That's great, the more people that have some idea of how the physical world works, the better. I actually did both mechanical and aero/astro for undergrad at Caltech, so you could say I'm a fellow aero/astro as well. Boring, really? I considered to go off and do business consulting when they came recruiting and enticing us with offers, but I was just not a fan of the business courses that I took. I have a friend here at Stanford in the MBA program that's often bitching about not enjoying the coursework (one did mechanical with me in undergrad). To each their own, though I'm sure business is the way to go for the $$.

But back to the gearing, it's not as isolated as you describe. This is evident from the above plot by comparing the x-axis distances where each gear set has the advantage. You can also make the same shift-time argument in the reverse case, where the 3.9 has the advantage and then shifts. But yes I never disagreed with the fact that on average the 3.9 will be faster, but I still contend that the advantage is not as dramatic as most like to think, and is still very dependent on initial speed.

I'll rig up some plots for the G35's torque curves and actual gear ratios so we can have a more meaningful argument over the average advantages.

 

i get most of my work done at 4 my ride by tony in ontario. his prices or low and his work is good.

 

trebien and MechEE- you both bring up some excellent points. A graph would certainly help clear up the confusion.

My understanding is that from a stop, this would be a mod that has a noticable difference in acceleration, but results from a roll (at any speed) are negligible. HOWEVER, I don't think that should lead or detract people from purchasing this mod. The main selling point is that you will have immediate power on tap in any gear. You will be 10% closer to the sweet spot in acceleration in every gear, and this significantly improves drivability, and most of all, the fun factor.

This is a mod I have been contemplating- just need to find a 6MT LSD pumpkin first...

 

This is exactly the viewpoint that I'm trying to combat. You will not be any closer to the sweet spot in any gear. Your engine output versus RPM has not changed. Your force to the ground in any given gear may be 10% higher, but at a lower vehicle speed. Power to the ground is constant (speed * force).

 

Say you are putting around town at 35MPH in second gear. You see some open road up ahead and decide to get on the throttle for a couple of seconds. With the 3.9 gears, you have effectively put yourself 10% higher in the RPM range, right? Instead of being at 3,500 RPM, you are at 3,850 RPM. With a once over of any dyno for a VQ35, it is evident that there is more HP and TQ available at 3,850 as opposed to 3,500. You are going to accelerate faster from that point (keep in mind, we are talking about a couple seconds- not entering speeds that require a shift into 3rd) with the 3.9 gears as opposed to the OEM gearing.

 

So you're trying to argue that with the 3.9 you'll generally be at higher RPM all of the time compared to stock? How do you figure? Not until you run out of gears. You're still going to rev, shift, rev, shift, etc. Sure for a fixed gear at a fixed speed you'll be at say 10% higher RPM, but that doesn't say anything since the 3.9 versus stock will often be in different gears. Your range of ideal RPM in each gear is still exactly the same as before since the relative ratios have not changed. On average you will not be any closer to the "sweet spot".

 

Here is an actual dyno of a stock 350z and the same car with the 3.9 gear and a Tilton flywheel.



This is a 5th gear dyno, so beside losing 15 mph at the end without the ECU mod your powercurve is shifted to the left. If the stock and 3.9 cars were going 80 mph in 5th gear, the 3.9 car will be making 175 rwhp at 4400 rpm. The stock car at the same speed will be making 145 rwhp at 4000 rpm. This is full time power any time you hit the throttle. The stock car will never catch up even though the 3.9 car will have to shift early at 115 mph. Get some top end breathing mods and the TS ECU rev limiter raised to 7100 and you can stretch it to 139 in 5th. Even chugging along at 50 mph in 5th gear yields 20 rwhp more than stock because the engine is that much closer to its sweet spot.

 

^^^ Interesting . . . Definitely worth some more time researching.

 

Again, you're comparing only same gear to same gear at the same speed. I'll post back with some G35 plots soon with more details. But I'm heading out to Tahoe for some snowboarding right now so it'll be a little while.

 

I think all of you just need to get laid...

 

I'm sure that would clear up any confusion regarding the 3.9FD.

 

No, but I'm sure it would lower the importance...

 

I'm sure you wouldn't be saying the same thing if it was a product you carried.

 

LOL . . .

This is gonna get interesting.

 

No not really, actually we on our way to stocking that part. I think the enthusiasm is terrific but I just don't understand the whole conflict of egos. It's great to have an intelligent debate about something but it's just sad to see people get personal i.e. where degrees are earned from and calling people names like idiot. The only reason I chimed in was to add a little lightheartedness that's all. C’mon guys save the low blows for when posting on Club Si!