The Ford Barn

Joe/Ct · 09-09-2014, 09:23 PM

We have a 1928 Model A roadster with a recently rebuilt stock engine. During an inspection, we found that we had a missing tooth on the pinion gear, which needs to be replaced (along with the ring gear). Our current setup is 3.78. Should we stick with that or go with the 3.54? What do we gain; what do we lose?

Purdy Swoft · 09-09-2014, 10:25 PM

I have always felt that 3.78 gears are the best for the model A . Henry must have also thought so because that is what most had. Model A's are under powered and needs the gear to have decent hill climbing ability, even if the engine is modified. With a higher gear ratio you may gain a few mph at lower rpm if you've got higher compression. Without higher compression it more than apt won't have enough power to kick up the RPM with the higher gear. Lugging the engine in the higher gears will beat out the babbit . You will do more harm to any engine lugging in the higer gears than you would at full throttle. It is much easier to lug the engine with a higher gear ratio.. I run 3.78 gears in all of mine .

Charlie Stephens · 09-09-2014, 10:39 PM

I run 3.54 in my '31 RDPU and am very happy with them. B cam and police head but otherwise stock.

Charlie Stephens

J Franklin · 09-09-2014, 11:31 PM

Since it is a light weight roadster I would use the 3:54 ratio.

Tom Wesenberg · 09-10-2014, 03:41 AM

I would even opt for the 3.27 ratio and the 5.5 head to help.
Not that I want to drive faster than 45, but I would like to keep the engine revs down a bit.

Patrick L. · 09-10-2014, 05:45 AM

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I would think it would depend on your surrounding terrain. If its really hilly the .78s may still be best. If its kinda flat maybe the 54s will be fine.
The point about the car being a roadster is a good one and maybe the 54s would work well anyway.
The 54s if my thinking/figuring is correct would increase your cruise speed about 4 mph for the same engine speed.

Mitch//pa · 09-10-2014, 06:33 AM

3:54's are a good overall ratio and that little extra top end is very helpfull even on a bone stock motor.

BRENT in 10-uh-C · 09-10-2014, 06:38 AM

Why not 3.27 gears?

Kurt in NJ · 09-10-2014, 07:25 AM

What are you going to use the car for ----if you mainly want to do parades, and around town driving keep the stock ratio, if you want to do a lot of driving over 50 mph and have a engine that can pull it choose one of the other ratios, if you want both high speed, and low speed get a Mitchell overdrive.---or other type of overdrive

modelAtony · 09-10-2014, 07:39 AM

Quote:

Originally Posted by Joe/Ct

We have a 1928 Model A roadster with a recently rebuilt stock engine. During an inspection, we found that we had a missing tooth on the pinion gear, which needs to be replaced (along with the ring gear). Our current setup is 3.78. Should we stick with that or go with the 3.54? What do we gain; what do we lose?

Get the 3:27 gears and never look back. You wont get 10 mph top end like they say but you will get 7 mph and it will be the difference of night and day. With the lower rpm's , you will be pretty happy. It won't be a Mitchell overdrive but you will like the outcome. have fun modelAtony tony white Lafayette, LA

Rex_A_Lott · 09-10-2014, 07:47 AM

You can do your own comparison pretty easily. Since you are talking about gears and ratios, that 's how you do it.
Say your present cruise RPM is 2000, with your 3.78 gears:
3.54/3.78=.9365 .9365 X 2000= 1873 would be your new RPM for the same road speed.
3.27/3.78=.8650 .8650X2000=1730 for the 3.27 gear.
This will hold true until you reach the point Purdy mentioned, where the motor doesnt have the power to pull the taller gear ( lower number).
Since gears are all about torque multiplication, what you gain in top end road speed, you lose on the bottom end taking off.
You can do the same thing with tires, but to a much smaller degree.
These are just general guidelines. As speed climbs, wind resistance becomes more of a drag....if you dont think so, stick your arm out the window running about 60, and then think about that brick you're trying to push through the air.
Hope this helps. Good Luck!

Purdy Swoft · 09-10-2014, 10:58 AM

It all depends on how the car will be used. If you go on long trips or cross country tours where you can cruise at high steady speed and run a higher compression head, a higher gear ratio will allow higher top speed at a lower engine rpm. My thought is a very large percent of model A owners use their model A for short trips, around town and ocaisional back road fun drives at low to moderate speeds. Even with a higher compression head, if higher gear ratios are used you will constantly have to be retarding the spark to avoid knocks and lugging. I run 5.9 heads and Winfield super street 3/4 race cams on some of my model A's and I can cruise at 50-55 mph with full advance and accelerate going up hill like a modern car with no knocks. Running a higher gear ratio will take away low and mid range power especially if you aren't running a higher compression head . Even if you run a 5.5 head and 3.54 gears you probably won't be able to run full advance without knocks on hills . Being forced to have to run with retarded spark looses power and causer higher engine temps . Most seem to run with retarded spark anyway either because they don't know any better or because they have to because of higher gear ratios . Many times these same people wonder why they run hot , don't have any power and have timing knocks on hills or the least amount of strain.

09-09-2014, 09:23 PM	#1
Joe/Ct Senior Member Join Date: May 2010 Location: Waterford, CT Posts: 250	Differential Gears We have a 1928 Model A roadster with a recently rebuilt stock engine. During an inspection, we found that we had a missing tooth on the pinion gear, which needs to be replaced (along with the ring gear). Our current setup is 3.78. Should we stick with that or go with the 3.54? What do we gain; what do we lose? __________________ Joe/Ct "The older I get, the better I was"

09-09-2014, 10:25 PM	#2
Purdy Swoft Senior Member Join Date: May 2010 Location: Alabama Posts: 8,099	Re: Differential Gears I have always felt that 3.78 gears are the best for the model A . Henry must have also thought so because that is what most had. Model A's are under powered and needs the gear to have decent hill climbing ability, even if the engine is modified. With a higher gear ratio you may gain a few mph at lower rpm if you've got higher compression. Without higher compression it more than apt won't have enough power to kick up the RPM with the higher gear. Lugging the engine in the higher gears will beat out the babbit . You will do more harm to any engine lugging in the higer gears than you would at full throttle. It is much easier to lug the engine with a higher gear ratio.. I run 3.78 gears in all of mine .

09-09-2014, 10:39 PM	#3
Charlie Stephens Senior Member Join Date: May 2010 Location: Southern California Posts: 7,033	Re: Differential Gears I run 3.54 in my '31 RDPU and am very happy with them. B cam and police head but otherwise stock. Charlie Stephens

09-09-2014, 11:31 PM	#4
J Franklin Senior Member Join Date: May 2010 Location: Oregon Posts: 5,966	Re: Differential Gears Since it is a light weight roadster I would use the 3:54 ratio.

09-10-2014, 03:41 AM	#5
Tom Wesenberg Senior Member Join Date: May 2010 Location: Mpls, MN Posts: 27,582	Re: Differential Gears I would even opt for the 3.27 ratio and the 5.5 head to help. Not that I want to drive faster than 45, but I would like to keep the engine revs down a bit.

09-10-2014, 05:45 AM	#6
Patrick L. Senior Member Join Date: May 2010 Location: Largo Florida Posts: 7,225	Re: Differential Gears Sponsored Links (Register now to hide all advertisements) I would think it would depend on your surrounding terrain. If its really hilly the .78s may still be best. If its kinda flat maybe the 54s will be fine. The point about the car being a roadster is a good one and maybe the 54s would work well anyway. The 54s if my thinking/figuring is correct would increase your cruise speed about 4 mph for the same engine speed.

09-10-2014, 06:33 AM	#7
Mitch//pa BANNED Join Date: May 2010 Location: Bucks County, PA Posts: 11,454	Re: Differential Gears 3:54's are a good overall ratio and that little extra top end is very helpfull even on a bone stock motor.

09-10-2014, 06:38 AM	#8
BRENT in 10-uh-C Senior Member Join Date: May 2010 Location: Eastern Tennessee Posts: 11,520	Re: Differential Gears Why not 3.27 gears? __________________ . BRENT in 10-uh-C . www.model-a-ford.com ...(...Finally Updated!! ) .

09-10-2014, 07:25 AM	#9
Kurt in NJ Senior Member Join Date: May 2010 Location: on the Littlefield Posts: 6,159	Re: Differential Gears What are you going to use the car for ----if you mainly want to do parades, and around town driving keep the stock ratio, if you want to do a lot of driving over 50 mph and have a engine that can pull it choose one of the other ratios, if you want both high speed, and low speed get a Mitchell overdrive.---or other type of overdrive

09-10-2014, 07:47 AM	#11
Rex_A_Lott Senior Member Join Date: Jun 2011 Location: Upstate South Carolina Posts: 794	Re: Differential Gears You can do your own comparison pretty easily. Since you are talking about gears and ratios, that 's how you do it. Say your present cruise RPM is 2000, with your 3.78 gears: 3.54/3.78=.9365 .9365 X 2000= 1873 would be your new RPM for the same road speed. 3.27/3.78=.8650 .8650X2000=1730 for the 3.27 gear. This will hold true until you reach the point Purdy mentioned, where the motor doesnt have the power to pull the taller gear ( lower number). Since gears are all about torque multiplication, what you gain in top end road speed, you lose on the bottom end taking off. You can do the same thing with tires, but to a much smaller degree. These are just general guidelines. As speed climbs, wind resistance becomes more of a drag....if you dont think so, stick your arm out the window running about 60, and then think about that brick you're trying to push through the air. Hope this helps. Good Luck!

09-10-2014, 10:58 AM	#12
Purdy Swoft Senior Member Join Date: May 2010 Location: Alabama Posts: 8,099	Re: Differential Gears It all depends on how the car will be used. If you go on long trips or cross country tours where you can cruise at high steady speed and run a higher compression head, a higher gear ratio will allow higher top speed at a lower engine rpm. My thought is a very large percent of model A owners use their model A for short trips, around town and ocaisional back road fun drives at low to moderate speeds. Even with a higher compression head, if higher gear ratios are used you will constantly have to be retarding the spark to avoid knocks and lugging. I run 5.9 heads and Winfield super street 3/4 race cams on some of my model A's and I can cruise at 50-55 mph with full advance and accelerate going up hill like a modern car with no knocks. Running a higher gear ratio will take away low and mid range power especially if you aren't running a higher compression head . Even if you run a 5.5 head and 3.54 gears you probably won't be able to run full advance without knocks on hills . Being forced to have to run with retarded spark looses power and causer higher engine temps . Most seem to run with retarded spark anyway either because they don't know any better or because they have to because of higher gear ratios . Many times these same people wonder why they run hot , don't have any power and have timing knocks on hills or the least amount of strain.