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[ANSWERED] Manual Transmission Gearing

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[ANSWERED] Manual Transmission Gearing

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Old 04-26-2017, 02:01 PM
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Default [ANSWERED] Manual Transmission Gearing

The original question is here.

Nosferatu asked:
On the manual transmisison Corvette (C6 & C7 included), what thought goes into how the specific gear ratios are spaced? I used to own a C6 and currently own a C7; both manual. In both generations, the Z51 package has short gearing for 1st thru 3rd gear. When you get into 4th gear the RPMs drop considerably as it is such a long pull.

On both models 3rd gear hits red line at approximately 100 MPH. 4th gear then makes a pull all the way to just under 150 MPH. As speed builds, the aerodynamic drag increases exponentially.

With that said why are there so many overdrive gears? I understand using the top most gear in their respective generations as an overdrive gear for fuel economy but closer spacing to match the lower gears would have likely gained better acceleration in the 100-150 MPH range.
Tadge answered:
Good observation, Nosferatu. A true "gearhead" question.

Before I worked in car design, I assumed that transmission gearing would be a lot like gears on a bicycle - you could match a set of gear teeth and get most any torque multiplication you wanted. Automotive transmissions are a lot more constrained. For several generations we have used 6 and 7-speed boxes from Tremec. There are few manual transmission suppliers left in the world and Tremec's unit is the one that fits best in a Corvette and is durable behind our engines. If you study all the ratio sets we have ever used with that transmission, you will notice that 4th gear is always 1:1. That is a foundational aspect of the transmission's architecture, essentially a fixed gear that we have to work around.

Gear selection involves a balance of performance, fuel economy, driveability and other metrics. Typically the standard car is biased towards taller ratios, better fuel economy and easy driveability. On more serious performance models we get more aggressive with the ratios to get higher performance off the line, better 0-60 and 1/4 mile times and quicker lap times on auto cross and most race tracks. If we could change 4th gear and get more even drops, we would, but that is just not possible. In the end, we decided not to ignore the performance potential of aggressive gearing in 1st - 3rd just to avoid a larger than optimal drop on the 3-4. Fifth gear is rarely used on the track but the major considerations there are optimization for top speed and end-of-straight speed at the Nurburgring.

Another way to accomplish the same end is to change the final drive ratio in the differential. Here again there is no free lunch. We use a variety of final drive ratios from 2.41 - 3.42 with the manual at 3:42. We already spec at the aggressive end of the range for all manual transmission cars. Why no 4:11, or more aggressive gear set? Durability and packaging. The final drive ratio is set by the relative size of the ring and pinion gear. To get more ratio, we would have to go down in pinion gear size or up in ring gear size. Smaller pinion gears are just not durable enough to stand up to a life in a Corvette. Larger ring gears make the whole differential larger, adding mass and making the whole transaxle longer. Since the location of the transaxle in the rear is pushed as far forward as it can be already, the impact of a larger differential actually make the wheel base and whole car longer! That would add even more mass. So you see, as usual, what we have in production is a carefully selected balance of competing constraints.

The question also specifically refers to "overdrive" gears. From a technical standpoint all that means is that the ratio is less than 1:1. Architecturally our manual transmission must have all gears above 4th as overdrive gears, but that is not a bad thing. Theoretically you could have a transmission with all overdrive gears and still have a perfectly driveable car - you would just need an extremely short final drive ratio. Mechanically, that is not practical or efficient so you don't see it in production on any mainstream cars.
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Old 04-26-2017, 04:38 PM
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Originally Posted by jvp View Post
The original question is here.
Great response from Tadge on this one!
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Old 04-26-2017, 04:58 PM
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Enlightening response from Tadge. I hope they keep the manual alive forever!

The gearing works really well in my 7speed Z06:
1st: For autocross using a slightly taller diameter tire it can be driven in first the whole time for best performance.
2nd - 4th: For track use I only need forth on two straights, it would be nice to pull faster here but hardly something to cry about.
5th is a good overdrive for going around town 55 mph speed limits.
6th is a good overdrive for going out of town for 70 mph speed limits. The car isn't geared well for top speed runs, 6th is too long, but I don't exactly have a need for top speed.
7th gear is useless so I just don't use it. Hopefully it works for the EPA.

Last edited by SBC_and_a_stick; 04-26-2017 at 05:07 PM.
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Old 04-26-2017, 05:26 PM
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Originally Posted by SBC_and_a_stick View Post
Enlightening response from Tadge. I hope they keep the manual alive forever!
You can be assured the team keeps fighting for it, but they have a lot of counter pressure internally to ditch it.

5th is a good overdrive for going around town 55 mph speed limits.
I actually top 4th on the big straight on Summit Point, and have to nail 5th. Elsewise I'm bouncing off the limiter for a decent length of the straight.
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Old 04-26-2017, 05:59 PM
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Originally Posted by Tadge
If we could change 4th gear and get more even drops, we would, but that is just not possible. In the end, we decided not to ignore the performance potential of aggressive gearing in 1st - 3rd just to avoid a larger than optimal drop on the 3-4.
I want to thank Tadge for taking the time to answer my question. It sounds like it's a supply issue from the transmission designer that doesn't allow the spread to be more uniform. Not all transmissions require 4th to be 1:1 nor do they require a specific 1:1 gear ratio. With his response, it appears there was never a real reason/need for the C7 to go from a 6 speed to a 7 speed manual. 7th pretty much equals 6th on the C6.

My old C6 was a non-Z51 to keep 3rd taller and closer in line with 4th gear since I went boosted with that application. It made more sense in that particular setup having 3rd redline closer to 120 MPH so it's only a 30 MPH pull in 4th vs a 50 MPH pull with a Z51 package.

Just to look at a different platform, my other car the Nissan GT-R, uses 5th gear as it's 1:1 and the ratios are snappy and always keeps the car buzzing along in the upper RPM band.

Food for thought this is the GR6 Transmission out of the GT-R with stock engine redline of 7000 RPM.

1st 4.056 (38 MPH)
2nd 2.301 (66 MPH)
3rd 1.595 (98 MPH)
4th 1.248 (125 MPH)
5th 1.001 (157 MPH)
6th 0.796 (197 MPH)
Final Drive: 3.70:1

Notice both 3rd geas end at around 100 MPH but see how 4th and 5th are much closer together to keep the car pulling along.

Anyways, this was awesome getting some feedback for my question thank you everyone again for voting mine to the top.
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Old 04-26-2017, 10:04 PM
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Originally Posted by Nosferatu View Post
I want to thank Tadge for taking the time to answer my question. It sounds like it's a supply issue from the transmission designer that doesn't allow the spread to be more uniform. Not all transmissions require 4th to be 1:1 nor do they require a specific 1:1 gear ratio. With his response, it appears there was never a real reason/need for the C7 to go from a 6 speed to a 7 speed manual. 7th pretty much equals 6th on the C6.

My old C6 was a non-Z51 to keep 3rd taller and closer in line with 4th gear since I went boosted with that application. It made more sense in that particular setup having 3rd redline closer to 120 MPH so it's only a 30 MPH pull in 4th vs a 50 MPH pull with a Z51 package.

Just to look at a different platform, my other car the Nissan GT-R, uses 5th gear as it's 1:1 and the ratios are snappy and always keeps the car buzzing along in the upper RPM band.

Food for thought this is the GR6 Transmission out of the GT-R with stock engine redline of 7000 RPM.

1st 4.056 (38 MPH)
2nd 2.301 (66 MPH)
3rd 1.595 (98 MPH)
4th 1.248 (125 MPH)
5th 1.001 (157 MPH)
6th 0.796 (197 MPH)
Final Drive: 3.70:1

Notice both 3rd geas end at around 100 MPH but see how 4th and 5th are much closer together to keep the car pulling along.

Anyways, this was awesome getting some feedback for my question thank you everyone again for voting mine to the top.
The Z06 seems to have the same top speeds in each gear except 1+2 are combined and then you get two more gears up top.

I actually like that first gear can do 66mph. How else would you have a successful launch with all that torque? It also saves time at autocross since there is no need to shift. The spacing is very similar except for 1st and 7th, which are setup to take advantage of the torque the car has in 1st to save shift times and 7th to save gas.
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Old 04-26-2017, 10:38 PM
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Thanks Tadge, phenomenal response. I love learning about these kinds of details of auto design. The M7 is really growing on me and I think you guys made a wise decision that probably saved quite a bit of money and helped keep the C7 attainable for us "working folk". Even the 7th gear I find useful, in Texas we have 75 and 80 mph speed limits. Add 10 mph to those and cruising in 7th is about perfect.
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Old 04-27-2017, 06:07 AM
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He said 'Nurburgring'!

Very informative response. Thanks.
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Old 04-27-2017, 08:02 AM
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Originally Posted by SBC_and_a_stick View Post
The Z06 seems to have the same top speeds in each gear except 1+2 are combined and then you get two more gears up top.

I actually like that first gear can do 66mph. How else would you have a successful launch with all that torque? It also saves time at autocross since there is no need to shift. The spacing is very similar except for 1st and 7th, which are setup to take advantage of the torque the car has in 1st to save shift times and 7th to save gas.
No, this is not true. I have owned both cars, and the Z06 is geared differently, with slightly taller 1-3 gears. The 3-4 shift is a *much* smaller RPM drop than in the Stingray/Z51. I noticed the difference in gearing the moment I first drove the Z06, and it's one of my favorite things about the car (among many).

In the Stingray with daily driving I would almost always skip a gear -- usually 2nd, but sometimes 3rd. They are just too closely spaced. In the Z06 I skip a gear only sometimes -- it's usually situational, like driving down an incline.
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Old 04-27-2017, 10:57 AM
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Good, informative response.

Thanks!
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Old 04-27-2017, 11:41 AM
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Good response to this one.
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Old 04-27-2017, 12:38 PM
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Originally Posted by Nosferatu View Post
Not all transmissions require 4th to be 1:1 nor do they require a specific 1:1 gear ratio.
And he never said that. He specifically called out TREMEC (previously Borg-Warner in the C5 before being purchased by TREMEC) as having that 1:1 4th gear requirement. If there was another manual that could withstand the abuse (input torque) of the small block Chevy, GM could consider it.

With his response, it appears there was never a real reason/need for the C7 to go from a 6 speed to a 7 speed manual. 7th pretty much equals 6th on the C6.
I'm not sure how you came to that conclusion. The 7th gear ratio is a silly-tall .45; the C6's 6th gear was a shorter .502. Might not seem like much, but every little bit helps with fuel economy. It also allowed them to shorten up 5th and 6th a little; not by much, but again: every little bit helps.

Last edited by jvp; 04-27-2017 at 12:38 PM.
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Old 04-28-2017, 08:54 AM
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Originally Posted by jvp View Post
And he never said that. He specifically called out TREMEC (previously Borg-Warner in the C5 before being purchased by TREMEC) as having that 1:1 4th gear requirement. If there was another manual that could withstand the abuse (input torque) of the small block Chevy, GM could consider it.



I'm not sure how you came to that conclusion. The 7th gear ratio is a silly-tall .45; the C6's 6th gear was a shorter .502. Might not seem like much, but every little bit helps with fuel economy. It also allowed them to shorten up 5th and 6th a little; not by much, but again: every little bit helps.
JP,

If I'm not mistaken aren't 5th & 6th exactly the same ratios from the C5 Z06 --> C6 Z51/Grand Sport --> C7 Z51/Grand Sport platform?
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Old 04-28-2017, 08:57 AM
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Originally Posted by jvp View Post
And he never said that. He specifically called out TREMEC (previously Borg-Warner in the C5 before being purchased by TREMEC) as having that 1:1 4th gear requirement. If there was another manual that could withstand the abuse (input torque) of the small block Chevy, GM could consider it.



I'm not sure how you came to that conclusion. The 7th gear ratio is a silly-tall .45; the C6's 6th gear was a shorter .502. Might not seem like much, but every little bit helps with fuel economy. It also allowed them to shorten up 5th and 6th a little; not by much, but again: every little bit helps.
The Corvette is not the only car that makes 465lb/tq in non-boosted form and 650lb/tq in boosted applications. There are plenty of other cars with transmissions that can handle this power level. Being forced into a 1:1 ratio for 4th gear from a performance stand point makes 7 speeds absolutely ridiculous and not necessary. It appears they went with the 3 overdrive gears to make 7 gears more for marketing than for performance. We can split hairs here the 6th in the C6 and 7th in the C7 are for all intensive purposes identical. They both are STUPID TALL and there purely for highway cruising.

Without that silly requirement of 4th needing to be 1:1 they could have made 5th a 1:1, make 6th a hybrid of the current 5th and 6th and then made 7th the stupid tall gear. I'll take your word for it on the exact gear ratios of the top-most gear but 0.45 or 0.502 are both ridiculously tall.

The idea that you have a 7 speed transmission and the car can get to top speed in 5th makes the car a glorified 5 speed car at the end of the day with gearing far enough apart that it's got the performance of a 1990s 5 speed manual in terms of gear utility from a performance stand point.

As the speeds get higher you're going to want the RPMs higher to keep the engine in it's power band. Given a 6500 RPM redline 2nd and 3rd keep it at 5000 RPMs or higher the majority of the gear. Shift into 4th bogs to around 4500 RPMs and it's a LONG pull. 5th gear bogs it even worse down to 4250 RPMs. With 7 speeds it should have kept the RPMs above 5200/5500 range so the engine can continue to scream.

Video I searched for briefly here we go...
1st done

2nd done

3rd somewhat done

4th, wait for it...wait for it, wait for it, okay finally hit red line after bogged the heck out of it.

Shift into 5th... forget it car has no oomph left since it's way out of the power band. It can't even get itself to 5000rpms.



Here's keeping the engine happy in the power band and using all your gears. The engine RPMs stay well above even 5500 RPMs as the car speed increases.
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Old 04-28-2017, 10:58 AM
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Originally Posted by Nosferatu View Post
The Corvette is not the only car that makes 465lb/tq in non-boosted form and 650lb/tq in boosted applications. There are plenty of other cars with transmissions that can handle this power level.
But can the transmissions of those cars fit into a corvette and meet the Corvette Team's cost requirements? Those three metrics must always be kept in mind. Performance, Cost, Packaging. From various responses from the team, it is quite obvious that if any one part fails any of those three metrics it does not go in the car.

"There are few manual transmission suppliers left in the world and Tremec's unit is the one that fits best in a Corvette and is durable behind our engines.
.
.
.
Why no 4:11, or more aggressive gear set? Durability and packaging."

-Tadge (Emphasis added)
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Old 04-28-2017, 01:41 PM
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Per the Corvette catalog, manual transmission gear ratios are as follows:

Base Stingray - 1st - 2.664 / 2nd - 1.783 / 3rd - 1.302 / 4th - 1.000 / 5th 0.741 / 6th - 0.503 / 7th - 0.424

Z51 Stingray and Grand Sport - 1st - 2.966 / 2nd - 2.066 / 3rd - 1.426 / 4th - 1.000 / 5th 0.709 / 6th - 0.567 / 7th - 0.476

Z06 - 1st - 2.289 / 2nd - 1.611 / 3rd - 1.208 / 4th - 1.000 / 5th 0.813/ 6th - 0.672 / 7th - 0.454

All three manual transmissions are paired with a 3.42 differential ratio.

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Old 04-28-2017, 03:01 PM
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Originally Posted by RPOC7R View Post
Per the Corvette catalog, manual transmission gear ratios are as follows:

Base Stingray - 1st - 2.664 / 2nd - 1.783 / 3rd - 1.302 / 4th - 1.000 / 5th 0.741 / 6th - 0.503 / 7th - 0.424

Z51 Stingray and Grand Sport - 1st - 2.966 / 2nd - 2.066 / 3rd - 1.426 / 4th - 1.000 / 5th 0.709 / 6th - 0.567 / 7th - 0.476

Z06 - 1st - 2.289 / 2nd - 1.611 / 3rd - 1.208 / 4th - 1.000 / 5th 0.813/ 6th - 0.672 / 7th - 0.454

All three manual transmissions are paired with a 3.42 differential ratio.
Thanks for posting this, it shows exactly what I was referring to earlier. The Z06 has MUCH more usable 1-3 gears in comparison to a Z51/GS car, and also confirms the much smaller drop from 3-4. I'm sure the massive torque of the Z06 would account for some of the difference, but also I'm sure gas mileage has a part to play too.
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Old 04-28-2017, 04:02 PM
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Originally Posted by SBC_and_a_stick View Post
Enlightening response from Tadge. I hope they keep the manual alive forever!

The gearing works really well in my 7speed Z06:
1st: For autocross using a slightly taller diameter tire it can be driven in first the whole time for best performance.
2nd - 4th: For track use I only need forth on two straights, it would be nice to pull faster here but hardly something to cry about.
5th is a good overdrive for going around town 55 mph speed limits.
6th is a good overdrive for going out of town for 70 mph speed limits. The car isn't geared well for top speed runs, 6th is too long, but I don't exactly have a need for top speed.
7th gear is useless so I just don't use it. Hopefully it works for the EPA.
Moslty! With our flat roads in Eastern SC I use 7th on the Interstate. Seldom use 6th, just skip it.

For an offramp usually just shift from 7th to 4th directly. The shifter springs are set to make that easy. Just pull back lightly with no side pressure and you're in 4th. Rev Match makes a perfectly smooth transition. Same choice when passing and 18 wheeler fast, reduces exposure time to truck tire throwing rocks (ask me how I know!). Also reduces exposure time to radar.

A neat feature of the C7 is if in cruise control at 75 mph, after the sprint to pass an 18 wheeler in 4th, just shift to 7th and no need to press the "Resume Button," it automatically returns to the preset speed! First standard shift I have owned that resumes cruise setting after the clutch is depressed and been driving only "sticks" as my DD since my 1st car in 1960!

Last edited by JerryU; 04-29-2017 at 07:18 AM.
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Old 04-28-2017, 04:09 PM
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Originally Posted by Nosferatu View Post
I want to thank Tadge for taking the time to answer my question. It sounds like it's a supply issue from the transmission designer that doesn't allow the spread to be more uniform. Not all transmissions require 4th to be 1:1 nor do they require a specific 1:1 gear ratio. With his response, it appears there was never a real reason/need for the C7 to go from a 6 speed to a 7 speed manual. 7th pretty much equals 6th on the C6.

My old C6 was a non-Z51 to keep 3rd taller and closer in line with 4th gear since I went boosted with that application. It made more sense in that particular setup having 3rd redline closer to 120 MPH so it's only a 30 MPH pull in 4th vs a 50 MPH pull with a Z51 package.

Just to look at a different platform, my other car the Nissan GT-R, uses 5th gear as it's 1:1 and the ratios are snappy and always keeps the car buzzing along in the upper RPM band.

Food for thought this is the GR6 Transmission out of the GT-R with stock engine redline of 7000 RPM.

1st 4.056 (38 MPH)
2nd 2.301 (66 MPH)
3rd 1.595 (98 MPH)
4th 1.248 (125 MPH)
5th 1.001 (157 MPH)
6th 0.796 (197 MPH)
Final Drive: 3.70:1

Notice both 3rd geas end at around 100 MPH but see how 4th and 5th are much closer together to keep the car pulling along.

Anyways, this was awesome getting some feedback for my question thank you everyone again for voting mine to the top.
This is a bit surprising to me.

I've always assumed that the ratios were also a large part of marketing efforts. Having third gear end at 98 mph means you have another shift to hit 100.

Since marketing usually talks about 0-60 or 0-100 times, it makes sense that the gears should get you just past those marks for the best magazine racing.

Last edited by WaxWeekly; 04-28-2017 at 04:09 PM.
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Old 04-29-2017, 01:18 AM
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Having owned and driven a number of sports cars, I really love the tall gears in the Corvette. It really doesn't give up much at the track, the it makes it so much more drivable on the highway, and of course there are the benefits of longer life with less RPMs and better gas mileage.
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