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Old 01-28-2019, 04:39 PM   #1
Dan 1939
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Default Want o go highway speeds ??? Need opinions.

My 1939 pickup overheats if I go over 60 MPH.
Do I change the rear gear to 3.25:1 ratio or, install a T5 transmission from a Chevy S10 with overdrive to be able to travel at highway speeds.
Which is the better plan and makes most since financially.
Thanks for your thoughts and guidance.
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Old 01-28-2019, 04:51 PM   #2
Charlie Stephens
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Default Re: Want o go highway speeds ??? Need opinions.

Why does your truck overheat, is it really due to the rear end ratio? What is your current rear end ratio?

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Old 01-28-2019, 05:44 PM   #3
drolston
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Default Re: Want o go highway speeds ??? Need opinions.

Pushing that truck through the air at 60 mph takes a certain amount of horsepower, regardless of what rpm the rear end dictates for that speed. The amount of heat generated by the engine at a given horsepower will vary a little with the rpm at which that horsepower is being delivered, based on the engine efficiency at a particular rpm. With the 4.11 gears normal for that truck, you are probably well over 3,000 rpm. With 3.25 gears the rpm will be down to maybe 2,500. The fan will be drawing less horsepower and the engine may be a bit more efficient at that rpm, but I doubt it will cure overheating.

Let's rule our a few things:
1) Water hoses not swollen or crimped
2) Take out the thermostats and try it. Maybe one or both are not opening properly.
3) Does your truck overheat sitting at idle, on a hot day? If so, check for retarded timing
4) Have you checked the water pumps; vanes intact and in good shape
5) Exhaust not restricted; crimped pipe or choked muffler?
6) After a hard run, check spark plugs to too-lean condition (white powder versus toast color)
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Old 01-28-2019, 07:12 PM   #4
1931 flamingo
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Default Re: Want o go highway speeds ??? Need opinions.

I'd start with the radiator....................JMO FWIW At 60mph the fan shouldn't come into "play", does it overheat if left at idling for any length of time??


Paul in CT
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Old 01-28-2019, 07:52 PM   #5
39pu
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Default Re: Want o go highway speeds ??? Need opinions.

Ignore the overheat concern what is most cost effective way to drop rpms at hwy speeds?
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Old 01-28-2019, 07:54 PM   #6
Kurt in NJ
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Default Re: Want o go highway speeds ??? Need opinions.

The 39 with a 60 had no problems going highway speeds of 65-70, it had 4.44 rear, ran cool flat out

My model A --stock 3.78 rear has traveled the whole length of the PA turnpike on 95 degree day at 65, didn't overheat

Start with the radiator, it's not the rear ratio
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Old 01-28-2019, 08:08 PM   #7
Phil Gillespie
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Default Re: Want o go highway speeds ??? Need opinions.

Quote:
Originally Posted by 39pu View Post
Ignore the overheat concern what is most cost effective way to drop rpms at hwy speeds?



Larger diameter tyres.
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Old 01-28-2019, 08:16 PM   #8
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Default Re: Want o go highway speeds ??? Need opinions.

Double check your timing. advance it a bit.
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Old 01-28-2019, 09:10 PM   #9
Charlie Stephens
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Default Re: Want o go highway speeds ??? Need opinions.

Quote:
Originally Posted by 39pu View Post
Ignore the overheat concern what is most cost effective way to drop rpms at hwy speeds?
What ratio is your current rear end? Are you sure your speedometer is correct?

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Old 01-29-2019, 08:17 AM   #10
fortyfords
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Default Re: Want o go highway speeds ??? Need opinions.

Do a Mitchel overdrive.
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Old 01-29-2019, 08:44 AM   #11
Juergen
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Default Re: Want o go highway speeds ??? Need opinions.

With stock tires and a 3.78 rear end, the Service Bulletins say at 60 you will be at about 2750 rpm which is a little high for prolonged flathead use. So a 3.54 will get you at about 3.54 / (3.78) x 2750 = 2575 rpm while a 3.25 gets you at about 2360 rpm. Finding a used 3.54 will probably be easier and cheaper than buying a new 3.25 and getting it set up. A bigger tire may be the cheapest. A 6.00 x 16 tire has a diameter of 28-29 inches. So one equivalent to a 3.25 (13% change) would need a diameter of about 32 inches. Of course use a turtle for a 3.25 (17 tooth I think)
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Old 01-29-2019, 11:11 AM   #12
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Default Re: Want o go highway speeds ??? Need opinions.

Most economical method, stay under 60. You will pick up increased fuel mileage too. 60 is fine for the highway.
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Old 01-29-2019, 12:18 PM   #13
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Default Re: Want o go highway speeds ??? Need opinions.

One of the problems I have seen with higher sustained speed in the 30s cars/trucks is fatigue cracking in the fenders.
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Old 01-29-2019, 12:35 PM   #14
woodiewagon46
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Default Re: Want o go highway speeds ??? Need opinions.

Do you have a fan shroud and all the sheetmetal that directs the air thru the radiator? I would do all that others have suggested before I do what you want to do.
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Old 01-29-2019, 01:12 PM   #15
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Default Re: Want o go highway speeds ??? Need opinions.

If your really determined to drive at higher speeds, I would go with the over-dive. That is the approach I took.
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Old 01-29-2019, 06:26 PM   #16
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Default Re: Want o go highway speeds ??? Need opinions.

All of the above are good suggestions but the root of the problem is the engine and/or cooling system. After that I would check the headlights, horn and steering wheel.
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Old 01-29-2019, 06:36 PM   #17
petehoovie
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Default Re: Want o go highway speeds ??? Need opinions.

Are you sure you want to drive on the highway? > https://www.youtube.com/watch?v=m_b_Pb6Ynjs
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Old 01-29-2019, 07:27 PM   #18
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Default Re: Want o go highway speeds ??? Need opinions.

Quote:
Originally Posted by petehoovie View Post
Are you sure you want to drive on the highway? > https://www.youtube.com/watch?v=m_b_Pb6Ynjs
LOL, good one!

JB
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Old 01-29-2019, 09:59 PM   #19
Tinker
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Default Re: Want o go highway speeds ??? Need opinions.

If you are on the 101 in LA you are either doing 100mph or creeping at 5mph.


Block might need a flush also, pull heads and dig out with a coathanger/long screwdriver/or both, to get sand out.


Lower rpms with power at any speed is a good thing. OD isn'e a bad idea. Running 7.00 tires, 3:78 rear, and a mitchell 36% od, cruise 70. Bit much at 36% OD, but my engine can pull it so far.


Mitchells aren't cheap, but no frame cut or open driveline conversions. Both take time and money. Put the mitchell in a day and still have my original torque tube and didn't need to do anything else.



Maybe a wash as far as money, I don't know. Don't know if it's better either. I like it.


Let us know what ya did. good luck.

Last edited by Tinker; 01-29-2019 at 11:18 PM.
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Old 01-30-2019, 04:09 AM   #20
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Default Re: Want o go highway speeds ??? Need opinions.

I had a 48 running hot all the time, rebuilt and advanced the ign 2 degrees, still hot,
then i ran 50/50 white vinegar and water for one day around the backroads, in the afternoon bottom tap was clugged all of a sudden, so i had to pull the bottom hoses of.
they were full of grit and crap from the entire cooling system, about 4 full hands of this stuff piled up in the pan, even what looked like casting sand. Flushed two times.
it stopped overflowing, it ran too cool all of sudden, and had a much more even heat distribution and dispatch.
The cheapest fix ever, about 5 Dollars. I`ve done the same on many cars with fascinating resaults every time. Try it
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