Bench racing - 3.23 VS. 3.91 gears in stock '70 318 car with a 3-speed manual.

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Alaskan_TA

Alaskan_TA

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Low mile stock car other than an electronic ignition conversion.

I have not seen a vintage type 'track test' on this engine & transmission combination, so I want to try it.

Advertised curb weight is 3025 pounds.

Rear tires are 25" tall, both gear sets are Sure-Grips & driver has decades of experience shifting a manual box.

So how much do you think a simple gear swap will change the ET & overall speed in the quarter from one gear set to the next?

Any predictions on each one?
 
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That teener is gonna trap around 84mph right? and the trans ratios IIRC are 3.02-1.76-1.00
Well with 25s, and 3.23s, 84 is;~3660 in direct,~6420 in 1.76 second gear, so you gotta use direct to trap with.
But with 3.91s second is way too high, and direct is 4420, close to perfect with the stock cam.

So the 3.91s are gonna go thru the power peak three times, while the 3.23s are just climbing the curve in third .
At the other end, the starter gears are 3.23x3.02=9.75 versus 3.91x3.02=11.81, so the teener is gonna blast thru the 60ft with 21% more authority, if you can work the gas and clutch just right.
OK so 3025 is about 3250 on the start line and if the 318 is making 150 hp, your P/W is 3250/150=21.7 and the mph for that is 84.
But with passing thru the power peak three times, Ima thinking the average power will increase enough to trap at say 86 mph.
Super stock unlimited chassis times for these numbers are; 15.5 and ~15.1.. Figure your street chassis on street tires to add about a second or a bit more.
I'm just guessing.
 

I was not planning on using first gear.

I still need to find the 'sweet spot' RPM for dumping the clutch without tire spin in this car.

As a guess, 1600 to 1800 RPM in second. I'll be hunting for the raw edge when the time comes, 'chirp & go.........'.
 
Alaskan_TA said:
I was not planning on using first gear.

I still need to find the 'sweet spot' RPM for dumping the clutch without tire spin in this car.

As a guess, 1600 to 1800 RPM in second. I'll be hunting for the raw edge when the time comes, 'chirp & go.........'.
Click to expand...
I'm missing something...… It's a 3 speed manual and you are leaving the line in 2nd gear???
 
Not yet, but I'm certainly willing to try it when I start hunting for the sweet spot for the launch.

I had a 4-speed 340-6 & if I launched in first, all it did was bake the tires. Launching in second, one quick chirp & then it hooked.

Every car is different though, so I'll have to learn what works best with this combination.
 
Well, if your planning on killing the gear by taking off in 2nd so you don't spin, then throw the gear swap out the window. Get some tires that will hook and leave the line in first gear...… lol
 
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If this is a 69-71 318 with 230 Hp and a good tune I doubt tire spin will be an issue. If you go to big on the tire it will slow you down. A set of good modern ( Michelin ex..) 205 / 70 14s will give you all the traction you need without the excess rotating mass.
My guess is high 14s @ 89ish mph with the 3.91s
 
mbaird said:
If this is a 69-71 318 with 230 Hp and a good tune I doubt tire spin will be an issue. If you go to big on the tire it will slow you down. A set of good modern ( Michelin ex..) 205 / 70 14s will give you all the traction you need without the excess rotating mass.
My guess is high 14s @ 89ish mph with the 3.91s
Click to expand...
If it's a 318 2bbl, with stock cam and exhaust manifolds, 14's will be a stretch... even with 3.91's
 
I am being optimistic. The manual should help it.
I have had some 318 cars that were pretty strong.
 
I've got a couple time slips that might help. When I first bought my 71 Dart it had it's original 318 (worn out and smoking), with an edelbrock performer intake, headers with 2.5 exhaust and a 625 CFM Carter AFB.

1st Time Slip - 15.537 @ 88.45 - This was with the stock 7 1/4 with 2.76 gears and worn out street tires (225/70/14) Spinning off the line.

2nd Time Slip - 15.224 @ 86.87 - This was with 8 3/4 rear 3.91 sure grip and 275/60/15 drag radials (0 Tire Spin)

Not sure if this even helps with your question because 1st run spun tires, and 2nd run was dead hook (Engine didn't have the power to spin the drag radials LOL)
 

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