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Old 05-09-2020, 09:13 AM   #1
Bob Bidonde
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Default What Defines Over-Revving The Model A's Engine

What defines over-revving the Model A's engine? What is its redline RPM, the speed a Model A's engine can operate without damage?

Ford advertised the Model A's engine to make 128 ft-lbs torque @ 1000 RPM, and 40.5 HP @ 2000 RPM. At 2800 RPM, Ford rates the HP at 40.9, so the engine does little over 2000 RPM. At 1000 RPM where the engine develops its maximum torque of 128 ft-lbs, the road speed is about 25 MPH in 3rd gear with 3.78:1 gears and 4.75x19 tires.

With 3.78:1 gears and 4.75x19 tires, each 5 MPH requires 214 RPM of the engine in 3rd gear. So at 65 MPH, a stock engine is running at 2785 RPM and 2999 RPM at 70 MPH.
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Old 05-09-2020, 09:35 AM   #2
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Default Re: What Defines Over-Revving The Model A's Engine

Wow! That's a lot of numbers. I think you answered your question! Thanks for sharing. Don't rev your engine!!
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Old 05-09-2020, 09:47 AM   #3
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Default Re: What Defines Over-Revving The Model A's Engine

How would you rate 450-21 tires at the same rpm.? Woody
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Old 05-09-2020, 09:50 AM   #4
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Default Re: What Defines Over-Revving The Model A's Engine

Isn't that defined by the condition of your motor?
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Old 05-09-2020, 09:52 AM   #5
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Default Re: What Defines Over-Revving The Model A's Engine



When you throw a rod out the side of the engine block , you have over-revved your Model A engine.

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Old 05-09-2020, 10:16 AM   #6
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Default Re: What Defines Over-Revving The Model A's Engine

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4.50-21 tires have a diameter of 30", so their circumference is 94.2". 4.74-19 tires have a 28.5 diameter, so their circumference is 89.5. The percent difference is +5.1% for the 2.50-21 tires, so that also is the percent difference in MPH. For example, at 1000 RPM where the car with 4.75-19 tires does 25 MPH, a car with 4.50-21 tires will travel at 26.3 MPH (25x0.051=1.275; using 1.3 thus 25+1.3 = 26.3 MPH).
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Old 05-09-2020, 10:25 AM   #7
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Default Re: What Defines Over-Revving The Model A's Engine

Jim,
I have not seen or know of a rod busting through a block on a stock engine. To the contrary, I have seen it on a modified engine.


I think you can run a Model A engine at full throttle with a stock Zenith carburetor, and being limited in RPM by the carburetor's breathing, the engine will not turn fast enough to throw a rod. I suspect that the engine's maximum RPM limited by the carburetor is between 3000 & 3500 RPM. In other words, it has by definition no redline.
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Old 05-09-2020, 11:12 AM   #8
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Default Re: What Defines Over-Revving The Model A's Engine

These curves also show they stopped testing at about 2900rpm. Redline?
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Old 05-09-2020, 12:49 PM   #9
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Default Re: What Defines Over-Revving The Model A's Engine

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Originally Posted by jim/ga View Post


When you throw a rod out the side of the engine block , you have over-revved your model a engine.

:d:d:d
HA!!!!!!!

Lugging the engine is bad on them too.

I always felt 40 MAYBE 45 was plenty good for a Model A Ford anything over that is pushing it.

NOW our 24 stud Flattie in the '36 Ford PU THAT is a different story it hums along at 55 w/o even working all that hard.
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Old 05-09-2020, 01:14 PM   #10
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Default Re: What Defines Over-Revving The Model A's Engine

The oil tray that feeds the dippers begins to suffer from 'windage' above 3200 rpm. That said, years ago modified A's raced with rod dippers in the 4-5000 rpm range.

Second point, even if you have a counter-weighted crank, with only three mains the weights are pretty far off-center from the actual rotational masses they attempt to counter. The offset opposing centripetal forces then bend the crank sections out of line in an RPM relationship that is exponential, not linear.

Third, an A has no front harmonic damper to prevent the severe 'ringing' shock waves that travel back and forth from front to rear along the crank. I wouldn't want to run all day at 3000+ rpm and heavy throttle unless you like swapping out broken cranks.
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Old 05-09-2020, 01:53 PM   #11
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Default Re: What Defines Over-Revving The Model A's Engine

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Originally Posted by Jeff/Illinois View Post
HA!!!!!!!

Lugging the engine is bad on them too.

I always felt 40 MAYBE 45 was plenty good for a Model A Ford anything over that is pushing it.

NOW our 24 stud Flattie in the '36 Ford PU THAT is a different story it hums along at 55 w/o even working all that hard.
I totally agree. I drive mine at 40 max 45 and feel safe and have no fear of damaging the engine. If you want to drive 60-65 buy a hot rod. I don't know a soul that's impressed to see a 90 year old car driving excessive speeds. IMO
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Old 05-09-2020, 02:09 PM   #12
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Default Re: What Defines Over-Revving The Model A's Engine

"....oil tray that feeds the dippers begins to suffer from 'windage' above 3200 rpm..." What's windage .. not come across the term before ( sorry I'm a Limey which explains a lot! )
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Old 05-09-2020, 02:33 PM   #13
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Default Re: What Defines Over-Revving The Model A's Engine

The 3" bore AF engine fitted to most English cars produced 28 BHP at 2600 RPM . The rear end had 4.55 ratio gears so in practise they were revved far higher than a stock 3.78" bore engine . The engine is way smoother than the 24 HP I guess due to less rotating weight . The crank and flywheel were the same for both engines . The AF camshaft was "hotter" the trick of the day was to fit this cam and the 14.9 head to a 24 HP engine and super cheap extra horse power . The Achilles heel of the small bore engine is that they ate their "Celeron" timing gears frequently so that did act as a rev limiter !!!

John in cold weather on the way Suffolk County England .
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Old 05-09-2020, 02:51 PM   #14
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Default Re: What Defines Over-Revving The Model A's Engine

Ok, I will throw a wrench into the works here. Ford did advertise that the car could do 65 MPH. Now, that being said, should you and for how long? I run down the freeway at 50-52 and she just purrs along without a care in the world. Granted we have better roads now and I did lighten the flywheel a tad.


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Old 05-09-2020, 02:53 PM   #15
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Default Re: What Defines Over-Revving The Model A's Engine

Henry Ford said every Model A would go 65 MPH, He did not say drive at a continuous 65 MPH. Any Model A done right is good for 45-50 MPH continuous and 55 if the occasion arises.

On my long trips I like to keep it at 1900-2000 RPM continuous all day "in overdrive".
Now that should evoke some comments.

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Old 05-09-2020, 02:54 PM   #16
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Default Re: What Defines Over-Revving The Model A's Engine

My tour thru the empty streets of the neighborhood today was one of the most pleasant ever. I did not need to keep up with other traffic thru the old back roads. I kept my Phaeton at 25-30 mph and she purrrred like a kitten.
Lots of folks walking, biking, wearing masks and lots of cheering as I drove past. I don't like to push my "A" faster than 40. Slow and steady wins the race.
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Old 05-09-2020, 03:46 PM   #17
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Default Re: What Defines Over-Revving The Model A's Engine

Some years ago Terry Burtz said that he was going to design his new engine to put out at least 100 HP at a significantly higher RPM than the usual 2200. He will be doing this with as counterweighted crank, carburation and perhaps other things. Tod is putting out a 5 main block with a counterweighted crank. With some carbs and other touches this engine should be good for more RPM's and HP. It looks like we need to put in an OHV assembly to get all of the value out of the new blocks that will be available soon. I am starting to put a few dollars away soon.


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Old 05-09-2020, 04:11 PM   #18
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Default Re: What Defines Over-Revving The Model A's Engine

Quote:
Originally Posted by johnbuckley View Post
"....oil tray that feeds the dippers begins to suffer from 'windage' above 3200 rpm..." What's windage .. not come across the term before ( sorry I'm a Limey which explains a lot! )
Windage is the resistance from the air/oil mist that gets churned up in the crankcase. Think about the air being moved inside the crankcase by the pistons going up and down, and the oil draining back from the block passing though this turbulent atmosphere. And then you have the oil in the pan being churned up. So that air in the crankcase gets saturated with oil and becomes "heavy", causing resistance to the spinning crank. At least that's how I understand it; there's probably more to it than this.
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Old 05-09-2020, 04:55 PM   #19
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Default Re: What Defines Over-Revving The Model A's Engine

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"....oil tray that feeds the dippers begins to suffer from 'windage' above 3200 rpm..." What's windage .. not come across the term before ( sorry I'm a Limey which explains a lot! )
In addition to all the things Tobey said about 'windage' in the crankcase, vehicles with dipper lube of connecting rod bearings have another concern.

In a Model A with dipper trays the torrent of churning air in the crankcase can blow all the oil out of the four dipper tray recesses. Normally the hit of the con-rod scoop into a liquid oil pool results in a pressurized forcing of oil up into the bearing. When the oil gets blown away from the tray by windage the bearing only gets secondary lube from the oily mist.

Modern cars solve the windage oil pickup problem from their pans with a 'windage tray' that keeps the air torrent from churning up the entire pan contents into a froth. It is shaped just like a dipper tray, except it is not intended to hold several pools of oil for the rod dippers. It just drains and shields the four quarts below.

In a Model A the oil in the bottom of the pan is also protected from windage, just like in a modern car, but nothing protects oil in the four dipper valleys.
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Old 05-10-2020, 07:48 AM   #20
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Ok, I will throw a wrench into the works here. Ford did advertise that the car could do 65 MPH. Now, that being said, should you and for how long? I run down the freeway at 50-52 and she just purrs along without a care in the world. Granted we have better roads now and I did lighten the flywheel a tad.


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Can you imagine doing 65 on 1930 roads? NO.
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