Quote:
Originally Posted by Cuttyman9
I did a bit more searching. 8v92 was the diesel it was under, the 92 was the cubic inches per cylinder equating to 736 ci. The blower is needed because it's a two stroke and it doesn't make enough compression or something
I'm going to need to find more info on how much air the blower pushes at a specific rpm so I can somewhat calculate the psi of boost.
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Detroit Diesel's nomenclature is the first number is the number of cylinders and the second number is the displacement in cubic inches per cylinder. Where a "V" is between the two numbers indicates it is a V type engine.
As these particular Detroits are two strokes, they aspirate every revolution. The reason for the blower in the first place is not for any supercharging, but to aspirate the engine as a two stroke does not have a dedicated intake stroke. Pressure to charge the cylinder must come from an outside pump.
The 2 stroke Detroits have been around since the late 1930's, so it didn't take very long for someone to figure out their blowers would make great air pumps to supercharge 4 cycle engines.
You really aren't going to need an 8V92 blower to pump up the 305. A 6-71, 4-71 or even a 6V53 would do the job. The smaller the blower, the faster you will have to turn it for any given amount of boost on your 305.
A few things to remember about the GMC blowers. The bigger they are, the more power they'll need to turn. Also, for any particular blower, the faster you turn it, the more power they absorb. (double blower speed, and the power requirement increases about 4 times depending on the internal leakage) Also, the GMC blowers become less efficient as you try to extract more boost from them.
I'm thinking a 4-71 (284 CI) or a 6-71 (426 CI) blower will give you the boost you want without absorbing to much power or becoming less efficient. Not to mention they are smaller and will probably package better on a narrow/shorter engine like the V6.
B&M used to make a couple of blowers, one was I believe 144 cubic inches for mildly boosted 350 sized engines and the other was a 177 incher for mild big block sized engines. Both of those were of smaller capacity than even a 4-53 GMC blower (212 CI) and they were of a nice small size easy to fit in most applications.
In theory, a blower providing 305 cubic inches of air per revolution directly driven at crankshaft speed would make 15 pounds of boost on a 305 cubic inch 4 cycle engine. In reality, the charge heating would probably reduce intake charge density to the equivalent of about 8 pounds of boost. If I recall, the maximum efficiency of the GMC roots blower is about 45 percent.
Between your engine size and boost desired, for any given blower size, you'll be able to easily calculate the needed drive ratio. I'm sure there is plenty of information and probably graphs of the various outputs of the many blowers out there, including the GMC's.