1932 900 Engine Cruise rpm

[petri]

I'm curious as to what the engine rpm is at a cruise speed of 55 mph is for my 1932 900 Light Eight. I would imagine all Packard straight eights from this era cruise at the same rpm.

I did a calculation using tire diameter, calculating tire circumference, rear end ratio of 4.36/1, high gear, 1/1, and a slippage/friction factor of 15%. I come up with something around 3,000 rpm at 55 mph.

I was a little surprised as I thought it would be a bit lower as these are long stroke, high torque, low rpm engines. Does anybody have an actual tachometer measurement of this parameter or know the answer to this question?

I know, but it's winter and inquiring minds want to know.

thanks,

Howard Petri

[Owen_Dyneto]

Packard published that kind of data rather routinely in their service letters and I expect you'll find it if you take the time to browse the data. But for comparison, my 34 Eight sedan with 4.69 rear axle turns about 2600 rpm at 53 mph. Your 900 should have 6:50 x 17 tires vs my 7:00 x 17. Given that and the different axle ratio you have, you should be able to work it out but your situation should be very slightly more advantageous.

Here's some other data from Packard's service letters for the year before your car:

Model 826/833 with 6.50 x 19 tires: 50 mph = 2302 rpm for the 4.38 axle, 2463 with the 4.69 axle.

Model 840/845 with 7.00 x 19 tires: 50 mph = 2219 rpm with the 4.38 axle, 2387 with the 4.69.

Edited January 15, 2011 by Owen_Dyneto (see edit history)

[petri]

Thanks Owen-Dyneto for the prompt response. I have the Service Letters but hadn't thought of looking there for this information.

If I do the calculation at 50 mph and leave out the slippage correction, which was only a wild guess, I get 2443 rpm. That's getting pretty close.

I wonder if anybody out there has a tach on their car and has actually measured this? I know Art Wennerberg of Blue Island, IL had a tach on his 900 but I never thought of asking him this question and sadly he passed away last year.

In fact, the 19 vs 17 in tire is approximately a 12% change in circumference. Well, one would actually have to measure the actual different tire diameters but that's close I would think. This would make those Model 800 numbers come out in the 2450-2550 rpm range, very close to what I now get. I doubt a calculation is going to get any closer than that.

Howard

Edited January 16, 2011 by petri
Added last paragraph (see edit history)

[K8096]

I know one person who put 1933 Packard Super Eight wheels on a 1932 900 car. They bolted right up with no modifications necessary. Not authentic, but the car rode and handled a lot better.