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Old 01-28-2021, 06:31 PM   #1
marty in Ohio
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Default Max RPM's

Please help settle a bet. What would be the maximum RPM's be for a stock Model A engine. Don't worry I'm not going to try it. Just want to settle a bet.
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Old 01-28-2021, 07:06 PM   #2
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Default Re: Max RPM's

Do you mean max revs while out of gear, revs at max speed or max revs in a lower gear? Whatever, things are getting pretty busy under there by about 2,500-2,600 rpm. With a little work, they will go higher.
Let's hear what others say.
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Old 01-28-2021, 07:45 PM   #3
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Default Re: Max RPM's

years ago when i was hill climbing, I woild not think twice about reving to 3600 - 4000 before popping the clutch, then winding back up to around 4000 before shifting into second gear. Dangerous - you bet, fun - you bet. Did I ever destroy an engine - nope.
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Old 01-28-2021, 07:58 PM   #4
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Default Re: Max RPM's

Hmmmm. so at 55MPH, with the tall tire, how many rpm?
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Old 01-28-2021, 08:24 PM   #5
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Default Re: Max RPM's

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Hmmmm. so at 55MPH, with the tall tire, how many rpm?
On 4.50 x 21 tyres, 55 mph is 2,440 rpm by my calculations.
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Old 01-28-2021, 10:11 PM   #6
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Default Re: Max RPM's

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Quote:
Originally Posted by kenparker0703 View Post
years ago when i was hill climbing, I woild not think twice about reving to 3600 - 4000 before popping the clutch, then winding back up to around 4000 before shifting into second gear. Dangerous - you bet, fun - you bet. Did I ever destroy an engine - nope.
Are you talking a totally stock engine ?
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Old 01-28-2021, 11:15 PM   #7
Kurt in NJ
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Default Re: Max RPM's

i have had mine over 3300 in second but i don't think it will go over 3100 in 3rd and have run it all day at that speed and i have held second till it wouldn't go any faster ---with stock zenith it is self limiting to a less than destructive speed I figure ---
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Old 01-29-2021, 12:41 AM   #8
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Default Re: Max RPM's

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i have had mine over 3300 in second but i don't think it will go over 3100 in 3rd and have run it all day at that speed and i have held second till it wouldn't go any faster ---with stock zenith it is self limiting to a less than destructive speed I figure ---
Big difference between 3300 and 4000.
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Old 01-29-2021, 12:42 AM   #9
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Default Re: Max RPM's

Quote:
Originally Posted by marty in Ohio View Post
Please help settle a bet. What would be the maximum RPM's be for a stock Model A engine. Don't worry I'm not going to try it. Just want to settle a bet.
Marty
So, what was the bet ?
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Old 01-29-2021, 07:29 AM   #10
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Default Re: Max RPM's

No, it was not a stock engine. Counterbalanced crank, custon cam. BF head, and most important - balanced engine. ken
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Old 01-29-2021, 07:47 AM   #11
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Default Re: Max RPM's

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Originally Posted by marty in Ohio View Post
Please help settle a bet. What would be the maximum RPM's be for a stock Model A engine. Don't worry I'm not going to try it. Just want to settle a bet.
Marty
If you are going to "settle a bet", how are you going to prove it? All you have is hearsay.

A stock Model-A engine that is rebuilt exactly as per Ford specifications will run out of carburetion before the 'bottom end' gives up. Especially if this happens in high gear driving on the road.
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Old 01-29-2021, 08:13 AM   #12
marty in Ohio
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Default Re: Max RPM's

OK. I must confess, it wasn't really a bet. Just two old Model Aer's wondering. Bob didn't know so he suggested asking on Ford Barn. I thought about 2,200 rpm's was tops. And yes, both of our cars are completely stock. I can always count on Ford Barn for an answer.
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Old 01-29-2021, 08:25 AM   #13
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Default Re: Max RPM's

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Originally Posted by kenparker0703 View Post
No, it was not a stock engine. Counterbalanced crank, custon cam. BF head, and most important - balanced engine. ken
Most Barners were very doubtful that you could achieve 4000rpm on a stock engine. Glad you clarified it.
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Old 01-29-2021, 09:19 AM   #14
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Default Re: Max RPM's

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Originally Posted by Kurt in NJ View Post
i have had mine over 3300 in second but i don't think it will go over 3100 in 3rd and have run it all day at that speed and i have held second till it wouldn't go any faster ---with stock zenith it is self limiting to a less than destructive speed I figure ---
Kurt, what do you have for a rear end ratio?
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Old 01-29-2021, 11:09 AM   #15
Bob Bidonde
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Default Re: Max RPM's

Here's a table of engine RPM per MPH for various differential gear ratios:
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Old 01-29-2021, 09:33 PM   #16
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Default Re: Max RPM's

You can do 2,400 rpm on a stock engine but not for long. Best to keep it about 2,100 max. Modified engines will do 2,500 to 3,000 on a regular basis but will break cranks without a dynamic dampener.
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Old 01-30-2021, 01:07 PM   #17
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Default Re: Max RPM's

I think nkhaminar is thinking theory, and has likely not run any performance engines. I run B engines with the 'C' cranks and no damper. The engine I have run the most still has babbitted mains, full pressure oiling, put together in 1960. I have run it at Bonneville, turning it 6,000 RPM for about 1 mile, and ran it in the quarter mile with a terminal RPM of 6200. Yes, the C crank is better than the A crank, but the A is still quite reliable. The most common breaking point is the rear flange, and the A is stronger back there than the B/C cranks.
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Old 01-30-2021, 01:17 PM   #18
Jack Shaft
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Default Re: Max RPM's

Herm Kohnke determined that a stock A crank flexed .001 at the center main bearing,2400 rpm due to torsional vibration,the tuning fork effect.Counterbalancing and thicker journals fixed that..the "C" crankshaft.
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Old 01-30-2021, 02:10 PM   #19
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Default Re: Max RPM's

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Originally Posted by Jim Brierley View Post
I have run it at Bonneville, turning it 6,000 RPM for about 1 mile, and ran it in the quarter mile with a terminal RPM of 6200.
Do you have any videos of it turning 6-6200 rpm. I would love to hear what it sounds like, perferably with open pipes!
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Old 01-30-2021, 02:13 PM   #20
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Default Re: Max RPM's

Jim, Short run time racing engines probably don't need the dynamic dampener. Performance touring engine do. Here is what a California friend wrote me: "One guy in our club has broken three or four cranks. He drives his Model A a lot and has been on a few long distance tours with us including one to Maine. He does push his engines pretty hard."


The problem is metal fatigue. It is high stress applied over time. The first order harmonic torsional vibration is right in the operating range of a high performance touring engine.

See: http://harmonicdampers.com/downloads...amper_Info.pdf
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