Fan Speed, Belt Slip, & Serpentine Belt kits

[Jadewombat]

I vague recall the stock set up is designed to limit the power transmission to the fan.

Not sure why VW would do that. I don't doubt that may have been the case, but to me slippage winds up as friction which translates to heat and vibration on the belt. While it's easy to spin the alternator/fan by hand at low speeds, things get a lot more difficult at high speeds due to rotational mass.

An extreme example, but I remember seeing a video a couple of years ago about the Bugatti Veyron. The tire cap which weighs all of 5 grams or so becomes something ridiculous like 300lbs. of rotational mass at its top speed of 250mph.

[Bruce.m]

I vague recall the stock set up is designed to limit the power transmission to the fan.

Not sure why VW would do that. I don't doubt that may have been the case, but to me slippage winds up as friction which translates to heat and vibration on the belt. While it's easy to spin the alternator/fan by hand at low speeds, things get a lot more difficult at high speeds due to rotational mass.
.

The tech article linked above explains it fairly well I think. The HP consumed by the fan robs too much power from the engine above ~4K rpm so the belt slip enables that power to go to the wheels with some heat lost to the belt friction, rather than the fan eating the power.

Fans have diminishing returns. To steal from the linked article....
15% increase in fan speed is circa 11% more cooling but 50% more power consumed by fan.

So at some point the situation is worse... if the fan actually turns faster.

[brewsy]

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.

I think Bruce has hit the nail on the head.
The stock fan has a limit to the RPM it can take without modification (welding the fins) BUT there isnt much point in revving it higher as the maths show that a) it takes poop loads of power (hahahah STF has spellchecked what I actually wrote - Id never use 'poop') to spin it faster and b) it doesnt end up pushing much more air beyond a certain point.
So Bruce's analogy of a clutch is perfect and it 'slips' when the fan trys to draw too much HP.

As I understand it most high rpm folk are concerned about the fan belt coming off rather than actually trying to push more air through the engine.
So in that case a belt tensioner/guide thingy is probably better as it will keep the belt on the pulleys but the 'clutch' will still slip when it has to.
Besides which if youre making a high HP engine and are doing everything to squeeze every last drop of power out of the old 2V AC engine why would you want to use up (lots of) precious HP spinning a fan when its not achieving anything?

There is a guy in the UK who has engineered an AC type clutch onto his generator assembly as he has a super high engine RPM combination. Before a run he engages the clutch to 'stop' the fan, does a 1/4 and then in the return lane engages the generator/fan again...
The belt stays on the pulleys as its not transferring any torque and slipping and for the 20-30 seconds of the run engine temps are within control.

[Jadewombat]

I had that thought this morning, if there is a point of diminishing returns you have an A/C clutch engage a smaller alternator pulley at high rpms or engage a peg on a solenoid to a gear that does simple switching to slow the fan speed and hp draw.

Again, we're talking about higher and high rpms (above 5K). For me this is not an issue or something I'd need to take on for my bus. Everything under 5K is just fine for around town. VW didn't change much in the fan design up to the 1970s where it was pretty much unchanged altogether since the last "old" new beetle rolled off the line a few years ago.

In the age small, cheap (Anduino anyone?) programmable control units, speed sensors, and hobby lathes/mills/makerspaces where we have more access to come up with a better answer it's not that far out of reach. My point is belt slippage just seems hokey and asking for something to break and isn't a solution all across the board.

I usually look to what Porsche did it situations like this...it looks like they're running 2 pulleys for that reason in 1994 (there's a tensioner on the inner one). Anyone know anything about how this works (will look it up when I have more time)? It may not be switching the fan speed, but that's what it looks like to me.

[brewsy]

Jade,
Good spot on the Porsche front.

Did a quick bit of digging and found this:
https://www.pelicanparts.com/techarticl ... vbelt2.htm
Looks (to me) that there isnt any fancy clutching going on with the dual drive belts on the fan. Have a look at the AC pump to the right for the size of (then) current? drive belt clutches.
My guess is simply that Porsche wanted a higher fan speed before any slip occurred and 1 drive belt simply couldn't provide that torque and a serpentine belt system wouldn't have allowed any belt slip and fan blow ups (Do Porsche fans explode?)...
Also different diameter belts probably have different vibration 'nodes' and might help cancel belt wobble and eventual flying off. Think of it like dual valve springs.

As for saving power etc. all the OEMs now do that kind of thing and PAS pumps, alternators etc. are all controlled by the ECU to minimise power loss to keep emissions down etc etc.

[Jadewombat]

Thanks brewsy.

I just had a thought, bear with me a second, why not use a turbocharger as a fan? It would have to be able to push enough volume at idle speeds, but again you could do an RPM dependent control of the wastegate. The biggest risk would be the absolute endless comments from everyone, "Why is that turbo not hooked up to the intake manifolds?"

[brewsy]

Thanks brewsy.

I just had a thought, bear with me a second, why not use a turbocharger as a fan? It would have to be able to push enough volume at idle speeds, but again you could do an RPM dependent control of the wastegate. The biggest risk would be the absolute endless comments from everyone, "Why is that turbo not hooked up to the intake manifolds?"

Hahahah I like your thinking but a turbo-supercharger is a pressure device and I think its nature means the output air is heated so youd need an intercooler before you used the air to cool the engine...

[Bruce.m]

Jade,
Good spot on the Porsche front.

Did a quick bit of digging and found this:
https://www.pelicanparts.com/techarticl ... vbelt2.htm
Looks (to me) that there isnt any fancy clutching going on with the dual drive belts on the fan. Have a look at the AC pump to the right for the size of (then) current? drive belt clutches.
My guess is simply that Porsche wanted a higher fan speed before any slip occurred and 1 drive belt simply couldn't provide that torque and a serpentine belt system wouldn't have allowed any belt slip and fan blow ups (Do Porsche fans explode?)...
Also different diameter belts probably have different vibration 'nodes' and might help cancel belt wobble and eventual flying off. Think of it like dual valve springs.

The twin belt set up appeared on the 959 and then again on the 964. It enabled the fan and alternator to be geared differently so the fan could run slower (than the previous generation) but the alternator wouldn’t have to also run slower (just as demands on electrics was increasing). The fan shaft has a hollow centre and the alternator shaft passes through the middle. Interesting that they actually slowed down the fan speed, compared to the previous generation.

[Tom in PA]

Wow, that's cool. Is there anything you guy's don't know??