yellow-fever

I have just ordered a Stage 3 200-4R w/ 12" 2400 stall converter from California Performance Transmissions to replace the TH-400 in my 74 BB coupe. I have a 3.08 now and would like to go to a lower rear gear for better performance. I am leaning towards the 3.70. Motor has approx. 450 hp. Suggestions?

Glensgages

'Assuming' you are running a typical 27"-tall rear tire, have full TQ-converter lock-up (1:1 ) in OD, and the OD ratio is .66:1 (a guess? ), 70 MPH is:

1750+ RPM w/ 3.08s
1910+ RPM w. 3.36s
2020+ RPM w/ 3.55s
2110+ RPM w/ 3.70s

yellow-fever

Thanks for the info. Yes, I have 27" tires, but my transmission will have a non-locking converter, which adds approx. 200 RPM. The 4th gear ratio is .67.

This puts the 3.70's around 2300-2350, so I think I'll be in good shape.

Glensgages

Today's non-locking TQ-converters are much-more effecient than the old pieces from the '70s (coming closer-to 1:1 ), which were generally about 10% slippage:
'assuming' we'd use that factor, and .67:1 OD ratio, the numbers become:

1980+ w/ 3.08s
2160+ w/ 3.36s
2280+ w/ 3.55s
2380+ w/ 3.70s

Using updated information, formula would be:

MPH x GR (Gear Ratio ), div/by .9 (10% slippage-factor ), div/by .006 (mathematic-factor, including pi ), div/by 1/2 TH (one-half tire height ), x ODR (overdrive-ratio ) = RPM

70 x 3.70 div/by .9 div/by .006 div/by 13.5 x .67 = 2380 RPM

7T1vette

I think you might have some trouble "hookin' up" with 450hp/auto and a 3.70 rear end.

Attfay Elleybay

I've got 4.11 and a 200r4 and see 3000 at 70. Better than 4000 at 60 I used to have.

yellow-fever

What would my RPM by in 3rd (1:1) at 120 mph, as in at the end of the quarter mile? I just want to make sure I cross the line in 3rd and not 4th.

yellow-fever

How can this be true, according to the above calculations? What size rear tires do you have?

Glensgages

If you are running 120 MPH in 1320' in a well set-up vehicle, this is indicative of a car running very-low 11-second Elapsed Times:
are you thinking this is what the car might run, on a typical 27"-tall street-tire?

120 x 3.7 (rear-gears ) div/by .9 div/by .006 div/by 13.5 = 6090+ RPM

BigBlockk

You may want to research this a little more. Any time the output shaft of an automatic transmission turns faster than the input shaft (like, whenever it's in overdrive) it works the torque converter VERY hard. Hard working torque converters create a LOT of heat. Why do you think the torque converter locks up at 35 MPH then the transmission drops into overdrive at 40?

There is a reason why the factory put lock up torque converters in overdrive automatic transmissions. Think about it!

BigBlockk

Later.....

Attfay Elleybay

Maybe my 4.11's are actually 4.54? The PO did the gear change. I've got 235/60 15s. If (when) I ever have to change the gears, I'd go with a 3.73. Only other thing is maybe my Tach or speedo is off?

Glensgages

These are all variables that need to be taken into consideration, especially when previous owners have made gear-swaps, and don't give us the correct information.

The 235/60R15s are possibly taller diameter than 27" (I believe my 255/60R-15s are 27", and the 225/70R-15s that came OEM on my '79 Z28 were 27.4" ), throwing the equations off, and who-knows if the tach/speedo is correct when we are discussing 25-30+ year-old cars?

Something-else that could be happening is the TQ-converter isn't going into full-lock, and is slipping-more than the normal 10%-slippage that older factory converters had?


P.S. ~ for-what-it's-worth, using the above formual with 4.56:1 rear-gears, a 27.2" tall tire (a 235/60R15? ), a 200-4R transmission that is not locking-up and a 10% slippage-factor, 3000 RPM works-out to just-over 72 MPH, close to what Attfay Elleybay says he is experiencing.

TheSkunkWorks

Didn't see this yesterday, but thought I'd post what I shared with the OP last night. The following formula should yield the desired ring and pinion gear set for a given rpm, speed and tire diameter...

Rear Gear Ratio = ((RPM @ finish line X Tire Diameter) / (MPH X 336))

If the transmission gear at the finish line is anything other than 1:1, divide the above result by the output percentage to obtain the corrected diff gear ratio required. To compensate for any transmission slip, you can theoretically factor that in by dividing the end result by (100% + slip%). Further, if you're on slicks, you'll need to account for potential tire growth by estimating the diameter at the finish line rather than using static height. (This can get complicated, no?)

Now, the answer probably won't quite match an available ratio, so you'll need to round off, but I've learned the hard way that it's better [in many cases] to leave a little room for more mph on the top end by going to the closest taller (smaller ratio) gear (or even skipping that in favor of the second closest taller one) rather than stretching things with the closest deeper gear (larger ratio), as once you're rpm is maxed out you have nowhere to go.

[NOTE: for a dedicated drag effort you'd really want to top out just over your max power rpm right at the line.]

Also, IMHO unless you're running 60' races I wouldn't get caught up in paying too much attention to 60' times, the exception being if they are inconsistent. The win light is at 1320'. FWIW, hope you can follow my logic there.

BTW, the calculation isn't exclusive to the drag strip, so you can substitute any desired RPM for any desired MPH into the formula.

Glensgages

I've seen that formula around, too, but I learned the following formula long-ago from a few drag-racers, and I'm too-old to change and remember the one you use:

GR = RPM x.9 x.006 x 1/2 TH (Tire Height ) div/by (desired ) MPH

(Really, the formulas I used earlier in this thread are just this formula inverted/bastardized..... )


As you suggest, since the optimal-ratio is often a number that is not found in any catalog, it is better to use the next highest-mecahnical/lowest-numerical ratio that is readily-available:
if the formula says the 'best-optimal' gear ratio is 3.81:1, you'll be better-off going with 3.73s if the car is street-driven, or drag-raced on street-tires, but if you have slicks, and/or aren't afraid to let the motor breeze at the finish-line, 3.90s might be best for your combination.

As-far-as tire-height growing when using slicks, a friend had a dragster he bought used with a special Goodyear slick, that was constructed to grow much-more than a normal slicks would (centrifigal-force ), and after discussing this phenomenon with the Goodyear Technicians at the NHRA Keystone Nationals, we were told that on only the quickest, fastest, and lightest door-cars would tire/slick growth amount to much:
when I asked them about extra-growth on my 3600-pound, 12-second Z28, they told me to 'go-away'.....

TheSkunkWorks

Good point there. Significant growth isn't that much of an issue for mere mortal cars on slicks. Still, there is a related point worthy of note about measuring slicks (or any mounted tire on the ground)...

I've heard arguments for measuring the tire's diameter (or radius, depending on which formula) thru the hub CL to the ground. However, doing so fails to account for the tire's true circumference, every inch of which rolls out with every revolution, wheel spin aside. Example:

We used to run about 5.25 psi in our 14x32W's, and with such low pressures the circumference calculated from static radius height measured from the ground to the hub CL was on the order of 3-4" smaller than actual measured roll out. The discrepancy there equated to several mph in the lights, not even factoring for any growth.

With higher inflation pressures and/or stiffer sidewalls the differences aren't so dramatic, but IMHO it's still more accurate to measure a tire's diameter or radius either off the ground or level with it rather than vertical to the ground so as to minimize errors.

Glensgages

I always use the circumference, and not ground-to-hub CL dimension:
ground-to-hub CL can be effected by a few things, but the entire circumference gotta-go-around each axle-revolution, regardless.

yellow-fever

I talked to Art Carr himself several times when I called California Performance Transmissions to order my new transmission. They do not have these transmissions already built and "on the shelf". Each one is built to a specific customer's application. CPT builds their own torque converters, and unlike some shops, Art prefers a non-locking converter with the 200-R4. He said if the car is not computer-controlled, the car has to use a "mechanical" mechanism to lock the converter, and this is apparently trouble-prone. The non-locking converters give up about 200 RPM at highway speed, but you can shift into 4th at full throttle, and the converter is designed to take it because it isn't trying to lock up. I still want to cross the 1320 in 3rd gear, and am leaning toward the 3.55 gears for my big block application, figuring 115 or so at the end of the quarter.

R96IMP

All the speedo calculations are useless if you do not have the correct(for rear gear)driven speedo gear.Tire size plays a factor but the gear at the end of the speedo cable is the largest adjustable factor.
Check with your local GM dealer parts dept and they will tell you what color and tooth count driven gear is available and what rear gear it was/is used for.
Example:You have 3.70 rear and a 2004R trans.
That trans never came in a Vette,so for ease of finding a comparable unit,use a Monte SS.They had 3.42 OR 3.73 rears.
You are in the ballpark.
Montes came with a 26.2"tire-215/65/15.