Is it a stock lower crank pulley? Or a smaller diameter aftermarket??

GSMopar couldn't keep a belt on his new hemi build and I told him I changed to a TopCog belt on my hemi and no more problem. He bought one and it solved his too he's running low 10's.

Could be that the reduction in mass helps that design work better at higher RPMs??

Joe

One thing that hasn't been mentoined yet is, once a belt comes off, its probably internally damaged and wants to come off again. I threw a belt at the track, put it back on, and made another pass. Checked it and it was still on, but now "V" up instead of down. New belt and its been years.

this is simple and works for me.

What I noticed is that the DAYCO Top Cog belt sits fully down in the pulleys when compared to the other standard v-belts.

It could be that the DAYCO Top Cog belt is actually narrower and actually the correct width since others have said that the standard generic v-belts belts available now are a wider width.

I thought many original belts were 11/32", just slightly narrower than 3/8"?

Not original, 136 Amp Denso on the 500" stroker for the '71 Charger using March crank and water pump pulleys.
I have had the engine at 7,000+ RPM (I think 7,200 on the chassis dyno) and no problems with the belt setup.

That's pretty slick, what's that bracket from?

I chased a belt issue for longer than I care to admit and it ended up being due to a wonky bolt hole. You'd torque it down and right at the end it would kick the alternator bracket out. I had to grind a spacer out of flat and never had another issue. It was so minimal you'd never would have guessed but it was enough. I believe the added friction, and thus the added heat, attributed to a premature alternator failure.

i just fabbed it up. been using something like that since the late '60's. that's a fat belt in the photo and they're not the best for rpm. a 5/16" wide belt is more user friendly.

One of your pulleys is bent, its not running true. At higher RPM it can and throw a belt. It can be hard to see by eye but if you've checked all the obvious this could be your problem.
It happened to me and drove me nuts until I checked and replaced the harmonic balancer pulley.

I had a 350 in a Camaro that would throw a belt almost every time I drove it. New belts didn't help. It turned out to be a defective alternator pulley. It lined up with the crank and water pump but had a section where it was slightly wider at one point. This was like a flat spotted tire where at that same point every revolution, the belt tension would change just enough to upset the contact and the belt would bounce out of position.

Whenever a belt rolls over in the pulley, which usually causes it to come off-it was one of the end belts on the slab it came from. It most likely does not have cord all the way thru the belt and should not have been used in the first place. So, when you buy a new V belt, flip it inside out-if there is a taper on the narrow part of the belt, get a different belt.

I want to thank everyone for their input. It gave me some ideas for things to check on.
I have used the Dayco top cog belts but that made no difference.
One thing I will try is a smaller crank pulley. The one on the car now is a large diameter.
I have watched the pulleys when the car is running and I didn't see any that were wobbling.
I mentioned earlier that I used a strait edge to align the alternator pulley with the crank and water pump pulleys so I don't think the issue is there.
I will try a smaller crank pulley and possibly a tensioner that I found on Mancini's website.
I will post what I find.
Thanks again for your help!

What is difference from stock?

If nothing then something is fubar with what you have. A dial indicator on the pulley while you rotate it by hand will tell you a lot more than your eyeball will. Balancer pulley might be a bit tricky in the car but you ought to be able to do it. Don;t forget to closely check the brackets as well since they could have a crack and while not obvious under a light load could cause the issue.

The factory setup didn't really have a rep for throwing belts. So unless something is bent/broken, you deviated from stock or are spinning it to crazy rpm's it should suffice.