Gear ratio for road course

[Performance nut]

I tried this in the transmission and drivetrain area but figured I would ask here as well.

I've been calculating gear ratios for the 10R80 and I'm sort of going back and forth on something. Going with a >3.15 will be great for courses where I stay less than 6th gear. Unfortunately my favorite course (COTA) I will shift into 7th on the straights if I go higher than 3.15. 7th is a deeper shift into the RPM range, 7200 to 5600 where as 3-6 drop down to 6000. With a street tire, I'm at 27.4" (295/35-19 and 305/35-19) which makes that back straight stay in 6th pretty much the whole time with the 3.15.

The question is whether or not that 6-7 shift is a big deal or not. Current setup I am in 6th or lower (haven't broken 150 mph on the long straight). Only part where I see having a detrimental effect will be the long straights were I'm 130+ mph. Anything above a 3.55 with my current tire setup or 3.31 with a 26" tire will force me to shift to 7th on the long straight. Another consideration is going with >3.15 also opens options up for things like Torsen differentials though.

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[Grintch]

It depends on the track.

But driving a track in 3rd through 5th with a tall final drive in practice should be little different than driving the same track in 4th through 6th with a shorter final drive.

Your not going to use the bottom gears on a road course, because you don't have standing starts. And you will never use the top end gears that are 200+. So with 10 to choose from, it's unlikely you will find any gaps that make a significant difference. Now if you had only 6...

 

[Performance nut]

It depends on the track.

But driving a track in 3rd through 5th with a tall final drive in practice should be little different than driving the same track in 4th through 6th with a shorter final drive.

Your not going to use the bottom gears on a road course, because you don't have standing starts. And you will never use the top end gears that are 200+. So with 10 to choose from, it's unlikely you will find any gaps that make a significant difference. Now if you had only 6...

Thanks for the response. This is a practice in theory as driving the course is truly the only way to do this so I appreciate your feedback. I had a feeling I was over thinking it but wanted to see if there was something I was overlooking or not considering. I appreciate the input.

 

[Grintch]

After looking at my gear chart:

What were they thinking, a tight 551 rpm drop from 9th to 10th, but relatively big 1595 & 1500 drops from 6th to 7th and 8th to 9th? It's like OE engineers pick ratios at random.

While the 6th to 7th gap is relatively big. It is still pretty tight by 6 speed manual standards. But avoiding it by using a taller final drive would probably be worth a little time. But I would stick with what I have unless you are to the point where spending $1-2K to gain a tenth or two is worthwhile.

 

[Performance nut]

After looking at my gear chart:

What were they thinking, a tight 551 rpm drop from 9th to 10th, but relatively big 1595 & 1500 drops from 6th to 7th and 8th to 9th? It's like OE engineers pick ratios at random.

While the 6th to 7th gap is relatively big. It is still pretty tight by 6 speed manual standards. But avoiding it by using a taller final drive would probably be worth a little time. But I would stick with what I have unless you are to the point where spending $1-2K to gain a tenth or two is worthwhile.

I was also reading up about the differentials. Lots of folks are saying the base differential is no where near optimal for the track which I can say it is pretty sketchy at best. Then again, I haven't driven any of the performance cars so I have no basis for comparison. Unfortunately no one makes a diff for the 3.15 aside from the stock on. Changing gears opens up options for a diff as well.

With all that said, am I looking at "small changes" that only advanced competitors do? I'm looking for a better experience at the track, 1/10 of a second isn't going to make a difference to me. 2 or 3 seconds will. COTA Is a really big track so minor stuff I don't think I will notice as much.

 

[Grintch]

Yes, Vorslag wore out the standard traction lock (?) in just a few events in their S550 TT car. If you want a limited slip diff that will last without a lot of maintenance, the Torsen is the way to go.

And you can probably buy a complete take out diff for about what a new final drive will cost with installation (lot of labor setting up the gears right). But that limits you to 3.55 or 3.73 options available with the Torsen. And the torsen (or a limited slip that works) will make more difference on a road course than a little improvement in gearing. I would go for the 3.55 as that avoids the 6-7 shift (with bigger RPM drop) on most tracks (but not likely COTA, Road Atlanta, VIR, or tacks with a particularly high speed straight).

 

[EFI]

Honestly, with a 10 speed auto you shouldn't have to worry much if at all about gearing. With so many gear choices, you should always be in the meat of you power band regardless of the speed.

Guys with 6 speed manuals where they only have 4 or 5 forward gear choices (3 or 4 really as 1st gear is useless) then rear end gear choice is much more important. You want to set it up where you have enough punch down low but still be able to carry enough top end without going into overdrive gears. With the 10 speed auto, you can have an aggressive 2nd gear for low speed corners and also a tall ass 7th gear for long straightaways, and 4 more gears in between.

 

[JohnD]

Honestly, with a 10 speed auto you shouldn't have to worry much if at all about gearing. With so many gear choices, you should always be in the meat of you power band regardless of the speed.

Guys with 6 speed manuals where they only have 4 or 5 forward gear choices (3 or 4 really as 1st gear is useless) then rear end gear choice is much more important. You want to set it up where you have enough punch down low but still be able to carry enough top end without going into overdrive gears. With the 10 speed auto, you can have an aggressive 2nd gear for low speed corners and also a tall ass 7th gear for long straightaways, and 4 more gears in between.

Totally right on. The MT-82 is effectively a 3 speed box at all the road courses I run. 1st is only for starting from stop, 2nd is not much better, 3-4-5 are useful and 6th is useless. I run out of revs in 5th gear all over the place with the 3.73 rear, thinking about going to 3.55's.

 

[EFI]

I run out of revs in 5th gear all over the place with the 3.73 rear, thinking about going to 3.55's.

Jeez what kind of tracks do you go to that you're constantly going over 150mph? Top of 5th in a stock geared stock tire GTPP is 154mph.

I find myself using 3 and 4 99% of the time, with just barely getting into 5th on the straightaways but these are short tracks where I top out at around 125mph. I would need a realllly long straightaway to find another 30mph and even then I would only be going to 5th only be once per lap. You seem to be doing this multiple times a lap.

 

[DB83]

Wait, if i'm reading this right, you can't run a torsen with a 3.31 rear end?

 

[EFI]

Wait, if i'm reading this right, you can't run a torsen with a 3.31 rear end?

Yes you can, the only odd-ball are the 3.15s

 

[DB83]

Phew! I find 1st and 2nd completely useless, and was hoping to make it a bit better swapping my 3.55 to a 3.31.

 

[EFI]

Phew! I find 1st and 2nd completely useless, and was hoping to make it a bit better swapping my 3.55 to a 3.31.

1st is pretty much always useless regardless of the gearing.

I would honestly go up to 4.09s and only use 3 - 4 - 5.

 

[DB83]

Yeah, but its also my daily, and i do lots of cross country club runs and cross Europe drives, so would also need decent highway gears.

 

[Grintch]

Wait, if i'm reading this right, you can't run a torsen with a 3.31 rear end?

You can, but you lose the option of a rear end swap because the torsen only came in 3.55 or 3.73 stock.

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