0Tom@Dewitt

I have read many threads over the last year touting MarkVIII fans. Some have even posted cfm and amperage ratings that I never could confirm or question, until now. Serveral months ago I sent a group of fans to a lab to have them tested. Amoung the fans was the MK8 and Spals' dual 11"s.
The lab results are shown below in a color curve. The MKVIII fan was tested at full speed and a lower speed (aprox. 75%). The left side of the chart represents air flow in cubic Feet per minute (CFM) and you would use only the solid lines when referencing the air flow. The right side represents amperage draw and you would use only the dash lines for this information. The bottom of the chart represents the resistance to the air flow in the form of pressure.
To start the comparison, we are going to focus on the solid red line (MK8) and the soild blue line (Spal dual 11's) with zero resistance or static pressure. Spal publishes this fan at 2760cfm and the chart confirms exactly that number while the MK8 tested just slightly below that or 2700 cfm. NOT THE 4000 CFM I have seen posted.
The amperage draw of the Spals is slightly higher, but that is understandable with a slightly higher flow. The amperage draws run paralell with each other until you hit the .6" pressure point (which is about what a radiator might provide) then the MK8 amperage runs up much higher.
My conclusion: using .6-.8" of resistance, these fans are about the same performance. That leaves price, packaging, and mounting and I feel Spal wins that comparision hands down. The Spal package incorporates (12) twelve free flow flaps for driving at high speed. When using our brackets, or even the Be-Cool brackets, the installation is much cleaner and the fans cover the core better.
The point of this thread is to provide all of you ACCURATE information. It is NOT to bash the MK8's or the users of them. They are good fans and I'm sure that everyone using them is happy, so no need to defend them or your decision to use them. They simply are not any higher rated that the Spals and they pull about the same amperage

Durango_Boy

Thanks for taking the time to do this test Tom. If you have the info, I would be interested to know the part number of the MRK8 fan that was tested.

I have always liked everything about the Spal setup but the price. If they were cheaper or more available in any salvage yard in the country I'll bet you'd see a LOT more of them in use.

As it is, ease of finding a cheap used one and even low price of a new one play a huge role in which one is chosen by most people when upgrading to electric.

Eddie 70

Great chart. I have been wondering about my 16 inch Spals current draw for sometime. So, with this 16 inch Spal installed on a DeWitts radiator, I should see .6 inches of resistance to airflow. This would give me an airflow of around 1000 CFM and about 18 amp draw. I have been wondering about it for a while now. Thanks for the chart.

0Tom@Dewitt

Hey Matt,
I don't have the Part number (or the fans) but they were the big ones from a Mark8. I doubt I'll ever see the test fans again as these labs tend to "loose" this stuff. lol
Regarding the price of the Spals, I think a lot of guys are comparing the junk yard price of the MK8 to the complete kit (fans, brackets, wiring, switch, etc) If you prcie the dealer bought fans to the new spal fans only, they aren't that different. We sell the dual fan for $248 but I don't know what anyone is going to do with just fans.
http://www.dewitts.com/pages/product...asp?ProdID=211

0Tom@Dewitt

No, the single fan in this test (black line) was our SP825. This is a 16" fan but it's the curved blade fan which is only 2070 cfm. We use the SP231 straight blade 16" and that one is 2360 cfm. You could use the curve as a reference and the numbers for your fan are going to be pretty close, just slightly higher.

0Tom@Dewitt

TTT,
Boy this sure died a quick death. Two (2) comments and one day later it's headed for the archives. It's pretty expensive to have lab testing done and I thought more people would be interested to see the facts on the famous MK8. Guess not

Durango_Boy

Fear not Tom. I've already made this thread into a PDF paper and hosted it online. If I ever see anyone ask about either the MarkVIII or Spal setup I will post the link to the paper so it won't matter if this thread is worked back a few pages.

By the way Tom, has Spal improved their fan controller units? I remember reading about a lot of problems with them.

S489

tom, it's hard to argue with results from bona fide lab tests, only the interpretation can be questioned - but not in this case.

i was surprised by the results just based on the current draw. i was convinced from reading others posts that the Mk8 required more current, maybe up to 2x that of spal and others, so assumed that the air flow rate had to be more even accounting for fan efficiency.

so were the fans tested in "free air", or were they mounted in something of a wind tunnel? did they provide you with dimensioned drawings of the test setup?

did the testing include a radiator/rad&condenser in front of the fans? how was the 0.6 - 0.8 in H2O value obtained? were louvers used to create the pressure drop? this is sort of the second half of the problem, i.e., the system curve. you have generously provided the fan curves!!! thanks . . .

SIXFOOTER

Good info Tom, thanks for posting. Pretty much verifies what my SWAG was. I have always preferred the dual setup for apearence and backup reasons. Two fans better than one, and I really like that spal shroud

S489

assuming the 0.6 - 0.8 in H2O values are based on the SPAL curve, and making some crude system curves yields:
deltaP = 0.6 in H2O
SPAL - 1800 cfm
Mk8 - 1700 cfm or about 5.5% difference

for deltaP = 0.8 in H2O
SPAL - 1300 cfm
Mk8 - 1150 cfm or about 11.5% diff

how about that!!! the real system curve(s) obviously would yield different values, but the trend is there . . .

SHAKERATTLEROLL

Tom that is some great info. And so now that we have the information that shows as well that you agree with that the flow is about the same between your system and the Mark VIII. Than the few things that will separate them are Fit,Reliability,Customer service after the sale, And then Cost. Not necessarily in this order for some lol.

If I am wrong with any of this please correct it Tom.

Thank You

roscobbc

On my L36 Vette the surface area of the radiator is 374 sq in. The swept surface area of the twin fans is 190 sq in. Even allowing for a small plenum effect between the fan blades/housing and the surface of the radiator it seems that these fans are going to be compromised in terms of performance with cooling air at best passing through 60% of the radiator surface. If the fans were mounted in a plenum sealed to the radiator and spaced further away from the radiator surface surely this would be far more efficient with the likelyhood of the cooling air being drawn potentially from over a larger surface area of the radiator. Obviously there are limits as to how far away the fans can be mounted. I wonder if the stock viscous coupled fan/shround assembly is more efficient than than normally thought. Does anyone know what CFM stock unit moves?

ghoastrider1

This is one of the best post made in some time! Thanks Tom D.

0Tom@Dewitt

The bottom of the curve shows the resistance, starting with zero or as you say "free air" then it is increased. I can only assume they used louvers or something to cutoff the inlet side, creating the restriction.


I don't really know. I quit selling them and haven't even thought of going back. I may later select another PWM from some other company but right now I'm sticking with the old tried and true relays.

carriljc

hopefully they did it at the fan suction...because if they were doing it at the fan discharge, and then we'd need to know the distance from the blades. Did they provide more info than just the graph?

DR.Jay

Thanks for the post Tom

I would have liked to see what the CFM of a stock fan would have been compared to the electric fans.

theoUK

Interesting stuff- but agree with some of the other posters that it would be good to know how they created the deltaP, and where in fact it was measured?? Tom - Does this lab give you a report on how the test is done, or just the "ultimate answer"?

But even without knowing how the test was set up, assuming the different fans were all tested in the same way, then just looking at the differences between them is still pretty informative.

And yeah, I'd like to know what the stock fan's CFM is too! Anyone?

S489

now that tom has offered some hard data on one of the most recent burning topics, i.e., mk8 vs dual spal, i also think comparision to a factory fan would be interesting. as i recall, some years ago someone compared a "stock" type fan with the thermal clutch vs popular flex fans. i dan't recall the details of that test setup, but the factory thermal clutch moved the most air when needed (lower rpms), used the least hp at cruise, and was the most quiet.

i would look for a test of mechanical fans to include required hp vs rpm, and in the case of the thermal clutch, hp vs rpm vs temperature (or something like this).

mrvette

IMO, I see no way in hell any clutch fan, even with 7 aluminum blades like came on old Caddys can possible pull as much air at lower engine speeds as any of these fan setups....ain't happening....forone thing, the blades have lots of clearance in the shroud, that alone cuts into efficiency something fierce....plus my experiences over some decades of using them says those clutches are about as effective or reliable as crap, even when new/replaced, hard to beleive but true...
it was so bad on one car, I strapped the fan to the pulley mount with some steel jumper brackets....forcing the fan to spin at belt/pully speed, THAT cured the overheating on that car....fan was hating life, but I got some freeking AIRFLOW, damnit....and that clutch was new too....all my cars have a/c in them for since my '61 Catalina....

shafrs3

Thanks Tom, no replacement for scientific results. I read other posts claiming 4200cfm out of these MK VIII fans.