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Old 04-29-2020, 10:19 AM   #1
rhphotord
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Default Flat head RPM range

Can any one give me what is considered a safe comfortable rpm range for a stock 42 flathead? Running a 4:11 gear set presently and rpm seems high at highway speeds. Love to install 3:78 gear set but not in the budget as of yet unless one falls into my lap at the right price.
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Old 04-29-2020, 10:54 AM   #2
deuce_roadster
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Default Re: Flat head RPM range

My 40 wagon (heavy car) has 4.11s and a small 221 ci stock engine. I don't drive it over 50 on the freeway as I don't want to over rev it. There is a Columbia in its future. I don't have a tachometer just going by sound.
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Old 04-29-2020, 11:27 AM   #3
drolston
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Default Re: Flat head RPM range

Highway cruising in the 2500+- range is economical and does not sound strained. You may need to go to 3.54 gears to get that at 60 mph.
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Old 04-29-2020, 11:40 AM   #4
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Default Re: Flat head RPM range

My avatar has 3.54s and is about 2600 at 60 mph. No worries on that engine though. drolston is correct.
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Old 04-29-2020, 06:50 PM   #5
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Default Re: Flat head RPM range

I had a 4.44 in my Ford when I first bought it, and kept up with the traffic at 65mph, although with a frown on my face. The flathead won't redline with a stock distributor, because you'll get point float first.


Edit: I should mention that my engine is a 59L, so it may be higher reving than others.
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Old 04-29-2020, 06:56 PM   #6
JSeery
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Default Re: Flat head RPM range

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Quote:
Originally Posted by rhphotord View Post
Can any one give me what is considered a safe comfortable rpm range for a stock 42 flathead? Running a 4:11 gear set presently and rpm seems high at highway speeds. Love to install 3:78 gear set but not in the budget as of yet unless one falls into my lap at the right price.
You can calculate the engine rpm, it is fairly simple math, all you need is the rear axle ratio (which you have) and the rear tire diameter.

RPM = (Rear Axle Ratio x MPH x 336.13) / Tire Diameter

As an example

(4.11 x 65 x 336.13) / 29 = 3096 RPM

That is not that may RPM. Original Torque/HP curves go to a bit of 4,000.

Last edited by JSeery; 04-29-2020 at 07:06 PM.
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Old 04-29-2020, 07:29 PM   #7
HDowse
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Default Re: Flat head RPM range

Here's a calculator I have used to check RPMs at different speeds/differential ratios:
http://www.advanced-ev.com/Calculators/TireSize/
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Old 04-29-2020, 09:27 PM   #8
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Default Re: Flat head RPM range

On my '51, with a slightly modified Mercury engine (Rochester 2G, Edmunds heads, and a Mallory dual point), anything above 2700-2800 starts to get uncomfortable. For me, with the tire size/ rear end ratio I have, that allows me to cruise about 65-70. It will easily go higher than that, and will run over 4000 "going through the gears".

I do not like running it over 4000. It doesn't seem to do any good anyway.
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Old 04-30-2020, 03:22 AM   #9
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Default Re: Flat head RPM range

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On my '51, with a slightly modified Mercury engine (Rochester 2G, Edmunds heads, and a Mallory dual point), anything above 2700-2800 starts to get uncomfortable. For me, with the tire size/ rear end ratio I have, that allows me to cruise about 65-70. It will easily go higher than that, and will run over 4000 "going through the gears".

I do not like running it over 4000. It doesn't seem to do any good anyway.

Where we're all headed, it's more fun to try and get there faster!
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Old 04-30-2020, 08:26 AM   #10
big job
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Default Re: Flat head RPM range

Then on the other hand my father bought a new F6 in Jan 1951 a leftover. Factory
governor says Ford on it probably made by King Seely. Governed at 3000 and yes
still has the 'lead seal' its a dump truck and will do 60 empty and just nurse the gas
pedal and it may just nick 62. Its purpose was only to kick it up momentarily to the
governor then shift it. Got a hard time that people need that 65 70 in any vehicle,
now, like from the 1950s 60s when many states with no speed limit. Around here in
Masshole takes you 20 min to go a mile in a half. Model T is perfect and I don't live in
a city either.
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Old 04-30-2020, 05:58 PM   #11
rhphotord
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Default Re: Flat head RPM range

I purposly installed a tachometer to monitor shift points on the flattie. Runs out to 4500 quickly in first gear and with the 4:11 and can practically start in second. At 4500 am I pushing the envelope so to speak? Thoughts?
Frankly at 55mph plus the old girl makes me kind of nervous but then again I'm used to driving my 34 street rod with modern independent suspension and disc brakes. Just saying!
Tomorrow I run the flathead out to 50 plus to check rpm's
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Old 04-30-2020, 06:29 PM   #12
Kurt in NJ
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Default Re: Flat head RPM range

The 39 with the 60hp (4:44 rear) didn't mind going 65, though it did bring the fuel mileage just under 20 mpg, kept to 55 it was 25 mpg----- and downshifted to second when speed on hills dropped to 40, the 60 hp needs to be above 2500 sometimes, and drove it over 70 on the parkway to keep up with traffic and get a head start on the next hill
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Old 04-30-2020, 09:27 PM   #13
tubman
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Default Re: Flat head RPM range

Quote:
Originally Posted by rhphotord View Post
I purposly installed a tachometer to monitor shift points on the flattie. Runs out to 4500 quickly in first gear and with the 4:11 and can practically start in second. At 4500 am I pushing the envelope so to speak? Thoughts?
Frankly at 55mph plus the old girl makes me kind of nervous but then again I'm used to driving my 34 street rod with modern independent suspension and disc brakes. Just saying!
Tomorrow I run the flathead out to 50 plus to check rpm's
This interests me. I have to ask : is this a stock or modified engine? My Mercury (with a stock cam) falls on it's face at anything over 4000. It'll rev higher, but unless you are already in high, grabbing the next gear is a lot more productive.
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Old 05-01-2020, 06:47 AM   #14
KGS
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Default Re: Flat head RPM range

Quote:
Originally Posted by rhphotord View Post
I purposly installed a tachometer to monitor shift points on the flattie. Runs out to 4500 quickly in first gear and with the 4:11 and can practically start in second. At 4500 am I pushing the envelope so to speak? Thoughts?
Frankly at 55mph plus the old girl makes me kind of nervous but then again I'm used to driving my 34 street rod with modern independent suspension and disc brakes. Just saying!
Tomorrow I run the flathead out to 50 plus to check rpm's
Here's a link to a chart on Van Pelt's site that shows the relationship of torque to horsepower and othings of interest as well.
Ken

http://www.vanpeltsales.com/FH_web/F...39-40_85hp.jpg
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