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What Ford 9" rear-end ratio should I go with?
#1418501 Wed Jul 28 2021 02:36 PM
Joined: Dec 2020
Posts: 37
C
ColbyG Offline OP
'Bolter
Howdy!

I am putting a SBC 350 (it was bored out 30 over by the previous owner), a Borg Warner T56, and a Ford 9" rear-end in my 1951 Chevrolet 3100. It's all going on a 1987 Chevrolet S10 frame.

The T56 gears are 2.66, 1.78, 1.30, 1.00, 0.74, and 0.50.

I plan to have 20" rims on it with tires of a 28" or 29" diameter.

I need to rebuild the Ford 9" and I am wondering, what ratio you guys would recommend I put in the rear-end?

I don't plan on racing it, but making it a cruiser that will have a little bit of get-up to it if necessary. I would also like to have decent gas mileage. As we all know, it's not cheap right now, so that is somewhat important. Being those things said, I was thinking a 3.73, but wanted to get y'all's opinions before moving forward.

Any advice?


colby

1951 Chevrolet 3100: Photos [photos.app.goo.gl]
–Engine: 219 to 250 to 350 (waiting to be dropped in)
–Currently: Readying a 1987 S10 frame for mounting the body.

1956 Chevrolet 6400
–Engine: moved the 250 from 3100 - newly installed
–Currently: Adjusting Brakes & Installing New Headlamp Switch

1963 Chevrolet C10 Fleetside
–Engine: EMPTY
–Currently: Sitting in Barn
Re: What Ford 9" rear-end ratio should I go with?
ColbyG #1418509 Wed Jul 28 2021 04:03 PM
Joined: Feb 2004
Posts: 23,018
H
'Bolter
HMMMMM- - - - -big rim diameter, Band-Aid tires, and two overdrive ratios- - - - - -I'd suggest a rear end gear no higher than a 3.73, and a 3.90 or 4.11 would probably be better. Getting the final drive gear too high will result in only being able to use the OD on the Bonneville salt flats, or somewhere else where you've got miles to accelerate, no hills, and no speed limits.
Jerry


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: What Ford 9" rear-end ratio should I go with?
ColbyG #1418513 Wed Jul 28 2021 04:20 PM
Joined: Mar 2007
Posts: 1,307
T
Crusty Old Sarge
I would imagine it all comes down to what RPM power band your cam develops. If the gear is to tall it will lug the engine, to low and it works to hard. Use an RPM calculator to help find that sweet spot everyone looks for .

https://spicerparts.com/calculators/engine-rpm-calculator

https://spicerparts.com/calculators/transmission-ratio-rpm-calculator

Last edited by TUTS 59; Wed Jul 28 2021 04:25 PM.

Craig

Come, Bleed or Blister something has got to give!!!
59' Apache 31, 327 V8 (0.030 over), Muncie M20 4 Speed, GM 10 Bolt Rear... long term project (30 years and counting)
Re: What Ford 9" rear-end ratio should I go with?
ColbyG #1418579 Thu Jul 29 2021 03:14 AM
Joined: Dec 2020
Posts: 37
C
ColbyG Offline OP
'Bolter
TUTS_59, how do I figure out more about the power band / cam? I actually know nothing about that.

And I don’t own any gasoline vehicles, so what RPM should I be shooting for on a SBC 350 around 70MPH?


colby

1951 Chevrolet 3100: Photos [photos.app.goo.gl]
–Engine: 219 to 250 to 350 (waiting to be dropped in)
–Currently: Readying a 1987 S10 frame for mounting the body.

1956 Chevrolet 6400
–Engine: moved the 250 from 3100 - newly installed
–Currently: Adjusting Brakes & Installing New Headlamp Switch

1963 Chevrolet C10 Fleetside
–Engine: EMPTY
–Currently: Sitting in Barn
Re: What Ford 9" rear-end ratio should I go with?
Hotrod Lincoln #1418581 Thu Jul 29 2021 03:17 AM
Joined: Dec 2020
Posts: 37
C
ColbyG Offline OP
'Bolter
Hotrod_Lincoln, you said you’d suggest “a rear end gear no higher than a 373” then you said “a 3.90 or 4.11 would probably be better.” Was there a typo in your original reply or am I reading that wrong? Thank you.


colby

1951 Chevrolet 3100: Photos [photos.app.goo.gl]
–Engine: 219 to 250 to 350 (waiting to be dropped in)
–Currently: Readying a 1987 S10 frame for mounting the body.

1956 Chevrolet 6400
–Engine: moved the 250 from 3100 - newly installed
–Currently: Adjusting Brakes & Installing New Headlamp Switch

1963 Chevrolet C10 Fleetside
–Engine: EMPTY
–Currently: Sitting in Barn
Re: What Ford 9" rear-end ratio should I go with?
ColbyG #1418588 Thu Jul 29 2021 04:53 AM
Joined: Apr 2002
Posts: 1,951
F
Fox Offline
A teacher, but always an apprentice.
No, it’s not a typo. Gear ratios can confusing for some. Think of a bicycle back gear. The little gear spins VERY FAST when you’re in that gear = high speed.
The big back sprocket= power.


Put it in low! You want power and a big gear, hence a low ratio.


Remember this:

Low and slow= power...a big back gear ratio(further away from 1:1 ratio) —-4.11 : 1, 4.56: 1a low (slow and powerful) gear ratio, but a higher number. The low can be thought of as power.

High and fast= speed...a small back gear ratio (closer to 1:1 ratio)—-2.73 :1 , 3.23 : 1, 3.41: 1 ratio... a high(fast) gear ratio but a smaller number. The “high” can be thought of speed.

That’s why he mentioned racing at the Bonneville. Some of those cars can’t get going by themselves because they have ratios that are high. No power, but LOTS of speed.

Heed Jerry’s words, you’d be wise to!

Confused much?

Last edited by Fox; Fri Aug 13 2021 04:40 AM.

In the Stovebolt Gallery [stovebolt.com]
More pictures here [photos.app.goo.gl]

1951 GMC 9430 1 ton dually—-Shiny!

1970 Chevrolet C10 - Grandpa’s- My first truck.—in progress to shiny
1972 Chevrolet C20- Rusty- the puzzle box lid for the C10.
1950 Chevrolet 1300- in progress to shiny.
1962 AMC Rambler American- my wife’s

Parts trucks-
1951 GMC 9300
1951-GMC 9430
1951- Chevrolet 1300
Re: What Ford 9" rear-end ratio should I go with?
TUTS 59 #1418589 Thu Jul 29 2021 05:01 AM
Joined: Apr 2002
Posts: 1,951
F
Fox Offline
A teacher, but always an apprentice.
[quote=TUTS 59]I would imagine it all comes down to what RPM power band your cam develops. If the gear is to tall it will lug the engine, to low and it works to hard. Use an RPM calculator to help find that sweet spot.


Think of the bike analogy again:

If you try to climb a hill with your chain on the little back sprocket (the speed gear) your legs are going to DIE because it’s so hard (engine lugging)!

If you try to go as fast as you can with the chain on the big back sprocket, you are going to be pedalling like a son of o’ gun and won’t be moving quickly at all (ENGINE SCREAMING)!


In the Stovebolt Gallery [stovebolt.com]
More pictures here [photos.app.goo.gl]

1951 GMC 9430 1 ton dually—-Shiny!

1970 Chevrolet C10 - Grandpa’s- My first truck.—in progress to shiny
1972 Chevrolet C20- Rusty- the puzzle box lid for the C10.
1950 Chevrolet 1300- in progress to shiny.
1962 AMC Rambler American- my wife’s

Parts trucks-
1951 GMC 9300
1951-GMC 9430
1951- Chevrolet 1300
Re: What Ford 9" rear-end ratio should I go with?
ColbyG #1418615 Thu Jul 29 2021 12:41 PM
Joined: Mar 2007
Posts: 1,307
T
Crusty Old Sarge
ColbyG,
it would certainly help if you knew what camshaft you had in the engine. Camshafts are developed for different load ranges across a given RPM band, you wouldn't want a race developed cam in a truck used for towing. Typically a SBC starts to make power at 1500-1800 RPM, with a midrange power band of 2400-3000 RPM. With an overdrive as high as you are considering it could drop the engine down to 1600-1800 RMP at speed. This setup would be fine on a flat open Interstate, now through in a few hills and your doing a lot of down gearing.

As an example, my 327 was built with a higher RPM cam (2200-6400 RPM), with a 3.08 rear and 27 inch and final drive of 1.00 my RPM at 65 MPH would be around 2500 RPM, change the rear ratio to a 3.42 and now were at 2750. Keep in mind that I built my engine for around town (traffic light to traffic light) and an occasional trip to our local drag strip.

The key to this is knowing what RPM range your engine was built for. It's time for HRL (Jerry) to come in and provide some knowledge.

Last edited by TUTS 59; Fri Jul 30 2021 12:07 PM.

Craig

Come, Bleed or Blister something has got to give!!!
59' Apache 31, 327 V8 (0.030 over), Muncie M20 4 Speed, GM 10 Bolt Rear... long term project (30 years and counting)
Re: What Ford 9" rear-end ratio should I go with?
ColbyG #1418617 Thu Jul 29 2021 12:56 PM
Joined: Feb 2000
Posts: 4,373
J
'Bolter
It takes a lot of torque to push a brick at 70 mph, your power band needs to peak at the rpm you are planning to run in while pushing said brick at 70 mph. Don't look at the horse power, look at where to torque is. I would want the motor be in 1800 to 2000 rpm range at highway speeds, so 3.73 - 3.90 - 4.11 would be about right. Carburetors also don't like low rpm in heavy trucks at 70 mph, the air velocity is pretty low so pulling the required fuel is pretty tuff. If you had a 454 or 500 inch Cadillac, then you could run around all day at low rpm due to the torque.

Re: What Ford 9" rear-end ratio should I go with?
ColbyG #1418640 Thu Jul 29 2021 04:23 PM
Joined: Mar 2007
Posts: 1,307
T
Crusty Old Sarge
Here's a list of Chevy 350 "Stock" torque ratings. Just food for thought.

Year HP@rpm Torque@rpm(ft. lb.)
1975 145@3800 250@2200
'76-'77 145@3800 250@2200

'78-'79 170@3800 270@2400 350 V8 w 4 bbl

Corvette w 4 bbl (from Chilton's 1963-1982 Repair Manual)
1969 300@4800 380@3200
1970 300@4800 380@3200
1971 270@4800 360@3200
1972 200@4400 300@2800
1973 190@4400 270@2800
1974 195@4400 275@2800
'75-'76* 165@3800 255@2400
1977 180@4000 270@2400
1978 185@4000 280@2400
1979 195@4000 285@3200
1980 180@4200 255@2000
1981 190@4200 280@1600
1982 200@4200 285@2800 (TBI)


There were of course factory performance options so these are not the only data specs out there. Many of the later year 350's had more power as fuel injection became the norm.


Craig

Come, Bleed or Blister something has got to give!!!
59' Apache 31, 327 V8 (0.030 over), Muncie M20 4 Speed, GM 10 Bolt Rear... long term project (30 years and counting)
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