New gearbox with 5.15 fd for racecar

jasonxr2racer · Post by **jasonxr2racer** » Tue Apr 05, 2016 3:10 pm

Well I've taken the plunge and bought an Accord Type-r gearbox to have re-built with a 5.15 FD

Should hopefully give me more speed on track and get my lap times down a bit

Fed up of struggling against lighter cars

Post by **Merlin** » Tue Apr 05, 2016 3:22 pm

The 4.64 is a big difference so the 5.15 will be crazy

jasonxr2racer wrote:Fed up of struggling against lighter cars

More Power!!!!!

Post by **NafemanNathan** » Tue Apr 05, 2016 3:43 pm

That'll be the only 5.15FD on the forum right? I'm intrigued to see the performance gains

Do you have a recent dyno graph for a before and after comparison?

Post by **newkid** » Tue Apr 05, 2016 3:44 pm

NafemanNathan wrote:That'll be the only 5.15FD on the forum right? I'm intrigued to see the performance gains

Do you have a recent dyno graph for a before and after comparison?

Dyno will show big fat zero bud

Post by **NafemanNathan** » Tue Apr 05, 2016 3:47 pm

Only if he plumbs in the new FD ratio.

Post by **newkid** » Tue Apr 05, 2016 3:50 pm

NafemanNathan wrote:Only if he plumbs in the new FD ratio.

Maybe I've been reading wrong but from what I understand the gears are the same ratios so doesn't effect the overall power on dyno it just moves the powerband. @merlin this correct?

Post by **NafemanNathan** » Tue Apr 05, 2016 3:56 pm

Surely if the dyno thinks it's a standard gearbox, being the resistance is delivered through the wheels (After the final drive) the final drive is going to impact the result giving the illusion the car has more/less power/torque.

Post by **newkid** » Tue Apr 05, 2016 4:05 pm

Possibly but mfactory do state on 4.64 final drive a standard car would need 9% more torque to accelerate at the same rate/speed and with a 5.15 it's 21% I took that as meaning on a dyno run both cars would still show the same power figures

Post by **Merlin** » Tue Apr 05, 2016 4:11 pm

I am not an expert on this as it pickles my brain a little

The power band is the same it is just the speed at which you go though the gear is quicker.

The dyno reads the power at the wheels and then uses the drag to calculate the flywheel power. The FD ratio wont make a difference to the outcome of the dyno. Just like if you changed from 15" wheels to 17", the flywheel and wheel hp are the same but acceleration times are changed. The measurement of torque Vs engine rpm is still the same, it is just with a shorter FD you will accelerate though the gear faster.

Post by **Merlin** » Tue Apr 05, 2016 4:17 pm

hotrod.com wrote: Will different final drive ratios affect the dyno readings?

This one's tricky. First, there are potential discrepancies because different gears have different inertia values, generate more friction, and change the amount of tire slip. Higher numerical gears tend to be more inefficient, so as gear ratios increase numerically, power levels tend to slightly drop, particularly on an inertia dyno. When torque is multiplied by steeper gears, tire slippage also tends to increase.

However, there's another, often overlooked, factor in the brew: rpm and torque are inversely related to calculating horsepower, so changing the rear axle ratio or testing in other than a 1:1 transmission gear seemingly shouldn't change the horsepower numbers. But this doesn't take into consideration the fact that changing gear ratios changes the engine's rate of acceleration. For example: We know that on an engine dyno, if you change a sweep test's acceleration rate from, say, 300 rpm/second to 600 rpm/second, the flywheel power number (bhp) drops due to the faster rate of acceleration. As an engine accelerates at a higher rate, the power required to accelerate the engine increases, and a greater portion is consumed before it gets to the flywheel. Going to numerically higher gear ratios-whether in the trans (testing in a lower gear) or in the rearend-is like increasing the rate of acceleration in a sweep test. Whether this actually changes a given chassis dyno's reported results depends on how the specific dyno manufacturer does its math. For the most consistent results, always test in the same trans gear (generally 1:1) and rebaseline the vehicle after a rear-axle ratio change.

http://www.hotrod.com/how-to/additional ... yno-guide/

What that article is saying (and I have heard this before) is that with a shorter FD you might actually see a small drop in power read by the dyno. This is due to more power of the engine being used to spin up the internals of the engine quicker, rather than being fed to the wheels.

The fact of the matter is that a shorter final drive increases real world acceleration.