Wow! Thanks for all of the excellent info.
I saw a duster slant six manual for sale and was dreaming of swapping out the trans to 4 on the floor to make a hotter performer with gas sipping at cruise speeds. I now see that there is a bit more work to be done with gears for what I was thinking. Would a 8 3/4 rear with 393 gears do the trick? Or would gears in the transmission also need to be changed?
Thanks again for sharing your knowledge.
B
I'm not understanding this exactly but here is some food for thought;
Typicall we use the 2.66box with 3.55s cus we can go zero to sixty with it and hit 60 revved right out in second gear, thus posting a good low-ET.
So we adjust the revved right out point with the rear gears, to make that coincide at 60.
Fot instance if you had an engine that power-peaked at 5000rpm, and carried that powerpeak out another 300rpm, then you would choose a rear gear of ;
60= (5300x tc)/ 1056* x the roadgear.
where;
tc is the tire circumference, somewhere between 80 and 88,usually,
1056 is a constant, to convert the final result to mph
and road gear is the second gear ratio times the rear end ratio
So if you had a regular 4 speed, second gear would be 1.91 ratio
Thus the formula would look like
60= (5300x 85)/1056X1.91 x Q the unknown rear gear.
Re-arranging to solve for Q, we get
Q=1/60 x (5300x85)/(1056x1.91)=3.722; rounds to 3.73s
Now, if you had the OD box, it has a second gear ratio of 1.67, so
Q= 1/60x (5300x85(/(1056x1.67)=4.258; rounds to 4.30s
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Now let's look at the cruiser gears;
In example 1, the cruiser is 3.73 x 1.00, and 65=3012rpm
In the second example, the cruiser is 65= 4.30 x.73od, so 65=2535; This is a good thing.
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Now let's look at the starter gears
In the first, it is 3.73x2.66= 9.92
In the second;4.30 x 3.09=13.29.
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Around town,I like to shift into second at 2800 cuz my engine likes 2800. It has lots of power there, and the pipes are singing nicely. And when it drops into second, it has plenty of power to merge with traffic.
Ok in the
second example, this would be 17mph, and dropping into second, the Rs would fall to 1513....... Do you see a problem with that in your engine?
Most engines, especially as they get smaller, don't have a lot of pulling power at that low an rpm. Furthermore; this gear now has to climb to 32mph/2855 rpm, a pull of 1342 rpm
Now in the
first example, 2800 is 22.7mph, and dropping into second, the Rs drop to 2010, about 500rpm higher. This is a good thing. This gear only has to pull 822 rpm to hit 32mph.
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Now lets look at trying to accelerate in second gear from cruising at 32 mph.
In the first example, 32 will be 2832 rpm, in the second it will be 2855.
Remember I set it up so both combos would be revved right out in second at 5300,300rpm past the power peak. So second gear will be the same everywhere in the speed range.
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And finally third gear.Lets continue to hiway speed.
First
combo 1, the 2.66box
Say you picked the following shift points; 2800,3300, and 3400 into 4th.
Your speeds would be 22.7,37.3, 52.8, and ending at 65=3012
at the shifts, the rpms would drop to; 2010, 2402,and 2445.
The mphs each gear would have to pull are; 22.7, 14.6, 15.5, and 12.2
Now notice the 2.66 box has fairly even pulls, and uses all 4 gears to get to 65, and most important,progressively higher rpm drop-in points.
Next
Combo 2 the 3.09od box, same shift points; 2800,3300,3400 into 4th
Your speeds would be;17, 37, 63.6,and ending at 65=2535
at the shifts the Rs would drop to; 1512,1976,and 2482.
The mphs per gear would be;17,20,26.6, and 1.4
Notice the od box has longer pulls in each gear and RPM drops quite a bit lower.
But what if we shifted the 3.09 at the same roadspeeds, namely 22.7,37.3, 52.8, and ending at 65. Ok that's fair.
The shift rpms would be; 3747, 3328, 2820. And the drops to; 2023, 1992, 2059. Now check out that final shift; you only revved it to 2820 in third, the Rs dropped to 2059, and now you're asking the slanty to pull that to 2535@65. That's a long hard pull, from 52.8 to 65.
And check out how high you had to buzz it up in first; 3747.
And finally, check out where the slanty has to pull from; idle in first,then from 2023, then from 1992, and finally from 2059. Tough life for the slanty.And this is with 4.30s in the back.
Ok but what if we revved it to whatever to drop in at say 2400, more or less like the 2.66box? Ok well that would make 4th worse, cuz currently it is pulling from 2482.
So how would I shift this box with a slanty? Fair enough.
Ok first, I would build my slanty to use those crazy splits, .54, .598, .73 This means she is gonna have to make good power below 2000rpm, cuz I don't want to rev her up to 4000 just to accelerate to hiway speed.
Ok then I choose 3500 as my shift rpm.
The speeds will be 21, 39, and 65.5 With drop-ins at 1890, 2096, 2555. And pulls of; 17.8, and 26.5; not too good as you can see. Again that's a long pull in third(direct), from 39 to 65. The only workaround is to rev second to a much higher rpm.
Basically, this trans is a 3 speed plus overdrive with a lousy 2-3 split.
OK so that's all I got for now.
