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Rusty O'Toole Quote:

Yes, but what's the fun in that???

Craig

This may be a US thing so I don't understand, but why blow through a carb with a Paxton or similar supercharger when you can suck through a better and lower profile manifold with a Rootes style unit?

Quote:

There are several possible reasons.

The Paxton or McCulloch supercharger was designed as an add on accessory. It was easier, or more convenient, to blow through the existing carb. Other centrifugal blowers have been set up to suck through but these were OEM installations.

Centrifugal blowers have several advantages over Roots type. One is they are more efficient, and produce more boost on less parasitic drag. Another is they are more compact.

The centrifugal job has the disadvantage that boost is not linear. They tend to have little or no boost at low speeds and overboost at high speeds. Paxton overcame this with his variable speed drive. It allowed the blower to run slowly when not needed, to go into high when needed, then back off to maintain a constant 4 or 5 pound boost at all speeds.

The Paxton was a successful effort to make a blower that was adaptable to all kinds of engines. Roots type installations are bulkier, more expensive, less efficient and must be tailored to each engine. I dare say that in the nearly 60 years since it was introduced the McCulloch and its successor the Paxton blower have been used on more engines, and more different engines, than any other.

Thank you Rusty - that explains the reasoning very well. On my own special that I am just starting I will be using a Rootes type blower as that is what I have experience in and also what I have 'in stock'. One of the other benefits of the Rootes over the Paxton style is, as I understand it, that the Rootes blower spins at much lower speeds, ideally only 1 to 1.5 times engine speed. Is it correct that the Paxtons spin much faster and that this, in the past, has perhaps caused some reliabilty issues or are these issues more likely to be service related?

Sorry for the hijack...

The impeller of the McCulloch supercharger is a pinwheel 4 or 5 inches in diameter. In operation it spins up to 50,000 RPM.

This was achieved by a double gearing up. First there was a variable speed pulley arrangement. This could go from 1:1 or less up to 2:1 or a little more. There was a fixed pulley on the engine crankshaft, a variable speed pulley on the end of the supercharger and a spring loaded jockey pulley. The variable speed pulley was controlled by an electric solenoid built into the supercharger, and internal air pressure.

Second was a sort of planetary gearing using 5 ball bearings about an inch in diameter in place of gears. The ball drive was arranged to give a step up gearing of 5:1. It worked by friction.

So, there was a possible total gear ratio of 10:1. The impeller never spun the full 50,000 RPM because the engine would have to be revving 5000 RPM and at that speed the supercharger would be backing off to prevent over boosting.

I don't know about reliability issues. They were considered reliable enough to be installed at the factory as optional equipment by Kaiser, Studebaker and Ford. Studebaker used them the longest, from 1957 up to the early sixties.

Incidentally it's a Roots blower not Rootes. It was originally invented by the Roots farm machinery company of Connorsville Indiana in the mid 19th century. Had nothing to do with the Rootes company in England.

More than you ever wanted to know about Roots blowers:

roystongroup.com/history.htm

Robert McCulloch developed the McCulloch centrifugal supecharger. His earlier major product was the McCulloch chiansaw which revolutionized the product by producing a one-man chainsaw. The supercharger was renamed the Paxton in the mid-50's as is name was Robert Paxton McCulloch.

In the 70's he bought 25 square miles in Arizona and developed Lake Havasu City, buying and bringing London Bridge from London as an attraction.

Coincidentally his initials are RPM

Wheels within wheels indeed Fred, and thanks again for the info. I was interested to note that J. Vincent used a roots type blower on his S8 Packard Speedster, but then drove it using some sort of hydraulic clutch arrangment. I intend to keep mine much more simple. Chain driven Roots blower sucking through a couple of carbs and pumping into a fabricated manifold.

Egge valves are not good in any kind of performance application the hardening they use on the tip of the valve will flatten out like a nail with any thing over stock, I've seen it way to much. The best (and most costly) option would be to get custom stainless ones made through Manly not cheap but the stainless will resist heat better and you will never have to worry about them. Tucker makes some really good seats in my experience.

I am very interested in your project. I, too, bought a McCulloch supercharger for my 1938 straight eight. It came with a spring-loaded belt tightener that pushes outward to tighten. I need to decide if I need the belt tightener. Photos on the internet of Packards show no tightener. I also need to drive an air conditioning compressor. I will mock this up, but right away I can tell that there is little room for a second belt. The fan is very close to the radiator.