given same 4 core rad, a proper shroud and hd clutch fan, which fan will cool better? The pitch of the 7 blade 18 inch and the 5 blade 20 inch appear to be the same - 2.5 inch pitch.
A 7-blade 20" GM fan will cool the best. Will have to go to the other divisions (Pontiac, Olds, Chev, etc) to find one as Buick never offered it. I have 2
The 68-70 fan shroud has a 20" opening. The 18" 7 blade fan is best for that shroud. The 71-72 shroud has a 22" opening. The 20" fans will be better for that shroud.
If your engine is experiencing high water temperatures? My 70 GS 455, did. I have tried everything. (1) 7 blades 18-inch fan with correct fan shroud. (2) SD Fan clutch because my 7-blade fan has a 2 1/4-inch pitch. A SD fan clutch is recommended and is NOT loud like the HD clutch fans are. (3) TA's high performance water pump. (4) High flow thermostat. (5) Evans waterless coolant. (6) The best solution for a hot engine. (I should have done first)> A Griffin 2 Core (1 1/4 " tubes) Aluminum Radiator. Turned out to cure my hot engine problem. I tried it ALL, including adding an MSD Ignition system with an adjustable vacuum canister. All because of a serious detonation issue that added to the high engine water temperatures issues. VET
Thanks for the replies. No over heating issues, just slowly finalizing parts of the 470 project. I have a 4 core rad, both sizes of shrouds, both fans. I do have the TA water pump, a 180 hi flow t stat, debating going to the Hayden HD clutch fan. The pitch of the 20 inch is 2.5 inches. Measuring the 7 blade 18 inch in the car, pitch appears to be 2.5 inches as well. I will double check the 18 inch pitch. Will do the engine swap over the winter.
Larry, as usual, is on point. The size of the fan has to match the size of the opening in the shroud (and vice versa) to maximize the pull on the air (and cfm).
The HD fan clutch will make a lot of noise. If you have loud exhaust, headers and a lumpy cam it might not bother you but the standard duty thermostatic clutch fan is more than sufficient if the fan and shroud are done correctly.
As im using a 5.4 inch crank pulley, i think the extra fan engagement at idle will be beneficial. So the HD clutch is likely the way to go.
I had an 18” fan and put in a large opening shroud meant for a 20” fan and had issues. Swapped in a 20” fan and all was good, make sure the fan matches the shroud
The real question is……. 18 inch 7 blade fan with matching shroud or 20 inch 5 blade fan with matching shroud…..my car being a 72 should really have the 20 inch 5 blade, but I just want the best cooling fan for the new 470 build. did Buick go to the larger 5 blade fan in 71 for a reason?
I would think because of the 22" opening in the shroud. One other thing to consider is the fit of the shroud to the radiator. If you look at the 71-72 shroud, there is a gap of at least a 1/2". It doesn't hug the radiator the way the 68-70 shroud does. So even though the 71-2 shroud has the larger opening, it may not be as efficient due to the gap. Now if you can modify the shroud to eliminate the gap, the larger opening/fan has to move more air.
I bought an old cadillac 7 blade x 18" with almost 3.5 " pitch. Search Ebay. And use a thermo clutch in good condition.
