Built 273, what sure grip ratio with a A833 3.09 first gear

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I've got an Aussie Hemi 265 6 cylinder that I'm building and will be turbo'd. I bought a 1969 b-body A833 and it's got the 3.09 gears.
Why is 3.09 good for 6 cylinders? and will a 3.54 rear be ok?
 
What are the other ratios in that 69 box

The 3.09 low is only good when the starter gear needs to be in a certain window for the engine to be able to come alive.
And generally ,an engine will like a high starter gear when;
it is of small displacement, or
has low power at low rpm,or you are looking for a quicker take off,or
the engine has to pull a trailer,or
operate at a higher altitude,etc.
Your 265 is getting up there in displacement, and has a better head-design than some others,lol. And it will be getting a turbo, so;
If the turbo starts building pressure soon enough, she may not need or want the 3.09 low. Your starter gear will be 3.09 x 3.54=10.94 So then if it only made 100 ftlbs at say 1800rpm, then the power to the rear axles would be 100 x 10.94=1094 and that is probably not enough to break the tires loose. So the solution is to rev it higher, say 2800, where she might be making 150 ft lbs and the new starter torque would be 150 x 10.94= 1641 and at sealevel that would be enough to, or should be/ could be enough to, initiate tirespin.

But then you come along with a little turbo that makes 200ftlbs at 1800 rpm and immediately pumps 200 x 10.94=2188 into the rear axles and this would be far in excess of what is needed to initiate tirespin. So now the engine revs up so quick in first gear that you can hardly get it into second fast enough. So the solution is a lessor first gear, say a 2.66. Now your turbo only puts down 200 x 2.66 x 3.54=1883 ft lbs, still enough to spin but it doesn't run outta revs as quickly.
All power numbers are fictitious and chosen to prove the point; your results will vary.

Now we get to point #2,the transmission's 1-2 split.
My ratios are 3.09-1.92-1.40-1.00 and the splits are .62-.73-.71.. So whatever I rev it to in each gear, the rpms will drop to those percents. Example if I rev to 6000 in 3.09 first gear, at the 1-2 shift, the Rs will drop to .62 x 6000=3720 If my engine falls off the power curve at 3720 with 3.54s then it will have to fight it's way back up to where the power is to increase it's rate of acceleration. Fortunately for me I have 367 cubes and it is well able. And fortunately for you, you have a turbo. And if engineered right, you will have V8 power, perhaps even more.
Since you already have the 3.09trans and the 3.54 rear gear, IMO, you are off to a very good start. Just try not to get too greedy in the turbo selection. And I can't help you with that as I have no turbo experience...... yet
 
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What are the other ratios in that 69 box[/QUOTE]

No sweat AJ, I just remembered I'd asked the question earlier in the year and re-read the thread and you gave me all the info I needed. The 3.09 is the way to go on the 6 you said.
I swopped the b-body housing for a '65 A-body ball and trunion. Weird that my box is a 3.09 B-body from 1969 right?
 
brewil said:
What are the other ratios in that 69 box
Click to expand...
No sweat AJ, I just remembered I'd asked the question earlier in the year and re-read the thread and you gave me all the info I needed. The 3.09 is the way to go on the 6 you said.
I swopped the b-body housing for a '65 A-body ball and trunion. Weird that my box is a 3.09 B-body from 1969 right?
Click to expand...

You lost me;
the 65 couldda/shouldda been the 3.09 short-tail box, with B&T output flange. I think this was a 1 or two year box
the 69B shouldda had the 2.66 long-tail box,with a slipyoke, and this box was around for many many years.
The next time we see the 3.09 low short-tail, is in the overdrive box with ratios of 3.09-1.67-100-.73od, which dates from about 1974 IIRC. It was also available in Chevy pick-ups about the same time with a long-tail and a Chevy 10 spline input gear.
How a long-tail 3.09low/direct 4th box with a 1x23 input made it into a B is anybody's guess, and I'd guess some Hotrodder made it and put it in there,lol; just as I did, into my 68 Barracuda
 
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have a question, what weight flywheel are you running behind the 273? also type of clutch?
 
update on the 3.91 gears I tried in my 65 cuda , In my opinion, it was way too much gear for the street . I went back to the 3.55 gears . just right now !
 
For me, and my 367, 4.88s and a 3.09low were way to much,(15.08 starter gear). Second gear was a sortof acceptable starter gear at 9.37,lol.
and so were 4.30s way to much;13.29 starter
But 3.91s with a 2.66, on a previous combo,had been pretty good; 10.40 starter.
So with 3.55s and a 3.09;that starter came out to 10.97......and that was just right.
So I agree,3.55s are just right; as far as a starter gear goes.
And they are almost acceptable on the hiway.
But when banging thru the gears,it takes a lot longer to wind her up, and the top of third is waaaaaaay up there in the mphs. But for me, I trapped well in third, so again,3.55s were just right,lol.
But if I had a 402, I'd probably not need the 3.09 low,lol.

As it is (10.97 starter), I know I could get away with 3.23s, for a starter of 9.98. which works out to a 2.66 low and a rear of 3.73s. I think that would be about as low as my 367 would put up with, for my dump-it-and-go driving style.
With a 9.44 starter,(3.55 x 2.66), I'd have to go back to the slip-it-out-and-try-not-to-sound-like-a-Mustang, way
 
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""9.5 compression 920 heads
ld340 intake manifold intake ports matched to heads""

Not really on topic, but how do you match an LD340 intake (large ports) to *920 heads (small ports)?...make the intake manifold ports smaller?

Just curious......
 
stroker402 said:
65 barracuda formula S streeter built 300 hp 273
9.5 compression 920 heads
ld340 intake manifold intake ports matched to heads
holley dp 650
1 5/8 semi fender well hooker headers
2.5 head pipes to flowmasters,
a A833 4 speed 3.09 low first gear
3.55 rear gears
Erson solid cam short dur. high lift. 510 lift , 286 duration advertised , 2000-7000 rpm range
72*overlap. Pretty snotty cam for a 273 teener. built similar to the 66 d-dart or 65 golden commando's barracuda "gold fish" specs .
Question is :
would there be any advantage going to a 3.91-4.10 rear gear ratio.
The starter gear with the present 3.55 ratio is 10.9 . 26" tall tire being used . I have to shift pretty quick with a 3.55 gear already. 4.10 would be Really be quick and has no advantage IMO . I'd like to take it on the freeways with 26" tall tires . It's not a race car by any means .
what are your opinions ?
video of 273 with open headers www.youtube.com/edit?o=U&video_id=Lur52zZxN9Y
Thanks .
Click to expand...
4.10 would be Really be quick and has no advantage IMO
With a 3.09 I sortof agree in as much as first gear is concerned, and that is very application specific. Where the pay-off is with performance gears, is threefold;
1) the engine blasts thru the gears quicker, reaching the shift-rpms sooner.
2) it will usually go thru the powerband twice on a zero to 60 mph run, putting more average power into the effort, and therefore should be quicker.
3) for a streeter, you can put your 2-1 downshift in the right place in your powerband to annihilate cars with mismatched and often, bigger combos.
4)But yes, hiway use is the sacrifice we often have to make.
____________________________________
For me, #3 was most important. I have a bigger engine but the point is the same. I wanted the 2-1 downshift to be at 32mph +/- 2mph. And my engine made peak power at about 5700(292/292/108 cam). Peak torque came in at about 4200. So in my naivete, I installed the 3.55s, giving me about 3600@32 mph with 28s, and the 2.66 low gear. And as you know 3.55s are a reasonable hiway gear. The thing is, my engine is too big, too powerful for the corner-carver type suspension it has, and so when I floor it, it instantly spins, and if I keep it floored it will not stop until I let up, which was near- speeding anywhere in Manitoba. I could easily give up a lot of camshaft, and so I did.
I swapped in a 270/276/110 that power-peaked at about 5000, torque-peaked at about 3600. So now with those same 3.55s, and on the 2-1 downshift, the Rs were still hitting 3600....... so it worked great! Except getting off the line with a 2.66x3.55= 9.44 starter gear and 28" tires was not making me smile. But I had 2 outta 3 so I drove it like that for a couple of years.
So I had the bright idea of installing a 3.09od box, and a GVOD to be used as a splitter, and gearing it to trap at 62mph, in first-overdrive. Daymit this was 3.55s again,lol. But now with the 3.09 low, the starter gear was 10.97 and that little 223*@.050cam pulled that easy. The 2-1 downshift was now 4200 at 32, and I just hit the splitter when it nosed over, and so now I was spinning all the way to 62 again....... and cruising at 65= under 1600. whoot! The bonus with that combo was; 32mph in second gear was a tic under 2300...... so I could sneak up on anybody, and shock the brown out of them tuner bums.
The downside was the ratios in the od box were too far apart and I was pretty much forced to always split gears and that got old in a hurry. I tried some other chunks to try and mitigate things but in the end I ditched the od box.

Long story short; What rear gear YOU run is up to you. Not everyone wants a tire-fryer right out of the gate. Nor wants a 2-1 blast-off. nor a 1600rpm cruise. If you like 3.55s and your engine likes 'em, let it be so.
At one time I had almost every chunk from 4.88 down to 2.73s, all lined up ready to go, and three different 4-speeds, plus an automatic, and a back-up 318. And I did a lotta lotta swapping. In the end, the 3.55s are back in there....... probably to stay,lol.
 
update from early this spring on my 273. well , the 273 cam up with a rod knock , the bearings were all toast and scored badly so bad I ended up putting that engine on the back burner . I found a real 273 commando engine with 67K orignal miles and stock bore. 10.5 to 1 compression dome pistons and built that with a smaller new isky cam ,used my closed chamber 920 heads and messaged them just a bit , put the proper ld4b intake on and my old 650 dp , headers of course ......... pertronix ignition with rev limiter , and curved the distributor , it runs great ! I started a post on "my 273 build" on this site if you like to review it.

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