Fact or fiction - final drives

chrismc · Post by **chrismc** » Wed Aug 10, 2016 9:18 am

No No No!!! Your understanding is flawed

The shorter FD car gets to 123 FASTER (top speed on the limiter in 5th!)
The taller FD car gets to 123 SLOWER - but continues on to a higher top speed

Basic terms.

Both doing 80mph. Both in same gear. Shorter FD car is pulling more RPM- maybe 4600 vs 4000. As such if they both accelerate the shorter FD car is making more power thus will pull away.

Alternatively- think of 2 identical cars side by side at 40mph. One in 2nd gear, the other in 3rd. If they both floor it which accelerates faster? Answer: The car in 2nd gear as its pulling more revs......

http://www.superstreetonline.com/how-to ... e-install/ Link talks about B-series but the theory applies!

Post by **newkid** » Wed Aug 10, 2016 10:18 am

I'm talking about from a standing start. The figures show its impossible for the 4.64 to beat the 4.266.

Your correct in if both cars are at the same speed (moving start) and same gear then the 4.64 has the advantage.

chrismc · Post by **chrismc** » Wed Aug 10, 2016 10:42 am

Standing or rolling!!. The shorter f.d effectively reduces the overall gearing. This gives more torque at the wheels at the expense of top speed.

Better acceleration but less top speed as the trade off.

How & why do deduce it's impossible for the 4.64 'to beat' the 4.2 from standstill yet better once side by side? Beat in what sense??

Post by **newkid** » Wed Aug 10, 2016 11:58 am

Its impossible for this reason, the 4.64 accelerates @ 9.19% faster, I'm not denying this.

First gear

standing still = 0mph
4.266 @1000rpm - 5mph to reach the same speed the 4.64 has to be @1092rpm that's 9.19% more
4.266 @3000rpm - 15mph to reach the same speed the 4.64 has to be @3276rpm again thats 9.19%more
4.266 @6500rpm - 32mph to reach the same speed the 4.64 has to be @7097rpm again thats 9.19%more

So even taking into account the 9.19% extra the 4.64 has to reach a higher rpm to achieve the same result (speed), the 4.64 in 1st then runs out of power while the 4.266 still has more to give.

The 4.64 is then into second gear so back down the power range while the 4.266 is still in its peak power range so therefor the 4.266 is pulling away.

Come both cars moving its different as if in the same gear we will say 2nd here

4.266@3000rpm - 26mph the 4.64 is @3276rpm (9.19% more) this means when moving the 4.64 is nearer the main powerband of the h22a

Its all in numbers and calculated out, from a standing start a shorter final drive is slowing you down, from a rolling start it speeding you up.

Post by **newkid** » Wed Aug 10, 2016 1:26 pm

@rattymcclelland Can you answer the below with a simple yes or no please

Both cars equal, one with a 4.266 final drive other with 4.64, will the engine rev up at the same rate ie from a standing start will both go from 1000rpm to say 7000rpm.

bennyboy · Post by **bennyboy** » Wed Aug 10, 2016 1:30 pm

I'm not Ratty, but no they won't. That's what I was getting at earlier with my RC example.
Gears are torque multipliers, the closer ratio will allow the engine to spin more freely as it were, there's less of a load/resistance on it.
I think I get where you're coming from, but the thing to remember is this sort of setup has positives and negatives, as most do, but the idea is that with certain scenarios, the positives will outweigh.
Even a supercharger for example, is a parasite on the engine and therefore 'drains' engine power, BUT the package as a whole 'produces' far more than it 'drains' = win.

Post by **RattyMcClelland** » Wed Aug 10, 2016 2:03 pm

newkid wrote:@RattyMcClelland Can you answer the below with a simple yes or no please

Both cars equal, one with a 4.266 final drive other with 4.64, will the engine rev up at the same rate ie from a standing start will both go from 1000rpm to 7000rpm.

No it won't. Because of physics and the different size gear or final drive. The engine effort is the same but the cog size of the final drive changes the speed it turns the wheels thus revving faster.

You are correct to some degree but that assumes both cars ate at constant speed same revs wtc. Acceleration isn't propositional. Distance covered is far greater on a shorter final drive meaning quicker acceleration.

chrismc · Post by **chrismc** » Wed Aug 10, 2016 2:32 pm

Think of riding a bike....

If you start in 1st gear & pedal as hard as you can your initial acceleration is good but your legs are soon maxed out & you aren't going very fast.

If you start in 7th gear & pedal as hard as you can your initial acceleration is slower but eventually you can pick up speed & will be going somewhat faster once your legs are maxed.

The lower gear is clearly better suited to the human body allowing your legs to get up to an efficient (for humans) ~100rpm cadence more easily from standstill. You then change gears to maintain cadence @ ~100rpm whilst increasing road speed.

On a car the shorter FD allows the engine to pick up rpm more easily as the gear is effectively a bit shorter.

Post by **newkid** » Wed Aug 10, 2016 5:35 pm

RattyMcClelland wrote:
newkid wrote:@RattyMcClelland Can you answer the below with a simple yes or no please

Both cars equal, one with a 4.266 final drive other with 4.64, will the engine rev up at the same rate ie from a standing start will both go from 1000rpm to 7000rpm.
No it won't. Because of physics and the different size gear or final drive. The engine effort is the same but the cog size of the final drive changes the speed it turns the wheels thus revving faster.

You are correct to some degree but that assumes both cars ate at constant speed same revs wtc. Acceleration isn't propositional. Distance covered is far greater on a shorter final drive meaning quicker acceleration.

Right so by this we go by what Mfactory say and that is the 4.64 accelerates 9.19% quicker than the 4.266?

mercutio · Post by **mercutio** » Wed Aug 10, 2016 6:21 pm

head mashed yet????