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- Thread starter AlaskaJeff
- Start date

Here's a calculator to figure RPM's at a given speed.

Spicer Engine RPM Calculator

All our Street/Strip a bodies run 3:91 gears

3.91 just gonna burn more gas to get to the same destination.

With as few miles (usually less than 1K) as I put on it in a season up here in AK, I'm not worried about how much fuel I burn! It's just nice to be driving it at all. Heck, it's been raining for over a month straight....I haven't had it out in a couple weeks I'd be happy to burn some fuel right about now!!3.91 just gonna burn more gas to get to the same destination.

3.91 just gonna burn more gas to get to the same destination.

So does driving any muscle or sports car if you really want to think of it that way.....

So does driving any muscle or sports car if you really want to think of it that way.....

Yup. And it's about the journey, not the destination

I have 3.91 SureGrip

Memories of an open 3.23 on my 68 Roadrunner. Long black stripe at least once a week...

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from Second to direct with a 727 is a split of 1.00/1.45=.69

Going from 3.23s then; 3.23 /.69=4.68

The point is that 3.23s in third gear equals 4.68s in second gear; like this;

3.23 /1.00=3.23 and 4.68/1.45 = 3.23

So then .69 is a full gear change.

Now, from 3.55 to 3.23 is 3.23/3.55= .91 or about a third of a gear change.

I know the math is right and I know I'm not presenting this very well, so let's try a different angle.

If One full gear change is .69, then from there to 1.00 being no gear change, divided into thirds is about; .9 to .8 to .7(.69 same thing)

So the gear choices for one-Third gear swaps would be;

3.23/.9=3.59s, rounds to 3.55s

3.23/.8=4.04s, rounds to 4.10s

3.23/.7=4.61s, rounds to 4.56s

Lets dig deeper; Top of First gear @5500, with 27"tires and

with 3.23s, this is ~56mph@ zero-slip, perhaps 50@WOT on the tach

with 3.55s, this is ~51mph@ zero-slip, perhaps 46@WOT, A/A

That's only 4 mph

But, cruising at 65mph @ say 3% slip, your Rs would be';

**2690** with 3.23s and

2960 with 3.55s;

That's 270 rpm. This is the SAME % increase as the gear change difference is, in %.

Example; from 3.23 to 3.55 is an increase of; 3.55/3.23=plus 9.907%

adding that to 2690 is; 1.09907 x 2690=2956

This % business works for all ratios.

For a streeter and a city car, I like to be revved out at 60/65 mph in Second gear. For a stock type engine, reving to 5000, that would take ~4.30s. But for a power peak of 5500, it could be 4.88s

Since yur not likely to run either, you might as well just run whatcha got, but stall it up so it gets off the line in a brisk enough fashion for you. 400rpm jumps make a big difference.

As to your question, 3.91s over 3.23s is plus 21% and your cruise rpm jumps the same plus 21%. And your rpm at any speed in any gear, also jumps 21%

Plus 21% is a fairly substantial jump .

Your rpm at 65 is likely to show ~3260 on the tach. This is plus 21% over your current rpm which should be ~2690.... on the tach; both are way too much for me.

yadayada......

There is a work-around for this.

Grab an A999 and stuff it in there. The gear ratios are 2.74-1.54-1.00 Here we go again

The 727/904 has ratios of 2.45-1.45-1.00.

First gear is thus 2.74/2.45=plus 11.8 %, and this will feel like 3.23 plus 11.8%=3.61s voila.

Second gear is 1.54/1.45=plus 6.21% so it will feel like 3.23 plus 6.21% =3.43s

And you get to keep your 3.23s, and the A999 has a lock-UP, so your cruise rpm will drop some 3%, or back to the Zero-slip number of ~2610

That's a win-win-win

Now if yur keeping up you might have caught that the Lock-up was worth about 3%. So theoretically you could add 3% to your 3.23s and cruise at the same rpm that you are now cruisin at, AND gain another 3% in gear Multiplication. ..... right? Well 3.23 plus 3% is 3.33s and there is no such gear available.

But hang on a sec, you can forget all thatchit, by upstalling .

Say your engine makes 300ftlbs at 2000 rpm. That would be 114horsepower.

but say she makes the same 300 at 2800. This is now 160hp. That is an increase of 40 fricken %. It's like a supercharger!

Lets see how this stacks up.

Using the 300@2000 thru 904 and 3.23s; At 2000 rpm you will be taking off with the equivalent of; 114hp x 2.45 x 3.23 /2 =451 hp into each rear axle.

Now lets move the stall up.

to 300@2800, and I get; 160 x 2.45 x 3.23 /2= 633hp per axle.. wait for it..... this is the same plus 40 fricken %, lol .

But hang on, stall does not work like gears. Gears are a constant % increase, If it's 10% at 10mph, it will be 10% at 110 mph.

Not so with the TC. It has an internal hydraulic multiplier that changes with the torque differential across it, and also with speed. So the multiplier is greatest at zero mph, diminishing with speed, and diminishing with any drop in torque that happens as a function of rpm or throttle-opening, or load.. So then, eventually the ratio ends up at it's minimum. This ratio may start out at 1.8, and finish at 1.1 (both at WOT) .

So the higher stall is a good kick in the butt for taking off with, but by the top of Second gear, is not doing much for you, unlike gears.

If yur just cruising around town, the Hi-stall can work for you in a different way. Firstly you get the peppy take-off; But say you are cruising in "Drive" at 30 mph, just minding yur own business. This might be 1250rpm. And you know yur engine is none too excited to pull from 1250. So it downshifts when you floor it, into Second and the Rpm jumps to 1800 mathematically, but to 2800 by the new Stall, so it blasts off fairly nicely in Second gear. If you let her downshift into First, the Rs would jump to say 3250, and the stall is more or less all used up

But what happens at 55mph when you go to pass someone?

You'll be cruising at 55=2280@3% slip, in Drive. On the downshift, the Rs will climb to 3500@10% slip, so well past the stall, and the stall-speed is no longer helping you much.

Whereas if you had more rear-gear, away you would go.

So, as you can see, the modest 2800 hi-stall is pretty much most useful for taking off with, and at lower rpms/speeds.

Now how about the combination of the A999 gears and a 2800 stall and a LU convertor, but still with the 3.23s? Now yer talking.

Going from 3.23s then; 3.23 /.69=4.68

The point is that 3.23s in third gear equals 4.68s in second gear; like this;

3.23 /1.00=3.23 and 4.68/1.45 = 3.23

So then .69 is a full gear change.

Now, from 3.55 to 3.23 is 3.23/3.55= .91 or about a third of a gear change.

I know the math is right and I know I'm not presenting this very well, so let's try a different angle.

If One full gear change is .69, then from there to 1.00 being no gear change, divided into thirds is about; .9 to .8 to .7(.69 same thing)

So the gear choices for one-Third gear swaps would be;

3.23/.9=3.59s, rounds to 3.55s

3.23/.8=4.04s, rounds to 4.10s

3.23/.7=4.61s, rounds to 4.56s

Lets dig deeper; Top of First gear @5500, with 27"tires and

with 3.23s, this is ~56mph@ zero-slip, perhaps 50@WOT on the tach

with 3.55s, this is ~51mph@ zero-slip, perhaps 46@WOT, A/A

That's only 4 mph

But, cruising at 65mph @ say 3% slip, your Rs would be';

2960 with 3.55s;

That's 270 rpm. This is the SAME % increase as the gear change difference is, in %.

Example; from 3.23 to 3.55 is an increase of; 3.55/3.23=plus 9.907%

adding that to 2690 is; 1.09907 x 2690=2956

This % business works for all ratios.

For a streeter and a city car, I like to be revved out at 60/65 mph in Second gear. For a stock type engine, reving to 5000, that would take ~4.30s. But for a power peak of 5500, it could be 4.88s

Since yur not likely to run either, you might as well just run whatcha got, but stall it up so it gets off the line in a brisk enough fashion for you. 400rpm jumps make a big difference.

As to your question, 3.91s over 3.23s is plus 21% and your cruise rpm jumps the same plus 21%. And your rpm at any speed in any gear, also jumps 21%

Plus 21% is a fairly substantial jump .

Your rpm at 65 is likely to show ~3260 on the tach. This is plus 21% over your current rpm which should be ~2690.... on the tach; both are way too much for me.

yadayada......

There is a work-around for this.

Grab an A999 and stuff it in there. The gear ratios are 2.74-1.54-1.00 Here we go again

The 727/904 has ratios of 2.45-1.45-1.00.

First gear is thus 2.74/2.45=plus 11.8 %, and this will feel like 3.23 plus 11.8%=3.61s voila.

Second gear is 1.54/1.45=plus 6.21% so it will feel like 3.23 plus 6.21% =3.43s

And you get to keep your 3.23s, and the A999 has a lock-UP, so your cruise rpm will drop some 3%, or back to the Zero-slip number of ~2610

That's a win-win-win

Now if yur keeping up you might have caught that the Lock-up was worth about 3%. So theoretically you could add 3% to your 3.23s and cruise at the same rpm that you are now cruisin at, AND gain another 3% in gear Multiplication. ..... right? Well 3.23 plus 3% is 3.33s and there is no such gear available.

But hang on a sec, you can forget all thatchit, by upstalling .

Say your engine makes 300ftlbs at 2000 rpm. That would be 114horsepower.

but say she makes the same 300 at 2800. This is now 160hp. That is an increase of 40 fricken %. It's like a supercharger!

Lets see how this stacks up.

Using the 300@2000 thru 904 and 3.23s; At 2000 rpm you will be taking off with the equivalent of; 114hp x 2.45 x 3.23 /2 =451 hp into each rear axle.

Now lets move the stall up.

to 300@2800, and I get; 160 x 2.45 x 3.23 /2= 633hp per axle.. wait for it..... this is the same plus 40 fricken %, lol .

But hang on, stall does not work like gears. Gears are a constant % increase, If it's 10% at 10mph, it will be 10% at 110 mph.

Not so with the TC. It has an internal hydraulic multiplier that changes with the torque differential across it, and also with speed. So the multiplier is greatest at zero mph, diminishing with speed, and diminishing with any drop in torque that happens as a function of rpm or throttle-opening, or load.. So then, eventually the ratio ends up at it's minimum. This ratio may start out at 1.8, and finish at 1.1 (both at WOT) .

So the higher stall is a good kick in the butt for taking off with, but by the top of Second gear, is not doing much for you, unlike gears.

If yur just cruising around town, the Hi-stall can work for you in a different way. Firstly you get the peppy take-off; But say you are cruising in "Drive" at 30 mph, just minding yur own business. This might be 1250rpm. And you know yur engine is none too excited to pull from 1250. So it downshifts when you floor it, into Second and the Rpm jumps to 1800 mathematically, but to 2800 by the new Stall, so it blasts off fairly nicely in Second gear. If you let her downshift into First, the Rs would jump to say 3250, and the stall is more or less all used up

But what happens at 55mph when you go to pass someone?

You'll be cruising at 55=2280@3% slip, in Drive. On the downshift, the Rs will climb to 3500@10% slip, so well past the stall, and the stall-speed is no longer helping you much.

Whereas if you had more rear-gear, away you would go.

So, as you can see, the modest 2800 hi-stall is pretty much most useful for taking off with, and at lower rpms/speeds.

Now how about the combination of the A999 gears and a 2800 stall and a LU convertor, but still with the 3.23s? Now yer talking.

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My heads full.

My heads full.

He said, rebuild the trans with a low first gear set..... and that is a groovy idea.

Run the 3.91s with a posi and enjoy it. I did in my 68 Cuda.

Everyone beats up on the 741.

Had a 741 case with 6.30:1 open in an old 1969 USDA Forestry service 10 foot bed stepside. Top speed was about 65 MPH but it never went on the freeways. /6 with granny 4 speed. First gear and I could walk faster but it would crawl over a foot high rock at a idle.

So, I'd do the 3.91 swap but keep the 3.23's for a road trip down south of the 49th.

You did not tell us Your tire diameter or camshaft specs

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I'm baaaak, lol.most of my driving is around town. Will the RPMs be too high or occasional driving @ 65-70 MPH?

3.91s are perfect for around town, especially with a higher than stock stall, because the engine is always ready to rock, right off idle.

But I'm no fan of 70=~

I like 3.23s with a 2800stall: in First gear, getting off the line, it will do almost whatever 3.91s will with a stock~2200 stall;

Have a look below; this is a 5.2Magnum, but you'll get the idea;

at 2200, the torque is ~280ftlbs/117hp

at 2800, the torque is ~292ftlbs/156hp

That's quite the increase in power;

At zero mph, and in 2.45First gear, that 292@2800 becomes 2311ftlbs into the rear axles with 3.23s. and

at zero mph, and in 2.45First gear, that 280@2200 becomes 2680 into the rear axles with 3.91s..

Now 2680 over 2311 is

However, at 2800 the engine is making plus 33% power.... so the TC is gonna increase it's Torque Multiplication business slightly.

Will it be 16%?

IDK

But I'd be willing to try it, just to keep the cruising rpm down to 70=~2900 versus 3510, that's about 610 rpm right there.

The other thing is this; 3.23s will get you 65mph at about 4400@15% slip in second gear.

but with 3.91s it will be closer to 5300, which is a win-win situation, namely; plenty of tirespin in first gear, and about the right rpm to ET well in the zero to 60 speed-contest.

So IDK

You'll have to make the call.

Happy HotRodding..........

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And agree, 741 reputation as a weak sister is undeserved. The weak area of the pinion is exactly the same on 741,742, and 489, and the 741 doesn't have the stress riser that the other two have.

However, Gear Ratios are much more limited, But if you can find

the gear you want the 741 is much cheaper.