Fan Speed, Belt Slip, & Serpentine Belt kits

[FJCamper]

Above: A "power" pulley on our Historic Sportscar Racing Ghia.

Fan Speed, Belt Slip, & Serpentine Belt Kits

The stock diameter VW pulley and generator/alternator pulley begins belt slip at about 5000 RPM. This is by design, because the fan itself meets greater and greater air resistance as the car reaches higher speeds. Fan air resistance eats horsepower.

Porsche 356's were allowed higher road speeds because they used a smaller diameter pulley which was the inspiration for the so-called VW "power pulley." The power pulley simply turns the fan slower. Belt slip begins at about 6000 RPM in 4th gear in a 356.

Mathematically, as road speed goes up, and air resistance increases, fan stall speed will occur and the belt begins to slip 100% of the time. This is why high performance VW engines so often throw fan belts. The belt slips, twists, and flies off.

To keep belts on, the serpentine belt kits were devised. They do keep belts in place, but use them with a warning. A nonslip belt keeps the fan turning against air resistance and really uses up horsepower, about 5000 RPM for the stock pulley diameter and 6000 RPM for the power pulley diameter.

This info is not anti-serpentine belt, it's tuning information.

FJC

[VW&MGman]

Nice write up FJC.

In my experience with a serpentine set up, the pulleys have to be perfectly aligned and there can not be any 'wobble' in the generator/alternator or else they will eat up belts at racing speeds, (6K-7K RPM). Also the serpentine system does not allow for any 'slippage' of the generator/alternator pulley so the fan speeds will be much higher, which could result in busting fans a part.

I changed my pulley system back to conventional set up with a power pulley and have not had any problems so far in racing conditions.

[Ol'fogasaurus]

I have seen idler setups used but they maybe confused as serpentine belts.

https://www.bing.com/images/search?view ... &eim=1,2,6

I have seen them using arms and horizontal sliding systems also. I agree with what you said about potential power loss.

Lee

[Jadewombat]

Another option. I used several of these at a place I was working at.

https://www.youtube.com/watch?v=c60gAN2s6X0

[John S.]

I've heard of exploding fans in unlimited street all blamed on serpenteen belt systems.

[Ol'fogasaurus]

https://www.bing.com/search?q=electric+ ... 9e204647dc

An electric fan system might be an option. You'd need to figure out a belt system for the charging system but the over revving of the fan causing it to fail might be solved this way (how close to being a propeller is an over revved fan ?).

Just a thought.

Lee

[FJCamper]

I don't like the "serpentine belt" description some of these tensioner pulley belt kits are marketed under. To me, a serpentine belt is a more complex affair. Lots of pulleys.

I personally think these tensioner pulley kits are a godsend to those guys who cannot keep a belt in place. Too many times they have a problem such as a alternator pulley not in alignment with the crank pulley because the fan housing is slightly tilted, or pulley spacers are added, missing, etc., due to parts being changed out. Aftermarket fan housings are sometimes to blame, their specs being way out from stock.

By the book, if you have the right belt, the right belt tension, and a straight alignment between pulleys, you should be able to touch 7k for short periods of time. I myself shim my crank pulley just a millimeter out of line with the top pulley. This creates a slight oblique dynamic tension that (to my reasoning) that combined with the normal static tension really holds our belt in the pulley.

FJC

[Jadewombat]

There's not a single major automotive manufacturer on the planet who runs anything but a ribbed belt system or a tensioner for that matter. V-belts went out with the 1980s because they flop all over the place. The only reason to keep a v-belt system is keeping the cost down. There's no advantage to a v-belt otherwise.

The place I worked at did not want to switch from the 3 foot long v-belts they're using on their heavy trucks. The vibrations were so bad without a tensioner system it was cracking the base welds, repeatedly.

https://media.licdn.com/dms/image/C4E12 ... J0kXmff3Kc

If you're talking high revs., a toothed belt and tensioner system would be the only way to go. Honda, new VW, Toyota, Subaru, Ford, etc. all could rev. to 7,000 rpms all day long on their engines no issues of throwing a timing (or accessory for that matter) belt.

And the fan would have to be up to snuff at those revs, which is of course a separate issue altogether.

[Bruce2]

An electric fan system might be an option.

Electric fans on a VW don't work. They don't push enough air to maintain constant head temps.
Dyno tests have shown that it takes 10-12hp to spin the fan. Even if you thought it was half that, try to power a 5hp electric motor in a car.
P = 746W/hp x 5hp = 3730W

I = P/V = 3730W/12V = 310 Amps.

I suppose you could rig up 6 stock alternators......

[Ol'fogasaurus]

The reason I suggested it was what happens to a fan (propeller so to speak even though this is a different kind of fan than a prop) at high RPMs. You also have to deal with the fan shroud and tin to distribute air. As the RPMs go up so does the power draw to push air though the semi closed system.

Thos of us who run in the sand usually do not use the bottom part of the shroud and run two of the exhaust pipes around the ends of the engine and the other two over the top of the shroud.

Bruce I hadn't heard of what you said before, I've only have heard mumblings of some advantages due to the high rotating speed air movement problem. I agree that the electric fan keeps a constant flow of air that does not go up or down unless there is a regulator to change the airflow. Until I hear differently I will assume your input is correct.

Lee

[Jadewombat]

I don't like the "serpentine belt" description some of these tensioner pulley belt kits are marketed under. To me, a serpentine belt is a more complex affair. Lots of pulleys.

I personally think these tensioner pulley kits are a godsend to those guys who cannot keep a belt in place. Too many times they have a problem such as a alternator pulley not in alignment with the crank pulley because the fan housing is slightly tilted, or pulley spacers are added, missing, etc., due to parts being changed out. Aftermarket fan housings are sometimes to blame, their specs being way out from stock.

By the book, if you have the right belt, the right belt tension, and a straight alignment between pulleys, you should be able to touch 7k for short periods of time. I myself shim my crank pulley just a millimeter out of line with the top pulley. This creates a slight oblique dynamic tension that (to my reasoning) that combined with the normal static tension really holds our belt in the pulley.

FJC

FJ,
SIr, you inspired me to take a crack at it:
https://youtu.be/zHHZ0HiHM2U
viewtopic.php?f=18&p=1285625#p1285625

[doc]

Pretty cool!

[brewsy]

Hi All,
This isnt MY webpage but I think this dude has done his research and seems to know of what he talks:
http://www.offroadvw.net/tech/wes/fan.html

Upshot is that serpentine can drive the fan faster but takes way more power and significantly increases chance of fan blowing up!
Interesting that 'power pulleys' reduce fan speed by 15% but you only lose 11% cooling airflow but reduces the power to drive the fan by 39%!!!

Thats a lot of 'free' power for not much loss of cooling and if youve got an external cooler you can easily lose that....

[Jadewombat]

Interesting brewsy, but again (as I mentioned above) if you're in the lofty rev. range of 6,000-10,000 rpms and fan slippage is critical--why would you not run a toothed belt? I didn't see any mention of that in the article you posted.

Blowers run toothed belts for that reason. The faster you turn something the less it wants to turn.

[Bruce.m]

I vague recall the stock set up is designed to limit the power transmission to the fan.
Once the force required to turn the fan any faster hits the limits of the single belt, it will slip so the fan rpm hits a “limiter” even if the engine turns faster. This keeps the fan within its design limits, otherwise it runs the risk of exploding due to the additional force on the blades. If you think of the stock setup as a form of clutch, then it can make sense to have brief belt slip at higher rpm as there is diminishing returns for a faster turning fan (requires more power than you get from the extra cooling) . Obv this will wear the belt and the stock set up is designed for a limited Rev range anyway.