suspect of any gear combination that requires more than 3 gears to complete the quarter mile [at near redline] because of the human shift time variable...........worse with automatics.

[Cam]

Any chance of seeing your analysis again, MrFox? I'd like to see if it 's anywhere close to my results from plugging numbers into an SCC Technobabble formula from a few years back...

[7thGear]

how would this work for FWD cars?

can i still use the s2000 calculations

[MrFox]

Hey, haven't been to the forums in a while. Anyways, the analysis only went up to the avaliable thrust at the drive wheels, so it doesn't matter which set of wheels are driven.

Now if we want to find out the actual delivered thrust to the pavement, then we'll have to factor in the available traction at the drive wheels. In this case it'll be different for FWD and RWD because of weight transfer due to acceleration - RWD getting traction bonus when accelerating, FWD in turn gets penalised. Factors like tire coefficent of friction, weight distribution, acceleration, CG height, and probably some other stuff will also need to be considered.

[s13sr20chris]

kind of ot but not really. i have been tossing around the idea of taller gears with laggy turbos to spend more time with gas pedal down, but i wonder if being at higher rpms would be more effective at spooling the turbo. any thoughts?