I would like to know what pulleys and belts people are using to keep them from coming off at about 5500rpms. Will the 3.625 pulley and belt from a L78 do it? I currently have a 2.750 pulley on my alternator. I would like to keep it as original as possible.

Brad
70 El Camino SS396

I still have the stock non-deep groove pulleys on my L79 and I can zing the engine to 6500 and above and I've never thrown a belt. If the belt is sufficiently tight and the pulleys are all properly aligned you shouldn't be throwing belts. If you are throwing belts, my guess is it is either too loose or one of more of your pulleys doesn't line up with the others. Or you could also have a pulley that is bent a little (not running true to the plane of the belt).

Lower ratio and/or deep groove pulleys wouldn't necessarily fix the problem if the belt is loose or if something is out of alignment.

I agree with Rich completely, I had a 1969 327 that I use to take to 7200 on a reg basis. It had a standard belt and pulleys on it and never had a problem. After I first put it together (many moons ago) the Belt would turn over or come off. I quickly found that the Alt was not in alignment. Once this was fixed, I never saw another problem with it. My brother takes his 383 up to 6500 with stock pullies, and I've never seen his belt come off. I would look for what Rich mentioned.

Brad,i have a 69/396/chevelle which i have spun to 5800+ rpms many times over the yrs on the stock pulleys/selts and never thrown a belt. I have also owned 3 z28's that saw 5800-6k on stock pulleys with no problems.

You must have an alignment issue of some sort going on between the belts,pulleys,and accessories or maybe your alt,pwr/st pump,or w/pump is going bad and starts to lock up at higher rpms throwing a belt.

Are there any bent/wobbley pulley's? If yes replace them.

Also,could your harmonic ballencer be going bad? If it's a stocker maybe the otter ring is slipping on the rubber inbetween inner/otter ring of the dampner at higher rpm's casuing the crank pulley to vibrate/shake/move a little casuing a belt to come off.
I have seen bad dampners that wobbled alot when they were bad.

Bottom line is the stock belts/pulleys should be able to handle the rpm's your turning with no problem so check everything out real well.

Good luck.

Scott

If I may add once the belt is thrown might as well toss it & get a new one.

Jim

Too tight of a belt can cause them to come off as well.

Later
Steve

man had this problem for years on my 396 bbc i was using stock pulleys on my engine and with a huge straight edge everything was perfect .....there was nothing out of line.....i was turning my bbc to 7000 rpms and would throw it almost every pass............my crank pulley and water pump pulley had about 1/4" between the two(top of the crank pulley and bottom of the water pump pulley).....i bought the deep groove alt. pulley and with no luck......i still threw them..... i changed pulleys even... i noticed i was sucking shut the bottom radiator hose when i was reving up the motor......so i replaced it with a heavy duty (wire coil molded in the hose)hose. and that didnt help infact made it worse.....


one day i got pissed and i tighten the new belt as tight as one can get it(i tried so many times with different tightnesses)and this finally made a huge problem......i blew the water pump off the block? just off the starting line.......fan took the radiator out and sent junks of metal threw the fiberglass hood......the only thing that was left on the front of the block was the 4 bolts that hold the water pump on....

come to find out i was hydro locking the water pump .......the pump couldnt push no more water thru the block and it was locking up the water pump and in turn throwing my belts.....but since i had it so tight the water pressure blew the pump off!

i didnt know this at the time....i replaced all my parts even with new pulleys and thought it was a freak deal with the factory water pump.....i left work one day and i stompped on it and done it again blew the pump right off again!.........

before in the past any time i ran a belt alittle tighter then it should be i would blow out the water pump gaskets........but since i cranked on the alt. to make the belt so tight it would just just force to much water to fast into the block with the water to have no place to go...my engine reved up fast 4500 stall and 4.56 gears

it came down to this the engine was to cold and the thermostate was not open and the water would "stack up" and lock up the water pump pulley in turn it would cause it to fly off....

i never put two and two together cause if i drove the car around town i wouldnt throw belts reving up to 7000 rpms.....but when i got on it before the temp gauge would come up it would throw it..

my fix cam down to this....i had to remove my thermostate and i never threw belts again going down the track......the water had a place to go now since i had a hole for the water to go thru....i even had the one bypass hose hooked up on top all the time.....

it just never made sense....til i thought about what was going on and pinning it down to when i would throw them and when i wouldnt....when the engine was warmed up it wouldnt throw it but if i sat to long between rounds and the engine was cold i would throw it....

might be something to look into......

just to add i have a daily driver too with a bbc 396 in it that i turn to 5900 rpms...i used some of the old pulleys i thought what was giving me problems with my race car...when i built the engine and right now i have no problems with belts staying on.....in fact if you look down the front of the motor you can see the alt. pulley sits and easy 1/4" ahead of the rest of the pulleys and it doesnt even fly off...but i also drilled 4 holes in the thermostate to help flow......but i make sure the engine is warmed up to 160 at least before i run it hard at the track also....havent thrown a belt yet in the 5000 miles i driven my daily fun driver.....

I went and checked the alignment of the pulleys
and they are off slightly about an 1/16. I have a
new alternator and new high flow water pump, but
the old harmonic balancer. I have been tightening it what I thought was tight. I use a small block of wood(protect the paint) and pry bar.I do not have any bypass holes drilled in the thermostat and the has not been totally up to operating temp,
about 170 deg. I should have put in my original post that it comes off after a full throttle to 5500 and then letting totaly off the throttle. It happens exactly at that time. It may be a water issue like DDennis suggests.

Brad
70 ss396 El Camino