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Help with sequential trans gear ratios

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803
1,291
TX
Hi all -

Would love a sanity check on gear ratios for the mustang since I'm getting really close to pulling the trigger. So HGT offers three different options for first

1678918435995.png

Plugged these into Tremec's site. Tire height is assuming 27.8 - good middle ground for the slicks I plan to run (Michelin/Pirelli takeoffs). Keeping the rear 3.73 for now (although I'd like to go to a quick change in the future). Planning on running new motor up 8, but will probably keep it at 7.7 for longevity.

This ratio makes the most sense to me. First gear is very usable. (Would keep engine above 4k RPM in the slowest corners)
1678918410117.png

This ratio could also work - but wondered if it would rev first out too fast and make the jump to redline in 2nd a little long
1678918638235.png

This felt a bit too long in first. I'm assuming if I had more torque, this might be helpful. But with a Coyote-levels of torque, don't think I need the taller gear? Plus, the run from 1st to second is REAL short
1678918792038.png


Would appreciate any thoughts/feedback/insights ya'll have. thanks!
 
42
46
Exp. Type
Autocross
Exp. Level
3-5 Years
Soddy Daisy, TN
I've got a dumb question. Do all sequential gearboxes/transmissions have to use an onboard air compressor? I've tried doing some looking into this and I haven't got a clear answer. It seems if they do, it would be just one more thing to fail.
 
98
113
Exp. Type
Time Attack
Exp. Level
Under 3 Years
So cal
I've got a dumb question. Do all sequential gearboxes/transmissions have to use an onboard air compressor? I've tried doing some looking into this and I haven't got a clear answer. It seems if they do, it would be just one more thing to fail.
Believe only if you are doing paddle shifters.
 
803
1,291
TX
I've got a dumb question. Do all sequential gearboxes/transmissions have to use an onboard air compressor? I've tried doing some looking into this and I haven't got a clear answer. It seems if they do, it would be just one more thing to fail.
Mine will be shifted with a traditional sequential shifter like the below

1679027253726.png
To shift with paddles, you need to move to a pneumatic kit which adds cost, but they're pretty darn reliable. Pretty much every GT4/GT3 car these days uses them. Lots of cool stuff you can do with the right ECU - prevent shifts that would overrev the engine, etc.
I start races in first on a 2.66 box - it’s a riot. I wouldn’t go too much longer especially if you are still doing the lockout thing
Will be back to VCT with the new motor. I want all the torque I can get.

Are you doing standing starts in first? Or you mean from a roll?
 
1,269
1,262
In the V6L
Hi all -

Would love a sanity check on gear ratios for the mustang since I'm getting really close to pulling the trigger. So HGT offers three different options for first

View attachment 84834

Plugged these into Tremec's site. Tire height is assuming 27.8 - good middle ground for the slicks I plan to run (Michelin/Pirelli takeoffs). Keeping the rear 3.73 for now (although I'd like to go to a quick change in the future). Planning on running new motor up 8, but will probably keep it at 7.7 for longevity.

This ratio makes the most sense to me. First gear is very usable. (Would keep engine above 4k RPM in the slowest corners)
View attachment 84833

This ratio could also work - but wondered if it would rev first out too fast and make the jump to redline in 2nd a little long
View attachment 84835

This felt a bit too long in first. I'm assuming if I had more torque, this might be helpful. But with a Coyote-levels of torque, don't think I need the taller gear? Plus, the run from 1st to second is REAL short
View attachment 84836


Would appreciate any thoughts/feedback/insights ya'll have. thanks!
I have no direct experience of these gearboxes, but if I had the problem of picking a first gear ratio, I'd look for an calculation that converts your engine's torque curve into a rate of acceleration through first and then second, and then pick the first gear ratio that gives you the fastest time to the top of second gear. It's basic physics. I used a similar approach back when I had a supercharged 2011 GT. I wanted to know how much HP I was producing in real world terms so I looked at my Racelogic data for speed vs time on the front straight at the Ridge. Combining that data with the weight of the car, I computed torque and HP. I was pretty happy when I got HP numbers that were consistent with Magnussen's claims for performance of the blower kit.
 

Dave_W

Cones - not just for ice cream
1,024
1,336
Exp. Type
Autocross
Exp. Level
20+ Years
Connecticut
Here's an example with a 42MPH start
You had to pull a shift pretty quickly there. I'd think you'd want to be more in the meat (or just below the peak) of the torque curve at the start to maximize thrust. Though that would take a longer gear in this case, which would reduce thrust at the contact patch at the same rpm/engine torque, so it's really a balance. You could build a formula in Excel that takes engine torque at a few rpm values, multiplies it by the gearbox & diff ratios to get wheel torque, then divides by tire radius to get thrust at the contact patch. You could work the same calcs backwards to get engine rpm for different gear ratios at a given mph (e.g., starting speed), and use that to feed the rpm inputs in the first formula.

@JAJ beat me to it.
 
803
1,291
TX
Okay all - @captdistraction shared his tractive force Excel doc with me. I've played around a bit to get numbers closer to my HP/torq and wheel size. Here's what I am looking

1679165657461.png

I am not sure what I am looking - but it looks like the extra wheel torque from the 2.66 set will get my to the top of second faster and nearly the same mph. Are you guys able to help interpret the graph?
 

Dave_W

Cones - not just for ice cream
1,024
1,336
Exp. Type
Autocross
Exp. Level
20+ Years
Connecticut
Hmm, not sure how that spreadsheet works, but I think you may be comparing a 2.44 ring & pinion to a 2.6, using the same gearbox ratios (gearbox A and R&P A vs gearbox A and R&P B). If there's only one set of gear ratio inputs, you probably need to make some changes so you can enter a second set of gearbox ratios, then tweak the R&P B formulas to use the gearbox B ratios. That would let you compare gearbox A using R&P A to gearbox B using R&P B. You can then use the same values for both R&P to do a straight comparison of gearboxes.

The other thing to look at is the Tractive Force values. If they are in pounds-force, you can divide them by your car's pounds-weight to come up with potential g's acceleration. But realize that you're traction-limited, so just because the gearing can put enough torque to the contact patch for 1.4 g's of acceleration doesn't mean you won't smoke the tires doing that. If you have some data logs, take a look at the g-g "friction circle" graphs and see what max acceleration you're typically able to use.
 
349
354
Exp. Type
Autocross
Exp. Level
20+ Years
So Cal
Hi all -

Would love a sanity check on gear ratios for the mustang since I'm getting really close to pulling the trigger. So HGT offers three different options for first

View attachment 84834

Plugged these into Tremec's site. Tire height is assuming 27.8 - good middle ground for the slicks I plan to run (Michelin/Pirelli takeoffs). Keeping the rear 3.73 for now (although I'd like to go to a quick change in the future). Planning on running new motor up 8, but will probably keep it at 7.7 for longevity.

This ratio makes the most sense to me. First gear is very usable. (Would keep engine above 4k RPM in the slowest corners)
View attachment 84833

This ratio could also work - but wondered if it would rev first out too fast and make the jump to redline in 2nd a little long
View attachment 84835

This felt a bit too long in first. I'm assuming if I had more torque, this might be helpful. But with a Coyote-levels of torque, don't think I need the taller gear? Plus, the run from 1st to second is REAL short
View attachment 84836


Would appreciate any thoughts/feedback/insights ya'll have. thanks!

I've got a lot of experience doing this. What tracks are you most interested in running? Generally speaking, you want to choose the tallest first gear as possible because you're generally traction limited at those speeds, anyway, and wider gear split on the lower gears means smaller ones in the higher gears.
 
349
354
Exp. Type
Autocross
Exp. Level
20+ Years
So Cal
Okay all - @captdistraction shared his tractive force Excel doc with me. I've played around a bit to get numbers closer to my HP/torq and wheel size. Here's what I am looking

View attachment 84891

I am not sure what I am looking - but it looks like the extra wheel torque from the 2.66 set will get my to the top of second faster and nearly the same mph. Are you guys able to help interpret the graph?
The shorter gears will accelerate faster, but you'll see a time loss by the time you get to the top of third because you've made the 2-3 shift longer.

If you're at 4k revs in first in slow corners, then you'll be traction limited through corner exit. That is too short of a gear, IMO. By the time you reach full throttle, you'll already have to shift., so you'd be better off with a taller gear and more advantageous rev drops in higher gears.
 
1,269
1,262
In the V6L
I did the math on @honeybadger 's gear options based on the torque curve he provided and the tire size and gear ratios he's considering. The result looks like this:

1679516996684.png
The time from 30 to 107 MPH looks really quick, but there are no driveline losses, shifting time delays or aero resistance in the calculations. For the fastest time, the times between points on the torque curve are computed for the speed change from 30 to 62.4 MPH in first, then exactly 62.4 to 86.3 in second, then exactly 86.3 to 106.8 in third. The second gearset is done the same way with its own specific shift points.

Bottom line? Given how close the two gearsets are are (0.036 seconds is about 5' 8" in distance at 107 mph) I'm not sure there's a lot to choose between them. There's also the risk that a 1.4G launch (with the shorter but faster gear set) on cold tires might not be a winning strategy. Better perhaps to figure out which gearset puts the shift points in the best locations on the track.
 
803
1,291
TX
Hmm, not sure how that spreadsheet works, but I think you may be comparing a 2.44 ring & pinion to a 2.6, using the same gearbox ratios (gearbox A and R&P A vs gearbox A and R&P B). If there's only one set of gear ratio inputs, you probably need to make some changes so you can enter a second set of gearbox ratios, then tweak the R&P B formulas to use the gearbox B ratios. That would let you compare gearbox A using R&P A to gearbox B using R&P B. You can then use the same values for both R&P to do a straight comparison of gearboxes.

The other thing to look at is the Tractive Force values. If they are in pounds-force, you can divide them by your car's pounds-weight to come up with potential g's acceleration. But realize that you're traction-limited, so just because the gearing can put enough torque to the contact patch for 1.4 g's of acceleration doesn't mean you won't smoke the tires doing that. If you have some data logs, take a look at the g-g "friction circle" graphs and see what max acceleration you're typically able to use.

@JAJ helped me work through the Excel sheet to make it more helpful (HUGE thanks, JJ)


I've got a lot of experience doing this. What tracks are you most interested in running? Generally speaking, you want to choose the tallest first gear as possible because you're generally traction limited at those speeds, anyway, and wider gear split on the lower gears means smaller ones in the higher gears.
Near term - will be mostly COTA with Road Atlanta, VIR, and Road America thrown in. Road Atlanta might be the new home track in the future - but generally i'm expecting to be running fairly high speed tracks that are all known to have a few low speed corners right before the fast straights.

I did the math on @honeybadger 's gear options based on the torque curve he provided and the tire size and gear ratios he's considering. The result looks like this:

View attachment 84971
The time from 30 to 107 MPH looks really quick, but there are no driveline losses, shifting time delays or aero resistance in the calculations. For the fastest time, the times between points on the torque curve are computed for the speed change from 30 to 62.4 MPH in first, then exactly 62.4 to 86.3 in second, then exactly 86.3 to 106.8 in third. The second gearset is done the same way with its own specific shift points.

Bottom line? Given how close the two gearsets are are (0.036 seconds is about 5' 8" in distance at 107 mph) I'm not sure there's a lot to choose between them. There's also the risk that a 1.4G launch (with the shorter but faster gear set) on cold tires might not be a winning strategy. Better perhaps to figure out which gearset puts the shift points in the best locations on the track.
I'm leaning towards the 2.44 first as it seems to be a good middle ground and doesn't lose too much. That said, the 2.16 might be worth a second look. It is slower on paper

1679528647893.png

Peak accel g looked to be about 1.15. This is probably more usable since my aero is always setup for high speed. And that initial 50ft out of the tight corners can be hard to put the power down. The higher first gear might be just the ticket. But might be worse all around on a race start.

I could also drop the diameter of the rear tire to get a bit more wheel torque if needed. Thoughts?

And @JAJ thanks again so much for your help working through the excel doc. It's AWESOME
 
349
354
Exp. Type
Autocross
Exp. Level
20+ Years
So Cal
@JAJ helped me work through the Excel sheet to make it more helpful (HUGE thanks, JJ)

Near term - will be mostly COTA with Road Atlanta, VIR, and Road America thrown in. Road Atlanta might be the new home track in the future - but generally i'm expecting to be running fairly high speed tracks that are all known to have a few low speed corners right before the fast straights.

So the slow corners at VIR and Road Atl are essentially the same and you want to be able to have the car straight before upshifting if you can, which asks for a taller gear. Road America is a similar speed at T5 and then T8 is a little faster than T5, but also usually first. It would be nice if the run from T5-T6 at Elkhart was only a single up/down. At COTA you have the hairpin, and a shorter gear can help in turning the car, but you also have the 13-17 esses. Trying to reduce shifts there can a good thing.

In then end, you really need to figure out what kind of speeds you'll actually see at these tracks to make an informed choice, but the decision shouldn't just be because a shorter gear accelerates faster.

Have you asked anyone else running this box what their opinion is?
 
1,269
1,262
In the V6L
@JAJ helped me work through the Excel sheet to make it more helpful (HUGE thanks, JJ)



Near term - will be mostly COTA with Road Atlanta, VIR, and Road America thrown in. Road Atlanta might be the new home track in the future - but generally i'm expecting to be running fairly high speed tracks that are all known to have a few low speed corners right before the fast straights.


I'm leaning towards the 2.44 first as it seems to be a good middle ground and doesn't lose too much. That said, the 2.16 might be worth a second look. It is slower on paper

View attachment 84983

Peak accel g looked to be about 1.15. This is probably more usable since my aero is always setup for high speed. And that initial 50ft out of the tight corners can be hard to put the power down. The higher first gear might be just the ticket. But might be worse all around on a race start.

I could also drop the diameter of the rear tire to get a bit more wheel torque if needed. Thoughts?

And @JAJ thanks again so much for your help working through the excel doc. It's AWESOME
If you look, the worksheet has a new page with shift points at 8K up to 6th gear. You can change the roll and gear ratios and it's all automatic now - it'll do the rest.
 
803
1,291
TX
Michelin S slicks in 68 height are 26.8" tall, not 27.8", 65 heights are 25.75" tall. Pirelli 650 and 680 are close.
I would have a 3.55 rear and a 3.31 rear gear set for normal road courses and Daytona. Love those ratios. Go with the 2.44 1st gear.
I was thinking I'd be running the 705/710s in the rear which come in around the 27.8 height. Do you recommend the 65/68 heights instead?
 

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