1968 Mso H.p. (sae) 57.8

[Donald B.]

Thanks to the NCRS release of the shipping data reports I now have a photcopy of the MSO. At the bottom left it says H.P. (SAE) 57.8. Can anyone shed some light on the meaning of this designation?
Thank you. Don.

[Wayne W.]

Re: 1968 Mso H.p. (sae) 57.8

It basically means its a 4.25 bore and 8 cylinders.

That said, here is the story. It is the N.A.C.C. (National Automobile Chamber of Commerce) formula for determining HP. It was used by some governments as a taxing formula. It makes a lot of assumptions etc. about engine speed, compression, etc., but boils down to a chart that considers the piston size and number of cylinders.

The formula is HP= (Dia)2 X #Cyls / 2.5

[Duke W.]

Re: 1968 Mso H.p. (sae) 57.8

It's usually referred to as "taxable horsepower". The formula was originally developed in the U.K. as a way to tax automobiles in the early 20th century, which is why most English engines had very "undersquare" bore/stroke ratios (less than one) well into the post WW II era.

I believe a few U. S. states may have also used it up until about the WW II time frame. It fell out of use in the U. S., but OEMs continued to include it in vehicle specifications for some time, thereafter.

One nice legacy is that it's incontrovertible proof of whether a Corvette was originally equipped with a small block or big block because for a given number of cylinders, it's a monotonic function of engine bore.

Taxable HP = (bore squared x number of cylinders)/2.5

So a 4-inch bore V-8 is 51.2 and a 4.25-inch bore V-8 is 57.6.

Why is it referred to as "horsepower"?

It was known at the time (and is still true today) that an engine's power potential is a function of cylinder bore. For a given bore and inlet and exhaust system flow restrictions, power doesn't vary much with stroke. Regardless of stroke, otherwise identical engines will produce about the same peak power at the same mean piston speed. It's the reason why most purpose built racing engines are extremely oversquare - like F1 engines that are believed to have extremely oversquare bore/stoke ratios of about 2:1.

At the time the formula developed in the early 20th century, it represented typical power that was produced by an engine of the given bore size and number of cylinders with the technology of the time, but as time advanced, improvements in materials and design yielded ever increasing specific output at ever higher mean piston speeds.

When a 350 CID engine became standard equipment in the Corvette in 1969 that used essentially the same heads, induction, and exhaust system with the same compression ratio as the previous 327 CID base engine, the advertised peak power did not change, but rated power speed was slightly less. Advertised peak torque increased because peak torque is primarily a function of displacement and compression ratio.

Duke

[Donald B.]

Re: 1968 Mso H.p. (sae) 57.8

My thanks to Wayne and Duke.

[Dale C.]

Re: 1968 Mso H.p. (sae) 57.8

How and where did you get the MSO copy? Looks like from that you could deduce or confirm some engine info.
Dale