Crab Distributor vacuum brake

[flatjack9]

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Originally Posted by Flathead Fever

I'll toss this out there too about the elimination of the vacuum brake. This is all theory; I have no actual experience on crab distributors. I was just out in the garage, I have a '66 Shelby GT-350 with a factory mechanical advance only distributor which reminded me not all cars had vacuum advances and vacuum brakes. Lots of high-performance Ford engines came that way in the 1960s with around 9.5:1 to 10.5:1 compression. You just had to buy premium fuel. They sold thousands of those Mallory dual point, mechanical advance only distributors for those 1960s-1970s V8s. I just bought one for a 289 in a Falcon I have. It was at a very small Redlands automotive swap meet advertised right here on the Ford Barn. It was brand new for $10.00, got the 289 Falcon headers too for $40.00. The idea on those high performance engines is you don't want the distributor backing off the timing under full throttle and killing horsepower. When it retards the timing it lowers the pressure in the cylinders, less pressure, less down force on the pistons.

What I don't know and I have meaning to research is what was the fuel octane in the 1930s and 1940s I know that Gilmore had "red" gas and "blue/green" gas so that when it went through the site glass on the pump you knew you were getting the octane you paid for but I have never seen what those octanes were? It may be that 87 is higher than what was available back when engines were 6.5:1 and you would do not need a vacuum brake at all with 87 octane. I just checked, I guessed close on the compression ratio, my 1949 Motors Manual says that a '46-'48 Ford V8 has 6.75:1 and 100 Horsepower at 3800 rpm. That compression ratio is a joke by modern standards. It might not ping at all on 87 octane. Back it off all the way and see what happens.

I worked with a mechanic that had worked for a research company formulating gasoline during the lead years, he taught me some stuff. He said, "you are wasting your money putting premium in a regular engine, your paying extra to actually lose power". The higher octane is only to prevent pinging/detonation in a higher compression engine. It burns slower to allow the mixture to be compressed further than lower octane fuel can. The slower burning speed has the same effect as retarding the timing. Even though the higher octane fuel does not produce as much energy that extra compression more than makes up for it.

I'll tell you a secret since were discussing timing. I had a CA State Smog License because we smogged the phone company vehicles inhouse. I couldn't do my own vehicles, I was in the same boat as you guys which just didn't fair. I should have been able to get away with all kinds of stuff on my personal vehicles. The allowable timing is plus or minus 2 degrees from factory or it fails the functional test. A lot of the time you can get a borderline hydrocarbon failure to pass by retarding the timing, but you can only retard it 2 degrees. We did everything by the book at work, no funny stuff. Whatever it cost to make it right we did it. I could not have done this at work but on a personal vehicle if you put premium fuel in it that has the same effect as retarding the timing because of the fuel's slower burn time. When the technician checks the timing, it will be within that plus or minus 2-degree window but it will actually be retarded further than the timing mark shows, with the premium fuel. That is perfectly legal, no rule about what octane you put in the tank. That just shows you how the octane effects the timing.

That is a total myth about high octane gas burning slower.Tests show that regular, premium, high octane produce the same power with the same timing when used in the same engine.