k24madness

iTrader: (6)

thumpetto007

iTrader: (15)

Keep the 410, shift faster.

codrus

The answer is that it depends on a whole bunch of things -- torque curve, tires, grip level, track configuration, ambient temperature, wind speed & direction, etc etc. I think in Real Racing they use simulations to determine the best option.

--Ian

LukeG

You'd almost be better going to a 3.6. The difference between a 3.9 and 4.10 (I've had both) is pretty small.

Use the FM gearing calculator to play with different options.
https://www.flyinmiata.com/tech/gearing.php

k24madness

iTrader: (6)

k24madness

iTrader: (6)

k24madness

iTrader: (6)

Midtenn

iTrader: (15)

Here is a gearing calculator that shows the speeds in each gear (not just top speed). It even allows you to compare multiple options (tire sizes, gearboxes, final drive, and rev limit). Gearing Calculator - Graf Garage.

acedeuce802

iTrader: (1)

Do you have a torque curve? Or at least someone elses torque curve that you think is close? I can plug all that into my gearing sheet that will look just like the one above, except it'll take torque at the wheel into account.

Have you taken datalogs at your most common tracks to know what your min and max speeds are? Going down to a 3.9 may eliminate a shift at the top end, but may also introduce a downshift at the low end. If you take a torque-at-the-wheels versus speed chart, overlay 4.1 and 3.9, then draw vertical bars at your min and max speeds, and factor in number of shifts between them, it'll be a good guess at which ever one is faster without doing simulation.

k24madness

iTrader: (6)

Here is the last Dyno. I will be limited to the local 3 tracks (Sonoma, Thunderhill and Laguna) in that order of preference. I only have data on Sonoma at the moment. With that in mind maybe a change is premature?

Savington

iTrader: (31)

Depends somewhat on the track, but mostly on the power curve of the engine.

Switching from a 4.10 to a 3.909 will take about 5% off engine speed for a given road speed in the same gear. If power is flat or nearly flat, then that could make no difference. If torque is flat, then power falls in lockstep with RPM, which means you immediately lose 5% power everywhere.

So, the tradeoff for avoiding that 5-6 shift into T10 is a 5% reduction in power everywhere. If I were a betting man I'd say there's no way that's worth it. I would be tempted to look the other way, in fact - can you go up to a 4.3 and keep the same number of shifts you have with your current 4.10s?

For a turbo/supercharged car, my litmus test for "too short" is whether or not you can get the power down on exit in 3rd gear. I have no problem hooking ~205wtq in Rover digging from the bottom of 3rd gear with 3.909s. With your torque curve and the tires you have on that thing, I suspect you have no problem with the 4.10s, and it might tolerate 4.30s on exit.

acedeuce802

iTrader: (1)

Here's the comparo chart. 245/40/15, Miata 6-speed, torque graph from above. Resolution isn't the best, which is why the shifts aren't all vertical (basically the way I have it set up, I have to choose the best cell to start the next gear from, sometimes it's a few mph too fast, sometimes too slow). The 3rd to 4th shift is a good example where resolution isn't a big factor. But your torque/power is pretty linear, so it's easy to imagine that every gear goes to redline, then it should be a vertical line down to the next gear. For example, the shifts from 1st to 2nd shouldn't cross, the red line is how it actually should be, the blue line should remain vertical, and you can imagine how 2nd gear would start more to the left on the graph. Something to take note is that when only comparing rear end ratio changes, only the details matter (such as slowest corner torque, top speed in 5th gear, number of shifts, etc). In a long drawn out (say 2nd to 5th drag race) the two cars will be nearly identical. Notice the squarish gap between torque curves at the 3rd to 4th shift, then notice the long skinny gap during the power band. The area of these gaps are equal, so the little torque you lose while both cars are in the same gear is completely made up for while the higher gear ratio'd car is still in the lower gear.

If you had a speed versus time plot, you could make a histogram of how many seconds you spent at each speed (in say, 1 mph increments). If you spent a good majority of the track between say 37 and 95 mph (and then occurrence dropped off significantly or to zero on either side of this range), then you have 3 long skinny gaps (during this time there's advantage to the 4.1) and only 2 short wide gap (which during this time means advantage to the 3.9), so that would mean 4.1 will be faster, since they also have the same amount of shift points. If the top end of that range was bumped up, so you actually regularly saw 37 to 105, now you're fast enough that the 4.1 would require either shifting (torque reduction and extra time) but the 3.9 can power to redline. In this case, both instances have 3 of each gaps (so average wheel-torque is basically the same), but the 4.10 has an extra shift, 3.9 wins. Go the other way, now your low end is actually 32 mph and the top end is back to 95 mph. There are equal number of gaps, but the 3.9 would have an extra shift (if you downshifted to 1st), so the 4.10 will be faster. If you didn't downshift in the 3.9, then you'd be at a torque disadvantage from the get-go, and the 4.10 would still be faster.

You could take it one step further, create a speed/occurrence histogram, assign each mph a wheel-torque value, then figure out your average wheel-torque throughout the course. Then also factor in shift points and how that would change with the new gear ratio.

Keep in mind that 3.9 to 4.1 is a very slight change and we're talking a few mph here and there. Deciding between these two is really just chasing tenths

Savington

iTrader: (31)

^^The flaw in that model is that you will never use 2nd gear, let alone downshift into 1st. In my experience, it is difficult to get more than about 1500-1600ttq (tire torque) to the ground in these cars while exiting slow corners. Delete your 1st/2nd gear data and expand 3rd gear down to roughly 3800rpm and that will represent the real-world usage of a 6sp car on track. That will shift things much more towards the 4.10's favor.

acedeuce802

iTrader: (1)

That's a good point. I always thought that my wheel-torque threshold was around 2000ttq, which would make 2nd gear usable in this car. Here's the graph if 2nd gear is traction limited. Agreed with Andrew that the 4.10 is much more likely to be faster.

k24madness

iTrader: (6)

I thought about the 4:30's. I have no problem putting down power in 3rd now. With the 4:30's I would likely introduce a shift mid track out unless I could pull 4th in those same corners though.

Working backwards from top speed what do you think I will top out at Thunderhill with 300WHP?
I gotta look at videos/data and see what's what.

k24madness

iTrader: (6)

Second gear is not a factor with any of the tracks I drive. The slower corners are third. This is interesting though. Thanks!

I am intrigued by the 4:30's. Any chance of doing the same chart for those?

Savington

iTrader: (31)

In a dead straight line ~2000ttq is pretty close. On corner exit, you have to drop that figure a bit. With a hot, sticky 245 200tw and ~350wtq, Acamas will just barely get 3rd gear hooked up in a dead straight line with 3.308s, and that's ~1900ttq. If I have the wheel turned at all, or if the tires are cold, or if the ground is cold, it's a tire fire.

Also remember that there are a lot of times on a road course where you might be down a gear from what is mathematically ideal, but the time wasted to downshift and then immediately upshift again just isn't worth it. At Sonoma, T2 and T6 are definitely in that category, where the lower gear is usable but forces an awkward shift on corner exit. It's usually faster to stay in the higher gear, which pushes the advantage even further towards 4.10s.

acedeuce802

iTrader: (1)

k24madness

iTrader: (6)

Thanks guys this all helps!!! Looking at videos I think the downside of a 3.90 outweighs the upside. The 4:30 looks to be a bit too short and will introduce unwanted shifts at mid track out.

With Tim driving he shift into 6th briefly 3 times. The rest of the time RPM's look damn near perfect! Thoughts?

Savington

iTrader: (31)

For comparison, this is Rover's powerband on the same chart. This explains my focus on going as short as possible without losing traction in 3rd and not worrying about the rest of it.