Regear the 6l80 (not the rear end)
01-31-2015, 11:10 AM
#21
Race Director
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.
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.
Since you won't believe me find a stock LS3 dyno chart and look where the peak HP is, it's below the 5250 crossover and Horsepower DOES NOT increase with higher RPM with a stock LS3. That's why peak speed comes in 5th gear instead of a higher or lower gear ratio.
01-31-2015, 11:39 AM
#22
Team Owner
But of course none of this has anything to do with the question asked by the OP...he wants to know if he can change gears inside the automatic so that 6th gear will run 3,250 rpm at 100 mph. Maybe a transmission builder can chime in and answer this question.
01-31-2015, 11:47 AM
#23
Race Director
Above is his original post. He wanted 6th at 200MPH, we have attempted to explain why that can't happen.
01-31-2015, 12:51 PM
#24
Team Owner
01-31-2015, 12:58 PM
#25
Race Director
Member Since: Sep 2007
Location: Peoria/Phoenix AZ
Posts: 16,555
Received 2,061 Likes
on
1,505 Posts
C6 of Year Finalist (performance mods) 2019
That 4942 RPM that you state above is very close to the PEAK HP RPM available for a stock LS3. The HP does not keep climbing with RPM past 5,000 RPM. Redline is not peak HP, it's a number the engineers have decided is the highest they want the motor turning.
Since you won't believe me find a stock LS3 dyno chart and look where the peak HP is, it's below the 5250 crossover and Horsepower DOES NOT increase with higher RPM with a stock LS3. That's why peak speed comes in 5th gear instead of a higher or lower gear ratio.
Since you won't believe me find a stock LS3 dyno chart and look where the peak HP is, it's below the 5250 crossover and Horsepower DOES NOT increase with higher RPM with a stock LS3. That's why peak speed comes in 5th gear instead of a higher or lower gear ratio.
HP always increases with RPM after the crossover point. It does on every one of my dyno charts, including the stock one, and on every dyno chart I've ever seen on this forum. Eventually they both drop and the torque drops first and fastest.
Here's an easy way to remember some results.
Oversteer is when the backend hits the wall.
Understeer is when the frontend hits the wall.
Horsepower is how hard you hit the wall.
Torque is how far you push the wall after you hit it.
01-31-2015, 01:23 PM
#26
Race Director
Member Since: Sep 2007
Location: Peoria/Phoenix AZ
Posts: 16,555
Received 2,061 Likes
on
1,505 Posts
C6 of Year Finalist (performance mods) 2019
I respectfully disagree with your interpretation of his question. He currently has 6th gear maxxed at about 300mph...it can't go that fast...we all know that. But that is the gear ratio that he currently has. He just wants a gear ratio for 6th that is lower such that 6,500 rpm is at 200 instead of 300. That equates to 3,250 at 100. This way no one will engage in telling him that he can't go 100. He just wants a different gear ratio, not to actually go 200.
dbratten's first chart (however he arrived at the ratios) was a perfect example to keep the RPM drops between the shifts in a good HP range for each subsequent gear. It just happens that when those ratios are used, the HP is available for a higher top speed. The ratios GM used were to meet MPG numbers at a lot lower speed.
01-31-2015, 01:35 PM
#27
Team Owner
He could have also asked if he could re-gear the trans so that 6th gear was 1.00 to 1.00 and all the gears below it lowered to maintain a good overall spread. This would have avoided all the talk of being able to go 200.
01-31-2015, 01:39 PM
#28
Team Owner
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 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!
01-31-2015, 04:51 PM
#29
Drifting
dbratten's first chart (however he arrived at the ratios) was a perfect example to keep the RPM drops between the shifts in a good HP range for each subsequent gear. It just happens that when those ratios are used, the HP is available for a higher top speed. The ratios GM used were to meet MPG numbers at a lot lower speed.
--Dan
01-31-2015, 07:31 PM
#30
Race Director
Hal, you're confusing HP with torque. Remember the link about the press release I posted about 10 posts ago? Scroll down near the bottom for the HP and torque specs. HP is 436 @ 5900 and torque is 428 @ 4600.
HP always increases with RPM after the crossover point. It does on every one of my dyno charts, including the stock one, and on every dyno chart I've ever seen on this forum. Eventually they both drop and the torque drops first and fastest.
Here's an easy way to remember some results.
Oversteer is when the backend hits the wall.
Understeer is when the frontend hits the wall.
Horsepower is how hard you hit the wall.
Torque is how far you push the wall after you hit it.
HP always increases with RPM after the crossover point. It does on every one of my dyno charts, including the stock one, and on every dyno chart I've ever seen on this forum. Eventually they both drop and the torque drops first and fastest.
Here's an easy way to remember some results.
Oversteer is when the backend hits the wall.
Understeer is when the frontend hits the wall.
Horsepower is how hard you hit the wall.
Torque is how far you push the wall after you hit it.
Still hold that the peak stock HP is not enough to support higher speeds with ANY gear changes. Top speed is aero limited and can only be increased by increasing HP.