Burtz vibrations

[Richard Knight]

Throwing a little more gas on this discussion. I am curious which flywheels are folks using, stock, modified, or Terry's 32 lb. Flywheel.

[Richard Knight]

Anyone have any thoughts? My thinking is for a bone stock or slightly modified Burtz is that the 32 lb. Flywheel is a little on the light side. For higher performance modern engines a light flywheel is part of the overall performance package. I think for most model A owners smoothness of the engine is more important than how fast it revs up. The factory set up with a 62 lb flywheel, 18 lb.pressure plate and disc has over 80lbs hanging off the back of the crankshaft. On a stock compression engine 4.2 to 1 this amount of weight does an excellent job of dampening out the combustion impulses. That is a big part of why a stock model A engine built to factory balance specs is so smooth. When we raise the compression ratio and lighten the flywheel I believe the dampening affect of the flywheel assembly is diminished to a point where more vibrations are felt in the car. My thinking at this point is that a flywheel of 40-45 lbs might have more dampening affect or even using a stock flywheel might be an option. That is why I asked what flywheels are folks using.

[johnneilson]

Some observations;
Flywheels, total weight is not the important factor, M.O.I.
Where is the weight in relation to the diameter.
Same goes for the crankshaft, TB has done the work and is as as close to perfect for 80% of the applications.

I will not discuss Front Harmonic Balancers, there are too many variables and quite frankly can cause premature crank failure.

Besides the mechanical balancing, rods and pistons weight there are two other factors that influence the "felt" vibration.
One is compression ratio and cylinder pressure and the other is ignition timing.

Both of these factors are performance and most often misunderstood.
Consider the original Model "A" was at a performance level of .20 hp per Cu/In displacement, not very impressive.
A mildly built OHV Mod"A" is at 100 hp, .50 hp per CI. That difference will be felt in the car regardless of mounts.

Ignition timing is critical the these engines and once you start pushing more performance the effects will become obvious.

John

[Richard Knight]

Agreed, on all points. That is my thinking on the 32 lb. Flywheel very little of the weight is out at or near the perimeter. the stock A and B flywheels have a lot of weight out at the perimeter. As you say timing has a lot to do with vibration.

[Flathead]

Harmonic balancer can cause premature crank failure???

[Terry Burtz, Calif]

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The "New Model A Engine" is manufactured in state-of-the-art factories that produce parts for many modern engines that rev to 6000 RPM, and every part is statically or dynamically balanced with modern equipment for 6000 RPM. This balancing is "First Order" and is required to reduce stress on the parts.

There is a "Second Order" vibration that occurs twice for every crankshaft rotation when pistons 1 & 3, and 2 & 4 suddenly change direction at TDC. There is nothing in a Model A Engine to mitigate this vibration.

Modern 4-cylinder in-line engines incorporate a pair of "Balance Shafts" to minimize second order vibration. The balance shafts rotate in opposite directions at twice the crankshaft speed to counteract the pistons' reversing direction twice at TDC for every crankshaft rotation.

There is no room in a Model A cylinder block for balance shafts. When a modern engine is raced, the balance shafts are removed because they are heavy and reduce horsepower.

The illustration shows the "Balance Shafts" in a modern in-line, 4-cylinder, 5-main bearing engine.

[Richard Knight]

Every thing Terry says is true. Unfortunately those secondary harmonics are what you feel when driving the car. Whether a stock model A, B, or a Burtz as has been said those harmonics are there. I my experience those harmonics appear somewhere between 1900 and 2300 rpm on all these engines. Unfortunately these rpms correlate with speeds model As are commonly driven at. If you run your engine below 2k with overdrive or above 2300 the vibrations are minimized. I am not sure when float o motors first appeared but it was a long time ago. I think these came about as an effort to minimize the affects of the harmonics. I am a long time user of floaters and for me they make my As more comfortable to drive. There are other factors that can increase or decrease the vibration one feels, primarily in the steering wheel such as compression ratio, timing, throttle position, effective gas pressure and a big one the mass of the car. These can be additive or subtractive. Something I found very interesting is that Kieth true says his model A that was put away in 1953 has floaters with and extra pad and D washers on the bottom of the frame bracket. That is something I added to my floaters on my Burtz powered 30 coupe and it does make a difference.

[Terry Burtz, Calif]

Originally Posted by Richard Knight

Every thing Terry says is true. Unfortunately those secondary harmonics are what you feel when driving the car. Whether a stock model A, B, or a Burtz as has been said those harmonics are there. I my experience those harmonics appear somewhere between 1900 and 2300 rpm on all these engines. Unfortunately these rpms correlate with speeds model As are commonly driven at. If you run your engine below 2k with overdrive or above 2300 the vibrations are minimized. I am not sure when float o motors first appeared but it was a long time ago. I think these came about as an effort to minimize the affects of the harmonics. I am a long time user of floaters and for me they make my As more comfortable to drive. There are other factors that can increase or decrease the vibration one feels, primarily in the steering wheel such as compression ratio, timing, throttle position, effective gas pressure and a big one the mass of the car. These can be additive or subtractive. Something I found very interesting is that Kieth true says his model A that was put away in 1953 has floaters with and extra pad and D washers on the bottom of the frame bracket. That is something I added to my floaters on my Burtz powered 30 coupe and it does make a difference.

Richard, thanks for your comments, vibration and harmonics are two different things. In any 4-cylinder inline engine, there are secondary vibrations. Secondary vibration occurs at all RPMs. Harmonic relates to natural frequency. If you strike a part like an original forged crankshaft, the original crankshaft will ring like a bell at its natural frequency. If run an original crankshaft at its natural frequency, the stress and deflections in the crankshaft are excessive, and this is where it will break. The Liberty Bell cracked when it was rung and vibrated at its natural frequency. The new Model A engine crankshaft is well supported with 5, 2-inch diameter main bearings and made from nodular iron that won't ring like a bell. An unbalanced engine will have excessive vibration at 2000 RPM, 4000 RPM, and 8000 RPM. With your car in neutral, rev the engine to 2000 RPM see how it vibrates. Next, rev your engine to 4000 RPM and see if the vibration is the same as it was at 2000. I do not recommend the engine being reved to 8000 RPM. I do not think you are feeling second-order vibrations. Consider driveshaft harmonics, where the skinny solid driveshaft of a Model A is unsupported for the length of the torque tube, whereas V-8 drive shafts are a larger tube within the torque tube. A heavy flywheel will dampen vibrations, but who wants a car with the performance of a heavily loaded cement truck.

[Richard Knight]

Terry, it is good we can discuss this. I imagine there are a lot of folks with a lot more time behind the steering of a model A than me but when I sold my 29 Murray with a modified B and overdrive in 2010 I estimated I had driven it about 40k miles. I had been driving that car since 1968 when it was bone stock until 2010 modified for highway driving. My first A that I drove was registered in 1965 which I drove to high school. So I do have some time driving As. Since 2020 when I got back into As I may have driven another 5k miles. I think you know I am an engine builder with a lot of engines under my belt, not just As but just about every old car you can think of and modern engines up to about the year 2000. So I disagree as I think I have been feeling the affects of the harmonics for the last 60 years. Sure when you have an engine balanced to within a nats ass what do you have left but harmonics.the A engine that was in that Murray for a time had counterweights added and was race balanced and turned 4k rpm in a vintage race car. There is no need to defend your 32 lb. Flywheel as it is just too light IMHO. A BURTZ with a stock or moderately lightened flywheel will RPM plenty fast for most model A drivers. So when are you going to get one of these engines in one of your cars and get some miles on it?

[BlueSunoco]

Interesting conversation. I'm learning a lot. Keep it coming

[AzBob]

Don’t have a Burtz engine in my Model A however, I don’t mind the vibration from my “B” engined 1930 sport coupe and find it therapeutic. The vibrations through the floorboards and pedals help relieve symptoms of my foot neuropathy.

[Richard Knight]

AzBob, I hadn't thought about that. I have neuropathy in my right foot which is somewhat annoying driving my model As. My B in my coupe is very smooth.