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11-20-2012, 01:43 PM | #1 |
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HP/torque vs rear end ratio
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?
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11-20-2012, 02:04 PM | #2 |
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Re: HP/torque vs rear end ratio
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.
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11-20-2012, 04:01 PM | #3 |
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Re: HP/torque vs rear end ratio
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 |
11-21-2012, 08:14 AM | #4 |
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Re: HP/torque vs rear end ratio
What body?
I run 3.54's in my Fordor with stock gears, and a 3.27 in the Tourer with close ratio gears. https://www.fordbarn.com/forum/showthread.php?t=85033 |
11-21-2012, 12:17 PM | #5 |
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Re: HP/torque vs rear end ratio
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.
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11-21-2012, 08:06 PM | #6 |
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Re: HP/torque vs rear end ratio
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11-21-2012, 08:08 PM | #7 |
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Re: HP/torque vs rear end ratio
dyno at 65 hp
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11-21-2012, 08:19 PM | #8 | |
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Re: HP/torque vs rear end ratio
Quote:
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11-21-2012, 09:20 PM | #9 |
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Re: HP/torque vs rear end ratio
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? Last edited by M2M; 11-21-2012 at 09:38 PM. |
11-21-2012, 09:44 PM | #10 | |
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Re: HP/torque vs rear end ratio
Quote:
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11-21-2012, 09:46 PM | #11 | |
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Re: HP/torque vs rear end ratio
Quote:
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.
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11-21-2012, 10:56 PM | #12 |
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Re: HP/torque vs rear end ratio
let us know how you make out
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'31 180A Last edited by tbirdtbird; 02-03-2017 at 10:49 AM. |
11-21-2012, 11:55 PM | #13 | |
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Re: HP/torque vs rear end ratio
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11-22-2012, 12:16 AM | #14 |
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Re: HP/torque vs rear end ratio
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.
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11-22-2012, 01:06 AM | #15 |
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Re: HP/torque vs rear end ratio
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.
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11-22-2012, 09:13 AM | #16 |
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Re: HP/torque vs rear end ratio
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
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11-22-2012, 12:21 PM | #17 |
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Re: HP/torque vs rear end ratio
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 .
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11-22-2012, 08:33 PM | #18 |
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Re: HP/torque vs rear end ratio
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
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11-22-2012, 11:42 PM | #19 |
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Re: HP/torque vs rear end ratio
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.
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11-23-2012, 01:06 AM | #20 |
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Re: HP/torque vs rear end ratio
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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