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Re: What Ford 9" rear-end ratio should I go with?
ColbyG #1418663 Thu Jul 29 2021 07:05 PM
Joined: Feb 2004
Posts: 23,001
H
'Bolter
Craig has done an excellent job of explaining torque, RPM, and why lots of people end up building a rig with totally incorrect gear ratios for the kind of driving they're expecting to do. "Lugging" happens when an engine is unable to accelerate when the throttle is opened wider because the torque the engine is developing can't overcome the drag caused by the weight of the vehicle, or the steepness of the hill it's trying to climb, etc. At that point, the driver needs to select a "lower" gear to get the engine back into the proper torque range to handle the load. Today's modern automatic transmissions with 8, 10, or more speeds allow the computer to figure out the proper level of RPM and torque to make the engine run most efficiently for the operating conditions and select the proper gear to keep the engine in its desired torque band. The person driving a manual transmission vehicle has to do that by experience, or "seat of the pants" driving, and it's easy to engage the wrong gear for the conditions at hand.

My father had a favorite saying- - - -"automatic transmissions are smarter than all drivers, and most mechanics!" That was true back in the 1950's and 60's when he was providing us kids with a place to live and food to eat by being the first guy in Nashville Tennessee to work on automatics, and it's definitely true today. Manual transmissions are entertaining toys for the guy who just has to do something with his right hand while driving other than scratch his personal equipment, but they're not the best at keeping the engine in the proper RPM and torque range.

A transmission with two overdrive ratios available where the driveshaft is turning faster than the engine will lend itself to overloading and lugging the engine down very easily. Don't go too "high" with the rear end ratio (numerically low, like 3.08- - - -2.73, etc. ) or you'll end up with an engine that's almost always running too slow to develop enough torque to pull the load. Performance (and gas mileage) will suffer terribly!

Dang it! I just fat-fingered a key on my laptop and erased two paragraphs! I was explaining how something as simple as changing the height of the rear tires on my daily driver (a Ram 1500 that gets 1k+ miles a week in several states) made a huge difference in being able to keep the transmission from constantly shifting in and out of OD and keeping the torque converter locked in. It also keeps me from getting speeding tickets, as the former 2K RPM "sweet spot" for cruising with the taller tires was 75-80 MPH! Now it's 65-70. I also get better gas mileage with the engine turning a little faster and staying in the proper torque band more of the time without being lugged down.
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 #1418671 Thu Jul 29 2021 07:49 PM
Joined: Jan 2020
Posts: 695
D
'Bolter
Ahhh, Fox, You and I have had a lot of experience with bicycle racing. A very good analogy, in my younger years I pushed a 52 front with a 11 tooth rear. This was track bike racing in Chicago (a track bikers dream), my knees are still in fair shape which is a miracle. I broke my back in Dec. but, I'm still training to compete in the Spring Senior's race in 2022. You have to keep going. PM me I would love to hear your history. Ed


Currently making 1954 3100 better than new and Genetics
Re: What Ford 9" rear-end ratio should I go with?
ColbyG #1418680 Thu Jul 29 2021 09:20 PM
Joined: Feb 2004
Posts: 23,001
H
'Bolter
Back to the original question- - - -your 350's best torque range is going to be determined by the camshaft profile and the compression ratio of the engine. A "torquer" engine is going to have fairly short time when the valves are open on each cylinder (called "duration") and a moderately high "lift", the amount of distance the valve moves from its closed position on the cylinder head, and its fully open position that allows the gases to move in and out of the cylinder. Cams with a duration of 270 degrees of crankshaft rotation or less on the intake and exhaust valves will have a smooth idle, plenty of low sped and midrange torque and good all-around performance at variety of engine speeds. As the duration and lift increases, the power band shifts toward higher speeds and low and medium speed power diminishes. The two biggest mistakes rookie engine builders make is "Too much cam" and "Too much carburetor". Doing race engine stuff on a street engine is an exercise in futility, no matter how the hotrodders protest that "It runs great"- - - - -compared to what? For a long time, Chevy dealers offered a new buyer a free STD- - - -so they could have something that "runs"!
smile
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 #1418688 Thu Jul 29 2021 10:38 PM
Joined: Apr 2005
Posts: 2,837
C
'Bolter
Years ago I stopped using high and low to describe gears. With ten people five would call 4.11's high geared and five would cal them low so I switched to calling them shallow or deep geared. With a 2nd overdrive of .50 and nothing radical engine wise I would go with a minimum of 4.11 and probably 4.56. Look up the rpm your engine is at maximum torque and then choose a gear that will hold it there at the speed you desire.


Evan
Re: What Ford 9" rear-end ratio should I go with?
ColbyG #1418734 Fri Jul 30 2021 12:06 PM
Joined: Mar 2007
Posts: 1,271
T
Crusty Old Sarge
As Evan and Jerry have said torque is what moves you down the road, horse power just gets you there quicker. My 327 was rated at 320 ft lbs of torque @ 2400 RPM, so a 3.08 puts it in it's sweet spot. A 3.42 would give my better off the line power but my cruising RPM would be higher, it's a give and take sometimes.


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?
TUTS 59 #1418857 Sat Jul 31 2021 11:20 AM
Joined: Jul 2008
Posts: 363
S
'Bolter
Reminds me of another circle track saying: Horsepower determines how fast you can hit the wall; Torque determines how far you can get through the wall.

SimS

Re: What Ford 9" rear-end ratio should I go with?
ColbyG #1418896 Sat Jul 31 2021 05:44 PM
Joined: Mar 2014
Posts: 2,475
J
'Bolter
I agree with Evan but I'd go with 4.56...with a little hesitation

With a 2.66 first and a .50 6th gear that's the only rear end ratio I can see working anywhere close to right. Unless you could find 4.71 gears. Those would be closer and once upon a time there were 4.86 gears for the 9 inch. 4.86 would be almost perfect in my opinion.

You can use the Tremec calculator (or there are about 100 other ones out there). Here's Tremec's:

https://www.tremec.com/calculadora.php

Plug in your desired rpm (I used 1900 and I believe that is an okay target in your case--2000 would be better for hills and wind), your tire diameter (I used 28), your axle ratio (I used 4.56) and the ratio for every gear in your transmission (you already posted those). With all the factors you mentioned and using a 4.56 rear at 70 mph you'll be turning 1900 rpm in 6th. That in my opinion is the lowest rpm you need to consider with a carbureted 350 and an AD pickup.

If you used 4.86 gears with 28" diameter tires, your rpm at 70 would be apx 2050 and at that point you're right where I think you need to be. However your starting line ratio would be 12.92 and that might be more than you want. However the original AD starting line ratio was 12.08, though so you are not too far off (that is in case you ever wanted to use it like a truck). If you don't care about using it as a pickup your ratio could run down to 10.5 or so. If you plan on putting 1000 pounds of "stuff" in the bed and driving without burning up your clutch, you ought to at least have an slr of 11.

If you want more reasons I think this is what you should use, please let me know.


Jon

1952 1/2 ton with 1959 235
T5 with 3.07 rear end
Re: What Ford 9" rear-end ratio should I go with?
ColbyG #1418987 Sun Aug 01 2021 01:00 PM
Joined: Feb 2000
Posts: 4,363
J
'Bolter
Those first four gears are going to go by really fast with a 4.56 rear ratio, you may not even need the first two gears. So you can gear it with low starting gears ratios, or low high gear ratios, you have to decide what type of driving you are going to be doing the most, or how much shifting you really want to be doing.

Re: What Ford 9" rear-end ratio should I go with?
ColbyG #1419175 Mon Aug 02 2021 08:51 PM
Joined: Apr 2005
Posts: 4
4
Moderated
I have my truck set up with similar components except the motor is a stock gen II LT1 with fuel injection and rear tires are 26". Rear gear is a 3.50 and that is just right. First is not too low and 6th is a highway only gear. With your 28 or 29 rear tires 3.9 or 4.11 seems about right.

Re: What Ford 9" rear-end ratio should I go with?
ColbyG #1420346 Thu Aug 12 2021 05:20 AM
Joined: Dec 2014
Posts: 815
M
'Bolter
3.73 will make you happy - good acceleration around town, but easy on the RPMs when you're in OD.


1951 Chevy Panel Truck
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