What? Why would you start off in 3rd while the other guy is in second? Does this make sense? By the way, the 3.9 gear will be faster on every occassion. With the 3.9, your engine or rpms will always be 10% higher than in stock form. This means that when the stock engine is running at 3k rpm, the 3.9 will be at 3300 rpm which is that much closer to peak torque and horsepower. When the stock engine is at 5000 rpm, the 3.9 will be at 5500 rpm...get the picture? If you superimposed a before and after dyno of this mod, you will see that the torque and hp curves for the 3.9 have been shifted to the left. Max torque for the 3.9 will happen at 4320 rpm instead of 4800 rpm. Max hp will happen at 6000 rpm instead of 6600 rpm. If you raised the redline to 7100 rpm and invested in some top end breathing mods, the car will continue to make horsepower past 6600. The 3.9 car with the increased redline will still shift at 7100 rpm before the stock car hits 6600 rpm and continue to pull away with each shift.

 

Right on f*cking point.

End of thread.

 

WTF?

MechEE - no offense, but you've been wrong on the technical merits in other posts (I'm not gonna dig em up)... and then you go on to back it up with a flawed explanation, which is even more humorous. If you don't know, don't post up. This is how misinformation gets spread around.

Under most circumstances, the 3.9 gears will yield faster acceleration up to their top speed (maxed out) due to a better gear ratio putting more power down to the ground from the engine. So unless you race to 165, and the 3.9s only top out at 155, the 3.9s will be faster.

The only time they won't be faster is when one car is in 2nd, about to redline, and the car with 3.9s has to be in third already (same redline). That acceleration advantage will only last for a split second, until the original car has to shift to 3rd. So, yes... for that minute amount of time, the original gears will be faster. But that is a very small subset of circumstances.

 

Uh yeah actually you were talking about gains, then went into %'s

 

Sorry, but you have no idea what you're talking about. Having different gearing does not shift the power or torque curves versus RPM for the engine output. That is always fixed. What does change is the force to the ground versus vehicle speed for a given gear. The plot below was for my last car that I generated for a 3.39 final drive versus a 3.94 final drive (in this case a supercharged 2.8L V6).



The x-axis is vehicle speed, and the y-axis is accelerating force to the ground for the five gears for each final drive. Areas in red are force advantages for the 3.94 final drive, and areas in blue are force advantages for the 3.39 final drive. As you can see, there are clearly speeds where the stock gearing as a large force advantage over the aftermarket (3.94) gearing. This advantage is sufficient enough to give a speed increase that is never overcome when a "race" is started from a speed with this advantage (for example, imagine staring a race at 65 mph, the car with stock 3.39 gearing will have a large advantage because it can stay in 2nd while the car with the 3.94 needs to be in 3rd, as I stated as an example before).

Who has the advantage a majority of the time is very dependent on the torque curve of the car. The curve shown above is for a centrifugally supercharged V6, so the the torque curve is very peaky towards the high RPM range. In the case of a naturally aspirated VQ35, the stock gearing would have even more of an advantage over the aftermarket gearing compared to what is shown above. Go ahead and generate your own plot for this before you start to argue.

If you don't know what you're talking about, you really should chime in. THAT is how misinformation is spread. Sorry, but everything I've said is pretty spot on. If you see instances in other posts where you think I'm technically incorrect, please, dig them up. I'll own your **** all day long if you want. In fact, if I am wrong I would appreciate you correcting me and defending your disagreement so we can all learn something. That 2nd/3rd or 4th/5th etc advantage that you speak of on average is in the avermarket's (3.94 in this case) favor, but not by that much. If you actually integrate the force to the ground versus time, you'll see that the person who puts the most torque to the ground first in the case of just different final drives will win the "race". They get a jump by gaining a speed increase initially, and that works to generate a distance delta that is very difficult to work off over time. I've written vehicle simulators that clearly show this. I replaced my 3.39 gears in my last car with 3.94, and then went back again to 3.39. In both cases I won or lost races with similarly modified versions of my same car depending on from what speed we started accelerating. There's no magic here, just simple physics.

 

3.39 to 3.94 gear swap is a 16% reduction and should not be compared to a 3.58 (stock g35) to 3.91 gear swap, which is a 10% reduction. You did know that the stock final drive is 3.58, didn't you? A 16% reduction would be unmanageable in the G with a redline of 6600 rpm. It might work for a S2000. Staying in gear with a supercharger is what you want because more boost is achieved. However, In a NA application, you want to get to where the car isn't making power anymore (6600) and then shift as soon as possible. The 3.9 gear allows the G35 to reach the 6600 rpm redline much faster than in stock form with 3.58 rear end. You last car sounds like a supercharged Grand Prix. I've heard stories about those cars only needing to be in 3rd to cross the 1/4 mile by doing a mod to hold the gear. That's great for a supercharger where engine speed equals boost. In NA applications, where the engine stops making power at 6600, speed to redline and a quicker shift is better.

 

Ummm...uhhh....stay in context...ummm...uhhh...ok!

 

why is it people always insist on being stubborn? this is a forum, guys!! so what if someone asks something once, twice, a thousand times?

 

Not really... but you're right about one thing. If I don't have the patience or time to dig up the old threads, I shouldn't bring it up. And I don't, so I rescind my comment. I just remember on past occasions thinking to myself "This guy seems like he knows what he's talking aobut, but is a little off on his assumption in this particular instance." I should have brought it up then, but I don't really care.

But since we are in the here-and-now... I may as well respond. I assume by your handle that you are a mech engineer or somehow related - good for you. I went to school for aerospace engineering, and then went the route of international business and an MBA because I was bored. Point is, I actually have some background in this arena. Not to mention a few years of experience in the aftermarket performance/racing area.

Anyhoo... you can argue force vectors and such all day long... but to put it into perspective so that WE ALL can learn, I'll go this route: Think of it this way... the vehicle makes so much power within a given speed range - basically from 0-165 MPH or so (theoretical redline). With a shorter drive, all that available power is being put down in a small field, from... say... 0-150 mph. You're "squeezing" that curve (the total area under the curve stays the same), and through the more aggressive gearing, the width of the curve is shortened (0-150 mph), so in order to keep the area the same, the height of the curve must increase (more power). The downside to shorter gears is worse fuel economy and lower top speed at theoretical redline. To achieve any given speed, you may have to shift more often... as long as the time taken to shift does not cause a temporary deficit in the total time-to-speed... the shorter gears will give faster acceleration. For instance, if shorter gears require shifting from 2nd to 3rd at 58, and taller gears cause a shift at 62... then a race to 60 mph will require 2 shifts from the shorter gears and only 1 shift from the taller gears. This time it takes may mean the taller gears win in that particular instance. Also, why most manufacturers make sure that 2nd gear goes up above 60 mph - for magazine tests. But again, this is a unique situation that presents itself much less often then the converse, of when the shorter gears would be advantageous.

We can also argue the OPPOSITE of what I am saying, and see if it holds true. So, by your theory, then taller gears makes a car go faster. Well, then... we should just all run 1:1 gearing in the diff and then the cars would be even faster. Right? Redline in first would be, what... about 120 mph? That's like starting out in 4th gear. So we should just use 4th gear and up, and we'll get better performance. Right?

And then I can bring in the real world with my experience. Ask anyone in racing - as long as you have traction, shorter gears (within reason) will give better acceleration. The limit is when the power overwhelms available traction... then the benefit of shorter gears is completely lost, and the additional time to shift is a liability.

 

The 3.70 gears are better for the 5AT IMHO. It's a better balance of MPG & acceleration. If you're like me you do a lot of freeway and surface street driving & don't want to be adding an extra $10-$20 a week into your tank. The stock 6MT gear set up is 3.53 while the stock 5AT is 3.36. A 3.90 set up on the 6MT is about the same upgrade ratio as the 3.70 on the 5AT (10%). I'm going to be installing the 3.70 gears in my 5AT within the next couple of weeks. I figure it's not a bad mod for the money regardless of what people say. I paid more for my exhaust & I'm happy with it but I'm just looking for a little more jump off the line & by theory this should do that for me.

 

If I race you and I'm in 2nd gear and you're in 6th gear. I'm going to win. It must be because of our gearing....idiot.

 

I don't think you have a full grasp of how the final drive is changing the dynamics of your gear to gear acceleration. Why do you keep focusing on RPM? Your relative gear ratios (2/1, 3/2, 4/3, etc) have not changed and ideal shift points stay the same, and my arguments for the benefits lying with either final drive depending on initial speed still hold.

That plot just happened to be one that I had on hand (a vortech supercharged VW VR6 engine at 10 psi, not a Grand Prix). Clearing the gearing is different, I only posted it to illustrate my general point. I'll look around for a good dyno of a NA G35 and do the same to try to aid you in your understanding.

 

That's great about the aero engineering. Did you at least finish a BS in that?

Anyhoo... you guys should read and understand my argument before you try and fight an imaginary one. I clearly stated that when accelerating from a stop, the 3.9 will have an advantage over the stock 3.54, which is congruent with your area manipulation argument for power under the curve for integrating this force from 0 to some common higher speed.

However, when you overlay these two maximum forces versus vehicle speed for the 3.9 versus the 3.54, there are points where the stock gearing is higher (as shown as an example in the plot above). If you start integrating these curves from some NON-ZERO vehicle speed to some other higher common speed (for example, 65 mph to 105 mph, a common "freeway race" scenario), then the 3.9 does NOT always have the greater area. And even in cases where it does have a slightly greater area (which represents a final velocity in this case), you really need to integrate it again to get a final position, and distance lost initially from a large initial force advantage (from the 3.9 being in a higher gear than the 3.54) is very hard to make up.

So in the real world, in a "street race" type of situation, it is NOT clear which car will always be faster, and is very dependent on that speed at which you start accelerating, as I initially summed up very shortly in my first post. So far I don't see anything I've said that isn't congruent.

 

Okay you have no ability to even begin to understand what we're talking about here, so just sit back and learn.

 

Yes - from Vanderbilt.

Yes - same thing I said - there are times when the 3.9 is not higher... isolated circumstances.

However, in general... the 3.9 IS going to be higher. There will be small sub-sections (a few MPHs in this case) where it is not, but MORE often then NOT, it will be. Law of averages.... it will be faster in more cased then it wouldn't be.

And you're taking that shift point into consideration. There is also the possibility that the extra shift time would be nullified by the extra acceleration.

Also, let's take 60-80 mph... a short distance. The 3.9 will be in 3rd, and the 3.5 will be in 2nd. The 3.9 may be able to take it up to 80 without shifting... the 3.5 will have to shift from 2nd to 3rd. So there are times then the original geared car will have to shift, and the shorter geared car will not.

So it works both ways....

But again, in general... the 3.9 will exhibit faster acceleration at more points along the curve than the taller gears, due to the increase in mechanical advantage (or whatever force increasing term you want to use).