5-blade vs 7-blade std duty.... [Archive] - Chevelle Tech

: 5-blade vs 7-blade std duty....

Nov 28th, 10, 11:30 AM
The set up would be in a 1966 Chevelle SS Big Block car. I am looking to install a 7-blade standard duty GM clutch fan # 3931002 in place of the original style 5-blade standard duty GM clutch fan. Both use the standard 1.5" fan clutch . The question is do you think it will make a differance in the cooling 5-blade vs 7-blade ? I will also be using a Griffin retro 6-564AM-BXX Aluminum radiator 2-tube 1.25" with a fan shroud. I am trying to avoid going with the H/D cluch as the clutch is 1.75" and the 772 fan as that just brings the fan closer to the radiator.

Nov 30th, 10, 8:37 PM
NoBody... just looking for some feed back

Dec 1st, 10, 1:10 AM
My understanding is the 7 blade will improve air flow marginally at the expense of more noise and a slight harmonic vibration (drone) at certain RPM's. The odd spacing of the 5 blade was done to lessen harmonics. Are you running a shroud? If not, adding a shroud is probably a bigger improvement than changing blades.

Dec 2nd, 10, 10:16 PM
Hi, Yes I will be running a Groundup Big Block shroud with a Griffin 6-564AM-BXX 1.25" 2-row Retro Aluminum Radiator they say there good up to 600 HP. I am running a 427 Big Block with a Comp Cam 1800-6000 rpm hyd cam with closed chamber GM heads 10.1 compression. I also was told I can move the spring on the fan clutch to bring it in a little early ? I wanted to go with the 772 fan and a hayden H/D clutch but that brings the set up closer to the radiator ?

Dec 3rd, 10, 11:47 PM
You also want the fan blades to be half in and half out of the shroud, if you weren't aware of that. :)

forever young
Dec 12th, 10, 8:17 PM
You also want the fan blades to be half in and half out of the shroud, if you weren't aware of that. :)

Dave, why is that? This is the first time I have heard about that. I am going to be assembling my BB with a 772 fanblade and the stock shroud that came in the 70 SS BB.

Dec 12th, 10, 9:42 PM
Roland - I've read that several different times, don't understand the principle exactly ...

Here is one example:


Then, I just read it should be 1/2 to 2/3 into the shroud. So now, I'm not really sure what is "optimum." :confused:

Dec 13th, 10, 6:05 PM
When the blades are 1/2 out, the rear non-shrouded edges of the blade sling the air outward radially, through centrifugal force. So the fan not only pulls air rearward through the shroud, but it also shoots it outward. This outward moving air also draws more air in behind it. It's this outward motion that causes the fan to draw more air through the shroud than it normally would, especially at high fan speeds.

Dec 14th, 10, 6:02 PM
Good info, Scott - thanks! :) So, that means that 1/2 in and 1/2 out of the shroud is optimum? Just to be sure. ;)

Dec 15th, 10, 10:55 PM
Slight change of plans sent the Griffin radiator back ordered an Alumi-tech from dave. smaller core should allow me to run the 772 fan with a hayden h/d clutch ?

Dec 16th, 10, 12:25 AM
Slight change of plans sent the Griffin radiator back ordered an Alumi-tech from dave. smaller core should allow me to run the 772 fan with a hayden h/d clutch ?

Clint, I'm running the 772 fan and H/D Hayden clutch on a 454 with short water pump in my 67. A repro 4 core radiator and factory shroud, and the fan is half in/half out of the shroud. Perfect fit.

Dec 17th, 10, 2:18 PM
On my old '66 ( 396, SWP), I used to run a stock 5-blade clutching (L78) . I switched to a stock GM 7-blade ( bought through Year One? years ago). I needed a different clutch ( fans bolt pattern was different).

IIRC, it did cool a little better. Used the factory shroud with both... I ended up going to an aluminum radiator, then to better pulleys and a better pump, but kept the 7-blade.

There were NO harmonic issues with the 7-blade, but it was a GM fan with differing spacing between the blades...

Dec 19th, 10, 9:59 AM
I Have a 1968 1002 GM clutch fan 7-blade thats standard duty that measures 2" Thick, I could use it with a GM fan clutch standard duty that would measure 1.5" ? I would like to use the 772 GM clutch fan and the Hayden H/D clutch 1.75" I am trying to find out how thick the 772 fan blade is..As that would be the best cooling for my Big Block with the blades being curved I am guessing it would help the fit. Any info or pictures would be great...Thanks Clint

Dec 25th, 10, 12:45 PM
Happy Holidays to all...
I received my Radiator from Don on the 24 of December put it on my radiator support it fit like a Glove perfect....fit. I know most of you say it should, but I also installed a Griffen earlyer this month and it was twisted from the factory. It would not fit flat against my radiator support and I know that my Car was never in a fender bender. I guess you get what you pay for...! Alumi-tech is owned and operated by Don here in the U.S.A. be American buy American when ever you can.
You will be glad you did.

Bill's 74
Dec 25th, 10, 12:53 PM
I friend had a correctly shrowded 5 blade and had issues on warm days and/or in traffic. Changed to a similar design 7 blade, with NO other changes, and all problems solved.