Mild 340 8 3/4 gear and possibly converter question

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nick455440

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Have a mild 340 that has a 8 3/4 rear end, I have recently over the last year or two did a /6 to V8 swap into a 65 Valiant and recently got it back on the read. The Converter and 3rd member I basically ran because i had them already from picking up parts ect.. The gear is WAY to much, its a 5.13 and looking for more ideal gear options. Info on the car below.
Engine - Cam is a Comp Xtreme 274/286, .488/.491, Heads are mildly ported X heads, Performer RPM with a 750 double pumper. 904 trans, The converter (was free so running it for atleast now) from what i can tell is a turbo action 10" stamped 19802S that looks to be a 3200 stall. The wheels are 17's and currently the wheel diameter is 25 inches, would like to go a little taller in the near future but will stick with the 17's so might be limited on tire height. This car will be mostly around town, hardly no highway and occasionally drag race for fun, not worried about gas milage as much as drivability. The 5.13 gear seems to blow through 1st and 2nd gear cruising around the block. Was reading the recent similar post about the 318 and was leaning towards 3.55's, was wondering thoughs and opinions, Thanks all for any input and info..
 
Yes, I think 3.55s would be my choice for your build/cam. In the light '65 you may even be happy with 3.23s. If your 904 is a later version with the low 1st gearset, I'd lean even more heavily towards the 3.23s, depending on what you ultimately end up with for tire height- but your current 25s would be a good match. Much taller of a tire would swing me back to the 3.55s
 
It kinda depends on the case number that you wanna put gears in. Very limited choice of new gears for a 741. Better choices for a 742 or 489 case.
If you can find a case with 3.23s in it I'd run them. If you wanna put new gears in your case, I'd go 3.55s I'm not sure if 3.23s are available new any more.
(And 5.13s ARE ridiculous. I had 5.14s in a 90% race car with 32" tall tires, and it was too much. It ran just as hard with 4.56s)
If you go with the 3.55s, and decide you still want less gear, you can add a couple inches of tire height, and it will feel like 3.23.
 
Its a 1973/74 904 trans i believe, the case is a 489, should have thrown that in there, also does the converter sound good/usable with this set up once gears are replaced to a 355 or 323?
 
Depends on the converter. A 3200 nine inch will probably work fine . A 3200 twelve inch won't. I'd sure try it after the,gear swap, before planning to replace it. 3200 is about right.
 
Seems like you don't mind gears, it's way over geared now but the lowest I'd go 3.91-4.10, if your gonna do some drag racing gear for optimal 1/4 or 1/8 which ever one you'll run.

tire diameter x rpm / 336 x mph = gear ratio
 
hardly any highway and for maximum around town shenanigans 3.91 would be the call. 3.55's would do great as well. maybe split the difference with 3.73's?

if you plan on driving it any kind of distance, 3.23's for sure.

you can probably trade your gears straight across for the set you decide on.
 
Best option? Well it really depends on what you plane to do, strictly around town? Some Highway? Lots of Highway/Freeway?

First off I'm a big fan of wide ratio gearsets in the trans, unless the engine is built to make power in a really narrow powerband...


Lots of Highway? Use 3.23's and find a wide ratio gear set for your 904...

With the wide ratio gear set your ratios would be

1st 8.85
2nd 5
3rd 3.23

Overall a good choice if your gonna be putting miles on the car
============================================
With the standard gear set your ratios would be

1st 7.9
2nd 4.7
3rd 3.23

That first gear jump from 7.9 to 8.85 matters a lot....
Most of us have run these ratios & it's okay, trading acceleration for comfort..
===============================================

More of an around town hotrod? 3.55 or 3.91's... 340's don't mind spinning 3000 rpm's so 3.91's are a viable option.. These days people are use to their daily drivers that have overdrive so 3000 rpm's freaks people out.... But that's how we rolled back in the 70's & 80's...

3.91 and wide ratio 904 Pretty much a hotrod, lots of tire spinning

1st 12.6
2nd 10.71
3rd 3.91
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3.55 and wide ratio 904 Excellent all around gears
1st 9.72
2nd 5.46
3rd 3.55
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3.91 & std ratio
1st 9.58
2nd 5.67
3rd 3.91
=========================================
3.55 & std ratio
1st 8.7
2nd 5.15
3rd 3.55
 
junkyardhero said:
hardly any highway and for maximum around town shenanigans 3.91 would be the call. 3.55's would do great as well. maybe split the difference with 3.73's?

if you plan on driving it any kind of distance, 3.23's for sure.

you can probably trade your gears straight across for the set you decide on.
Click to expand...
99% of the people with 3.23s would have no use for 5.13s. I've had 5.14s (9"gear ratio) in a car. They were awful!
 
33IMP said:
99% of the people with 3.23s would have no use for 5.13s. I've had 5.14s (9"gear ratio) in a car. They were awful!
Click to expand...
that's true. but in a wheeling and dealing scenario if you're moving parts they'd be good trade fodder.
 
With most any of the ratios mentioned,monitor trans temp closely.Those loose converters really build some heat,especially with 3.23’3.55 Even 3.73.Not a problem just plan for it.
 
Dropping down to 3.23 or 3.55 from 5.13 is 1 & 1/2 to 2 full ratio loss, even though 5.13 over geared I don't think your gonna like losing that much pulling power it's gonna be pretty noticeable. Especially since you don't care about the highway.
 
3.55 x 1.45 (2nd gear) = 5.14, 2nd with 3.55 is the same with 5.13 in 3rd,
basically gonna feel like you lost first gear and gained a 0.69 OD.
 
That converter will act much better on the street with less gear. For what your doing with the car, i'm in the 3.91/4.10 range.
 
I tend to err on the side of caution these days. If it is a street car with the occasional trip to the track I would run 3.55 gears. With your short tires it will be more like 3.73.
Even with taller tires it will pull fine and the converter will put it on the cam.
Remember it is a light car and will be plenty quick.
 
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