My 1070 350 300 hp car came with a 4 blade fan w/o clutch. At the shop I found a & blade fan and I need to do is buy a clutch. Do I need a 17.5" fan or an 18"? Will it affect my fan schoud? I want a long clutch right?

Brian

Save yourself the hassle of clutches and so forth. Try checking out the 17" fans from Derale #17217 heavy duty flex series. These are great products and move more air than the BB models. Also will take less hp up top. About $60 from Summit.

You want me to run a 6 blade fan without a clutch? that seems foolish
i don't want chrome/ stainless
I have access to 5/6/7 blade fans for free why buy a new one

Well...things are always what they seem to be!

Fixed bladed fans can take up to 40hp+ to fling around and even 30ish with a clutch. The new technology Derale takes less than 7hp at full tilt because it gives up it's "biting" attitude at high rpms. Pretty effective really and they come in black too. Here is a similar link to an older test we did several years ago on fans etal. http://carnut.com/ramblin/dyno.html

Clutches are an enigma. You NEVER CAN TELL IF THEY ARE WORKING and most are "universal" design. When GM was making clutches for specific applications (such as small block, Bblock, air, heavy duty cooling, towing, truck, heavy car, light car...well you get the idea) they had over 37 differing part numbers for each specific year of manufacturer. These would address such things as working temperature, fan type,size and pitch, peak rpms of motor, retention settings, idle "meshing" and other specific design applications. With your now moreover "universally" designed clutch (and I don't care who it's made by), they are guessing at a range of performance issues such as temperature range engaging/disengaging points, upper rpms design (this point is significant whereas most of these cars in the era that they were created were only turning 5500 rpms max! unlike todays performance engines that can easily wail thru 6500 rpms with ease). I know of of at least one manufacturer that states their upper rpm limit is 5800rpms. They are also "reversed" engineered as I said and IMO cheap replicas of the originals. I recently have seen more clutched fans come apart and start to depart parts than any high quality flex fan such as Derales 17xxx series which are good for 8000rpms and made better than the "fixed" OEM ones. So, I report...you decide.

I am not sure what you are trying to accomplish here. Originality or more cooling or what. Electric fans are a better alternative for big cooling issues tho if that's where your going.

Did it have a shroud and not a fan clutch and only 4 blade? That sounds strange. If I was trying to upgrade my stock system, I'd just get the parts that were used stock for the "with AC" or towing/HD cooling sytem option. Stock systems work just fine if in good condition. Check assembly manual or parts manual for p/n's. There's a guy selling parts manual on CD, Check restoration Forum here.
My $0.02

hi steve,
summitt has part#17017 for about $32. just so its clear, you feel thats a better choice for a big block powered 454 chevelle than the stock 7 blade with a clutch? i have a 7 blade gm fan with a new clutch i was going to use but its 19" and won't fit in my 69 shroud. if you feel it would be a better choice i would rather have the non chrome version especially if it would not rob as much power as the 7-blade gm with a clutch. i spin my motor to 6K pretty regular. i had a bad experience with a rebuilt water pump with a mint 7 blade fan and replacement clutch. it appears the piece on the water pump that the attaches to came unwelded and took out clutch, blade , shroud and new4 core radiator. i replaced all the compoments with the gm P-30 step van radiator setup(aluminum, plastic side tanks), new hd water pump but used an old 6-blade flex fan i had lying around. in heavy traffic it goes to 220 and thats too hot for me. if in your opinion that fan would help for the cheap price i'd give it a shot. thanx mike

ratuned........absolutely. I think a 18" will fit and would be better in fact if you can get it in but a 17" will work as well. Make sure you position the fan about 2/3 the way into the shroud. Usually this takes a 2" spacer to make up for the stock clutch. But depends on model year etc so maybe two one inch spacers can be purchased and use what you need.

I have converted so many clutched vehicles with these fans I have lost count. All cannot believe the difference.

thanx again steve,
one problem that i notice on the 69 and 70 big block chevelles is that the shroud only covers about 2/3 of the radiator. also it is very shallow where the blade sits into it. it appears to be poorly designed. i should look into if the 70-72 shrouds cover the radiator better. thanx again for your advice! mike

how does the derale fan use so much less HP?

Which would use less horse power/torque
1. derale fan
or 2. stock 4 blade fan

I have seen some very cheap flex fans and I heard of them loosing fins from spinning so fast. But theses derale units are solid as or better then stock?

While I have not tested myself the Derale unit on a dyno, the four bladed stock I am sure will use more hp than 7hp, but I am not familar with your particular fan etal. There are several differing four-bladed fans out there on GM marks. The Derale has high tensile strength stainless blades that are "progressively" formed to give up most of their "bite" over 2500rpms. It's amazing how much it becomes literally inert at higher rpms where you don't need the fan cooling per se.

Bri I have just installed a 7 blade fan from Ground up with a NON Thermal clutch at Mr 4 speeds recomendation. I had a running hot problem at idle with a thermal type clutch. Have not seen hot weather yet but gauge seems to run a lot cooler. The Non thermal type disengauges at 1800 RPM or so and pulls like crazy at idle. So the Non thermal seems the way to go!
Gary

According to a 1970 Pontiac brochure that was heavy on the engineering dope (entitled "For people who think driving should be fun...") they state:

"Fans. They cool. They also eat horsepower,. Conventional fans slurp up 20-24 horses @ 6000 fan rpm. Flex fans, about 5 or 6. And a clutch fan, about 4 or 5 horses. Take your choice.
Why the difference? The flex fan blades flatten out at high rpm. Less resistance. Less drain. The clutch fan is thermostatically controlled. Cold, it runs no faster than 1000 rpm. Hot, it engages and picks up to 1800 rpm. All because of a little clutch."

Flex fans were recalled many years later because, with flex comes metal fatigue eventually (or fiberglass fatigue) . Spinning shrapnel.

Fan clutches can either seize up and lock up solid, or they can just refuse to lock up at all and just freewheel. Certainly a lot safer in terms of inertia, but not infallible either.

Electric fans that engage on demand seem like a viable alternative.

The Pontiac brochure shows the available 19½ " clutch fan (7-blade) and the 19" Power-Flex fan (5-blade) which was listed as standard on the GTO & G.P.

I thought that once GM switched to the crossflow radiators (1968 Chevelles & Novas, 1967 Camaros, 1971 full size Chevies) the radiator shroud was made standard on all. True, Corvettes used the crossflow style and the L88 models did not use a shroud, but in general terms the shroud was standard.

so...
Stock fans use a lot of horse power
Clutch fans are not reliable or accurate
Flex fans can fatique and fall apart (doesn't look stock)

So can we agree
Buying an electric fan will save you the most horsepower, move alot of air acurately.
Whats the downside to electric fans? non stock appeance?