It's Finally Available! The 981 GT4/Spyder Shorter Final Drive Ratio
10-28-2017, 03:21 PM
#61
10-28-2017, 05:19 PM
#62
This is the best solution so far for the 981's long gearing problem that a lot of people don't want to face upfront....including Porsche.
The shorter FD increases engine torque to the wheels commensurately because of the higher ratio torque multiplier effect and therefore you accelerate faster.
The shorter FD increases engine torque to the wheels commensurately because of the higher ratio torque multiplier effect and therefore you accelerate faster.
There are many speeds where the higher final drive results in less thrust than stock, and many where it is higher thrust.
Last edited by Mech33; 10-28-2017 at 06:59 PM.
10-28-2017, 08:44 PM
#64
10-29-2017, 10:51 AM
#65
Not so much... thrust is simply power divided by speed, and gearing doesn’t change the engine power. Gearing simply manipulates what speed ranges put the engine at higher power parts of the power curve, thus increasing thrust.
There are many speeds where the higher final drive results in less thrust than stock, and many where it is higher thrust.
There are many speeds where the higher final drive results in less thrust than stock, and many where it is higher thrust.
i don't know what you are talking about and I think you don't know what you are talking about.
The extra thrust came from the increase in torque to the wheels because of the higher multiplier ratio from the lower final drive. That's why 1st gear has more thrust than 2nd and more thrust than 3rd..etc. Simple mathematics and simple logic.
10-29-2017, 12:01 PM
#66
Nordschleife Master
i don't know what you are talking about and I think you don't know what you are talking about.
The extra thrust came from the increase in torque to the wheels because of the higher multiplier ratio from the lower final drive. That's why 1st gear has more thrust than 2nd and more thrust than 3rd..etc. Simple mathematics and simple logic.
The extra thrust came from the increase in torque to the wheels because of the higher multiplier ratio from the lower final drive. That's why 1st gear has more thrust than 2nd and more thrust than 3rd..etc. Simple mathematics and simple logic.
10-29-2017, 12:21 PM
#67
Rennlist Member
10-29-2017, 01:27 PM
#69
Originally Posted by Jimmy-D
If i were keeping my GT4 I would do this upgrade with the LWFW. Car would be amazing with all my other Mods.
10-29-2017, 04:52 PM
#70
i don't know what you are talking about and I think you don't know what you are talking about.
The extra thrust came from the increase in torque to the wheels because of the higher multiplier ratio from the lower final drive. That's why 1st gear has more thrust than 2nd and more thrust than 3rd..etc. Simple mathematics and simple logic.
The extra thrust came from the increase in torque to the wheels because of the higher multiplier ratio from the lower final drive. That's why 1st gear has more thrust than 2nd and more thrust than 3rd..etc. Simple mathematics and simple logic.
But at 4700 RPM in 2nd gear, a GT4 w/ a 4.62 final drive is going 42 mph while a stock GT4 is going 50 mph.
Let's look at the 4700 RPM in 2nd gear example for the 4.62 final drive GT4 at 42 mph. The car just shifted to 2nd gear because 1st gear tops out at 7700 RPM going 40 mph. The stock motor puts out about 252 wheel HP at 4700 RPM. Crunch the numbers, and you'll see the car has about 2261 lbs of thrust at this speed (depending on the dyno curve you use).
What about a stock GT4 cruising next to you at 42 mph? The stock GT4 is still in 1st gear at 42 mph, at 6700 RPM. The motor puts out 323 wheel HP at 6700 RPM. Calculate the thrust, and the stock car has 2899 lbs of thrust.
So the stock GT4 has 28% more thrust than the modified GT4 with the 4.62 final drive at that 42 mph.
Do this same analysis at every speed and you'll see that the two cars trade which has higher thrust over various speed ranges.
So my point was your "higher final drive ratio means more torque and more thrust" comment is overly simplified, and it's a common misconception that this somehow translates to more acceleration for the car at every speed.
10-29-2017, 07:44 PM
#71
Originally Posted by Mech33
Do this same analysis at every speed and you'll see that the two cars trade which has higher thrust over various speed ranges.
10-29-2017, 08:22 PM
#72
I was always under the impression the intent of a different final drive ratio is to match it to your use... I.e. to maximize the thrust and gearing for a particular track layout. Simplified... If your track front straight max speed attainable is 140 mph... does it make sense to have gearing that allows for 186 mph if you never use it? Vice versa is true... you don €™t want to run a ratio that tops out at 140 if you can achieve 150. This simplified example doesn €™t take into account shift points etc around a course.
Really depends on the specific power curve, gear ratios, and application. Not as simple as a general rule, in my opinion...
10-29-2017, 11:40 PM
#74
Three Wheelin'
10-30-2017, 09:55 AM
#75
It's true that a GT4 with a higher final drive in the same gear at the same RPM will accelerate faster (at that instant) than a stock GT4. Perhaps that contributes to the perception that the higher final drive car feels much faster, because you are remembering what the acceleration used to feel like in 2nd gear at 4700 RPM now vs. before.
But at 4700 RPM in 2nd gear, a GT4 w/ a 4.62 final drive is going 42 mph while a stock GT4 is going 50 mph.
Let's look at the 4700 RPM in 2nd gear example for the 4.62 final drive GT4 at 42 mph. The car just shifted to 2nd gear because 1st gear tops out at 7700 RPM going 40 mph. The stock motor puts out about 252 wheel HP at 4700 RPM. Crunch the numbers, and you'll see the car has about 2261 lbs of thrust at this speed (depending on the dyno curve you use).
What about a stock GT4 cruising next to you at 42 mph? The stock GT4 is still in 1st gear at 42 mph, at 6700 RPM. The motor puts out 323 wheel HP at 6700 RPM. Calculate the thrust, and the stock car has 2899 lbs of thrust.
So the stock GT4 has 28% more thrust than the modified GT4 with the 4.62 final drive at that 42 mph.
Do this same analysis at every speed and you'll see that the two cars trade which has higher thrust over various speed ranges.
So my point was your "higher final drive ratio means more torque and more thrust" comment is overly simplified, and it's a common misconception that this somehow translates to more acceleration for the car at every speed.
But at 4700 RPM in 2nd gear, a GT4 w/ a 4.62 final drive is going 42 mph while a stock GT4 is going 50 mph.
Let's look at the 4700 RPM in 2nd gear example for the 4.62 final drive GT4 at 42 mph. The car just shifted to 2nd gear because 1st gear tops out at 7700 RPM going 40 mph. The stock motor puts out about 252 wheel HP at 4700 RPM. Crunch the numbers, and you'll see the car has about 2261 lbs of thrust at this speed (depending on the dyno curve you use).
What about a stock GT4 cruising next to you at 42 mph? The stock GT4 is still in 1st gear at 42 mph, at 6700 RPM. The motor puts out 323 wheel HP at 6700 RPM. Calculate the thrust, and the stock car has 2899 lbs of thrust.
So the stock GT4 has 28% more thrust than the modified GT4 with the 4.62 final drive at that 42 mph.
Do this same analysis at every speed and you'll see that the two cars trade which has higher thrust over various speed ranges.
So my point was your "higher final drive ratio means more torque and more thrust" comment is overly simplified, and it's a common misconception that this somehow translates to more acceleration for the car at every speed.
If you use speed to compare thrust then of course it might be different. You neglected the time factor if you only measure thrust at a specific speed. That is the reduction in time to get to a particular speed with the lower diff.
I am trying to simplify the acceleration matter because not everyone here is a mechanical engineer.
The end result is with 2 identical cars, the one with the lower diff will get to any particular speed faster as long as gearing permits. Period.