833 trans, OD

1olddmf said
just looking for rpm drop on the hiway, currently has 391 gears,

The 340 with a manual trans, really needs the 3.91s to get off the line, especially if it is down on cylinder pressure. And even more so if the trans is a T/A ratio.
The 3.91s also help it blast thru the gears.
The od trans solves getting off the line, and solves the cruise rpm. Unfortunately, it kills Second gear.
When analyzing why this is, you have to look at the "overall gearing" which I call road gears, which is nothing more than the rear gear times the trans gears.
Supposing that you have a 70Up A833, the trans gears are
2.47-1.77-1.34-1.00 and with 3.91s, the Road-gears are
9.66-6.92-5.24-3.91, and 65=3165, with 27" tires
This is a great set-up, except for the cruiser, and 6.92 is a great Second gear, if a lil high..
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The od gear ratios are;
3.09-1.67-1.00-.73od. with the 3.91s,
the Roadgears would be
12.08-6.53-3.91-2.85, and 65=2300. compare this to your current
-9.66-6.92-5.24-3.91,
But that 12.08, while fun for a while, is gonna get old fairly soon. and
third gear is a lil far away from Second. Too far for my driving style.
>The bottom line is this, if your 340 has a "bit of a cam" in it, the pressure is likely gonna be down some, which softens the power at low-rpm. If that is true, then she may not like the Rpm drops between gears.
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Forget running 3.23s with the od box; the Roadgears are
9.98-5.39-3.23-2.36
First is nice now, but Second sucks quite a bit, and the cammed-up 340 is not gonna like 65=1910 rpm, nor will the low rpm save any fuel , unless you install an Ignition Timing Computer.
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If I had a manual trans 340, it would get a 5-Speed, with gears something like
3.09-1.92-1.40-100-.78od, and with 3.55s the Roadgears would be
10.97-6.82-4.97-3.55-2.77, and 65=2240
Compare this to your combo;
>9.66-6.92-5.24-3.91, and 65=3165, with 27" tires. Notice the first three gears are very similar to what you now have, except First is a little lower. This helps take-off and parading
This 2240 is about as low a cruise rpm as you can run, and still get a decent cruise timing. This combination of gears does not require a lot of horsepower to have fun with. The stock 340 with/or without headers is all you need.
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but I gotta tell you, cylinder pressure is worth gear multiplication.
From my years of experience as a streeter, I'm gonna guess that every 20psi is worth at least 8% gear. So going from 120psi to 160, is 40psi is worth say 16% gear, so 3.91s less 16% is 3.28s, rounds to 3.23s .
if you already have 150psi then you'll need 190psi to drop down to 3.23s and you cannot run that with iron heads.............. but you can with alloys.
With 3.23s and 27s, 65=2600 and
2600 is a heckuva lot better than 3165.
However,
3.23s will make your take-off suck! and introduces a new condition, namely, a lack of acceleration thru the gears because of the additional time spent spent in each gear, due to the loss of Torque-multiplication.. ....... but you said you were not driving it that way nor racing it.
The new Roadgears with 3.23s and the T/A trans, would be
7.99-5.72-4.33-3.23 compared to the current
-9.66-6.92-5.24-3.91
And that is why my 340 would get a 5-speed.
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But if your 340 is of the hi-compression variety, and it no longer has the factory pressure-plunging 114Lsa cam in it, well then, the pressure is likely to be up, which is good for the bottom-end, and so will do well with 3.55s and would only be a lil sucky with the 3.23s.
And if it has the older 2.66 low trans in it, then the roadgears would be
8.59-6.20-4.52-3.23, and you are back in action. If you can find a Commando-Box which has the 3.09 in it, then the Roadgears with 3.23s are
9.98-6.20-4.52-3.23, and problem solved. Compare this to yours
9.66-6.92-5.24-3.91, Badaboom!
Happy HotRodding.