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1971 Chevelle. Carb’d 6.0 LS Th350 3.90 12 Bolt. 1972 Greenbrier Wagon 489, 700r4 3,73 12 bolt
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Discussion Starter · #1 ·
Getting ready to order an S60 for the Chevelle. Wondering what gear ratio I should pick...here is what is available:

Gear Ratio Selection
3.54 / 3.73 / 4.10 / 4.30 / 4.56 / 4.88 / 5.13 / 5.38

I'm currently running 3.90 in the 12 bolt....275/60/15 tire....4500 stall nitrous converter, 150-250 shot of nitrous.....Havent got to run the car yet with the 383 but it's in the car now. More strip than street....was thinking either the 3.73 or 4:10....

Thanks for any thoughts.
 

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How fast are you looking to go? Figure out what your MPH will be for your ET and gear it for that.
 
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Is the 650 the cfm recommended? What rpm do you think your going to shift at? The calculators can get you in the ball park. Ray is right gear to MPH.
 

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Were is peak power made?
 

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I would go for the 4.10.
You have good heads and a large cam and it should RPM if everything is right.
Between the 3.73 and 4.10 there will be about 500 rpm difference at 110 mph.

Math 336 is a given.
336 X Gear Ratio X MPH then Divide by Tire diameter = RPM Allow something for converter slippage.
10% would be very safe I would think.

With 28" tire and 120 MPH you would be 5900 rpm or so with the 4.10 then allow for slippage.

Still pretty conservative gear.
Your car is heavy and probably needs all the help you can give it.
Buddy borrowed my 3.70's for his 9" and 355" with the flat tappet 292H.

After about 2 weeks he wanted his 4.56's back.
I really like the 4.56 gear for the small block that see's 6800-7500 rpm.

But if you have issues with traction taking some gear out may help it.
 

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1971 Chevelle. Carb’d 6.0 LS Th350 3.90 12 Bolt. 1972 Greenbrier Wagon 489, 700r4 3,73 12 bolt
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3,280 Posts
Discussion Starter · #6 ·

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1971 Chevelle. Carb’d 6.0 LS Th350 3.90 12 Bolt. 1972 Greenbrier Wagon 489, 700r4 3,73 12 bolt
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3,280 Posts
Discussion Starter · #7 ·
Is the 650 the cfm recommended? What rpm do you think your going to shift at? The calculators can get you in the ball park. Ray is right gear to MPH.
No thats not the recommended carb, just what I had running on the previous 355. It's pretty small for this 383....going to get with jmarkaudio and have him put one together....thinking something like a 950HP would be good..

Probably shift around 7000 area....
 

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1971 Chevelle. Carb’d 6.0 LS Th350 3.90 12 Bolt. 1972 Greenbrier Wagon 489, 700r4 3,73 12 bolt
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3,280 Posts
Discussion Starter · #8 ·

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1971 Chevelle. Carb’d 6.0 LS Th350 3.90 12 Bolt. 1972 Greenbrier Wagon 489, 700r4 3,73 12 bolt
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3,280 Posts
Discussion Starter · #9 ·
I would go for the 4.10.
You have good heads and a large cam and it should RPM if everything is right.
Between the 3.73 and 4.10 there will be about 500 rpm difference at 110 mph.

Math 336 is a given.
336 X Gear Ratio X MPH then Divide by Tire diameter = RPM Allow something for converter slippage.
10% would be very safe I would think.

With 28" tire and 120 MPH you would be 5900 rpm or so with the 4.10 then allow for slippage.

Still pretty conservative gear.
Your car is heavy and probably needs all the help you can give it.
Buddy borrowed my 3.70's for his 9" and 355" with the flat tappet 292H.

After about 2 weeks he wanted his 4.56's back.
I really like the 4.56 gear for the small block that see's 6800-7500 rpm.

But if you have issues with traction taking some gear out may help it.
Thanks Jeff...I'm going to work on the weight some, nothing too crazy just the basics....lighter wheels, fiberglass hood (need a bigger cowl anyway) that sort of stuff. The 4:10 was what I was thinking of going with...wasnt sure if less gear would be better with nitrous or not...just driving it around the 3.90's don't quite seem to be enough....but I also don't have alot of timing in it yet...need to get some better fuel first to be safe.
 

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4.10 if more strip than street NA, you can always bring it down with a taller tire. On a large shot of nitrous maybe 3.73 or less.
 

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For a more strip than street and shifting at 7k I would consider the 430 or 456 . I run a 456 with 28" tire both on the street and the track, with a decent converter I go thru the lights around 7 k at 126.

Brian
 

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If working on losing some weight, why not keep the 12-bolt? Built right, it should be plenty strong enough for a hot small block and lighter than a Dana 60.
 

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1971 Chevelle. Carb’d 6.0 LS Th350 3.90 12 Bolt. 1972 Greenbrier Wagon 489, 700r4 3,73 12 bolt
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3,280 Posts
Discussion Starter · #13 ·
4.10 if more strip than street NA, you can always bring it down with a taller tire. On a large shot of nitrous maybe 3.73 or less.
I'm not sure if I can fit a taller tire under there...mine sits kinda low.
For a little nitrous motor, I would go 4.30 or 4.10. Nitrous makes tq, but still have wight and a small motor that needs some "help".
I wasn't sure with the nitrous...maybe the 4.30's wouldn't be bad...had 4.10's in my first Chevelle, but I think the tires were pretty short....

For a more strip than street and shifting at 7k I would consider the 430 or 456 . I run a 456 with 28" tire both on the street and the track, with a decent converter I go thru the lights around 7 k at 126.

Brian
How are the 4.56 on the street? The farthest I tend to drive is to Norwalk, bout an hour and a half give or take. I have a PTC converter that was gone thru and loosened alittle by a local guy...see how it works in a few months..
 

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1971 Chevelle. Carb’d 6.0 LS Th350 3.90 12 Bolt. 1972 Greenbrier Wagon 489, 700r4 3,73 12 bolt
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Discussion Starter · #14 ·
If working on losing some weight, why not keep the 12-bolt? Built right, it should be plenty strong enough for a hot small block and lighter than a Dana 60.
I went over beefing up my 12 bolt and by the time I have the ford ends put on it, buy axles, pinion yoke etc it's not that much more to just buy the S60. Figure I can sell my12 bolt and cover a lot of it. The S60 is supposed to only be about 25 lbs heavier and going with a lighter , better driveshaft should even it out. I'm kinda planning for the future too :thumbsup:
 

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I went over beefing up my 12 bolt and by the time I have the ford ends put on it, buy axles, pinion yoke etc it's not that much more to just buy the S60. Figure I can sell my12 bolt and cover a lot of it. The S60 is supposed to only be about 25 lbs heavier and going with a lighter , better driveshaft should even it out. I'm kinda planning for the future too :thumbsup:
I still use a 12 bolt.
 
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1971 Chevelle. Carb’d 6.0 LS Th350 3.90 12 Bolt. 1972 Greenbrier Wagon 489, 700r4 3,73 12 bolt
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Discussion Starter · #17 ·

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1971 Chevelle. Carb’d 6.0 LS Th350 3.90 12 Bolt. 1972 Greenbrier Wagon 489, 700r4 3,73 12 bolt
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Discussion Starter · #18 ·
I would consider discussing it with the folks who built the convertor as well to get a total picture before making the final decision.
With my old converter and 3.31's it stalled to 4000....when going to the 3.90's the stall dropped to 3800. New converter stalls at 4500 without spraying, should go up some with the nitrous I think...with 4.10's probably not much difference? Maybe the 4.30's would drop it some?
 

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What all is done to it?
Its got 33 spline Moser Axles with 5/8" studs, Strange spool, Richmond 4.33 pro gear, and Strange Billet caps. The axles have C-clip eliminators. The axles and spool are 13 years old.
 

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With my old converter and 3.31's it stalled to 4000....when going to the 3.90's the stall dropped to 3800. New converter stalls at 4500 without spraying, should go up some with the nitrous I think...with 4.10's probably not much difference? Maybe the 4.30's would drop it some?
Sometimes hard to determine and convertor testing can get expensive...this is why I would consider input from every angle. The guys at PTC will be able to answer many of your questions...just be honest with them about your combo and the tweaking of the unit since it left their shop.

The 12-bolt in our Camaro is a Moser housing w/35-spline axles, 5.13 Pro gear/spool and c-clip elims. The car currently has a 383 SB which runs low 10's on 30x9 radial slicks...convertor and gear are a less than ideal holdover from a former Stock Eliminator set-up.

Current build of a '67 El Camino has the factory 12-bolt narrowed w/late Ford housing ends, 33-spline axles/spool and will sport radial slicks as well, but likely a little shorter. Another 5.13 gear is used in this as it will be a genuine Stock Eliminator set-up initially.



 
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