My "68 Barracuda is very stable at 120/130 ...... in a straight line....lol. no spoilers. If it wasn't illegal to cruise up there, I would.
My math says 130 with 3.73s and 27s is 6024 (zero-slip) in direct, 4156 in overdrive (zeroslip). Estimating and adding 5% for slip, gets you 6325/4365. Will your engine survive that and for how long?
20% more rear gear would get you 20% more rpm in overdrive, so now 5240, and maybe your engine would have enough power at the new rpm to continue accelerating. Or
10% less rear gear would get you 10% less Rs in direct, so now 5700, and maybe your engine would have enough power at the new rpm to continue accelerating. 10% less gear will get you 10% more Rs and so 10% more speed (if the power is there), so adding 10% to 130 you get 143mph.
.9x3.73=3.36s and there ain't such a thing in a vintage Mopar ; that is between gears.
With 3.23s,27s and in direct;145=6110@5%slip
With 2.94s,27s and in direct; 145=5560@5% slip
Adding 20% to use od, we get 1.2x3.73=4.56 , and those we have.
So with 4.56s and 27s,in overdrive;145=5950@5% slip.
And with 4.30s,27s and in overdrive; 145=5612@5% slip
The biggest problem for this exercise is the .69 overdrive ratio. It might be hard to get into it and continue accelerating because of the very large rpm drop;eg; .69x6000=4140.
Consider this; the A998 non loc-up with a GVOD(.78od) with ratios of 2.74-1.54-1.00-.78od ..
With the .78 od; We get .78x6000=4680 which is 540 rpm higher.
145 at 5800@5% slip would take 3.91s to get 5770@5% slip. To get into overdrive your engine might like say 6300@124mph and that would drop to 6300x.78=4900 in overdrive, still a heck ofa job.
To find YOUR optimum ratio;
First determine your engine's peak power, where you want to buzz it up to. Second, subtract the TC slip, and third;here is your formula;
mph= (rpm x Tr)/1056xR1xR2
and finally; calculate the reciprocal of the result
where;
Tr is the Drive-tire roll-out (diameter x 3.1416), R1 is the trans gear,R2 is the od, R3 is the diff.
>In the following example, I picked peak power out of a hat for purpose of calculations, same as the 85" roll-out, and the 5% slip; You will have to sub in your own numbers.
Example; for peak power at 6000 less 5%slip =5700, so
145=(5700x85)/(1056x1.00x.78xR3)
145=484500/823.68xR3
rearranging, we get
R3=145x823.68/484500=.2465 , and the reciprocal of that is 1/.2465=4.057
So 4.06 is the gear you need to hit 6000 at 145 with 27s
to verify;
6000 less 5%=5700, and
(5700x85)/(1056x1.00x.78x4.057)=144.99 mph
>of course there may not be 4.057s available so you may have to choose between 3.91s and 4.10s in this example.
> and you can work the same numbers with your current trans, just stick in your own numbers.
