Identifying 6/71 Cases

[805moparkid]

i have not found a single thread other than this post that breaks down the visable differences between a 6-71 Small Case/Bore, 6v-71, and a 6-71 Large Case/Bore.

The small bore 6 has 3 reinforcement cross bars on the inlet side, and tops of all of them are machined.
(no pic)

The stock Gimmie 6 big bore case has has 3 ribs also, but only the center one has the top of it machine.

After market cases don't have the outer cross braces and sometimes the blower builders will remove the outer ones from stock Gimmie, but all stock GM manufactured cases had the 3 bars because the blowers are mounted on their sides when they are used on a Gimmie diesel.

anything to add?

 

[805moparkid]

bump

 

[70Duster440]

The difference between a standard size 71-series blower and the later "small diameter" blower is barely perceptible, and would make little difference in performance on a street engine. However, you don't want to mix these rotors and cases. Here's a quote from the book, Street Supercharging:

In approximately 1978, GMC introduced a new line of 71-series blowers, labeled simply "4-71" and "6-71," which have smaller diameter rotors than previous 4-71's and 6-71's. (The reason for this reduction was to reduce horsepower required to drive the blower, to reduce fuel consumption.) The difference in rotor size is hardly perceptible, even with a pair of these blowers side by side. The exterior of the cases, and the end plates, gears, etc., are the same. However, the large (early) rotors will not fit in the small (late) case; on the other hand, the small rotors will fit the in large case, but will give clearances that would make the blower nearly ineffective. Either size of 4-71 or 6-71 could be used for an automotive performance blower, with aftermarket end plates, drives, manifolds, etc. as long as the rotors match the case, and are clearanced properly.

In other words, the reason it's so hard to tell externally is because the walls of the later small rotor case are thicker.

The 6V-71 has an oddball case requiring a less common style intake. Supposedly Dyer's makes (or made - past tense) components for the V-71 series. I'd stay away from them.

 

[805moparkid]

The difference between a standard size 71-series blower and the later "small diameter" blower is barely perceptible, and would make little difference in performance on a street engine. However, you don't want to mix these rotors and cases. Here's a quote from the book, Street Supercharging:

In approximately 1978, GMC introduced a new line of 71-series blowers, labeled simply "4-71" and "6-71," which have smaller diameter rotors than previous 4-71's and 6-71's. (The reason for this reduction was to reduce horsepower required to drive the blower, to reduce fuel consumption.) The difference in rotor size is hardly perceptible, even with a pair of these blowers side by side. The exterior of the cases, and the end plates, gears, etc., are the same. However, the large (early) rotors will not fit in the small (late) case; on the other hand, the small rotors will fit the in large case, but will give clearances that would make the blower nearly ineffective. Either size of 4-71 or 6-71 could be used for an automotive performance blower, with aftermarket end plates, drives, manifolds, etc. as long as the rotors match the case, and are clearanced properly.

In other words, the reason it's so hard to tell externally is because the walls of the later small rotor case are thicker.

The 6V-71 has an oddball case requiring a less common style intake. Supposedly Dyer's makes (or made - past tense) components for the V-71 series. I'd stay away from them.

it does matter, the difference is more between a small and large case than a 6 and an 8/71...

Here are some general displacement figures for standard helix (30 degrees of twist) roots blowers: A small rotor 6-71 displaces approx 339 cubic inches per revolution, a standard rotor 6-71 is 411 cubic inches, a B&M Megablower is 430 cubic inches, and an 8-71 is 438 cubic inches.
One thing to keep in mind, due to air leakage between the case and rotor tips AND between the ends of the rotors and the ends of the case, these blowers will not be able to move that exact amount of air/fuel per revolution, especially when the boost comes up. It will be less.
Another dimension to note is that a 6-71 rotor is 15” long, the 8-71 rotor is 16” long, a 10-71 rotor is 17" long, a 12-71 rotor is 18" long a 14-71 rotor is 19" long and the 16-71 rotor is 20” long. All rotors are the same diameter with the exception of the small rotor 6-71.

 

[70Duster440]

Don't shoot the messenger - just passing along one author's opinion. Your question was how to tell the difference between large and small rotor units - presumably without disassembling them. The 6V has angled mount holes - very obviously different. The fact that the large rotor units move more air per revolution as do the 8-71 and up is a given. They're going to be more efficient on large displacement engines.

What, exactly, are you concerned about?

 

[805moparkid]

Don't shoot the messenger - just passing along one author's opinion. Your question was how to tell the difference between large and small rotor units - presumably without disassembling them. The 6V has angled mount holes - very obviously different. The fact that the large rotor units move more air per revolution as do the 8-71 and up is a given. They're going to be more efficient on large displacement engines.

What, exactly, are you concerned about?

if i buy a 6/71 i want to know im buying a large case not a small case...

 

[70Duster440]

Well, then other than the difference in ribs across the inlet (as you've pointed out), and assuming that the extra ribs have not been machined out, measuring the case thickness at the inlet or outlet would be the next best answer. Judging by the small photo I have of both cases with the end plates removed the small rotor cases are upwards of 1/8" to 1/4" thicker.

I've also attached an article you might find useful, if you don't have it already.

 

[805moparkid]

Well, then other than the difference in ribs across the inlet (as you've pointed out), and assuming that the extra ribs have not been machined out, measuring the case thickness at the inlet or outlet would be the next best answer. Judging by the small photo I have of both cases with the end plates removed the small rotor cases are upwards of 1/8" to 1/4" thicker.

I've also attached an article you might find useful, if you don't have it already.

i use firefox so i cant open any PDF's anymore! GRR!