MTFCA Forum

Engines have vibration "periods" which are speeds at which they vibrate, in one way or another, more than at other speeds. These periods are more noticeable in many antique vehicles with more or less rigid engine mounts and no crankshaft counterweights or harmonic balancers. Does anyone have information on what RPM range(s) or road speeds in high that a stock Model T engine in good condition would be expected to show the strongest vibration when running at a steady speed on level ground?

We who run the Montana 500 experience this usually between 48 to 51 mph we refer to it as the thrum.Not all engines do this.

I seem to see it from about 1600 rpm to 1800 or so. The car is very smooth at 1500, and it seems smooth and happy at 1900-2000. I avoid going any faster than that. Some people with late model Jeep Wranglers complain of a thrumming sound around 63 MPH in high.

It's not unusual to experience vibrations in high gear when first shifting from low to.high. These can be difficult to trace. I had this problem with a 1912 roadster that resulted from the crankshaft being 22 grams out of balance. Problem disappeared when I had it dynamically balanced.

I realize you want "stock" motor info, but this could be helpful. 13-40 gears, high compression head, stock crank, everything balanced . 53-55 gets the rattles & wont accelerate thru the spot unless I retard the spark. Motor will pick up speed & I can readvance spark.
Same engine with a scat crank : That harmonic rattle hits around 58. Same procedure to get thru it. I dont normally cruse that fast, so the change in harmonics is perfect.

Mine has vibration (not extreme) in the range of about 1650-1750 indicated rpm. There may be some slight power loss associated with it, but it's not much. The car accelerates through that particular speed range with no problem. There is some noise associated with it, but it's hard to tell where the noise is coming from It could be the exhaust system or sheet metal resonance or the internal parts of the transmission. The exhaust pipe is touching the hogshead, but it is not loose at the manifold, and it does not leak. Slowing down or speeding up eliminates the effect. Touching the pedals has no effect. Pulling the hand lever back back slightly has no effect. Adjusting the spark has no effect, or very little. The car is smooth as an inline Buick 8 at 1500 rpm/35 mph.

My 1926, factory stock engine starts vibrating at around 32mph, so bad that I cannot see anything in the rear view mirror. When I accelerate to above 35mph, it gets less. Jim Patrick

It looks like the vibration/noise is normal. Model As have similar vibration periods.

Mine use to do that at 28 mph, I had it dynamic balanced and the vibration is gone now, Had a friend Bud Delong ask me if knew anyone that could do this found a local speed shop that could and have never looked back since!

Looks like it's normal. Balancing helps a lot, but you can't balance it all out at all speeds,, even in a modern 4 cyl.

38 MPH. Smooth as silk at 40.

you can't balance it all out at all speeds,, even in a modern 4 cyl.

well, it is a 180 degree design, so the right shop sure can. Getting things balanced well enough at the natural frequency will balance it well in excess of the RPM the engine can manage. Now, if you mean that if you raced the engine in the garage at full speed available at the throttle, it might indeed vibrate at some high RPM, but if the build components were well selected, and the balance properly done, the car cannot reach that RPM necessary to excite oscillation/vibration on the road. Again, all assumes that the engine was balanced by a knowledgeable shop after weight-matching reciprocating components were used in the build.

Scott. What constitutes a knowledgeable shop? Due to the age and nature of the Model T engine, I would not trust my engine to just any shop for this kind of balancing work. Are there shops out there that specialize in the Model T engine that you can recommend? I would trust it to George King III of Connecticut where I sent my engine in 2006, when he had “Connecticut Antique Engine Restoration” in North Franklin, Conn., but he has long since retired. Jim Patrick.

Inline 4 cylinder engines vibrate at certain speeds; even modern ones do it. Old inline 4 cylinder engines with limber crankshafts and no counterweights certainly vibrate. Production engines can be, and usually are, out of rotating balance to some degree, and they have torsional vibration and "rocking" motions at various speeds and loads.