Alternator max rpm? Fan explosion

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grego
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Alternator max rpm? Fan explosion

Post by grego »

Hi,

I took my caddy for a Sunday drive yesterday.
While giving it some stick an almighty bang erupted from the front end, followed by my grille flying off and into the windscreen.
This happened at around 7300rpm.

There was no loss of power, but there was a scraping sound coming from the engine bay afterwards.
Pulled over at the next safe opportunity to check.
Turns out the metal fan on the alternator (external) had exploded... Smashed the front grille, severed the bonnet release cable, huge dents in the valance and front edge of the bonnet.
A piece had also got stuck between the crank pulley and lower timing belt cover, puncturing the cover and chowing the outer edge of the cam belt in the process.

Got off very lucky! Damage could've been way worse.
Has anyone experienced this before?
What is the max rpm that an alternator can handle?
Should i need to change pulley ratios to reduce the alternator speed if revving the motor to 7800rpm?

Look forward to hearing your thoughts on the matter.
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grego
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Re: Alternator max rpm? Fan explosion

Post by grego »

smashed valance and severed bonnet cable
Image

fanless alternator
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valance front
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destroyed grille
Image

missing piece
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bonnet damage
Image
Image
Last edited by grego on Mon Oct 15, 2018 8:56 am, edited 1 time in total.
HendrikCTI
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Re: Alternator max rpm? Fan explosion

Post by HendrikCTI »

Maybe just a weak park somewhere.
There are Datsuns in Australia that frequently see 8000rpm around tracks. I know they change ratios on the water pump pulley for this but it is more to prevent the water pump impeller spinning too quick to flow the radiator water.
Main problem will come in if the alternator will still charge at idle and low rpm if you change the ratio. You will have to change so much on the ratio that I think it will have a impact on charging at idle in high traffic.
Is this a dedicated track car?
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Re: Alternator max rpm? Fan explosion

Post by Howie-WP »

On most of the Challenge race cars, we fit oversized pulley to the alti to reduce the rpm of the unit at high engine revs.
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Re: Alternator max rpm? Fan explosion

Post by PapaJo »

This might have been a bad fan with a crack from long before. You were lucky, with the amount of damage you had. Coukd have been worse.

As Howie said, a larger alternator pulley reduces rpm on alternator for high reving motors. This does affect the idle charging. I’m sitting ina similar situation, where my nain crank pulley is smaller (100mm) diameter, thus turning water pump and alternator slower. On idle, I get the battery light blinking and going off over 2000 rpm. Idle charge voltage on mine is around 12,4v and not over 13 volt. This will in idle traffic cause a drain on battery. Water temp seems to be the same as with the std crank pulley.
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Re: Alternator max rpm? Fan explosion

Post by grego »

Hi guys,

The car is a weekend toy, not my daily drive.
Changing pulleys could work, but would rather avoid that I possible.

Does anyone have experience running the alternator without the fan? Can it be done?
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Re: Alternator max rpm? Fan explosion

Post by ICE King »

Some alternator's has fans while other's don't, the alternator's with alot of vents in the casing allows air to flow through it and in turn cool it down with out the need of a fan.

Your alternator looks pretty sealed hence there was a fan.

Considering your motor rev's high , I doubt your alternator will last without a fan. I wouldn't risk it
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Re: Alternator max rpm? Fan explosion

Post by Howie-WP »

grego wrote: Thu Oct 18, 2018 2:10 pm Changing pulleys could work, but would rather avoid that I possible.
Why? It will solve your issue, and the resultant charging issue (related to low rpm) is moot as this is a weekend toy.
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Re: Alternator max rpm? Fan explosion

Post by PapaJo »

Go to an engineering shop and give them your current alternator pulley and ask them to increase the diameter size by 20%, 50mm to 60mm. Install with a new steel fan and Bob’s your uncle.
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Re: Alternator max rpm? Fan explosion

Post by HendrikCTI »

Don't know the price of these things, but for engineering costs and steel fan fitment, can't he maybe look for one of those fan less alternators and then install that?
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Re: Alternator max rpm? Fan explosion

Post by PapaJo »

What was the outcome of this? Is this fixed now?
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