I did simplify a bit, no?

Yes, ideally, you would fit a polynomial to the dyno curve and integrate that to find ideal shift rpm in each gear.

That would give you a good starting point to start track testing.

 

Thanks Gary - very informative.

 

You guys and these big words!
Yes, you need a chassis dyno to find your power peak. Then, you need to know your rpm drop between gears. Go past that much on shifts, and calculate rear gear to hit peak power at the finish line in transmission gear of your choice.

 

Ha, a well-deserved rebuke.

I do try to keep the geekiness under wraps. So, I'm saying this was Koda's fault for starting the big word cascade.

 

I guess, my interpretation of "Redline" is DON'T go past it, or risk engine damage.
Without a doubt, revving past "Peak Power" is needed to fall within the peak torque/power range for maximum acceleration. To me, "Redline" is not "shift point" (RPM). The advantage to a.close ratio four speed (M21) is the engine is kept in the desirable power band in every gear.

There was some "theory" about rear end gearing to have the engine revving 10% over peak power when you went over the finish line, as opposed to hitting peak power going over the finish line. Refresh my memory on this ? Seems like 1/4 MPH was the goal.

 

Guilty, your honor.

 

That theory would simplify gear selection. But each engine has a different torque curve and a one-size theory could let you down. It's always best to do a little bit of calculation based on your specific dyno curve.

A friend made a presentation to the racers in his club and this is one of his slides. It's about when to shift to the next gear. But it could easily be tweaked to show how the average wheel torque in 4th gear changes as you add revs. You'd choose a rear gear to take you to where the average torque in 4th stops rising (limited by redline, of course).

 

Here's a great calculator to plug in all your #s: https://www.tremec.com/calculadora.php

I plugged in all my info and it really gives you a nice baseline to look at your shift points and RPM drops for each gear. Here are my #s for my 442 with the 6 speed and 4.10 rear:

Transmission: Magnum 2.66 speed (2.66, 1.78,1.30,1.00, .8, .63) W: 4.10 gears & 305/40/18 Nitto DRs: {27.72” tire}
POWER RANGE = 3900 – 5800 (1900 RPM Power band)

1st = 4.10*2.66= 10.91/ 2nd =4.10* 1.78 = 7.30 /3rd =4.10*1.30=5.33 4th =4.10*1.00=4.10 Shift points:

1-2 44 mph 5800 RPM (RPM drop to 3,886 67%)
{zero to 60 = 2nd gear at 5400 RPM}

2-3 65 mph 5800 RPM (RPM drop to 3880 67%)

3-4 89 mph 5800 RPM (RPM drop to 4243 73%)

{ Cross ¼ mile traps in 4th gear (112 to 117 MPH) 117 mph = 5850 RPM }

4-5 116 mph 5800 RPM

2400 RPM = 60 MPH in 5th and 76 mph in 6th

65 mph in 5th gear @ 2500 RPM (W/27.72”)

70 mph in 6th gear @ 2200 RPM (W/27.72”)

75 mph in 6th gear @ 2355 RPM (W/27.72”)

80 mph in 6th gear @ 2512 RPM (W/27.72”)

150 mph in 6th gear @ 4710 RPM (W/27.72”)

 

3.91 or 4.33 gears.

 

I still say and would use a M20 for ANY application. If you have a high winding Z28 with no torque,then a M21 might work.

 

And you can. With 3.91 gears you're definitely going to go like a raped ape through third gear and you'll be shifting quick. But an M-21 with 3.91 gears has virtually the same speeds at 800rpm as an M-20 with 3.42 gears through third gear. I used 800rpm because it seems like a typical relaxed take off rpm. Just saying there is a reasonable use for an M-21. I have 2.78 gears in my car. I couldn't get it rolling without cooking the clutch if I had an M-21. I understand what you're saying, but if you have low rear gears, an M-21 is a good gearbox. I'm going to buy an M-22Z so I can get rolling reasonably and retain my low cruising rpms. 🍻

 

Imho...I think you that maybe you should rethink that.
You would have to slip the clutch an insane amount to be at 800 rpm, if it's even possible, which I don't think it is

 

So what do you think is more realistic?

 

Agreed. 800 is basically idle. I would start around 1500-2000

 

It DID work and was the transmission of choice in that application.

 

I guess I should have explained myself a little better. I chose 800 rpm as the rpm I'm shooting for at the point where the clutch is fully engaged (foot off the clutch pedal). If my rpm is 1500 at the point I get the clutch fully engaged, I have slipped my clutch more than I like. My rpm does not reach 1500 until I'm completely off of the pedal and am accelerating away. If I'm going for a more aggressive launch, then my rpm will be a little higher, but not much. Maybe 1000-1100. The higher your rpm as you start to engage the clutch is just slippage. I just prefer not to do that. But everyone has their own style and technique.

My point to this was the fact that the speeds of an M-21 with 3.91's are the same as an M-20 with 3.42's through third gear. So the M-21 has a purpose and is a good gearbox. It just depends on your rear end gearing.

Cheers guys.🍻

 

The concern here is getting the car moving, which is 1st gear and rear ratio. The torque load will decrease once in motion, so the 2nd and 3rd gear speeds are less of a concern in normal street driving. Now, it may make more difference in 1/8 vs 1/4 mile drag racing. Then, you end up in a Frod truck situation where 1st is really steep (granny gear), 2nd and 3rd are close together, and 4th almost becomes OD, but isn't because it's 1:1.
There are a bunch of factors to consider. That's why I went with the 22W after trying to launch a straight 22 and frying clutches. It fit my needs the best.

 

I agree completely. Which is why I made the comparison of the M-21/3.91 and M-20/3.42. The gearbox ratios and rear end ratios net the exact same speeds per rpm until you get to 4th gear. So if you are trying to get the car rolling, you will see no difference between the two combinations mentioned. One has no advantage over the other.

 

I didn't crunch your calcs but assuming they are correct there is still the factor of different differential ratios on your driveline. The 3.91 will be easier on your transmission and harder on the diff...
To me, that's a important factor to consider

 

Engine rpm will be the same. Revolutions of the tire will be the same. The only difference will be the revolutions of the driveshaft. It will take less torque from the motor to get the M-21/3.91 car rolling than the M-20/3.42 car.

In normal driving you will not notice a difference.