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Opeler
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Discussion Starter · #1 ·
Last month I drove my GT almost 1700 miles to Charlotte and back. The car performed nicely through the heat in NC and heavy rain, fog and cold weather on the way back. Upon returning home, I noticed that the alternator belt was unusually loose. I am using cog belt (timing belt) which put less stress on alternator and water pump bearings.

Close inspection discovered cracked alternator bracket. I built that alternator bracket in 2015 when I had to relocate alternator to the R.H. side of engine due to installation of modern A/C system. Due to limited space, I used 70A Denso mini alternator.

The bracket was built using ¼” aluminum plate and it was properly braced for additional strength. Well, it cracked after almost four years and 25,000 miles of use. You can call it Murphy curse; I prefer to think it was material fatigue.

Last weekend I made new bracket but this time I used steel. I also supported alternator with three “wings” rather than just two. I also took an opportunity to improve alternator charging at low rpm (when idling) by increasing pulleys ratio from 1:2 to 1:2.5. The picture shows the difference between old and new (smaller) alternator pulley.

That’s it. Always something to improve…
 

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Über Genius
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You made a good looking bracket but the fatigue wasn't from design or materials. The fatigue was due to the fact that you had no antivibration measures involved in the mount.
Without rubber bushings you create a micro whiplash effect that, over time, will fatigue metal. Aluminum just doesn't like to be bent so it broke quickly, relatively speaking. In fact, aluminum is weird because the more you flex it, the more brittle it gets.
 
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Can Opeler
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I agree with first Opel. I’m not a mechanical engineer, but I am a certified civil engineer as of last week! Woohoo.

That looks a lot like block shear, but I do agree if you had vibration dampening somewhere this wouldn’t have happened. I would also suggest plug welding the U shaped piece that holds the bushing onto the main plate. This will add a lot more strength than just the perimeter fillet welds.
 

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Opeler
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Discussion Starter · #5 ·
Denso mini alternator proved to be of very good quality. The only negative is ridiculously narrow base foot (just over 1"). Any rubber bushing there would cause significant changes of alternator angle and its square position. In order to stay square, that alternator must be literally sandwiched between tabs. That is the main reason that I am using cog belt as it does not require as much tension comparing to V-belt, hence less stress on the bracket, not counting that the belt is so near the lower radiator hose that you must be sure that nothing will move there.

Keith was obliged to use similar method for that alternator on one of his A/C projects, except the bracket was cut out of one piece. My new bracket will be fine not only because it has been made of steel but because alternator is now supported much wider through extra tab.
 

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Can Opeler
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Yep I have Keith’s design AC compressor. I do think it will break on the upper mounting point sometime, but I think Keith is working on a better upper mount.
 

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Non Civilian
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Most modern cars no longer have anti-vibration measures built into their mounts either. They do have an idler pulley though.That might be something to look into.
 

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Just Some Dude in Jersey
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I wish someone would make a lower alternator bracket that sticks out farther and maybe at an upwards angle, but still has the oem rubber cushions. This would be for non-A/C cars. Maybe just a spacer under the oem bracket would do the trick. Also, it would be nice if the lower bracket's pivot bolt didn't stick out so close to the belt. I was able to use the oem upper and lower alt brackets, but I had to change my pulleys and grind some of the lower bracket away to make it work.
 

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Your Noble Friend ;-)
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I also took an opportunity to improve alternator charging at low rpm (when idling) by increasing pulleys ratio from 1:2 to 1:2.5. The picture shows the difference between old and new (smaller) alternator pulley.
Just to make sure I understand this correctly: The new (larger) pulley, is it on the Engine side or on the alternator???

Dieter
 

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Can Opeler
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Just to make sure I understand this correctly: The new (larger) pulley, is it on the Engine side or on the alternator???

Dieter
The crank pulley is the same, the alternator pulley is now smaller. The larger pulley is the older one.
Think about it like this if the alternator pulley was the same size as the crank pulley it would turn the same speed this is 1:1. But if you make the alternator pulley smaller it has to turn faster to cover the same distance as the larger crank pulley.

My alternator stops charging below 900rpm with AC on so I need to make my alternator pulley smaller too.
 

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Opeler
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Discussion Starter · #11 ·
Yeah, as Kyler explained. On that subject, I did some research and most sites are recommending ratio 1:3 crankshaft vs. alternator pulley. That could be right for slow revving diesel engines but it cannot be right for fast revving car engines. Most DENSO alternators are rated to max. 16,000 rpm. If I went to 1:3 ratio, alternator speed would reach 19,500 rpm at 6500 engine rpm. That would probably cause major alternator damage. At 1:2.5 I will be fine.
 

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Just Some Dude in Jersey
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I'm using the small crankshaft/large water pump pulley combo. Combined they slow the water pump a lot AND the alternator a little. At idle, stuck in traffic, this can cause a problem for me because the water pump is turning too slowly and I get overheating. The alternator spins good enough. So this Winter I'm planning on either changing the large water pump pulley back the smaller oem size or putting the larger oem size crank pulley on instead of the smaller one. I used to have the toothed belt pulleys that PJ has, but they made noise that I didn't like, so I put the other combo on. I think I had a clearance issue, using the oem size pulleys and a modern alternator on the oem lower bracket, that made me opt for the smaller/bigger combo I mentioned.
 

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Senior Contributor
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Thanks PJ. I have a chrome Denso on the shelf that I am planning on using to try and create enough space for A/C. I don't know if it is possible with my EFI setup. I am in the middle of getting settled in my home in Florida and once I am done I can start to play with the cars again. I have to build a new shop in the back of the garage that I built too. I have it part way completed but I need to hang pegboard and I can't do that until I replace the input driveshaft seal on the truck.
 

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Your Noble Friend ;-)
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The crank pulley is the same, the alternator pulley is now smaller. The larger pulley is the older one.
Think about it like this if the alternator pulley was the same size as the crank pulley it would turn the same speed this is 1:1. But if you make the alternator pulley smaller it has to turn faster to cover the same distance as the larger crank pulley.
Yeah, as Kyler explained. On that subject, I did some research and most sites are recommending ratio 1:3 crankshaft vs. alternator pulley. That could be right for slow revving diesel engines but it cannot be right for fast revving car engines. Most DENSO alternators are rated to max. 16,000 rpm. If I went to 1:3 ratio, alternator speed would reach 19,500 rpm at 6500 engine rpm. That would probably cause major alternator damage. At 1:2.5 I will be fine.
Thanks guys. I'm not a stranger to the physics, but I was confused reading the original post. Reading it again, I guess I just overlooked the word "ratio" in "by increasing pulleys ratio".

Dieter
 
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