Need help with gear choice

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rlk211

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Location
Murfreesboro,TN
Car info
73 Scamp
360 Magnum stock internals
Hedman Headers, Air Gap intake and Eddy 600 carb
2800 stall converter in 727
26" tires

I have a 8 1/4 with 2.45 gears now

I just pulled a 8 1/4 (27 spline) out of a dakota that is suregrip with 3.90 gear. I also have a set of 3.21 gears that I could use. The gear I have now is a dog out of the hole and I have converter slippage. I mainly cruise around town but occasionally on the interstate for a few miles. The dilemma is I don't want to overheat the tranny but I don't want the rpm's thru the roof either. What would you guys do?

BTW I know the carriers are different and I don't have 3.55s so it will be 3.21 or 3.90
 
With those two choices, I would use the 3.90. The RPMs will be high, but the 3.21 is not a good match for that converter, either. The 3.90 is. If the engine is stock, why do you even have such a converter in the first place? It would run much better without it.
 
StrokerScamp said:
With those two choices, I would use the 3.90. The RPMs will be high, but the 3.21 is not a good match for that converter, either. The 3.90 is. If the engine is stock, why do you even have such a converter in the first place? It would run much better without it.
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Thanks for the reply! To be completely honest I am cheap. I needed a converter to get the car on the road and a friend sold it to me for $50. I do plan on replacing the stock cam sometime this coming winter.
 
Since you're the one that's going to live with it, you make the choice. Obviously you know the 3.90 is going to launch you off the line better, the 3.21 is going to be more highway friendly. Either is going to give you better acceleration than what you have now.
Here's some information for you to consider. :read2:

A 235/60-15 tire properly mounted and inflated is 26.1" high. With this as the set-up, the following is true:
3.21:1 gear, engine rpm @ 60 mph = 2480, engine rpm @ 70 mph = 2894.
3.90:1 gear, engine rpm @ 60 mph = 3013, engine rpm @ 70 mph = 3516

If you accept a 5500 rpm red line on the engine, the following is true:
3.21 gear @ 5500 rpm, vehicle speed is 133.05 mph.
3.90 gear @ 5500 rpm, vehicle speed is 109.51 mph.

Top speed data assumes no aerodynamic drag. Hope you find this useful.
 
rlk211 said:
Will I notice that big of a difference between the 2.45 and 3.21?
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I think a 30% difference would be noticeable. 8)

rlk211 said:
Will the converter be alright with a 3.21?
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Off hand, no. A stock converter will be better in your application.

A good cam for this axle and the rest of the setup is going to have 195-214º duration at .050". The power band should reun in the 1500 - 4000 rpm range, with torque from 1500 - 3500. Your converter isn't ideal for this kind of thing.

IMO, and from other posts, it is easy to over cam and over carb the LA. Smaller numbers on these might not be good for bench racing, but will give you better throttle response on the street. (It's cheaper, too.)
 
2 Darts said:
I think a 30% difference would be noticeable. 8)

Off hand, no. A stock converter will be better in your application.

A good cam for this axle and the rest of the setup is going to have 195-214º duration at .050". The power band should reun in the 1500 - 4000 rpm range, with torque from 1500 - 3500. Your converter isn't ideal for this kind of thing.

IMO, and from other posts, it is easy to over cam and over carb the LA. Smaller numbers on these might not be good for bench racing, but will give you better throttle response on the street. (It's cheaper, too.)
Click to expand...

Thanks 2 Darts. After doing a little more thinking, I going to go with the 3.90 gears for now. If it is to much I will replace them later. Hopefully that will work better with that converter. What would be a good cam to run?
 
Your converter will be fine with either the 3.21 or 3.90. Just make sure you have a good cooler. Not familiar with any hyd roller cams so i can't really recommend anything other than to say something in the 220 at .050 range. A member here(joedust451 i think) has a good combo worth copying.
 
I have a very simular build and a 3.21 8 1/4. I would go with the 3.90. Mine is a dog off the line. Which is where I like to have the power, not on top end.
 
forphorty said:
Your converter will be fine with either the 3.21 or 3.90. Just make sure you have a good cooler. Not familiar with any hyd roller cams so i can't really recommend anything other than to say something in the 220 at .050 range. A member here(joedust451 i think) has a good combo worth copying.
Click to expand...

Correct...Your converter is mild and fine, i run a dynamic 9.5" 4.500rpm stall with 3.55 and 28" tires. BTW a stock 70/71 10.5" 340 converter would stall at about 2200rpm if the engine was running good back in the day.
 
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