russell13

Ok so I have a 2004 base Z. And i put a motordyne plenum spacer on it, with a injen cai. Ive started to hear lots of these 3.9 final drive gears. I know the Central 20 is a lot cheaper than the evo350 one. Im wonder if there is a big reason for this. Id like to get feedback from owners of Z's that have both if possible. Im wondering how much of an increase in acceleration will be felt? Also anyone if anyone has the evo350 throttle body let me know if its worth the cash. Thanks

Russell

Cannysage

iTrader: (29)

you may gain a bit over 20ish lbs of torque. and 10ish hp at the wheels.

priscilla ls1

iTrader: (15)

Its not about feeling HP as it is feeling more low end tq. But I have seen hugh 1/4 et gains with 3.9 gears and some drag slick. Like a second off your time.

helldorado

iTrader: (4)

I personally opted for the 350EVO gears for two reasons. First, they're generally more readily available than the Central 20 gears. Second, even though the price is higher, I prefer to support 350EVO since they are a US based company that uses all their parts for full out track racing. So by supporting a company like that, I feel it'll help to make even more parts of this nature available to us.

Its an interesting mod because it doesn't generate any more power form your engine. It just changes the way its applied to the wheels.

We're looking to do a dyno before and after once we do the swap. That's still a couple weeks out, waiting on a couple parts before we can do the build and need to schedule time to do the actual swap and dyno.

taurran

iTrader: (18)

o rly????

Cannysage

iTrader: (29)

check out this chart that helldorado posted. I'm still trying absorb all the graphs and understand. I've always thought final drive only gave you shorter gearing and reduce drive train loss, but there are actually HP and Torque gains across the power band? zweet

scroll down and see the charts.

http://forums.freshalloy.com/showthread.php?t=149710

taurran

iTrader: (18)

I didn't read any of it, but understand where you got the idea that it would gain power.

You don't actually gain any power from shorter gearing, rather for each rotation of the crank, the wheels are turning to a higher degree. This is due to the change in the gearing ratio. You aren't actually reducing drivetrain loss, but rather making up for it by rotating the wheels more for each turn of the crank. This translates to putting torque to the ground sooner with a more abrupt torque curve, but loss in top end power and potential top speed. Horsepower is a function of torque, so you will also see an increase in bottom/midrange power on the dyno.

Keep in mind that you will also fly through the gears faster, which will also negate the effect of the shorter gearing depending on your driving skills.

priscilla ls1

iTrader: (15)

lol you cant gain hp by switching out your FD gears. PERIOD!

taurran

iTrader: (18)

Right, but you will build power differently (faster) than with taller gears. You won't see any gain in engine output, or peak power.

helldorado

iTrader: (4)

To set the record straight, that HP chart is unrelated to a change in FD. It was just a file that contained a calculator I created for charting HP - TQ changes that also included the calculator for tracking theoretical wheel speed changes via different FD's.



On a properly calibrated dyno, there shouldn't be any changes from your power output. If you notice what I posted above...
Your engine still makes xxx ft-lbs of torque at xxxx rpm. That hasn't changed. That force and speed of revolution is transferred through the crankshaft to the clutch or flex plate and into your transmission. Its at this point the basic principals of gear reduction take place.


Assuming lossless power transfer to the wheels, this is the formula for torque applied to each rear axle.So if we compare 3rd gear on a 6MT with 3.538 and 3.917 final drive ratiosYou can see there is almost a 60 ft-lb increase in torque being applied to the axles. This is the force that a chassis dynamometer measures and uses as a basis for the calculations it uses. So as I said, if the dyno is improperly calibrated, it can show an increase in torque. Its no different than running in a different gear and getting an improper reading.

What makes this such a great mod in my mind, is that to match that power on the 3.358 FD, you need to increase your engine power accordingly to make the same improvement. The 3.917 gear ratio is a 10.7% reduction. So if you had a 10.7% increase in power on a stock transmission, you would see comparable power.Now as I said, this doesn't take into consideration the inherent drivetrain losses from the inertia of rotating mass, but you can see the basic principals in effect. But I'm inclined to believe that a 10% change in your drivetrain would be more effective than a 10% increase in power because its further down the chain and there are less places for loss in efficiency to occur between the place of change and the point of contact between the tires and ground.

This is why you get better acceleration from the new FD, but lower top speed. Basic gear reduction principal states that the decrease in output speed is offset by an increase in torque.

helldorado

iTrader: (4)

While I'm thinking about it, if you do go with this mod, you should seriously consider swapping in a VLSD while you're at it. The Nismo LSD would be the easiest and most cost effective for an open 3.5. The improved traction bias will help. The additional wheel torque I cited above will only exacerbate your ability to spin your rear wheel.

Cannysage

iTrader: (29)

aside from lsd,the final drive doesn't alter the drive train loss % at all?

helldorado

iTrader: (4)

The losses are the effect of the power required to rotate the mass that is every piece of your drivetrain from the crankshaft to the wheels. So no, the FD will not reduce those losses. Only reducing the weight of the individual pieces will reduce those losses.

sentry65

iTrader: (11)

the 3.9 final drive is as if you were running in a lower gear, but not quite a full 1 gear lower - 4th gear would feel as if it was 3.66th gear for instance, or 3rd gear would feel like 2.75th gear

tienlo

It's such an easy concept guys. for every meter your car rolls, you fire more cylinders.
It's like taking off in 1st gear compared to 2nd gear, lower gearing is more engine rpm's per unit distance.

Doc25398

iTrader: (5)

Sorry I know this is a newb? but would this mean for instance that on the freeway in 6th gear the rpms would be above 3k wheras stock its at or slightly below 3k rpm? Because I see the correlation of having the rpms higher per gear to allow response/faster power transfer. I am basically asking if theres a trade off between having better fuel economy (lower rpm in 6th) vs. more power (staying in a lower gear longer instead of shifting conservativly). Sorry if that wasnt clear.

Alex

sentry65

iTrader: (11)

thats correct

ZPirate

That's correct. I have noticed with mine that the increase is about 300 rpm over the stock gearing at any give speed.

You probably do loose some top end, but with a car capable of doing over 150 mph that really isn't an issue for me. The biggest advantage of 3.9 to me is having more torque available in every gear. I can also now use 6th gear without feeling like I am bogging the engine.

As you stated the disadvantages are slightly lower fuel economy and higher rpm at any speed. To me the benefits outweigh the negatives. Not everyone wil see it the same way.

Dildorado

How long is the installation? Will the staff at my dealership notice it if they go play under my car?

helldorado

iTrader: (4)

Generally, 6-8 hours depending on the shops familiarity with rebuilds of that nature. That doesn't include taking the pumpkin down and putting it back in the car. Anticipate another 4 for that.

That only way the dealer would know is if they drove your car. There's not a single thing about the mod that would make it readily apparent to anybody as its wholly contained within the pumpkin.

BTW, dildorado, we related somehow?