Flathead 8BA RPM

M

msstring

Senior Member
What are your opinions on max RPM for a fresh 8BA engine? I have a 1950 F3 4 speed, not sure what rear axle ratio is other than I'm sure it is original. I can cruze comfortably at 60+ MPH. However, am I over winding the motor at that? It'll do more, there is plenty of power, but I do have to keep coaxing it. Going to order a tach to put on it. Haven't done that yet.

I never use 1st gear, never need it. I can idle along around 25 MPH in 4th.

Looking forward to your feedback.

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Take Sand Paper to rear axle. On bottom, front edge , a flat spot approximately 1” wide there are numbers.
 
ssffnomad said:
Take Sand Paper to rear axle. On bottom, front edge , a flat spot approximately 1” wide there are numbers.
Click to expand...


Sometimes EASY to read, other times NOT so easy to see. Coop/AMERICAN


I just realized that this is an F3 truck, for which my pics likely DO NOT apply...SORRY!


316388



306142
 
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I remember riding with my uncle in his 49 Mercury M68 (3/4 ton) and the flathead sounded like it was really screaming at highway speeds. Probably in the 50 mph range. At 60 I'd guess it would be right up there at maximum hp rated rpm (3600?) Stock axle ratio and 7.50x 17 tires.
 
My preferred cruising rpm is about 2600.
If your truck has the factory Timken rear axle, and noone has re-geared it, it will be either the very rare 4.11 ratio or the very common 4.86 ratio. From your description you either have the higher speed 4.11 ratio, the axle has been swapped for a higher ratio Dana 60, your ear is terrible at determining idle speed, or your speedometer is way off. A factory 4.86 F-3 in my experience is comfortable to about 45 mph and will idle in high gear at about 15 mph.
 
Thanks for the feedback everyone. I'm pretty sure about the Speedometer because I am using my Garmin and the Speedometer ties right out to that. It could be any of the other suggestions though. 25 might be a little more than idle but I don't know if I could keep it running at 15. Now I wanna go try it! But it will be the weekend at best before that can happen. Hopefully on the weekend I can find the marking on the axle.

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I think a lot of the fearsome screaming "it's gonna blow" high rev stories are simply due to fan noise. Try a brief run with the fan belt off. A flathead is an extremely quiet engine due to no "monkey motion" in the valve train (assuming the lifters are set correctly). Also factor in the use of a non-stock air cleaner. The factory air cleaners are big and fat for a reason-to quiet the full or near full throttle intake roar.
 
I have a 3600 rpm diesel generator engine in my 47 tonner with the stock 4.86 and 7.50 x 17 rubber. It does 65 on the freeway just fine. The tires are 34" in diameter. My I phone speedo app isn't making stuff up. I also have a discreet little digital tach called a 'tiny tach' and we are below the engine's rated 3600 (at 120 hp). On another note I have a very old book a very old guy gave me 20 years ago called 'hopping up the ford V8'. Note they didn't call it a 'flathead V8', duh... Like there was an overhead ford V8 when the book was written.? And the last paragraph on the back side of the last page is a poem. 'A man is a man who will fight with a sword, or tackle Mt Everest in snow, but the bravest of all has a '34 ford and will try for six thousand in Low'.....I think it was a Floyd Clymer book.
 
GB SISSON said:
.....On another note I have a very old book a very old guy gave me 20 years ago called 'hopping up the ford V8'. Note they didn't call it a 'flathead V8', duh... Like there was an overhead ford V8 when the book was written.? And the last paragraph on the back side of the last page is a poem. 'A man is a man who will fight with a sword, or tackle Mt Everest in snow, but the bravest of all has a '34 ford and will try for six thousand in Low'.....I think it was a Floyd Clymer book.
Click to expand...


I have that same book, "How to hop up Ford & Mercury V8 engines" by Roger Huntington, published by Floyd Clymer, 1951. Have read it cover to cover many times!
 
My '48 Operators Manual, lists only one standard axle ratio for for both F-2 & F-3, 4.86. F-4's had 5.14 and optional 5.83.

Doing the math, a 4.86, with 7.50x17 tires, gives 19.6 mph, per 1000 rpm or almost 74.5 mph @ 3800. That would be almost 65 mph @ 3300, for the 6 cyl.

My '48 F-3 was originally a 6 cyl.

I can't say if they might have had optional ratios in '50 but, the OP would be running at 3200-3300 rpm with 4.86's @ 60+.
 
I guess on a related question, are there six volt add on tachometers available for those of us still running six volt systems?
 
Thanks for doing the math frnkeor. I appreciate that!!! I will see if I can find some numbers on the axle this weekend.
The question remains, if it is running 3300 RPM at 65. Is that too fast for a stock 8BA motor It runs super smooth but is it damaging itself in the process?

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As long as the engine is in good condition, it shouldn't hurt it. By good cond, I mean the bearings, mostly. Watch for any oil pressure drop.

BTW, what is your oil pressure now, at 60+.
 
I'll have to look at oil pressure again don't remember for sure but I'm thinking around 40PSI. Motor was just rebuilt, has around 100 miles on it.

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I searched the differential and I can't find anything resembling a ratio. Have posted some pictures maybe someone can ID from that or direct me where to look.

But I also did some testing:
- One wheel revolution = 94.5"
- one drive shaft revolution = 18.5"

If someone can help me with the formula on how to translate that to ratio and RPM I would be very grateful.
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There are old formulas I have seen for 40 years like 3.14 x tire diameter x blah blah that determine what your RPM is at 60 mph. I used to plug the numbers into the equation when messing with old landcruiser FJ series tire sizes, axle ratios etc. Now days it seems there are online calculators that do all the math. Look it up. For me, It's friday night after a long week of work... zzzzzz
 
using your circumference, I get a 30" tire diameter. Dividing the drive shaft revolution into the tire rev, I get 5.1, close enough to the stock 5.14 ratio. Using a 30" tire and 5.14 ratio, The calculator shows 3450 at 60 mph.
 
To the OP, What is your tire diameter? I have seen 7,50 x 16s from 30.5 to 32.5 and 7.50 x 17s from 32 to 34.5. Don't forget a small difference in diameter is a big difference in circumfrence. And circumfrence matters...
 
Chuck's Trucks has been supplying a 4.12:1 set of gears in kit form for the Timken-Detroit rear axles or at least they were. Counting the teeth on the ring and pinion is the best way to divide out the ratio. I don't know if they can be seen through the fill hole or not. They didn't stamp the tooth counts on the Timken-Detroit rear axles like they did the Ford banjo rear axles. Chuck's shows 4.12:1 and several taller ratios for the Timken-Detroit. I thing the 4.12 gear set is modified from a Franklin quick change set so it has to have the kit to put that set in.
 
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