How do you do it? Since i am going to be adding nitrous ive been askign myself where would i want the pills to be and where would i like to shift and from what rpm would i like to go. I usually take my car up to redline as is, but i understand with nitrous or any other seroius mods that affect the power curve of the car there must be a better starting point or a better ending point.

I was thinking i could go from a 3800 RPM in 2nd or 3000RPM in third. I would have a bunch more TQ at 3000 RPM but i will be in a higher gear. I dont know if this is a good idea, but how do i make sure?

 

 

someone please answer

 

What you are asking for is not easy to calculate. Optimal shift points should put the trans at or near the peak of the torque curve after shifting. Take your gear ratios from the trans, calculate the difference between each and then calculate from there. Example first gear is 3.0, second is 2.3, third is 1.7 and fourth is 1.0 Peak torque is 4600 rpms and peak HP is 6000 and rev limiter is 6500 rpms, tire diameter is 26", and rear axle is 3.42. Second gear at 4600 rpms=26*4600*.002975/3.42 *2.3 = 45 mph

First gear is 3.42*3.0*45/.07259 = 5970 rpm. Hope this helps.

This allows you to be at max torque in each gear after the shift.

 

After you get NO2 you need a dyno printout showing the HP/Torque curves. The ideal shift points will always keep you in the max Torque range. The chart will show you that if you shift to early you will lose HP/Torque, shifting too late has a similar effect.

Say for example that your max Torque is at 4200 RPM, you'll want to shift into 2nd at the rpm that is equal to 4200 when you're in 2nd. Shifting into third at a point where the 3rd gear RPM is 4200.

This is the way I would do it; Drive the car in 2nd until an rpm of 4200 is reached and note the speed, say for example it's 45. Put the car in 1st and when you reach 45 MPH note the RPM, that will be the shift point because at that point shifting into 2nd you would have 4200RPM.

There are probably better ways but this is how I would do it.

 

I understand where your coming from. But arent what we are shooting for is maximum power? if so then i may not need to be so close to peak TQ just as long as i am in the area where i would get the maximum area under the graph correct? I say that becuase i know that if i short shifted first to get to my peak TQ in second i would lose a lot of ground. Most drag racers advise you to shift as close to redline as possible. I have no idea why.

Keep your thoughts coming guys.

 

Chevy High Performance mag did an article about this subject a few months ago. The best point to shift at is when the HP/rear end torque (their curves are the same) in your current gear equals the HP/RET at the next gear. So, probably the best way to find your best shift points is to Dyno it and note where your HP is in each gear. On a stock C5, I believe it was found (by other members using the above method) to be right around 5,800 RPM. But if you've made any significant changes (espcially with a new cam), you might want to make your own calculations.

Good luck,
Dave

 

Every time I have seen people go through this it seems to come back to the same thing. It's more important where you are, in the RPM range, after the shift. Each upshift puts you in a gear with less torque multiplication so it becomes more critical to stay in the best part of the power band after the shift. With most gearing I have seen that means upshifts at red line.

 

any more?

Thanks for those that contributed.

 

This is actually very easy. An A4 car will neve reach the optimum shiftpoint due to the large gaps between each gear(3.06-1.63-1.0). You want to shift where your HP(not torque) in 2nd gear meets or excedes that of the hp in first gear. With the stock 6200 rev limiter you pretty much can not reach this point. You're lowest quarter mile time will be achieved by having the highest possible average horsepower through out the run.

Math: 6 spd regular1st 2.66 2nd 1.78 3rd 1.30 4th 1.00
Examples from my car:
HP@4000 RPM = 230
HP@4500 RPM = 260
HP@5000 RPM = 280
HP@5500 RPM = 300
HP@6000 RPM = 300
HP@6300 RPM = 300
HP@6500 RPM = 260 - guestimate -
HP@6700 RPM = 240 - guestimate -

Total HP 1st gear = 2.66 * 300@6000RPM = 798 HP
Total HP 2nd gear = 1.78 * 230@4000RPM = 409 HP

So as you can see...shifting to second definately won't help your times.
Now let's assume you have a hell of a drive train. And you can shift out at 6700 RPM.

Total HP 1st gear = 2.66 * 240@6700 RPM = 638 HP
Total HP 2nd gear = 1.78 * 260@4500 RPM = 462 HP

So as you can see the most important factor is 1.78/2.66 then 1.3/1.78
and lastly 1/1.3

These ratios determine what RPM you are at after the shift and how much less power you are making.

2st = 66.9% 3nd = 73% 4th = 76%

If you are at 6600 RPM in 1st and you shift to second you go 6600 * 66.9% to determine what RPM you will be at in second gear.

So basically a good rule of thumb...if you're still making over ~220ish horsepower on the dyno graph don't shift. Assuming normal power numbers....this has to be adjusted upwards for more powerful engines.

 

Power! that's all that matters when accelerating If you multiply torque out through the gear ratios to get actual force applied by the tires to the ground, it maps out to be the same thing as power. So the trick is to simply take your rear wheel horsepower dyno graph, and map it through gear ratios and wheel radius to speed, and overlay all 6 gears together. Where the graphs cross is where you want to shift. But as others have mentioned, you can't reach that point with our motors, except for the higher gears.

For instance, here's the graph for a stock Z06:



You can see that for all gears but 4th, you want to shift at 6500rpm, while 4th to 5th crosses over at about 6300rpm. 6th is worthless as a performance gear 5th goes to 180something, and 6th goes to 270, but the car will be aero drag limited before the top of 5th anyway.