Max RPM's

[Jack Shaft]

Funny thing is Ford engineering caught the torsional vibration issue and wanted a larger journal crankshaft..Mr Ford preferred the heavy flywheel to dampen it. It wasn't till 1932 that engineering won that battle.

[Pete]

You can not feel torsional vibration so the only way to know if a damper helps is whether the crank breaks soon or not at all. A simple way to tell if you have any is, take a fairly long (10 mile) drive with varying driving conditions and then stop and feel the damper. If it is warm, you have torsional vibration.

This car had a 12 lb. rubber bond damper, no flywheel or clutch. Ran 3 seasons with about 300 laps per season. Turned 6100 rpm. (flathead B) Don't forget, all driveshaft and rear end drive parts are part of the flywheel effect.

[nkaminar]

Getting back to the maximum rpm for a stock Model A, I drove my bone stock 1930 coupe all over the west coast in the 1960's at a maximum of 50 mph and mostly slower. Only rarely did I go 55 and only for a short time. With the non counterbalanced crank it vibrated a lot at 55.


I had put the 3.78:1 gears in the rear end but did not have an overdrive so at 50 mph the engine was turning about 2,140 rpm. If you have a stock Model A you can sense when the engine is being hammered by the vibration.



Traffic in the 60's was different. I now drive my Toyota at 70 mph on the freeways but get passed all the time by people driving 80 mph or 90 mph or more. Back then 65 mph was fast.

[Bob Bidonde]

Torsional vibrations in combustion engines are repetitive impulses due predominantly to the combustion and compression strokes. Impulse is a product of force and time:
Force x time duration = mass times change in velocity.

Perhaps a better explanation is that torsional vibrations in combustion engines are repetitive twisting shocks in the crankshaft. So in the Model "A" we have approximately a 32 pound crankshaft trying to instantaneously accelerate (shock) an 80 pound flywheel & clutch.

The massive 80 flywheel & clutch react very little to the crankshaft impulses, so the crankshaft suffers from windup torsional reactions that work to shear the crankshaft. A modern type crankshaft damper that has a small mass works well with a modern lightweight flywheel, but would have little to no value in the Model "A" or Model "B" that have massive flywheels and clutches.

The impulses in the crankshaft not only repeat, but the combustion impulses twist the crankshaft in opposite directions. This action is not good for the fatigue life of the crankshaft.
As the RPM of the crankshaft increase, the time for the impulse force to fully develop decreases. Thus the go fast guys have it right, increasing speed dimsinishes the impulse effects. To test this, accelerate your engine while the gearshift is in neutral. You should notice the vibrations become smoother, but the noise increases. Do we associate the engine's noise which is mostly valve clatter with failure???

[old31]

So Bob, what happens, if anything with torsional vibrations when we change the stock flywheel with one that has been lightened by 50%?

[Jim Brierley]

Sponsored Links (Register now to hide all advertisements)

Ruth, no video, sorry. I am not technologically advanced. My Bonneville car is rear engined, and turbo charged, but still sounds pretty good.

nkaminar, I understand fatigue, but I drive mine fairly often, more than many as I'm 40 miles away from the nearest Model A guy. I have friends that have driven their A to many MAFCA meets, never heard of any of them breaking a crank. Racers yes, touring no.

[nkaminar]

Jim,



I am glad you have had good success with your car. The person who has broken several crankshafts has had the opposite.



Bob, regarding heavy vs. light flywheels,



The torsional vibrations are there all the time when the engine is running. It is like ringing a bell. The hammer hits once but the bell continues to ring. At certain rpm's the impulses match the natural frequency so the torsional vibrations increase quite a lot, up to 10 times. The dampener opposes the natural frequency of the crankshaft to reduce the torsional vibration. The dampener has to have a natural frequency that is designed to fit the crankshaft of a particular engine. Depending on a number of factors, including how fast run you engine, you may not need a dampener, but the torsional vibrations are always there to some degree.



The heavy stock flywheel means that the node point is just at the flywheel so that the torsional vibrations are not felt at the flywheel. A lightened flywheel moves the node point forward so now the flywheel has some torsional vibration. In both cases there is still torsional vibration in the crankshaft.



The impulses come from the power stroke, the inertia of the pistons and rods, and is a complex signal. They "ring" the crankshaft like ringing a bell. A dampener is like adding a rubber band to the bell to keep it from ringing.



I encourage everyone to read the link I provided in a previous post.

[Jack Shaft]

^^^ Neil is right ^^^ the best way to describe it is picture it as a tuning fork,as the crankshaft vibrates it deflects or basically bends, it is a reason for center main bearing failure

[wwirz]

I was told from a reliable member of my Model A club...that with a stock engine, no counterbalancing, ...driving it no more that 40 mph...it will last forever. So I do not drive mine over 40...all considered.

[40 Deluxe]

Did you know that torsional "twist" is necessary in a modern computer controlled vehicle? That's how a misfire is detected by the computer. The system is so sensitive that it measures the slight increase and then decrease in crankshaft speed every time a cylinder fires. If it doesn't detect this minute variation, it knows a cylinder has misfired, and it knows which one. After a certain amount of misfire, the "check engine" light comes on.

[alexiskai]

Originally Posted by 40 Deluxe

Did you know that torsional "twist" is necessary in a modern computer controlled vehicle? That's how a misfire is detected by the computer. The system is so sensitive that it measures the slight increase and then decrease in crankshaft speed every time a cylinder fires. If it doesn't detect this minute variation, it knows a cylinder has misfired, and it knows which one. After a certain amount of misfire, the "check engine" light comes on.

Neat article here on how those work:
https://www.samarins.com/glossary/crank_sensor.html

[BRENT in 10-uh-C]

Originally Posted by wwirz

I was told from a reliable member of my Model A club...that with a stock engine, no counterbalancing, ...driving it no more that 40 mph...it will last forever. So I do not drive mine over 40...all considered.

Specifically, how long is 'forever' in this scenario? I have serious doubts that a Model-A engine will last "forever" just because it was driven under 40 mph. If you want to argue the point, I would suggest you go drive your Model-A in first gear however stay under 40 mph and see if it lasts forever!!

[Jim Mason]

https://www.youtube.com/watch?v=7OZtR2LWKmw
I believe it says 2800 max in this video.
2400 would be more comfortable

[alexiskai]

Originally Posted by BRENT in 10-uh-C

Specifically, how long is 'forever' in this scenario? I have serious doubts that a Model-A engine will last "forever" just because it was driven under 40 mph. If you want to argue the point, I would suggest you go drive your Model-A in first gear however stay under 40 mph and see if it lasts forever!!

Hey, hey, keep this thread on topic. The thread for arguing about what is the proper speed to drive is over here:
https://www.fordbarn.com/forum/showt...9326&showall=1

[Ruth]

Originally Posted by BRENT in 10-uh-C

Specifically, how long is 'forever' in this scenario? I have serious doubts that a Model-A engine will last "forever" just because it was driven under 40 mph. If you want to argue the point, I would suggest you go drive your Model-A in first gear however stay under 40 mph and see if it lasts forever!!

Let's stay sensible in our discussion. I don't think he meant to drive 40mph in first gear.

[BRENT in 10-uh-C]

Originally Posted by alexiskai

Hey, hey, keep this thread on topic. The thread for arguing about what is the proper speed to drive is over here:
https://www.fordbarn.com/forum/showt...9326&showall=1

Exactly WHAT is the topic?

It appears we have gone from maximum RPM of a stock engine question, ...to citing an answer using modified engines as the example, to talking about Bonneville-specced engines, to using harmonic balancers, to torsional flex in a computer controlled modern engine ...and someone even chose to mention crank sensors and link to some video supposedly relating to that topic.

So exactly 'when' did this thread go off-topic, ...and by whom?? (-I'm pretty sure my comments both in post #11 and in #32 pretty closely relate to the topical question(s) the O/P was asking. )

Originally Posted by Ruth

Let's stay sensible in our discussion. I don't think he meant to drive 40mph in first gear.

You're likely very correct as he also did not really mean the word "forever" too, since we both know that would not be sensible either!!

[alexiskai]

The difference is that I'm pretty certain a digression on Hall-effect sensors isn't going to hijack the thread, whereas a digression on how fast to drive a Model A definitely would. You know this crowd, there's certain topics that are chum to the sharks and that's one of them.

[BRENT in 10-uh-C]

Originally Posted by alexiskai

The difference is that I'm pretty certain a digression on Hall-effect sensors isn't going to hijack the thread, whereas a digression on how fast to drive a Model A definitely would. You know this crowd, there's certain topics that are chum to the sharks and that's one of them.

Well I will yield to your wisdom in that regard ...however I am definitely confused as to why I am being blamed as the one who was diverting or hi-jacking from the original topic when my only statement was to refute a subjective answer posted in MPH instead of RPMs (-as the Original Poster had asked for.).

Maybe your point is that it really does not matter what the Original Poster's specific question is since this forum really is just a springboard platform to blurt out any opinion or comment, no matter whether it has relevance to the original question or not! If that is indeed the case, then I now understand the protocol and will try to be more diligent in providing similar content as my peers.


.
.

[Y-Blockhead]

Nevermind... back to the OP's question.

[alexiskai]

Originally Posted by BRENT in 10-uh-C

If that is indeed the case, then I now understand the protocol and will try to be more diligent in providing similar content as my peers.

I'm going to ignore this for the simple reason that I don't believe you'd ever take my advice about anything.