[Teknikalei]

Hello-

My name is Todd, and I'm an automotive student at the College of Southern Nevada. There was a discussion in my auto electrical class a couple days ago that I'd like to get your input on.

We were discussing OAPs, (over-revving alternator pulleys) which I think are equipped on some late model GM vehicles.

My teacher seemed to think these pulleys aren't a very good idea (and I tend to agree), especially if they're put onto vehicles like Corvettes, that can hit 5500 RPM with the stomp of a throttle. It would seem that an OAP might be slightly stressed out by a lead footed driver, maybe even have some kind of catastrophic failure.

So my questions - is there such a thing as an aftermarket OAP for hotrodders? If so, has anyone here used one, and if so, for what vehicle? Do you (or would you) have any concerns about failure of a component like this at high revs?

Your feedback on this subject would be appreciated - my teacher would be interested in getting some more info on the subject as well.

Thanks in advance...

[Frisco]

Originally Posted by Teknikalei

We were discussing OAPs, (over-revving alternator pulleys) which I think are equipped on some late model GM vehicles.

My teacher seemed to think these pulleys aren't a very good idea (and I tend to agree), especially if they're put onto vehicles like Corvettes, that can hit 5500 RPM with the stomp of a throttle. It would seem that an OAP might be slightly stressed out by a lead footed driver, maybe even have some kind of catastrophic failure.

Discussions in or out of classrooms can and are great learning opportunities. You just have to think and listen.

As to your question...OK. I'll bite.

I've not heard the term OAP before. Sounds like a smaller drive pulley on the alternator. Since most automotive alternators do not begin to "put out" until the RPM is above idle, an "OAP" pulley should solve that and be an advantage. Also due to the lower engine RPM's being utilized today at cruise speeds the alternator output might be increased with a smaller drive pulley. This would further enhance the electrical system because of the higher electrical demand on todays vehicles.

As to being subject to a "catastrophic failure" at higher engine RPM's, I don't think so. Even if the unit with the pulley was slightly out of balance, the resulting RPM at the high end would not be enough to hurt the unit. If you are thinking that the electrical output of the alternator would be excessive and possibly cause a problem, this is controlled by the regulator.

Your teacher doesn't seem to be very knowledgeable. Why does he think the pulley is not a good idea? If these pulleys are in fact being installed on late model vehicles (GM or others), don't you think that their engineers and research & development teams have tested this product before allowing them to be installed?

Think about it.

[OneMoreTime]

Highly unlikely one of those could be an issue except for us reactionary ole guys that hate complexity..with the demands on the electrical systems of the new cars tech such as that will be more common..

Sam

[OLNOLAN]

Hey Frisco, I know, that you know what you meant to say but, a larger alternator pulley will slow down the alternator, a smaller alternator pulley will speed it up.olnolan

[Teknikalei]

Hi Frisco-

Thanks for the reply.

Originally Posted by Frisco

Discussions in or out of classrooms can and are great learning opportunities. You just have to think and listen.

I agree, I am enjoying the class discussions and am learning a lot. This question just happened to be one my teacher asked for a little more info on, as OAPS don't seem to be all that widespread, and he was curious if anyone out there had experience with them.

Originally Posted by Frisco

I've not heard the term OAP before. Sounds like a larger drive pulley on the alternator. Since most automotive alternators do not begin to "put out" until the RPM is above idle, an "OAP" pulley should solve that and be an advantage. Also due to the lower engine RPM's being utilized today at cruise speeds the alternator output might be increased with a larger drive pulley. This would further enhance the electrical system because of the higher electrical demand on todays vehicles.

As to being subject to a "catastrophic failure" at higher engine RPM's, I don't think so. Even if the unit with the pulley was slightly out of balance, the resulting RPM at the high end would not be enough to hurt the unit. If you are thinking that the electrical output of the alternator would be excessive and possibly cause a problem, this is controlled by the regulator.

Your teacher doesn't seem to be very knowledgeable. Why does he think the pulley is not a good idea? If these pulleys are in fact being installed on late model vehicles (GM or others), don't you think that their engineers and research & development teams have tested this product before allowing them to be installed?

Think about it.

You make a good point, and I will be presenting your response to him with a big grin on my face. Since he's very close to becoming a world class GM tech, I don't think lack of knowledge is the issue here. That impression is likely due to my ham handed approach to passing on the concerns behind his question. Maybe I'll ask him to clarify. Thanks again for your input.

[carsavvycook]

Originally Posted by OLNOLAN

Hey Frisco, I know, that you know what you meant to say but, a larger alternator pulley will slow down the alternator, a smaller alternator pulley will speed it up.olnolan

Yes I agree.

A case in point, was a Crower Fuel Injected 481 CID BBC, in a drag boat. I video taped the bow dropping down at the 3/4 mark, the problem was a lack of voltage. I used the same Crower mechanical pulley's and belt, but the smaller pulley was adapted to fit the crankshaft, the larger pulley was then adapted to the smallest import alternator I could find. (35 amp output) I also used the early Chrysler voltage regulator to charge the battery.

This engine was being twisted to 8200 RPM, and I was concerned it would overdrive the alternator.

We won the high points championship after that addition, so I figure it was the correct decision.

[Frisco]

Originally Posted by OLNOLAN

Hey Frisco, I know, that you know what you meant to say but, a larger alternator pulley will slow down the alternator, a smaller alternator pulley will speed it up.olnolan

Thanks for pointing out my error. You are, of coarse, correct. My statement would still apply. Just swap the word smaller for where I typed larger.

I will go and edit what I originally posted. My two fingered typing is even faster than my brain sometimes.

[Frisco]

Originally Posted by Teknikalei

You make a good point, and I will be presenting your response to him with a big grin on my face.

You are welcome. Just giving my opinions on this as I have not seen what you call an OAP.

Please look at my edited and corrected response.

[OLNOLAN]

Originally Posted by Frisco

Thanks for pointing out my error. You are, of coarse, correct. My statement would still apply. Just swap the word smaller for where I typed larger.

I will go and edit what I originally posted. My two fingered typing is even faster than my brain sometimes.

I type sooo slooow that sometimes I forget what I was gonna say. But I take that with a grain of salt, you should too. I like talkin with ya'll. olnolan

[75gmck25]

I think that either you or your instructor may misunderstand the questions about the use of an OAP. Its main purpose is to reduce noise and vibration, and it has nothing to do with changing the size of the pulley.

Based on my quick google search, an OAP is an alternator pulley with a one-way clutch, which allows the alternator to coast to a stop when you shut the engine off.

The other possibility (more complicated) is an Isolating Decoupler Pulley (IDP), which has one-way clutch and a spring. It's main purpose seems to be reduction in Noise, Vibration and Harshness (NVH), regardless of speed.

This site has additional info from the manufacturer. http://www.decouplerpulley.com/basics.cfm

Bruce