differiential gear ratios

[Duke W.]

Re: differiential gear ratios

Your calculations are correct.

You might feel a difference of 5 percent, but it can be mostly recovered by optimizing the spark advance map with a quicker centrifugal curve for more low end torque.

Ten percent can definitely be felt.

760 is the nominal tire revs/mile for the OE 6.70-15s. For a direct drive transmission (1:1 top gear) RPM at 60 MPH is merely tire revs/mile times axle ratio and you can linearly proportion from there.

Being as how the 283/230 is docile and has high low end torque and unless you like to race guys off stoplights, the WR with a 3.08 axle is the best combination.

It won't be "fast" compared to modern cars, but it will be pleasant to drive - either around town or on road trips at legal freeway speeds.

When the 327 was introduced in '62 the base axle ratio with the WR four-speed was 3.36 and 3.08 was optional. Of course, even the 327/250 was peppier around town due to more low end torque, but a 283 /230 should have no problem with a WR and 3.08 unless you live in an area with a lot of steep hills.

Duke

[Michael F.]

Re: differiential gear ratios

Heres a calculator that may help.

http://www.snoman.com/HTML/axlecalc_5a.html

[Duke W.]

Re: differiential gear ratios

These "calculator sites" that use tire diameter rather than specfied revs/mile won't give accurate results.

Pneumatic tires deform under load and don't yield the same revs per mile as a rigid body.

They'll work okay for steel railroad wheels.

Duke

[Michael F.]

Re: differiential gear ratios

These "calculator sites" that use tire diameter rather than specfied revs/mile won't give accurate results.

Pneumatic tires deform under load and don't yield the same revs per mile as a rigid body.

They'll work okay for steel railroad wheels.

Duke

I am not saying you are wrong.
But I plugged my real info into it. It gave me the exact ratio and speed I am running correctly. 26.7 diam tires, 3300 rpm and 3.55 rear end=75mph. Where did it go wrong?

[Duke W.]

Re: differiential gear ratios

If you get the mfgs. revs/mi spec for the exact make/model/size tires you are running and run the numbers you will get a slightly different answer.

The difference may be speedo and or tach error.

Revs/mil specs are for new tires. When they are down to the treadwear indicators, revs/mi increases about two percent.

A portable GPS probably gives the most accurate speed.

Duke

[James G.]

Re: differiential gear ratios

Best ratio I enjoyed was wide ratio with 3:55 in one of my 65 fuelies. Has 9.01 off the line (vs. 9.04 with a 4:11 and CR 2.20) and at 60 cruise your only turning 2698.

Used that combo in 57 original 283 engine cars as well and they all worked well with good gas mileage as well.

[Donald H.]

Re: differiential gear ratios

As mentioned, you CAN obtain a much more hiway compatable rpm range, but to do so will render your car NCRS incorrect.
So, you kind of have to decide whether you want correctness-----------or practicalness.
As mentioned, you can change the internal gears in your T-10 to convert it to a wide ratio, which will work better with a 3.08 or 3.36, or conpletely replace the tranny with a later wide ratio T-10 (or even a Muncie ).
I have a wide ratio Muncie and 3.08 posi in the 56, and I wouldn't change back for anything. I enjoy driving it both in town and on the Interstate. For the lower speeds in town, the 3.08 is often too much gear, so I simply compensate by shifting into whichever gear is more compatable.
With the 3.08 in the 56 and 225/75R15 radials, at 76mph the rpm is 3000.

Also, I feel that a 3.08 or 3.36 would not be a problem in an early Vette with a 283. Remember, the FULL SIZE Chevys with low perf engines got a 3.36 in 57 and then the 3.08 in 59. So, a somewhat lighter Vette should be just fine with a 3.08 or 3.36, even with a base 4bl 283.

Tom, if I rebuild my close ratio T10 from close ratio to wide and use a 3.36 rear, why does that make it NCRS incorrect. I realize technically it would be incorrect, however outward appearance of the tranny does not change, and to overall 1st gear ratio is almost the same as a close ratio and 3.70 rear.

Thanks
Don

[Tom P.]

Re: differiential gear ratios

Don,
Here is why.
IF you have it judged, and during performance verification, if you have a judge onboard who is very familiar with rpm ranges vs speedometer reading, he will be able to recognize that the engine rpm is not compatable with the indicated speed.
Like you pointed out, the OUTward appearance may be 100% correct, but when going down the road, let's say at 60mph, and the tach is 500-1000rpm less than it should be at 60mph, he may and may not count that against you. A judge with only 25 judging points may never notice this, but someone like Mike M. with zillions of judging points would probably pick up on it after only going 50ft.

[Donald H.]

Re: differiential gear ratios

Don,
Here is why.
IF you have it judged, and during performance verification, if you have a judge onboard who is very familiar with rpm ranges vs speedometer reading, he will be able to recognize that the engine rpm is not compatable with the indicated speed.
Like you pointed out, the OUTward appearance may be 100% correct, but when going down the road, let's say at 60mph, and the tach is 500-1000rpm less than it should be at 60mph, he may and may not count that against you. A judge with only 25 judging points may never notice this, but someone like Mike M. with zillions of judging points would probably pick up on it after only going 50ft.

Sorry for being dense, but does this mean that since my car is supposed to have a 3.70 rear, that anything else will cost points in judging?

Thanks,
Don

[Duke W.]

Re: differiential gear ratios

Flight judging cannot determine the installed gear set ratio.

But if you are going for a Duntov, you have to have a PV, and if you have a different than OE available axle ratio the PV judge will probably pick it up by observing the speed-rev relationship in top gear, and if it's not proper, you fail the PV.

If you don't intent to do a PV on the road to a Duntov Award, the installed gear ratio will never be a judging issue.

Duke

[Duke W.]

Re: differiential gear ratios

I just pulled the dif out and rebuilding it. Trying to decide which gear to use, choices are 3.08 or 3.36.
Its a 4-speed, 230 horse 1960 corvette
any thoughts out there

You may have another option. I'm not sure, but you might be able to install the Super T-10 guts in the original T-10 case. It's worth a call to Richmond if you're interested in this approach.

The wide ratio Super T-10 gearset puts the big gap between first and second, which is where it is best to be instead of between third and fourth as is the case with the original T-10 and Muncies. Most modern (and vintage) transmissions are designed so that gear spacing becomes closer as you work you way up the gears.

If you check this out, let us know the answer.

Duke

[Tom P.]

Re: differiential gear ratios

IF, IF, IF during judging, a KNOWLEDGEABLE judge rides in the car who is accustomed to looking at the tach and the speedometer and they don't corrospond to what to what is known for a given speed/engine rpm, they may comment at the least and deduct at the most. I'm familiar with judging practices, but I DO NOT do judging simply because I have, and build, cars my way. I AM NOT a numbers fanatic, although I THOROUGHLY enjoy and appreciate what is involved, and the result of, the efforts to return a car, boat, WWII plane or whatever to its original configuration. It's also gratifying to see what it really was like when it was brand new. But I, like many other hobbiests, sometimes prefer to personalize things beyond what they were when new. A pure stock, 265 56 Corvette engine is NOT my desire for a kickass early model Corvette. So, I drive a hotrod Vette, and admire an NCRS correct Vette. And I don't have mine judged, but I know what judges are looking for, and believable rpm vs indicated speed is one of the things SOME judges may notice.
I'm not sure how else to explain it.

[Tom P.]

Re: differiential gear ratios

What Dave Perry said is what I tried to express.

[Duke W.]

Re: differiential gear ratios

If the PV judge can tell the difference between a CR and WR transmission, which is pretty easy from the 3-4 rev drop, and if he knows that a '60 230 HP four-speed should have a 3.70:1 or shorter axle, which means that revs at 60 should be at least 2800; and if the car exhibits the typical WR rev drop on the 3-4 shift and turns about 2550 (3.36 axle) at 60, it is a reason for the PV judge to fail the car because it does not behave as expected for an originally configured car.

There is an item on the scoresheet under Manual Transmission (I'm looking at the second edition '63-'67 Ops Manual and PV Test Guide. I don't have the applicable version for a '60, but it's probably the same) - Type and Appropriate Application - pass or fail. Failure of any one line item is a failure of the PV test.

The trip odometer is set to zero prior to the road test, and it must read within five percent of the official course measured distance, and there is a specific line item for this on the scoresheet.

One could get around the speedometer-tach relationship by retaining the speedometer driven gear for a 3.70 axle with a 3.36 ring and pinion installed, but then the speedo and odo will read about 10 percent low. If the PV judge can't detect the speedo error in normal traffic, the odometer check will tell, and there is no way to get around the difference between the CR and WR 3-4 shift rev drop.

So if you have a 3.70 speedometer drive gear with a 3.36 axle, a failure would result based on specific evidence - excess odometer error. If you have a proper speedometer drive gear for a 3.36 axle the judge could still check the fail box for Transmission Type and Appropriate Application if he recognizes the big 3-4 gap.

Duke