Packard Forum Index » Pre-War (1899-1942)

Change in tire size nomenclature between 47 and 48, mine should have had 8:20 x 15, but (a) they were one hell of a close fit in the rear, and (b) I didn't have the money for the right ones anyway, the Dunlop G-78 were (at the time) about $45 each, and lasted 20 years.

I've done quite a lot of checking on this subject for my Speedster engine, and came to the conclusion that 4000 rpm is the max I'd ever want to take out a 356. It gets interesting when you start researching piston speeds, and how fast things like to fly outside the block when you exceed the max. I'd start with a Piston speed calc to determine what the piston speed at a given rpm is per your stroke of your particular engine. Then, keeping in mind that 3000 fps is the max you want to go with a stock engine, you can figure out what your max safe rpm is on a stock engine.

Take the 356, with it's 4.625" stroke, running at 4000 rpm. The piston speed of the engine at that rpm is 3083.333. This is why I will rev-limit my engine to 3800 rpm, to be under that, which puts it to 2929.167.

Now, you ask where did I come up with the arbitrary 3000fps number? Everything I've read on the subject says that 3000fps is the safe limit for a stock engine. Example
Keep in mind that our old straight eights are no where near as short stroke, short rod type of engine like the Ford engines in that example. Knowing this, I wouldn't take a stock rod bolt, stock piston straight eight past 3500rpm, just to be safe. I won't even mention the poured babbit engines...

Guys

When I was little, I practiced reading on old Road & Tracks that my Dad had laying around. Actually I hoarded them as soon as I got my hands on them.

They used to have a calculated data panel in the road test results. One item was piston travel in feet/mile, which should be easily found if you add the stroke into your figuring described above. I think the optimum figure is about 2250-2500 feet per mile.

In case the point wasn't made, there was a calculated wear index expressed as a percentage which I think took into account the piston travel and vehicle weight.

I believe these useful figures were dropped about the time John Bond sold the magazine. I'm fuzzy on the math as I was less than 10 at the time.

These issues came up, I'm sure, because of the number of British cars people were driving in the 50's & 60's with long strokes and wall climbing rear axle ratios.

Figuring out the math on these again would be useful for us fossils driving fossil cars top figure cruising speeds.

Happy Thanksgiving

John Harley