Fact or fiction - final drives

jasonxr2racer · Post by **jasonxr2racer** » Tue Aug 09, 2016 1:37 pm

I fitted a 5.15 FD to my racecar and to be honest didnt make a great deal of difference in lap times, just meant I was in VTEC more.

Probably 1-1.5 secs a lap quicker around Anglesey

It will have pros and cons at every circuit in the country but unless your willing to change gearboxes to suit each circuit its not worth the messing

Maybe once I get used to it my times will drop further.

chrismc · Post by **chrismc** » Tue Aug 09, 2016 2:01 pm

Put very simply- as others have done!

The shorter f.d car will accelerate quicker, but will top out at a lower top speed.

A 1/4m won't really show huge benefit as the effective shorter gearing means traction might be more of an issue off the line & you might need 1 extra gearchange over a 1/4m

With all things being equal- put 2 identical Ludes (aside from FD) side by side on a dual carriageway & accelerate at the same marker- the car with the shorter FD will edge away. It won't be night & day difference but a shorter FD helps keep the motor more on the boil as it will be pulling more rpm for a given gear/road speed

Whilst the gear 'ratios' are unchanged (same drop in rpm between gearchanges). The shorter FD effectively lowers ALL of the ratios..

Post by **RattyMcClelland** » Tue Aug 09, 2016 3:01 pm

It will rev faster because the engine has to pass it's rotation through the final drive. So the final drives determines the rate in which engine rotation is transferred to acceleration via the gears.

Copied from another forum as it explains it perfectly.

The final drive determines the ratio of how many revolutions the driveshaft makes to turn the differential or tires one turn. So if you have a final ratio of say 4.11 then that means that the for every 4.1 turns of the drive shaft the wheels will do one full turn.

With a ratio of say 3.23 for every 3.23 turns of the driveshaft the wheels do one full turn.

How does this affect acceleration?

The reason a higher gear number such as 4.11 will help accelerate a car faster is because the engine can spin up faster, getting into and through it's powerband faster and creating more leverage to turn the wheels(this applies to front wheel and 4WD as well) The higher the number the higher the torque multiplication factor.

If you have a larger wheel, then it will rotate less (less acceleration, higher top speed) ... if you have a smaller wheel, then it will rotate more (greater acceleration, lower top speed) ...

Post by **newkid** » Tue Aug 09, 2016 4:30 pm

See this is the part that makes no sense. Even if the engine does rev up faster which I don't get how it does the final drive still needs to turn more to complete one full wheel revolution.

A 4.64 if the figures are to be believed accelerates @9.19% quicker but to achieve one full wheel revolution the drive shift has to turn 9.19% more. Sounds counter active to me.

Post by **RattyMcClelland** » Tue Aug 09, 2016 4:41 pm

newkid wrote:See this is the part that makes no sense. Even if the engine does rev up faster which I don't get how it does the final drive still needs to turn more to complete one full wheel revolution.

A 4.64 if the figures are to be believed accelerates @9.19% quicker but to achieve one full wheel revolution the drive shift has to turn 9.19% more. Sounds counter active to me.

Yes you got it.

The engine revs faster because the final is turning the momentum faster thus the car is moving faster thus the engine revs quicker. Don't think of the engine separate to the box. They all are limited by each other.

Same as a light fly wheels. Less mass, revs up faster.
Shorter fd revs up faster through the gears because the speed pickup is quicker per rev.

Post by **newkid** » Tue Aug 09, 2016 5:10 pm

Yea but as said it needs to turn 9.19% more to achieve a full wheell revolution. So kind of counter acts against the extra it's giving. Balances out in a way but with a slower top speed.

Really should have looked into it more.

mercutio · Post by **mercutio** » Tue Aug 09, 2016 5:27 pm

Just as a question just in case what would people pay for a gearbox rebuilt with all the best gear with the Mfactory diff and final drive remember this would be a bolt on Mod not just the parts ????

Post by **RattyMcClelland** » Tue Aug 09, 2016 6:38 pm

newkid wrote:Yea but as said it needs to turn 9.19% more to achieve a full wheell revolution. So kind of counter acts against the extra it's giving. Balances out in a way but with a slower top speed.

Really should have looked into it more.

It turns 9.19% faster per wheel rotation due to its size. It's simple cog physics and gearing. Changing cog size in one area changes the effects further down the chain.

Post by **RattyMcClelland** » Tue Aug 09, 2016 6:40 pm

mercutio wrote:Just as a question just in case what would people pay for a gearbox rebuilt with all the best gear with the Mfactory diff and final drive remember this would be a bolt on Mod not just the parts ????

Close to £2k fitted I paid iirc.
That's mfactory final drive, stage 1 upgrade diff, rebuild kit inc syncros etc

Post by **newkid** » Wed Aug 10, 2016 12:23 am

RattyMcClelland wrote:
newkid wrote:Yea but as said it needs to turn 9.19% more to achieve a full wheell revolution. So kind of counter acts against the extra it's giving. Balances out in a way but with a slower top speed.

Really should have looked into it more.
It turns 9.19% faster per wheel rotation due to its size. It's simple cog physics and gearing. Changing cog size in one area changes the effects further down the chain.

Based on 1st gear with ratio 3.307 with 205/55/15 tyres

Column 1 is gear rotations, same on both 4.266 and 4.64
Column 2 is 4.266 rotations
Column 3 is 4.266 speed reached
Column 4 is 4.64 rotations
Column 5 is 4.64 speed reached

1000rpm 302 71 5mph 65 5mph
2000rpm 604 141 10mph 130 9mph
3000rpm 907 212 15mph 195 14mph
4000rpm 1209 283 20mph 260 19mph
5000rpm 1511 354 25mph 325 23mph
6000rpm 1814 425 30mph 390 28mph
7000rpm 2116 496 35mph 456 32mph

Based on 2nd gear with ratio 1.905 with 205/55/15 tyres

Column 1 is gear rotations, same on both 4.266 and 4.64
Column 2 is 4.266 rotations
Column 3 is 4.266 speed reached
Column 4 is 4.64 rotations
Column 5 is 4.64 speed reached

1000rpm 524 122 9mph 112 8mph
2000rpm 1049 245 17mph 226 16mph
3000rpm 1574 368 26mph 339 24mph
4000rpm 2099 492 34mph 452 31mph
5000rpm 2624 615 43mph 565 39mph
6000rpm 3149 738 51mph 678 47mph
7000rpm 3674 861 60mph 791 55mph

In 1st the 4.64@6958rpm reaches 32mph, the 4.266@6398 reaches 32mph. That's 560rpm less to reach the same speed and is roughly 9.19% less rpm to acheive the same result.

1st then has that extra 560rpm on the 4.266 as an extra powerband so even counting in the 9.19% faster acceleration from a standing start a 4.64 will never match that of a 4.266 if both cars are 100% equal on weight, power, etc and the only difference is that final drive.

In short the 4.64 NEEDS THE EXTRA 9.19% increase in acceleration just to keep up from a standing start, due to the the gear change the 4.266 then shoots in front.

So yes mfactory are correct as in its a 9.19% increase in acceleration but that's just to keep up! The shorter final drive then loses out.

So identical cars one with 4.266 final drive other with 4.64 and a 7000rpm limit, the 4.64 will reach its max speed of 123mph quicker than the 4.266 will reach its max speed of 134mph (note max speed) but will actually reach 123mph slower than that of a 4.266.