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With 26.5" tires, if you could get to 6500 RPM in 5th gear with a 2.56 rear, you'd be doing 231 MPH. The aerodynamics is what will require more HP to get to that RPM than what a stock LS3 has available.
However, the 4-speed in the '05 Vette combined with a 2.56 gear and 26.5" tires is exactly right to hit 200 MPH in 3rd gear at 6500 RPM.
Gears won't get an LS3 to 200MPH. Horsepower is the only solution. You need HP to overcome aerodynamic drag.
the Z06 can hit about 196/198mph and has a massive amount more drag than the base model .289cd vs .34iirc . With 470 to 500 hp 200mph is very likely in a narrow body car. I have no need nor desire for overdriven gearing; if I have 6 forward gears, I want all of them to be useful.
I found out PATC makes new planetary gearsets for the 4l - Im sure they could do them for the 6l. Just one more thing to add to my list when I get this thing rebuilt...
the Z06 can hit about 196/198mph and has a massive amount more drag than the base model .289cd vs .34iirc . With 470 to 500 hp 200mph is very likely in a narrow body car. I have no need nor desire for overdriven gearing; if I have 6 forward gears, I want all of them to be useful.
I found out PATC makes new planetary gearsets for the 4l - Im sure they could do them for the 6l. Just one more thing to add to my list when I get this thing rebuilt...
Read your own statement above, "With 470 to 500 HP 200mph is very likely in a narrow body car".
HORSEPOWER gets you 200 MPH, not gears. With no body modifications it takes a lot of HP to overcome the aerodynamic drag of a C6. You won't overcome drag by changing gearing, only by adding HP.
The supposed 186MPH top speed in a C6 is in 5th gear of a manual trans, not 4th, not 6th. Gearing doesn't make any difference.
Read your own statement above, "With 470 to 500 HP 200mph is very likely in a narrow body car".
HORSEPOWER gets you 200 MPH, not gears. With no body modifications it takes a lot of HP to overcome the aerodynamic drag of a C6. You won't overcome drag by changing gearing, only by adding HP.
The supposed 186MPH top speed in a C6 is in 5th gear of a manual trans, not 4th, not 6th. Gearing doesn't make any difference.
The goal isnt to get the car to 200mph - thats just the max I want the gearing to get to - if the car makes it to 200mph then great...
The real goal is to regear it so all 6 gears are useful. For me the only purpose of the car is having fun driving it fast. Having 6 useful gears is more fun; plain and simple!
The goal isnt to get the car to 200mph - thats just the max I want the gearing to get to - if the car makes it to 200mph then great...
The real goal is to regear it so all 6 gears are useful. For me the only purpose of the car is having fun driving it fast. Having 6 useful gears is more fun; plain and simple!
You're saying that you want a 6 speed auto that keeps the same 1st gear ratio and has a 6th gear that gives you top speed?
An A6 w/2.73 gears reaches top speed @ about 5500RPM in 5th gear and won't go any faster in 6th. If you had the HP to rev to 5500RPM in 6th you would run 235MPH.
If you leave the 1st gear as is and then change the 6th gear to the current 5th gear ratio (and adjust the gears in between the old 1st and the new 6th)then you would still reach the low 180's but in 6th instead of 5th.
To reach the goal of 200 mph in 6th gear of your current A6 trans, you could do it with 26.5" tires at 6600 rpm and a 3.90 rear axle ratio.
Or you could do it in 3rd gear of an A4 trans with your current 2.56 axle and still have 4th as OD.
But it's a lot easier to change the rear axle to 3.90 and retain 6 gears, than swap transmissions. However, the overall ratio in 1st gear of 15.72 would produce great clouds of tire smoke at WOT.
If you want to retain the 2.56 rear axle ratio and regear the trans to have 6th gear be 1 to 1 and 1st gear to stay at 4.03 to 1, then you also need to change 2nd through 5th. As a WAG, I'd suspect the cost would be in the $25K or above range. A $6K supercharger would probably get you just as much fun.
The goal isnt to get the car to 200mph - thats just the max I want the gearing to get to - if the car makes it to 200mph then great...
The real goal is to regear it so all 6 gears are useful. For me the only purpose of the car is having fun driving it fast. Having 6 useful gears is more fun; plain and simple!
You should have just asked if 6th gear in the 6L80 could be changed so that it produced 3,250 rpm at 100 mph...this would have avoided all the discussion about whether your car will go 200 or not. Maybe someone like RPM will chime in. I don't know if it is as easy as changing out some gears. It would seem to me though that all the gears might need to be changed in order to make the overall gear ratio structure make sense.
I am running 700+ ....e-force charger/ARH 3" Corsa/close throw shift etc etc. I am an acceleration junkie....not a high speed enthusiast but I have had mine to 165 with much nervousness for police cars and at that point I was still in 5th so 200 would be pretty easy I think....but......I have been told the aerodynamics of the C6 make it so the front lifts at 190-200 if you are running a stock body. I guess a foil on the front may help but the wind pressure at 200 is immense so the foil would have to be incorporated into the front suspension....yes? No? This is all third party for me so opinions would be appreciated......
As far as gear change within the tranny.......guess if ya have $$ you don't know what to do with you could go that way but rear end change is quick and low cost in comparison. $25,000 vs $1500
To reach the goal of 200 mph in 6th gear of your current A6 trans, you could do it with 26.5" tires at 6600 rpm and a 3.90 rear axle ratio.
Or you could do it in 3rd gear of an A4 trans with your current 2.56 axle and still have 4th as OD.
But it's a lot easier to change the rear axle to 3.90 and retain 6 gears, than swap transmissions. However, the overall ratio in 1st gear of 15.72 would produce great clouds of tire smoke at WOT.
If you want to retain the 2.56 rear axle ratio and regear the trans to have 6th gear be 1 to 1 and 1st gear to stay at 4.03 to 1, then you also need to change 2nd through 5th. As a WAG, I'd suspect the cost would be in the $25K or above range. A $6K supercharger would probably get you just as much fun.
Gear changes will not give him a 200MPH top speed. If it was that simple then everybody would be doing it.
Your example of 26.5" tires and 3.90 gears still would not give a 200 MPH top speed because the engine doesn't have the HP to reach 6600 RPM with that combination.
A stock manual trans in 5th gear only has the HP to reach 5500 RPM or so, in 6th gear you'll be lucky to get to 4100RPM.
Gears alone won't work! It takes HORSEPOWER for top speed.
The OP, as I understand what he is asking, does not want to go 200 and is not suggesting that a trans gear change will allow his car to then go that fast.
If correct, I can offer this example of gearing with an A6 trans and base 2.56 gears and OEM tires to accomplish OPs goal. Finding someone to make this change, if even possible, is another matter. Also, gas mileage is gonna suck. I took a pretty good hit going to 3.15 gears with my A6.
--Dan
Re-geared A6 trans ratios with lower 1st, fairly even spacing, and a 6th gear hitting just over 200 mph at 6,500 RPM.
Gear changes will not give him a 200MPH top speed. If it was that simple then everybody would be doing it.
Your example of 26.5" tires and 3.90 gears still would not give a 200 MPH top speed because the engine doesn't have the HP to reach 6600 RPM with that combination.
A stock manual trans in 5th gear only has the HP to reach 5500 RPM or so, in 6th gear you'll be lucky to get to 4100RPM.
Gears alone won't work! It takes HORSEPOWER for top speed.
Wow! I said in post #3 that he would need HP to hit 200. Then I never mentioned HP again, because his goal was only to gear for 200. His car is currently geared to hit 300, so changing the ratios to allow for 200 at redline (totally irrespective of the HP needed) is all I mentioned.
Actually, he probably has enough HP for 200, if he had the right tires and cleaned up some of the details on the aero. But he still couldn't get there with the current gearing because he wouldn't be at the highest part of the power band. The required rpm is too high for 4th gear and not high enough for 5th.
The Z06 manages 198 using stock tires, since it's geared for 208 in 5th gear and that's in spite of the .34 Cd and due to more HP available at a higher rpm.
You cannot gear a stock A6 LS3 to run past 182 MPH.
You cannot gear a stock M6 LS3 to run past 186 MPH.
The aero of a C6 requires more than 436 HP to reach more than 186 MPH with any transmission gears. Or tires. Or rear gears.
dbratten; your example above won't work. The engine doesn't have the HP to run past 186MPH regardless how you gear (transmission or rear gears) it. There is an aerodynamic wall at about 186 MPH that you cannot climb without added HP.
You cannot gear a stock A6 LS3 to run past 182 MPH.
You cannot gear a stock M6 LS3 to run past 186 MPH.
The aero of a C6 requires more than 436 HP to reach more than 186 MPH with any transmission gears. Or tires. Or rear gears.
dbratten; your example above won't work. The engine doesn't have the HP to run past 186MPH regardless how you gear (transmission or rear gears) it. There is an aerodynamic wall at about 186 MPH that you cannot climb without added HP.
dbratten's chart with the revised gear ratios has very sound principles included. As the ground speed increases, each gear change incrementally moves the lower end of the RPM range higher, which compensates for the increased drag.
With the stock A6 gearing, the gear change from 6500 RPM in 4th at 176 MPH would drop the RPM to 4800 in 5th. Using the ratios in dbratten's chart where he has the same 1.15 ratio for 5th as the stock one has for 4th, means that the gear change to 6th only drops the RPM to 5600. Since the rated HP is at 5900 RPM, the engine is producing more power at 5600 than at 4800 and therefore has the HP you're looking for to exceed your theoretical 182 MPH wall. Gearing DOES make a difference. It's why transmissions with multiple gears exist.
BTW, what source are you using for that 186 MPH limitation? That sounds more like a tire speed rating than anything.
Hal, in the interest of reality I have made the following table of gear ratios that I hope you approve. 6th gear remains the same as OEM to preserve gas mileage. --Dan
Hal, you've explained it very well and no one is arguing with you. FYI -- both charts that I have posted are made up of fictitious gearing and are only presented to the OP to show possibilities of gear changes. I can plug in any ratios unlike in a real transmission.
FWIW -- my C6-A6 can only go 176 MPH as it was limited by length of available road at the time. I firmly believe that cars are owner limited -- that being the top speed of what its owner is willing to take it to. Many C6s rarely reach 100 and are limited not by HP or drag but by their owner. In that case, top speed discussions are BS.
My C6 was limited within the OEM tune to cut off at 186 MPH. This is probably a factor of tires and also of bragging rights that 300 KPH can be achieved -- maybe, downhill, with tail wind. The limit can be set higher but that doesn't make the car any faster without less drag, higher HP, or owner's will. I recall someone running for top speed who tucked in the mirrors and taped some of the front end to gain several MPH as they reduced drag.
I'm evidently not doing a good job of explaining aerodynamic drag. Top speed is drag limited, not gear or RPM limited.
The drag doen't change because you changed gears either in the transmission or the differential. It's a constant due to the body shape.
At 436 flywheel HP aprx 186 MPH is all the car can do. Add HP and you can add top speed but it take a lot to gain MPH.
A stock C6 doesn't reach peak HP at 6700 or 6200 RPM, the HP peak is lower than that.
Per dbratten chart above, a stock C6 A6 has a 1.15 4th gear, see how fast it'll go in 4th.
Hal, I'm evidently not doing a good job of explaining what portion of that 436 peak HP is available at any MPH point in any gear.
I fully understand the drag is a constant at any given speed. The difference here is that the HP is not a constant. The gear ratio dictates the RPM at a given speed and the RPM dictates the HP available.
With a stock A6 and 2.56 axle at 182 MPH (your number) and 26.85" tires, the engine is turning 4942 RPM in 5th gear. At that RPM the engine is producing about 8-10% less HP than at it's 5900 RPM peak. Or in HP numbers about 396 at the crank. Now if we change the gear ratio in 5th from .85 to 1.00, the same 182 MPH will have the engine spinning at 5830 RPM or roughly about 1% below it's peak. Or in HP numbers about 431 at the crank. With the difference of the gears, we increased the RPM, which allowed the engine to produce 35 more HP. The HP increase now has the capability to push the car past that 182 MPH number. After reaching peak HP it will fade about 3% by the time it gets to 6500. That's why it's important to keep the engine in the 5500-6500 RPM power band at higher car speeds and why when you hit the highest attainable speed in a stock A6 in 5th gear, it will slow down when you shift to 6th, because the power is not there at the lower RPM. It's also why you downshift to go up steep hills, because the engine doesn't develop as much power at a lower RPM.
With a stock A6 and 2.56 axle at 182 MPH (your number) and 26.85" tires, the engine is turning 4942 RPM in 5th gear. At that RPM the engine is producing about 8-10% less HP than at it's 5900 RPM peak. Or in HP numbers about 396 at the crank. Now if we change the gear ratio in 5th from .85 to 1.00, the same 182 MPH will have the engine spinning at 5830 RPM or roughly about 1% below it's peak. Or in HP numbers about 431 at the crank. With the difference of the gears, we increased the RPM, which allowed the engine to produce 35 more HP. The HP increase now has the capability to push the car past that 182 MPH number. After reaching peak HP it will fade about 3% by the time it gets to 6500. That's why it's important to keep the engine in the 5500-6500 RPM power band at higher car speeds and why when you hit the highest attainable speed in a stock A6 in 5th gear, it will slow down when you shift to 6th, because the power is not there at the lower RPM. It's also why you downshift to go up steep hills, because the engine doesn't develop as much power at a lower RPM.
Excellent. Both you and Hal have added a lot to our understanding. Thank you.
01-28-2015, 12:41 PM
Regear the 6l80 (not the rear end)
