AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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EDIT See below
You won't like it(the o/d-GV combo). Go with a regular 4spd, an auto, or since it's a pick-up, an A518.
And heres why; To build a bruiser implies a big cam cuz that's what most guys think. To be effective you will need big meats. Big meats means little or no tire spin. Engine married to road means engine married to trans ratios. Big cam 400 pulling from 54% to 60% of redline,means 400 stuck in basement, while SBM motors by.
OK but you said with GV Well that's just dynomite big fun, if you use it as a splitter.I forsee at least three problems.
#1) the GV does not shift particularly fast when going through their computer. I discovered that if you slam battery volts straight into it through a push-on-push-off switch then it shifts like lightning.
#2)You cannot backshift through the GV, it tears it up. You can decouple the engine from the tranny and outshift the GV, and then re-couple. That's ok. But you can't do any roadwork that way.
#3) If you accidentally leave the GV engaged and start backing up, the GV is history, and they ain't cheap to rebuild. That's what their computer is designed to prevent.
Edit; Oh boy, I screwed up. I see you wrote REGULAR 4spd. And I read overdrive. So the above story is for those that might have been thinking about the A833 O/D and a GV.
Now, here's something to think about;
You have 400plus cubes.That could make a lot of torque down low. If you exploit that,you only need 3 gears as a brawler.You need one to get moving, and one to cruise with, and one to brawl with. Your brawling speeds will normally be from mid 35mph to 65mph cuz nobody want's a speeding ticket, cuz that usually leads to loss of driving privileges.Plus they say;speed kills.
So all you need, is to be ready at 35mph and to be done at 65. That's exactly one gear if you engineer it right. If you put a shift in there, it can cost you .2 of a second.If you blow a shift, it's overe.If you blow through the tires(all to easy with a 400plus), that pesky SBM is blowing by you, again.
So zero in on your intended usage, and engineer the gearing involved. And then build a powerband into that 400 that will cover the bases. Then select whatever tranny hits the most bases.
I'll throw something at you.See if it sticks.
-Let's explore the A833 o/d box,, 3.09-1.67-1.00-.73od,since I am confident about the three gear requirement. Since this box has the o/d let's go with 4.30 for a rear, which could then cruise around 2300.
Now you are probably gonna run some big meats, so let's go with 30inchers(which have about a 94 inch roll-out) .The 3.09 low will combine with that set-up pretty good. But I'm more interested in what's going on from 35 to 65.
35 to 65 is usually 2nd gear, so that would be a 1.67 ratio.And the math says 35mph is 2824rpm. Math also says 65 is 5244. so with 4.30s your powerband will need to be from 2824 to 5244.That would be 2420 rpm. That is sooo doable. But you say theres no power in that zone, how can you win.
Ok then, lets work it backwards. Say you think you need 6200 to make a good show.
The formula is (rpm x TC)/ (1056 x R1 xR2) = MPH
We know the rollout(TC) is 94, and the tranny-gearR2 is 1.67, and the mph is 65, and the RPM we want it to be; 6200.
Lets plug it in; (6200 x 94)/(1056 x R1 x 1.67) = 65
Doing the math and rearranging we get;R1 = 5.08, so you will need 5.13s to make that target. Cruising will be 2734. And the starter gear will be 3.09 x 5.13 =15.85, which even with 30s will be rediculous; taking off like 5.38s, with a 2.66 box and with 27inch tires. But we are not quite done yet. RPM at 35 mph in second will be 3368. With a 400, screaming along at 3368, you will fool no one. And the powerband from 3368 to 6200 is 2832 wide. That's a bit of a stretch with a cam that wants to hit 6200 with full power.So this tranny only hits 2 bases and even one of those is not very good and 5.13s on the street will beat the oil to death; never mind the driveshaft whipping about under your azz.
But is it really the tranny? Or is it that cam that doesn't make power until 5800ish.
Remember the 4.30s? They hit all the bases with a reduced powerband.
Decisions, decisions.
But we did learn one valuable thing. The 6200 demands a gear ratio of 8.48; which being 1.67 x 5.08 = 8.48. Well that is priceless. So all we need to do is come up with a second/rear combo to make 8.48 and we are set!
Or are we.
Remember the powerband requirement of 2832 for this redline? Suppose we make a compromise and run up the middle and hit 65 at (6200 + 5244)/2 =5722. This will then require a similar trick with the powerband requirement; (2420 +2832)/2 =2626.
So what gear-combo do we need to hit 5722 at 65mph? Going back to the formula, the math says we need 7.836; and we are in business again. So throwing that O/D box into the weeds, what tranny/rear combo can get us a 7.836 2nd combo and a reasonable starter combo.Glad you asked!
-Lets try the regular box.Ratios are 2.66/3.09-1.92-1.40-1.00.. To get the 7.836 second requires 7.836/1.92nd = about 4.10s; bang-on good and 4.10 x 2.66 = a 10.91 starter gear. Well that's not too snappy with 30inchers, so lets go with a 3.09low. That gets us in the ballpark;3.09 x 4.10 = 12.67 Perfect! So we now have 3 bases covered; the rpm at 65, the powerband of 2626, and the starter gear. Lets go for 4th base; the cruise rpm. The math says 2994 at 65mph. OUCH!But hang the GV on there and cruise becomes 2335 at 65.
And one more thing. A bonus.This 3.09/direct 4th box lends itself to gear-splitting. You can go 1st-1od-2nd-2od. The ratios are 3.09-(2.41)-1.92-(1.50). with the above 4.10s you hit all the bases. Now the splitter can keep you on the pipe with rpm drops of about half of what it would be without the splitting. I figure that feature is worth one or two cam sizes. So now, instead of that 284* cam, you might show just as well with a 276* cam. And you know what that means; more brawling with less gas money.And as an afterthought, 2nd-od will get you down the 1/8 to the tune of 94mph at 6500;with just one stick-shift...... Just one Pull
Ok one more. The Passon box ; 2.66-1.59-1.00-.80 We already have the one requirement of 7.836 in 2nd, so; 7.836/1.59 = 4.92 rounded to 4.86s. Well 4.86 x 2.66 = a 12.93 starter, and the od gets us 2839 at 65. Thats 3 bases again.And the powerband was previously determined to be 2626. That's all 4 bases! Hooray!!
Now all you have to do is build a 400 stroker with a 2626 powerband, nosing over at 5722. Easy-peasy. If you don't thoroughly trounce all comers, start the truck on a diet until you do.
And now for the icing; If you put the GV behind the Passon box, the cruise rpm drops to 2214.
This box is a natural for the GV. The ratios would be 2.66-(2.07)-1.59-(1.24)-1.00-(.78 )/.80-.62 and the splits are .78-.77-.78-.81-.80-.78 That makes a powerband requirement of about 1200rpm (4500 to 5722) or maybe 1350ish on the bigger cam.
Just remember you cannot backshift under power.
'Course I made a lot of assumptions in this story, but the theory is sound.
Your result with different inputs, WILL vary. The formula works for all inputs.
And now a word about the 3.09Low/direct 4th/GV combo. From post #28;
You will have to pry this combo out of my cold dead fingers!
