Pete
BANNED
You were told wrong.
Model A flywheeel lightened
- Thread starter charlesea
- Start date
You were told wrong.
Tom Wesenberg
Well-known member
A radical cam can make an engine run like crap, like those rough running, loud machines at many car shows, but it shouldn't make excessive vibration. I haven't heard of any Model A cams that radical though.
BillLee/Chandler TX
Senior Member
Anxious to see an answer to this!
Last edited:
Pete
BANNED
A long duration race cam in a model A engine can be made to idle quite smooth
at 600 to 700 rpm. Smooth enough to balance a nickel on the deck lid.
I had a flathead B engine with a 4 ported block and a 305 degree cam at the Riverside hill climb in 2002 that I drove all around LA for a week.
There are several long duration high lift cams available for A/B engines but the cost to install is WAY beyond most peoples budget. That is why you don't hear about them much.
Mike V. Florida
Well-known member
as far as vibration, James Rogers of DreamWorks engines has stated that as of a couple of years ago he has done probably a hundred and have never had a complaint, in fact quite the opposite. He says they work great on a balanced crank or a stock one. He has found that, most of the vibration in a Model A comes from bad workmanship or flywheel housings that are broken or warped and not re-conditioned.
There are two separate types of vibration in regard to a rotating crankshaft/flywheel/drive line. Lateral vibrations and torsional vibrations. Lateral vibrations are caused by mismatch of weights (pistons, rods, unbalanced flywheel, etc). Lateral vibrations (at 2 x RPM) are also caused by the vertical acceleration at BDC being different than that at TDC, which are inherent for a 4 cylinder in line engine. Torsional vibration is oscillatory twisting of the crankshaft and drive line. For example (when looking at the front of the crankshaft) #1 rod journal will have a different oscillatory “clocking” than #4 rod journal at a given load and RPM. You cannot usually “feel” torsional vibration, although you can sometimes hear it at a given RPM if the clutch pressure plate springs resonate with the torsional vibration. However, the crankshaft journal fillets DO “feel” the vibration in the form of oscillatory stresses (fatigue stresses) that, over time, can cause fracture. The magnitude of these oscillatory stresses is related to the firing impulses and to the natural frequency of the rotating assembly. The natural frequency (there will be several of them) will change if the flywheel weight (or more specifically the moment of inertia) is changed. It will also change if the diameter of the crankshaft journals is changed (which changes the torsional stiffness). The maximum magnitude of the oscillatory stresses will occur at a resonant condition where the firing impulse frequency is at or near a natural frequency of the rotating assembly. This is called a “Critical Speed”. On many engines, a harmonic damper is installed at the front of the crankshaft (as a part of the pulley) to reduce the oscillatory amplitudes and associated fatigue stresses at critical speeds. So, changing the flywheel weight will change the critical speed at which the maximum fatigue stresses occur and may increase or decrease their value, as well as the location they occur (#4 main journal, #3 rod journal, etc). Also, the torsional vibration characteristics for a Model A engine vs a Model B engine are different – since the Model B has larger crankshaft journals and therefore more torsional stiffness. This may be the reason Ford went to a lighter flywheel on the Model B.