Discussion in 'Mopar Performance Issues' started by loganscuda, Jan 10, 2019.
Thanks just got me a new hood have not painted yet.
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 going into 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. For direct 10% less gear will get you 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 For overdrive Adding 20% to use .69od, 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, and the accompanying power drop. 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 and od to get 5770@5% slip. To get into overdrive your engine might like say 6300@124mph in direct,and that would drop to 6300x.78=4900 into 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)=MPH=144.99 >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 formula with your current trans, just stick in your own numbers.
Where do you buy the hood from?
I went a buck 50 in my 440 powered 71 Charger in the early 80's on interstate 90 in Washington state, tried it in the 67 fastback and gave up at a buck 15 after a slight wisp of wind from the wrong direction made the car want to steer by itself.lol
Thanks professor, this is exactly what I was looking for. I looked for a calculator on line but was confused by a few of them. Thanks for well thought out calculations and information.
I have to check that out, I had 2 AAR's bought the first one in 76 the other in 83 sold both about 5 years ago, wish I had them back.
That sounds a lot like a speed-limiter issue in the engine computer, even with a stock 5.7 Hemi the car would literally start to fly before you'd be overcome by drag. I've had my Duster up to 110 a couple times and there was still plenty left with 2.94 gears in the back, I was barely getting into the secondaries IIRC, definitely not flooring it for sure and it was "only" a 360 with about 320 HP. I'd argue the front end on those cars are even less aerodynamic than a Cuda like yours, it's totally flat and the top of the grille is canted forward slightly with nothing below the front bumper like a valance panel (is that what it's called?). This Spring/Summer I want to see about getting my Duster into 1/2-mile racing (and road racing) so I'll be trying to improve the aero for sure. And this time around it'll be a hotter 360 with closer to 450 HP.
The factory computers limits the MPH to the tire rating, if the tires are rated to 110 MPH then the computer limits the car to that speed, you have a aftermarket controller so I doubt that is the problem.
I have factory ECU but have never seen a tire rating for ECU only tire size
ECU's are programed to tire speed rating that the car left the factory with.
I was gonna say, how does the computer know what type of tires are on the car when you change them? Lol
An air dam under front end will reduce drag and make more stable. See thread Awesome A-body air dam for under $30!
Definitely getting one of those for my car, that is indeed awesome thanks!
Separate names with a comma.