Inline six cylinder engine harmonic characteristics 1

[strokersix]

In the early '60s General Motors intorduced the 194/230/292 series of engines. The crankshafts have seven main bearings and six counterweights. The counterweights are located next to main #1, in front of main #3, both sides of main #4, behind main #5, and next to main #7. With the introduction of the 250 engine in the late '60s both the 250 and the 292 crankshafts had 12 counterweights.

My question is: Does the 12 weight design have different torsional harmonic characteristics as compared to the 6 weight and what effect does this have on crankshaft durability?

Thank you,

Mike

 

[GregLocock]

Yes it will affect the torsionals, although it is hard to say which way it will go for sure, generally a 12 will be less mass effective than an 8 counterweight design, so it will have lower torsional frequencies.

The durability will be affected by too many other considerations, but if the first mode is under the red line then it will be excited more often. On the other hand, dropping the frequency tends to reduce the energy of excitation a bit (maybe, can't see why off the top of my head) so you may get lower torsional amplitudes so it may be better.

When they next designed the crank how many c/w did they go to?

From my point of view a 12 cw crank is often a cop out - the engine designer is basically saying that he can't be bothered to analyse the inertia forces and bearing loads properly, so he just treated the engine as 6 single cylinder engines. You might have to do this if your crank is very floppy.

We had a 12 c/w crank for a few years but went back to 8, the weight saving was worth more than the arguable improvement in sound quality. We went to it to reduce the amplitude of the bending resonances in the crank, which are always at around the same frequencies as the TVs, and tend to introduce a harsh sound when they are both excited simultaneously.

Cheers

Greg Locock

 

[strokersix]

Greg,

Thank you for your comments!

The design history of the 292 engine crankshaft as I understand it is:

'63-'64 forged steel, 6 weights, 7/16 inch flywheel bolts
'65-'67 nodular iron, 6 weights, 7/16 inch flywheel bolts
'68-present nodular iron, 12 weights, 1/2 inch flywheel bolts.

This engine series also includes a 194, 230, and 250 inch displacement engines. The 250 was introduced in '68 I think and it has 12 weights. The 194 and 230 cranks I have seen all have 6 weights and were produced in steel and nodular iron through about '70.

All crankshafts are interchangeable with the appropriate flywheel bolt pattern. There was at least one design change to the damper also. The earliest dampers had two vee belt grooves and the later ones have three and appear to be more massive. This design change may simply be an accessory drive need rather than a torsional damping need, I don't know.

 

[GregLocock]

Were they dampers, ie with rubber between the inner and the outer section, or just heavy pulleys? It is surprising how often a bit of mass solves vibration problems.

Cheers

Greg Locock

 

[strokersix]

Greg,

Yes, they were dampers. Typical center hub/elastomer/outer ring construction.

Mike