HP/torque vs rear end ratio

M2M

M2M

Well-known member
So let's say you have a Model A with a rebuillt stock standard 40HP engine that has a 3.78 rear end.

From what I understand, with a 3.78 rear end a Model A will do well in the hills.

Question:
If you take the car above and change the rear end to 3.27 and add to the engine a hi-comp head, B carb, bored inlet manifold and an aries muffler will the extra HP/torque allow the car to climb hills as good as did with the 3.78 rear end and a standard engine?
 
too many variables, including how you're going to adjust your shift points. One thing for sure, none of the modifications mentioned do anything to strengthen the rods or bearings that will have to transmit your extra torque.
 
By my rough calculations, you would need about 15% more power out of your engine in all ranges of RPM to pull the hills as good as it did before modifications. (Or at least at the RPM you are attacking the hills at.) I think you would easily get 6 more HP (or 15%) from the modifications you describe. Comment: A new touring cam would help with the power you desire but would require opening up the engine. I suspect you are trying to avoid opening the engine as all the mods you describe are bolt on and go.
I have dyno records of the modifications you are describing and my opinion is based on my records showing about 15 hp gain with the addition of a mild touring cam and 1.7" intake valves to your list. Check the results from Piranio's Antique Automotives Model A Ford engine Dyno tests. The results will be interesting to you.
http://www.modelaparts.net/dyno.htm/dyno.html
My advice to you if you make these changes...be careful to avoid timing advance beyond 28 degrees as it could take out your bearings/babbitt.

Good Day!
Dave in MN
www.durableperformance.net
 
I feel that the 3.78 gear ratio is best for model A cars regardless of any other mods. Any time that I would drive my A's over 50 mph would be only for a short distance and not worth the loss of low and mid range power that a 3.54 or 3.27 ratio would cause . I don't think that any engine mod that could be made would overcome the power loss that a 3.27 ratio would cause.
 
M2M. I have 31 tudor. 3.27 gear, mitchell od 26%[ you can order different gearing] inserted engine , weber carb, brilery cam,electronic ign, Brumfield 5.9. Been running the 3.27's since 2001 and all the other stuff since 2003. on open road 2900 rpm 96 mph . dyno shows 96 mph @ 2900 rpm, 117 lbs torque. normall 60 mph at 1800 rpm but I can get it to 60 mph at 1760 with different fiddling around. Running hiway at 63-64 mph in 430 miles fuel averages 22.5 . I average about 1200 miles per month. have fun modelAtony Lafayette,la
 
modelAtony said:
M2M. I have 31 tudor. 3.27 gear, mitchell od 26%[ you can order different gearing] inserted engine , weber carb, brilery cam,electronic ign, Brumfield 5.9. Been running the 3.27's since 2001 and all the other stuff since 2003. on open road 2900 rpm 96 mph . dyno shows 96 mph @ 2900 rpm, 117 lbs torque. normall 60 mph at 1800 rpm but I can get it to 60 mph at 1760 with different fiddling around. Running hiway at 63-64 mph in 430 miles fuel averages 22.5 . I average about 1200 miles per month. have fun modelAtony Lafayette,la
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Damn I went 80 in mine once when it had a stock motor, with a 4 speed overdrive. 96mph would have me grinning from ear to ear and crapping my pants at the same time.
 
Thanks for your replies.

I have no doubt the 3.27 will give a more relaxed 60mph and better mpg on flat roads, but I'm more interested to hear what a 3.27 it's like in the hills if you have a mildly strong engine (hi-comp head, B carb, bored inlet manifold and an aries muffler) with say a phaeton or roadster body.

Purdy, you're saying that in the hills the extra 20+% HP and torque you'd get from the above mentioned modifications would not compensate for the 3.27 rearend...how big a difference are we talking about?
 
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Logan said:
Damn I went 80 in mine once when it had a stock motor, with a 4 speed overdrive. 96mph would have me grinning from ear to ear and crapping my pants at the same time.
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Is there any truth to the rumor that at 81 MPH the front end comes off of the ground due to the airplane wing like fenders? I bet 80 MPH felt like it had power steering.

Charlie Stephens
 
M2M said:
So let's say you have a Model A with a rebuillt stock standard 40HP engine that has a 3.78 rear end.

From what I understand, with a 3.78 rear end a Model A will do well in the hills.

Question:
If you take the car above and change the rear end to 3.27 and add to the engine a hi-comp head, B carb, bored inlet manifold and an aries muffler will the extra HP/torque allow the car to climb hills as good as did with the 3.78 rear end and a standard engine?
Click to expand...

It will depend on multiple factors such as weight of the vehicle (body type), tire size, but most important in my pinion is the cam. Reground cams will raise the torque curve a bit requiring you carry loads at higher RPM levels which is less conducive to higher ratios. Just how much will depend on how it was ground but it will make a difference. A well designed new cam with full size lobes is FAR more conducive to what you're proposing.

With that said, attention to timing (spark advance) is far more critical for the longevity and survival of the engine with low end torque and high compression. Some folks are very adept at knowing and reacting instantly by feel if they are in the wrong gear for the circumstance and similarly if the timing is too far advanced or retarded. If this is not one of your attributes I'd be inclined to go with a centrifugal advance distributor.

I strongly regret NOT going with 3.27 gears in one of my cars. My driving habits and personal preferences (and considerable experience) dictate that I only use new cams. Some folks don't share those views/preferences.
 
M2M said:
Thanks for your replies.

I have no doubt the 3.27 will give a more relaxed 60mph and better mpg on flat roads, but I'm more interested to hear what a 3.27 it's like in the hills if you have a mildly strong engine (hi-comp head, B carb, bored inlet manifold and an aries muffler) with say a phaeton or roadster body.

Purdy, you're saying that in the hills the extra 20+% HP and torque you'd get from the above mentioned modifications would not compensate for the 3.27 rearend...how big a difference are we talking about?
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I've never ran the 3.27 gear ratio, so this is more less my opinion. I don't believe that the 20+hp would compensate. It would allow higher top speeds at lower rpm but you have less torque at lower rpm. It wouldn't have nearly the low and mid range power that you would get with the lower 3.78 ratio whether the engine is modified or completely stock. My modified roadster with 3.78 gears will go over the steep hills with ease at 50 mph that would slow my stock 31 sedan to 30 mph. It has power and will accelerate to 65 mph or better , about as fast as a modern car. We like cruiseing at 50-55 mph with power to accelerate and pass if necessary. Some like cruiseing at higher speed with low rpm. It is a matter of choice . I run the super street Winfield 3/4 race cam ground by Bill Stipe. It is a popular regrind and has 314 lift . I run a slightly modified original distributor in the roadster and have no problems using the spark. I keep the momentum up and don't allow it to lug and very little use of the spark is needed. You can get down to lower speed without lugging with 3.78 gears than 3.54 or 3.27 gears would allow.
 
Charlie Stephens said:
Is there any truth to the rumor that at 81 MPH the front end comes off of the ground due to the airplane wing like fenders? I bet 80 MPH felt like it had power steering.

Charlie Stephens
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I wouldn't say the front end picks up off the ground. But you can feel it getting a little light. It's pretty sketchy going that fast in one even with hydraulic brakes. I only did it once just so I could say that I did it.
 
I have two A's. A 30 coupe with 3.78 and stock motor, and a 31 roadster with 3.54 and a warmed up B motor. The roadster will climb faster but only if I use second gear and overdrive. The coupe will climb hills in high gear that the roadster won't climb unless I use second and over. It's a matter of torque. All the hop-ups you describe will increase horsepower, but only at a higher rpm. Torque is what you need to climb hills, and torque may even be reduced by the hop-ups at a given rpm. All that increased horsepower will only come into play at high rpm. Leave the rear end gearing alone if you want to climb hills in high gear.
 
My 31 tudor did great race 2003 with paris,tx school as x-cup. Ozark and SMOKIE mts were a piece of cake . High school shop teacher, Billy Copeland and the 4 kids said the car blew the doors off and really couldn't believe how it can climb. I have no problem pulling grades and if I do , DROP TO REG 3RD OR GO TO HIGH 2ND. my 31 WILL PULL OFF AT STOP SIGN IN 2ND GEAR WITH 2 ADULTS AND NOT SLIPPING CLUTCH TO GET IT GOING. HAVE FUN MODELATONY - TONY WHITE LAFAYETTE,LA
 
With the 3.78 gears I have never needed to down shift on any hill with my roadster . It will burn rubber from a stop light if I would let it . Reground cams do cause power loss if the core is worn out . The core needs to have a lobe diameter of 1.225 and a center bearing diameter no less than 1.557 . I measured lots of cam cores before I found a suitable core . Most blocks have wear in the cam bearing area. A new cam with oversize bearings would be better and would compensate for worn block cam bearings . As long as I can find suitable cores and have Bill regrind them I'm happy with a regrind . Lift can be increased by grinding off the heel and using longer or adjustable lifters , that is basically what was done with the B cam .
 
I put a 3.25:1 ring and pinion in my stock 85 hp 35 Ford. Big mistake. Engine would overheat and the car just pulled a lot harder than it did with 3.78 gears. I put the 3.78 grears back in and now it's all happy once again. Maybe if you ran an overhead valve head on your Model A that puts out 100 hp. Then maybe those 3.27 gears might be OK
 
I run 4.11,s in both my A,s along with 28.5 % OD,s & they work great.One of them has been in use since 1960 with over 100,000 miles and have never regretted the change.If you hit a steep hill at 45 in 3,rd,you will do the same or better at the top.It will pull 8% grades in OD with no problem.I am running 33 B,s with a 5.9 head.Around here there are a lot of hills & I can pull most of them in OD without lugging.I could not imagine using 3.27 gears AND OD in my area.Perhaps in the flatlands you could get away with it.
 
The force needed to carry the higher gear ratio is torque, not horsepower, which is itself calculated from torque. The extra amount needed can be calculated. For example, at a given rpm, assume 100 ft/lbs torque. The 3.78 gear ratio (in high gear) will deliver 378 ft/lbs of torque to the hubs, minus friction and other losses. Divide the new 3.27 ratio into 378, and you need 115.6 ft/lbs of torque at the engine to deliver the same 378 ft/lbs of torque to the rear hubs. That is the 15% more "power" noted in Dave's Post #3. This ratio is the same any any rpm, although the torque curve will change with the mods.
Multiply the stock torque at any rpm by 115.6% to see what torque you need for an equivelent torque with the 3.27 rear end. Check the dyno tests link in Dave's post and see what modifications do to the torque produced at the rpm you chose.
 
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