What Defines Over-Revving The Model A's Engine

[Bob Bidonde]

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.

[Gary WA]

Wow! That's a lot of numbers. I think you answered your question! Thanks for sharing. Don't rev your engine!!

[Woodys29]

How would you rate 450-21 tires at the same rpm.? Woody

[Curtis in MA]

Isn't that defined by the condition of your motor?

[Jim/GA]

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

[Bob Bidonde]

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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).

[Bob Bidonde]

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.

[Jim Mason]

These curves also show they stopped testing at about 2900rpm. Redline?

[Jeff/Illinois]

Originally Posted by jim/ga

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.

[MikeK]

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.

[McMimmcs]

Originally Posted by Jeff/Illinois

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

[johnbuckley]

"....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! )

[john charlton]

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 .

[1930artdeco]

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.


Mike

[Richard in Anaheim CA]

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.

Richard
Anaheim CA

[al's28/33]

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.

[Vic in E-TN]

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.


Vic

[Tobey]

Originally Posted by johnbuckley

"....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.

[MikeK]

Originally Posted by johnbuckley

"....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.

[old31]

Originally Posted by 1930artdeco

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.


Mike

Can you imagine doing 65 on 1930 roads? NO.

[old31]

Originally Posted by Richard in Anaheim CA

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.

Richard
Anaheim CA

Richard, with OD at 55 are your rpms at 2k? Do you ever do 60?

[daren007]

Holy cow a lot of math here. Just listen to the engine it will tell you. If you are in a hurry you are driving the wrong car.

[Patrick L.]

"He did not say drive at a continuous 65 MPH. "


When my Grandfather sold them new the salesman were to take them on the only local paved road and show they would go 65. It was a sales gimmick.

[Kevin in NJ]

Some comments.

First off you all need to do some historical reading and not just guesses and opinions not based on facts.

If you read period magazines and newspaper articles you will find that people talked a lot about driving their full out on all the roads of the day. In some cases that included no roads. My favorite was an oil company in the mid 30's or so doing a reliability run. The used a tudor with some miles on it and the tore the engine down and measured it. Then they put it back together with no new parts. Filled it up with their oil and drove from Phoenix to San Diego (pretty sure I have the towns right) been a while so some details will be off slightly. They drove as fast as they could. They averaged total time like 60 MPH. That is distance from Az to CA over total time including fuel and food stops. Going through mountains and areas where roads did not really exist.

You can also read about people setting records in their A's for going cross country and to Fl from NY. Then there are the people the drove backwards across country.

Then you talk to the people who used the cars everyday and they have story after story about flying down the roads 50 to 60 MPH.

So Yes they really did drive the cars hard when the cars were just everyday cars.

So to answer the question about top end. Really the answer lies in how well balanced is the engine and how good are you at setting spark. The two things that kill the engine are balance and pre-detonation. Most of the engines out there today are out of balance in weight and poor machining. So even getting the engines safely up to 2500 rpm is a chore.

My experience driving a stock well balanced (not as well balanced as I would make it today I should add) A is they really cruise fine up at 55 to 60 MPH and get over 20 MPG.

Quite frankly you can beat the crap out of a properly built A and it will just keep working. The biggest issue is knowing you are not causing pre-detonation. They will do jumps and drive around the stuck 4 wheel drive trucks in the woods.

[Licensed to kill]

35 years ago I read in a hot rod mag where someone asked "How do I know if I've over reved my engine?'. The answer given was, "localized pains in the left rear pocket and the realization that your engine will now fit loosely in a 5 gallon bucket". Personally, I like the answer given "your engine will tell you". With mine, I don't know what RPM it's running at but I do know that if i go over 45-47 MPH it starts to vibrate and does not sound happy. If I stay at 43 or less it does sound happy.

[Jack Shaft]

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Your crankshaft bearings are protected by the hydrodynamic principle.It is a hard and fast law,.001 clearance per inch of shaft diameter,which is why model a rod and main bearings are ideally clearanced at .0015.

The stock model a crankshaft is dynamically balanced but does not have offset weights to counter balance power stroke impulse.

At maximum rpm,this power stroke impulse results in a flexing of the crankshaft (according the the late Herm Kohnke) as much as .001 flex at the center main bearing,resulting is a loss of the hydrodynamic principle and a failure of the bearing.

If you run an harmonic balancer it helps reduce this potential damage,also a lube line directly from the oil pump with a full flow filter down stream helps pressurize the center main bearing,providing some protection from the loss of the hydrodynamic principle that occurs.

[katy]

In 1927 when Ray Dahlinger drove a new Model A around the USA as a Ford publicity stunt, what speed did he drive at?

[Patrick L.]

"the late Herm Kohnke" ???????


pre-detonation ??? no such thing. pre-igntion and detonation which are 2 different animals.


The main problem I see with driving at 65mph is not the driving part, but, the stopping part. [ even with perfect brakes] That said anyone can operate their vehicle any way they wish. Over the years some say they drive/cruise at 65 and others say they run theirs at WOT in all gears until they reach the speed they want. To each their own.

[Jack Shaft]

Herm passed last week or so

[redmodelt]

Originally Posted by Jim/GA

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

LOL I was going to say just at the point or just after the point it blows up!

[Licensed to kill]

Originally Posted by Patrick L.

"the late Herm Kohnke" ???????


pre-detonation ??? no such thing. pre-igntion and detonation which are 2 different animals.


The main problem I see with driving at 65mph is not the driving part, but, the stopping part. [ even with perfect brakes] That said anyone can operate their vehicle any way they wish. Over the years some say they drive/cruise at 65 and others say they run theirs at WOT in all gears until they reach the speed they want. To each their own.

While I agree that everyone is free to drive as they wish, I think (at least for me) the point of these "cruising speed" discussions is to determine what is considered best for the health/longevity of the engine and why (the "why" part is important to me). I don't want to drive around at 20MPH thinking that I'm saving my engine if 40 or 50MPH is just fine and I don't want to be driving at 50-60MPH thinking that it's just fine to find out down the road (no pun intended) that my engine pound3d out the bearings prematurely because of it. So, yes, anyone can operate their "A" anyway they choose but it's best if that choice is an informed one. JMO.

[Hotrodfil]

Hope this helps with the number crunching. I tend to run along around 2-2500 rpm which with a 3.27 rear end is around 50-65 mph.

https://docs.google.com/spreadsheets...gid=1208129397

[Jim Brierley]

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.

The above is from MikeK. This is true, and is what causes broken cranks. It is called 'nodulation', the rear flange is constantly forced to 'wobble', and eventually breaks. a friend was in the navy in WWII, and drove his A roadster every weekend from LA to San Diego, always with his foot solidly on the floor, never any problems. I run my speedster to 4100 RPM on hill climbs. It is a B block, C crank with stock oiling except for a full-flow filter. I do have a high volumn oil pump but not sure that makes any difference?

[Patrick L.]

Originally Posted by Jack Shaft

Herm passed last week or so

Sorry to hear that. He posted here often.

[Patrick L.]

Originally Posted by Licensed to kill

While I agree that everyone is free to drive as they wish, I think (at least for me) the point of these "cruising speed" discussions is to determine what is considered best for the health/longevity of the engine and why (the "why" part is important to me). I don't want to drive around at 20MPH thinking that I'm saving my engine if 40 or 50MPH is just fine and I don't want to be driving at 50-60MPH thinking that it's just fine to find out down the road (no pun intended) that my engine pound3d out the bearings prematurely because of it. So, yes, anyone can operate their "A" anyway they choose but it's best if that choice is an informed one. JMO.

I understand. Its tough to figure out what to do sometimes isn't it. So how does your 'A' feel ? It'll have a comfortable speed and it'll let you know what that is. I think the majority are comfortable in the 40-50 mph area.

Our family business started in 1918 and I was brought up in it. I grew up hearing that these Fords shouldn't be driven over 50 mph because they would then pound the bearings to death. Many do exhibit the secondary vibration at around that speed, but, I'm not sure I believe they will do that. I've seen quite a bit of bad/ruined babbitt but think it was just due to poor babbitt repours or old age. I've seen only one broken crank and believe it was due to a poor regrind.

[daren007]

It looks like few people bought a model A for a model A. I bought my A to slow down, I have enough stress in my life

[Jeff/Illinois]

Originally Posted by katy

In 1927 when Ray Dahlinger drove a new Model A around the USA as a Ford publicity stunt, what speed did he drive at?

As fast as he could to get back to Evangeline if Henry was lurking about

Kevin I read that article years ago about the endurance run LA to Phoenix you are right about that. The Model A was a '28 or '29 Roadster as I recall, and I believe it was promoting Mobil oil. They drove that Roadster really hard with no ill effects.

I wouldn't do that myself, today. As someone here said earlier 'If you are in that big of a hurry, you are driving the wrong car'

[burner31]

Grandpa once told me "If your engine is screaming loader then the guy in the trunk, you are over revving your motor"

I'll always remember my Grandpa’s last words before he died “Pints! Gallons! Litres!”
That spoke volumes to me.

[bdtutton]

I have a 1930 Tudor and it cruises nicely until about 50-52 MPH. As I exceed those speeds the noise and vibration greatly increase. I am not sure if I have the original 3.78 gears or if they have been upgraded to 3.54 gears. I am thinking about getting a tachometer so I can figure out what gearing I have, but I am not sure it matters because anything over 50mph is overrunning the brakes anyway. But, I would really like to know what RPM is considered the maximum RPM a model A engine can run over long periods of time.

[ryanheacox]

I'd say on a stock engine redline is around 2500RPM and the never exceed speed is around 3000RPM. Like Bob says in his first post that is roughly 60 and 70MPH respectively. This is only my opinion.



I feel comfortable cruising at 50 (~2200RPM) all day long and the car seems to run nice and smooth at this speed. A little slower and I get an annoying vibration and much higher I'll be well over most posted speed limits around here and there's no need for that in an A.

[BRENT in 10-uh-C]

Originally Posted by old31

Can you imagine doing 65 on 1930 roads? NO.

Yes, ...and it was not that difficult back then to find roads to travel 65 mph on as there were many paved highways & roads in 1930. Also, the steering and suspension system on Model-As back then wasn't worn out like we find on 'restored' Model-As today, ...and most other parts were within specification including the engine.

Originally Posted by Richard in Anaheim CA

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.

Ummm, while he did not use the word 'continuous', I'm pretty sure he implied that it was built for continuous speeds.

Below is a 1930 Salesman Brochure;




Another sales brochure offers similar verbiage claiming much the same;
"Speeding along at fifty-five to sixty-five miles an hour, or ..."




"The new Ford has unusual speed. It will do 55 - 65 miles an hour. So well does the new car hold the road that you can travel at high speed for long stretches ...."




I have other Ford brochures that state similar phrases that I can show. This proves this was not just a 'one-time' bogus advertising ploy meant to deceive. They were serious when they advertised the Model-A could travel at speeds such as the 55 - 65 mph for sustained lengths of time. Model-A's that are truly 'completely restored' in today's modern time (-with such add-ons such as a 3.54 ratio, better camshaft, higher compression, Stipe shocks, shortened pitman arm, etc.) should not have any excuse for not being able to maintain 65 mph all day long on the open road.

.
.
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[40 Deluxe]

Originally Posted by burner31

I'll always remember my Grandpa’s last words before he died “Pints! Gallons! Litres!”
That spoke volumes to me.

Good one! I read this last night but didn't 'get it' until just now!