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EFV-8 Club Forum / Light Commercial Truck Discussion / '51 8BA RPMs

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Discussion Topic: '51 8BA RPMs -- page: 1 2

Big Red 51

11-28-2009 @ 6:19 AM

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I run my stock '51 PU on the hiway with a HD 3 speed and 4.27 rear end. I usually run around 50 MPH which sounds like a "comfortable" speed but I do not want to over burden the engine on a 600 mile run. Is there a formula for calculating the RPM and what is a good speed or RPM with this setup for sustained running? If a tach is required, what is the best way to hook one up?
Thanks, Don

Stroker

11-28-2009 @ 8:02 AM

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(0.00595) * (RPM * r) / (R1 * R2) = vehicle speed in
miles/hour

where:
RPM = engine speed, in revolutions/minute
r = loaded tire radius (wheel center to
pavement), in inches
R1 = transmission gear ratio
R2 = rear axle ratio

supereal

11-28-2009 @ 8:25 AM

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Most truck gearing is for power, rather than hiway speed, so they are set up to place the maximum engine speed close to the power band, where horsepower and torque are at peak. That occurs around 2,500 rpm. Most of the trucks I've seen hit that at about 50-55 mph. You will need access to a tachometer to determine where you are. The sticker is that to alter the engine speed, it can only be done by a change in rear axle gears, for a large change, or wheel and/or tire size for modest adjustments. Many of us have installed aftermarket overdrives for modern hiway driving. I have one in my '47 that reduces the engine speed by 33% when engaged. I use it above 40-45 mph. Mitchell and others make them, but you will have to do some searching for your '51. As to the tach, most diagnostic types hook up between the input terminal of the distributor and ground, and are calibrated to number of cylinders.

Big Red 51

11-28-2009 @ 3:33 PM

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Thanks for the formula Stroker and the comments Super.
Looks like 2500 rpm puts me at about 49 MPH with my setup. My rebuild has about 3000 miles on it. So I have been running about 55 which is out about 2800 rpm. Is 2800 harmful?
Super, about how much is a Mitchell OD if I can find one?
Thanks, Don

ford38v8

11-28-2009 @ 6:22 PM

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Don, I don't believe the Mitchell OD will work on your truck. It's for torque
tubes, not open driveshafts.

Alan

supereal

11-30-2009 @ 9:38 AM

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I believe Alan is right, but they may be able to point you toward a unit that will work. We have swapped rear axles in light trucks over the years when we were able to find them in salvage yards. Most will fit with some minor revisions to the spring pads and the driveshaft. We used that approach when we dropped a V8 into an Econoline pickup a couple of years ago. I don't think 2800 will harm your truck, but it gets really tiresome to hear the roar and feel the vibration. Most old trucks are not subjected to the loads they were designed for. Even a 10 percent reduction in rear end ratio would help. That would require a ratio in the order of 3.85, more or less.

Dale Fairfax

12-01-2009 @ 7:47 AM

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Assuming you have a 1/2 ton or F-1, you should be able to bolt in any '49-'59 Ford/ Merc B-W Overdrive transmission with only moderate difficulty (shortening the driveshaft; intalling some means to shift the side shift box; wiring). Or: install one of the later Ford 4 speed transmissions with the integral O.D.(4th gear is O.D.). Or: on the more complex side install a late T-5 5 speed which requires some sort of adapter plate plus shortening the drive shaft. Finally, you can search for a later rear axle assembly of the right width with a more favorable gear. (Changing the gear ratio in the existing axle can be a problem due to lack of available parts and the difficulty of working on the type of rearend end in your truck.)

Big Red 51

12-02-2009 @ 6:46 AM

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Thanks Dale and others, I may explore some of these options. Of course, I can continue to stay under 55 on the hiway. These trucks are not all that stable at higher speeds anyway. If the old 4.27 fails I will replace with a lower gear ratio.
Thanks, Don

supereal

12-02-2009 @ 9:16 AM

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Trucks usually have a high center of gravity which amplifies any bounce or sway. A set of really good shocks can go a long way toward making the vehicle safe on the road. If the kingpins have any slop, or the steering box is loose, keeping between the fences can be a chore. Another important factor is proper toe-in adjustment. If not correct, it can set up a dangerous shimmy that requires coming to a stop to regain control. I love old trucks, but getting them in shape for long trips is a very good idea.

wmsteed

12-03-2009 @ 6:17 PM

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In 1951 my father purchased a new Ford F-1 with V8, three on the three, and overdrive. The truck was a custom cab with radio, heater and 16" Firestone split ring wheels...

That truck was the best truck my dad ever had, I had the pleasure of driving the truck all over the western states. With the overdrive, the truck would cruise very nicely at 60 mph plus, when hills were encountered, or we had to pull a trailer, disengaging the OD gave the lower ratio we needed to pull the hills.

It is a little known fact that factory equipt vehicles have a lower axle ratio than non OD vehicles. In the mid 1950's Ford started using 3.89 as the standard axle ratio for OD's, where-as non OD vehicles were somewhere around 3.73 unless the vehicle was ordered with 4.10 or lower axle ratio.

As mentioned in the earlier posting to this topic, you can use the OD unit out of just about any '50/60 Ford car/truck, so long as it is compatible with your trans. Your will have to shorten the drive shaft and have the correct linkage. Sometimes the easiest way to get everything you need for the conversion is to purchase a vehicle that has a factory OD.

You can also use either a Gear Vendors, Mitchel, or one of the other after market units. Your 4.27 axle ratio will still be a little low for an OD, 4.10 or 3.89 would be better.

Don't let someone talk you into a gear ratio higher than 3.89 with a stock engine and a OD because you will not have enough engine to pull the higher ratios.

Bill
36 5 win delx cpe

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