VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1912-1928 What is considered "Normal" engine vibration

During my 7 year restoration of my 24 Touring the engine was completely torn down, meticulously inspected and reassembled with new rings, reground cam, new lifters and all clearances maintained. The engine starts easy and runs smooth but when the RPM is raised it seems to me to have a fair amount of vibration. It doesn't shake the car violently but certainly vibrates the car. I realize the engine is hard bolted to the frame members and is not balanced like a modern 4 cyl engine.

As I have no frame of reference and have never seen another Chevy 4 cyl run in person what is considered normal? Would appreciate FB from other 4 cyl owners.
Thanks, Tom

Each vehicle will have some engine and road speeds that will vibrate more than others. The severity depends on the frame structure, engine balance, wear, tires (type, inflation, balance, roundness), road surface among others. The better condition and balanced the entire vehicle the less the vibrations. The best will still not match modern vehicles but don't act like threshing machines either. It will take some time and work to both understand your particular vehicle and minimize the vibrations. All my old Chevys are different and each has their own sweet spots where we both are more comfortable.

Chipper,
The car rolls and drives quite well. it is primarily the engine itself that seems a bit rough. May need to go back though and re-verify timing and fuel settings.
Thanks Tom!

Hey Tom;
I am also new to these old 4 cylinder chevys.I just got mine last summer. I have also questioned as to if my car was normal as I have nothing of that vintage to reference it to. I thought mine seemed awful noisy and rattly. Your description pretty much describes the way my car is. I would love to go for a ride in another '24 just to compare! Tom

I am also pretty new to the 20’s 4 cylinders but I have a 27 Chevy and it is in my opinion pretty smooth all around. I am working on a 28 truck that has not run since the Eisenhower administration. And while I have the motor getting redone the machine shop “balanced” the flywheel and clutch assembly and I was surprised how much drilling they did to get it to balance. ( I have no reference as to how it ran before the balance because it never ran).
I worked in a service dept. at a Chevy dealership in the early 2000’s and one thing I learned is what one person thinks is a shake another thinks is smooth as glass. It was amazing what people would complain about and want “fixed” when it is normal. Also what people thought was normal and never complain about but was in fact really had something wrong. So it is real hard to know without “feeling” another car. You could use the membership search and find a member with a car like yours close by and shoot them a message. I looked up the Wisconsin members by me and there are a few with my same car so I could drop them a line if I will be in there area to compare vehicles.
As I mentioned I am new to this and just recently joined the VCCA and my first issue of the G&D magazine there was an article about the “new wave” of us newbies to the early Chevrolets and the VCCA club as the circle of life has seen many members pass away and then the cars get sold to a new area people…..us.

Didier,
Thanks for the response, I know exactly what you mean. For a reference point I am a retired USN Aerospace Maintenance Officer with more than 30 years of engineering background and have done my own mechanic work for the last 40 years. While I don't profess to have the skills of some of our more talented folks here at VCCA, I would say I have considerably more experience than most. Having said that, most all of my experience has been restoration/rebuilding cars/trucks from 1965 on, certainly a lot different than 1924. The other negative for me is I have been unable to find anyone relatively close that has a similar car and having just recently retired from the Navy, I have had little time over the last 5-6 years as I was deployed to some rather dusty parts of the world. Hence the 7 year restoration project. I plan on going to Hershey this year and certainly will have a lot of my questions answered there.
Best regards, Tom

Tom,
4 cylinder engines are not as smooth as 6 cylinders.

For those inclined the following was copied from Wikipedia. It explains the imbalance characteristics of the two types of engines. Might help folks to understand why some vehicles don't run as smoothly as others. Also understand that modern engines have several engineered anti-vibration modifications.

Normal inline-4 configuration has very little rocking couples that often results in smooth middle rpm range, but the secondary imbalance, which is undesirable for high rpm, is large due to two pistons always moving together. Rotational vibration on the X axis, which is often felt during idling, tend to be large because, in addition to the non-overlapping power stroke inherent in engines with 4 or less cylinders, the height imbalance from connecting rods centre of gravity swinging left and right[note 2] is amplified due to two connecting rods moving together. Intake and exhaust pulse on ordinary inline-four engines have equal 360° spacing between the front-most and the rear-most cylinders, as well as between the middle two cylinders. So an equal-length (longer-branch) four-into-one exhaust manifold, or two 'Y' pipes each merging exhaust flows from #1 and #4 cylinders, as well as #2 and #3 cylinders are required for evenly spaced exhaust pulse. Older twin-carburetor setup often had each carb throat feeding the front two and the rear two cylinders, resulting in uneven 180°-540°-180°-540° intake pulse on each throat. Modern inline-four engines normally have four equal-length runners to a plenum (which is fed by a throttle at 180° evenly distributed frequency), or four individual throttles (at 720° equal spacing on each throttle).

Inline 6 normally has crank throws at 120° phase shift to each other with two pistons at about equal distance to the center of the engine (#1 and #6 cylinders, #2 and #5, #3 and #4) always moving together, which results in superb plane balance on reciprocating mass (4.) and rotating mass (6.) in addition to the perfect phase balances 3., 5., 13. and 15.. Combined with the overlapping torque generation at every 120° of crankshaft rotation, it often results in a very smooth engine at idle. However, the piston pairs that move together tend to make secondary imbalance strong at high rpm, and the long length configuration can be a cause for crankshaft and camshaft torsional vibration, often requiring a torsional damper. The long length of the engine often calls for a smaller bore and longer stroke for a given cylinder displacement, which is another cause for large secondary imbalance unless designed with otherwise-unnecessary long conrods that increase engine height. Moreover, 4-stroke inline 6 engines inherently have 14. (Plane imbalance on torque generation) and 16. (Plane imbalance on compression), which are typically more or less balanced on V12 and Flat-12 configurations.

In terms of firing spacing, these typical inline 6 are like two inline 3 engines connected in the middle, so the firing interval is evenly distributed within the front three cylinders and within the back three, with equal 240° spacing within the trio and 120° phase shift to each other. So three-into-one exhaust manifolds on the front and on the rear three cylinders, with each of them then connected with a two-into-one pipe results in 120° (240° if not merged in a dual exhaust system) evenly distributed exhaust pulse.
Jaguar XK inline six with 'three' SU carburetors, which cause irregular intake pulse at the front and the rear carburetors

Intake pulse, which is also important to have equal spacing for evenly filling the cylinders with the same volume and mixture of intake charge for 11. (uniform amount of torque) and 12. (uniform timing in torque generation), is formed the same way, so two carburetors or throttle bodies on two one-into-three intake manifolds each on the front and the rear three cylinders (strictly speaking when the three runner lengths are equal) results in evenly spaced intake pulse. Jaguar XK inline 6 had three SU carburettors each serving the front two, middle two and the rear two cylinders in the later models, which resulted in unevenly distributed intake pulse at the front and the rear carburetors (the middle carb gets an evenly spaced pulse at 360° interval). This configuration, while resulting in higher power due to the increased total flow capacity of the carburetors than the earlier evenly-spaced-pulse twin carburetor configuration, may have contributed to the later 4.2 Liter version's "rougher running" reputation compared to the legendary 3.4 and 3.8 Liter versions.

Modern inline six engines with fuel injection (including Diesels) normally have equal length intake runners connecting the intake ports to (often protruding into) a plenum (See Inlet manifold for parts descriptions) to keep intake pulse evenly spaced.

I think the flywheel is used to dampen some of these vibrations. It is important to fit the flywheel in the same location as when it was originally fitted. I think there is a section in the service manual that makes a reference to this.

In my experience, all early Chev-4 motors have different levels of vibration. Some motors just purr across the rpm range while others feel like they're on the verge of flying apart. I believe it to mostly be a result of prior service. For example, a con rod babbit fails, a replacement con rod is fitted without respect to balance with the other three. I've been into quite a few Chev-4 motors and rarely do all four con rods have the same forging number.

Also, not all crankshafts or flywheels are created equal. I have taken cranks for spin balancing and some were so bad they almost flew off the machine. Others were almost perfect. In my experience, the motors that retain all the original parts they left the factory with have less vibration than those that have been put together from spare parts.

I don't think some realize the importance of balancing the entire rotating assembly when rebuilding a Chev-4 right down to weighing the big ends and small ends of each con rod separately. A well balanced motor is a joy to drive. Hopefully your mechanic balanced your motor and what you feel is normal. Cheers

Thanks for all the inputs! Some very good info. The "Mechanic" was yours truly! HA! Therein may lie the problem... Seriously though, When I dissembled the engine I could tell the engine had been overhauled not too long before its sudden stoppage in a parade in 1971. My guess it was late 60's as I got the car from my uncle along with a lot of its history. All of the Babbitt bearings looked like new with very, very minor wear. I had the block machine honed and the crank polished and checked for balance and straightness. I carefully assembled the engine ensuring every rod and piston went back to the respective bore. I also installed new valves, reground cam, NOS lifters. While I didn't balance the entire reciprocating assembly I weighed each piston and rod and all were within an ounce of each other. While this would be high for a modern engine I felt with the RPM of these old engines it would be ok and probably normal for a bunch of cast iron pistons. The car runs quite smoothly at idle, it's when the car revs up above ~1200 RPM that the car shakes a bit.

Hopefully I will get this sorted out and I will not have to take the engine back apart. Not going to do anything major until I can locate and listen/feel another 4 cyl engine.

Thanks for all your inputs!!!

You should look on the 28 site and see what the Australian member did to MONTY. My new project 28 speedster will get new improved motor mounts. Bill

I've been thought his entire blog quite a few times and missed that mod... I will have to go back and look.

Problem with modified mounts is it changes the car from original. I have been very strict ensuring that the car is as close to original as I can possibly make it. The only deviation to date is the carburetor which came on the car when I got it but was not original. The carburetor was made by Stromberg (OE-1) specifically for the 490/Superior. I actually have the original manual that came with the carb.