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EFV-8 Club Forum / General Mercury Discussion / 47 Merc 3.78 gear

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Discussion Topic: 47 Merc 3.78 gear -- page: 1 2

len47merc

12-04-2013 @ 7:51 AM

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Having just brought this '47 79M Merc back to life after a few decades of storage am surprised (probably shouldn't have been) to find driving at current highway speeds of 65-70 mph results in rpms in the 3,250-3,500 range, respectively, driving up cabin noise from the old flathead as well as significantly and adversely affecting fuel consumption. Am targeting 2,000 - 2,250 rpms for 60 mph long-term. Being quite uncomfortable pushing this 66 year old motor (37,725 original miles) at these higher rpms for even short periods of time I am considering a change in the rear pinion set gear ratio from the original/current 3.78 to 3.54 as advised by multiple sources. Doing the math though suggests the improvement to be marginal at best and has me questioning the value of the investment in time and expense. The Columbia OD cost/value is not worth consideration at this point. Am soliciting input from other members regarding experience in this regard and thoughts. Thanks in advance.

This message was edited by len47merc on 12-4-13 @ 7:53 AM

ford38v8

12-04-2013 @ 8:04 AM

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Len, to expand on my reply at the fordbarn, I'll agree that you'll lope along at a more comfortable RPM with the 3.54, which would be a significant improvement over a ratio of 4.11, but not so much from a 3.78 ratio. If you are able to do the job yourself, you should save some coin, but it would still take you 50 years to recoup in gas savings the investment in the lower ratio. Also, your stop & go in town driving experience will suffer, as well as reducing the life expectancy of your clutch.

I changed my ratio from 4.44 to 3.78, but again, that's a much biggerr difference than from 3.78 to 3.54. Your change is not worth the effort and money.

Alan

len47merc

12-04-2013 @ 9:45 AM

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Thanks Alan - your response and clarification here is excellent and greatly appreciated. I was sensing the 3.54 value proposition was not worth it and your and others' responses have confirmed it. The car is too enjoyable a drive around town to lose the feel that a change to 3.54 will create, and I suspected the change from 3.78 to 3.54 would be far too marginal at speed to be of value.

Btw - I will clarify my signature. My name is Steve.

Thanks again Alan!

supereal

12-04-2013 @ 11:29 AM

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Alan is right on, as usual. The 3:78 was Ford's choice for a compromise between power and speed back when average hiway speeds were in the mid 1950's. I like to drive my '47 on longer trips but, like Steve, found that the engine had to roar to keep up with the now common 65-75 mph. I opted for an overdive unit, which allows me to keep up on the road. It is a 33% unit coupled to the 3:78 rear end. This meens that at 60, the engine is loafing along at 40. If I had it to do over, I'd choose a 30% OD, as it would function better below th 45 mph that is the point where I engage the OD. Installation is fairly simple with a Mitch*ll OD. Mine is a Ryan, but I don'tknow if they are still in business. My first choice was a Columbia, but all I could find was overpriced junk.

carcrazy

12-04-2013 @ 8:32 PM

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Provided you are willing to accept having the shift lever on the floor, the best solution might be installing a 5-Speed Borg-Warner T-5 Manual Transmission. The best of these for use in a Flathead powered vehicle is from a 1985 to 1986 Chevy S-10 Pickup. Low gear is 3.76:1 while the high gear (O.D.) ratio is 0.72:1. This will give you a 28% reduction in the overall gear ratio going down the highway. Based on your current engine speeds, your new engine speeds with the 3.78:1 rear axle would be:
2160 RPM at 60 MPH
2340 RPM at 65 MPH
2520 RPM at 70 MPH.
This overall ratio in high (O.D.) of 3.78 X 0.72 = 2.72 is just about perfect for cruising down the highway as the engine will be running close to the torque peak speed of 2,000 RPM.
This is not an easy swap but everything required to do it is either available or can be readily built. You will have to convert to an open driveshaft system which will require changes to the rear suspension system. Adapter bell housings are available as are clutch discs with the proper diameter, hub features, and splines. If you are interested in pursuing this course of action there is a lot of information available to you.

TomO

12-05-2013 @ 8:03 AM

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Before you buy any parts, check the rear end ratio, stock ratio for the Mercury was 3.54:1. The ratio should be stamped on the banjo web.

I find that my car is much happier going 55-60MPH and I get my best mileage at 45-50MPH. If I drive 55-60 on the Interstates and the slower speeds on the secondary roads, in the right hand lane, I find that most of the other drivers give me the space that I need to drive safely. It is also less tiring to drive at the lower speed.

I considered a Columbia back in the early 80's and decided against it due to the cost and the added problems that I had with one in the 50's.

Because you said in your other post that you are restoring the car, keep in mind that a Columbia is the only Ford authorized overdrive for your car. The other options may require modifications to the body or frame to install.

Tom

This message was edited by TomO on 12-5-13 @ 8:07 AM

len47merc

12-05-2013 @ 9:07 AM

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Thanks Tom for your response here and on the other thread. I have the original owners manual and 'VIN' numbered specification sheet (equal to a modern day 'window sticker'), as well as the Ford/Mercury specifications for RPM by speed and 3.78 gear set. The rear is referenced as a 3.78 on all documentation and my rpms by mph & 3.78 gear are almost perfect matches (am running slightly larger tires than standard). The standard for the early '47's was 3.78, whereas after April of '47 Mercury changed to the 3.54 and started touting the virtues of the new 'higher speed' or 'high speed' '47 Mercury. Perhaps the national speed limit was being raised at the time and they saw this as a preemptive marketing opportunity (?). When and if I move forward with an OD unit I will reconfirm and re-validate the reference numbers as you mentioned - sincere thanks for the good advice on this.

Steve

supereal

12-05-2013 @ 11:47 AM

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While many overdrive units come with a floor shift lever, I use the cable made for a truck power takeoff. I mounted the knob on the lower left edge of the dash so when the knob is out, the unit is disengaged, and when it is pushed in, it operates. The OD bolts onto the front of the banjo, and is furnished with its own driveshaft and torque tube. No frame or floor modification was necessary. The car can be restored to stock any time.

This message was edited by supereal on 12-5-13 @ 11:52 AM

len47merc

12-08-2013 @ 6:44 AM

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All,

In speaking with Early Ford V-8 Club technical advisers it appears either some or all of my research and representations made by perhaps non-qualified resources regarding the 'early' versus 'late' '47 attributes may have been in error. The car was originally and is now titled as a '47, the grill's chrome perimeter is '47 though most if not all other attributes appear to be from '46. Glass codes are from '47, VIN is late '46/early '47. The 3.78 vs 3.56 is being pursued further as well and, while in play here, may not have been represented correctly by me earlier in this thread (this titled '47 Merc is confirmed to have 3.78 gears). Not sure where this one will end up but am glad to now be a Club member working with the qualified resources we have. Apologies if the info given earlier in this thread is in error. Depending upon outcome may post a new thread if found to be of interest and value.

Sincere thanks to all on the great overdrive info!

Steve

MikeRhodes

01-04-2014 @ 12:58 AM

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I have a 34 roadster with a Borg Warner overdrive that does not work at the moment. It appears to be the same as that mentioned by Supereal (noted above). It's attached to the front of the differential. In all fairness, I have spent a grand total of 0 time looking into it but will share my experience once I get under the car and grease my knuckles a bit.

The car drives like a dream and I run it for short local trips around town, …. but would love to get the overdrive operational. Stay tuned.

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