So, you wanna bang gears.And you want the 408.
I can help you with that.
The 408 will not need the deep-low tranny, and the od box is out too.
Or is it?
I think I can make this od box work for your application, but you are gonna have to bear with me.
The od box has ratios of 3.09-1.67-1.00-.73--(You can buy these for $50 to $150, and the first three ratios are every bit as strong as the regular box.) The split percents are 54-60-73. This means that at whatever RPM you outshift at, the RPMs will drop by those percents. So for me, outshifting at 6800, the Rs drop to 3672 and 4080. So my powerband requirement,to use this tranny is 3128rpm and 2720rpm. That's not gonna put a lot of average hp down. This is the slow way down the track. But I have a GVOD that I use to split those ratios. The new ratios are 3.09-2.41-1.67-1.30-1.00-.78-.57 GV in red. So, now my split percents are 78-69-78-77-78-73. Notice that the splits are now hovering around 78%. This makes my rpm drop from 6800 to just 5304, for a powerband requirement of 1496. If I put the rpm of peak power in the middle of this,I will put a huge amount of average power down during the run. In fact, on a engine with a small cam,the power might fall just 4% on either side of this. Say you had a stout 360 with a 223ish cam that made 380hp@ 5400 . You would probably shift this at 5900, to drop in at 4500, for a powerband requirement of just 1400. If the power falls from 380 to 365 at the shiftpoints, You are gonna put down an average HP of around 370 or more! That is the fast way down the track. You would be able to keep up with much more powerful engines that are stuck with the earlier splits.Maximum average hp wins races.
So what does this have to do with you?
Nothing
I just wanted you to understand a little about this transmission.
Now, let's apply it it to your 408.
We already know that this box is not a hi-rpm box cuz it drags the rpm waaaay down. But it does work very well on a low rpm engine with a lot of torque.Like a 408 could be.
So for your application, which will end at about 85mph for freeway cruizing. Lets say you were willing to put a small cam in that 408, to make a really insane amount of low-speed torque. And you were willing to observe a much lower than typical shift rpm. Let's fit that to this tranny.Lets say you picked a cam that power-peaked at 5200. This engine will want to be shifted around 6500 with this tranny, to drop into second at 3510, for a powerband requirement of 2990. The 408, at 3510 will have a huge amount of torque; nearly it's max torque number. With a powerpeak at 5200, the torque peak will come in at around 3700 to 3900. Ima guessing it will make torque in the range of 450 to 475 ftlbs with this little cam. So now this tranny is working in first gear just fine; for racing.How about as a streeter?
>Well I like to shift at 2800 with my combo. This is high enough to motor briskly, yet not so high as to be buzzy.And it doesn't sound like an old 5.0 that has to rev to nearly 4000 to find torque. So in this case your Rs will again drop to 54% or 1512rpm. Now here's where it gets interesting. Will your 408 have torque down at 1512? This is where reason #2 comes in that I picked a tiny cam that peaks at just 5200 rpm. I saw this coming, having been caught with my pants down the first time,lol.So,the answer is; YES,the 408 will have torque at 1512. Certainly more than a 318 with a 268 cam, or a 360 with a 292 cam,lol. ( I tried those combos, with this tranny;bad idea). OK so the 1-2 shift is nailed, how 'bout the 2-3 shift?
>Well out-shifting second at 2800, better than the 1-2 so;will drag the Rs down to 60% so 1680, nearly 400 rpm it will be alright too.
So having determined that it is possible to use this El-Cheapo $100. box, let's marry it to the chassis.
>The first thing to think about is that 5200 rpm power-peak. This can only be achieved with a fairly small cam. That cam will torque peak very early, and pull a lot of vacuum. This means it will pull a very small hiway gear with ease.
>What about the starter gear? Again,because this 408 has such a small cam, and accompanying big low-rpm torque, it will easily pull a small starter gear.
>This leaves us free to set the chassis up for the 2-1 down-shift. We can gear it in such a manner that on the 2-1 downshift, the engine will drop down into a happy place, and put the hurt on the tires,big-time. Since peak torque will come in at around 3700 to 3900, lets set 32mph at 3800rpm in first gear. No surprise, this is 3.23s with 28 inch tires (you will need the tallest fattest tires that you can find). 32mph will be 3832 with 28s and that 3.09 first gear,Shazzam!!
>Those 3.23s will make a starter gear of 9.98, which the 408 will love. 65mph will be 4200 in second, a little low perhaps. And 85 in od will be 2405. 65mph will be 2519 in direct,1839 in od;now yer talking fuel economy.
If you want to perk it up a bit, 3.73s would put the top of second at 65=4850, and 65 in overdrive will be 2123, and 85 will be 2776. All still very good numbers. The sacrifice here is the starter gear, which climbs to 11.53. You will need to baby it out a bit.
I like the 3.73s. The 2-1 downshift at 32 mph, will get you a smoking hot 4426rpm.So perhaps lower the downshift speed to 30mph@4150rpm
Now
This combo,with a powerpeak at 5200 will not get you any bragging rites as to hp and tq. It will not get you a big trapspeed or a low-ET.
What it will do, is make a fabulous hi-torque streeter, that for it's displacement, gets fabulous fuel mileage. Tune it up right, and that will be something to brag about. If you can make the tires stick, you could make a really strong zero to 60mph number, and with a little more gear,I think I can see a little air under your front tires.
So, now, about that 5200 power peak, what say you?
I'll tell you a little secret.
The 360, will for you,hit all these targets too. With just a little less tirespin,lol, and a little more mpgs. I know because I already built this combo.
And I have the recipe.
