Since I'm looking at pulley combinations for the engine I'm building, I bought an alternator that will generate 145 amps. It occurred to me that alternators have pulleys that are much smaller than the crankshaft pulley. I suppose this is so when idling at 600-800 rpms, the alternator will turn faster and still generate enough ampherage to charge the battery. Without measuring, I'd say its about a 3-1 ratio between many crank/alternator pulleys. So 700 rpms at idle means the alt is actually turning about 2100 rpms. Thats good. But what about when the engine is reved up and turning 5,000 to 7,000 rpms? That same ratio equals 15,000/21,000 rpms at the alternator. Now those higher rpms are somewhat "momentary", but it still seems awfully high. I'm going to switch the pulley on my new alternator from a serpentine to a V and was thinking about maybe changing the diameter. Anybody have any thoughts on changing the pulley size? Anybody know what engine this bracket came on....
If you're gonna be spinning it to 5,000 - 7,000 rpm on a regular basis, put a deep groove, large diameter pulley on it. You may have to rev it on start up to excite the alternator, but once it starts charging, you'll be fine.
I had a related situation when doing calculations for my driveshaft driven alternator setup. Look up the spec sheet for your alternator and it should list the minimum RPM for it to kick in and charge and also the maximum RPMs for sustained use.
I've read a fair amount on this subject in the last ten years. I do off topic stuff with a lot of amp drain. Some of the new alternators I purchased had a dyno sheet with them. Every one was engine rpm. I was ***uming they used the pulley ratio for the intended application but don't know 100%. Unless you have a tester at your disposal with accurate info, you will have to test and measure. If you have added up your amperage requirement at idle, the quick test is to measure out put voltage. You will see conflicting opinions on this but a minimum of 13.5 volts under load at idle would be my standard. In the past, I've had solid lifter engines that I reved to 7 grand. I have no idea what the pulley ratio was but I didn't have one come apart. I haven't looked but maybe some company like Powermaster has maximum rpm listed on their website. Sorry I'm not more help. Instant after thought. look at the ratio on a modern coyote with a 7 grand redline.
I've got a pretty large pulley on a Ford alternator, about the same size as the crankshaft pulley. At idle it is not charging, but once it gets to about 1200 rpm the voltage gauge shows around 13.8. Edit: I just tested the car again, the engine needs to speed up to 1600 rpm to get to 13.8 volts. It's borderline in the winter with headlights and heater when just puttering around town, but fine in the summer.
Unless your racing it I'd rather have it charging at idle. jump start a car it's nice to get idle and charge . Especially yesterday in winter at 20 degrees like my work van. Electric fans, AC , big stereo with amps and sitting in traffic at idle with a small battery is no fun . Alternators seldom come apart these days. 145 Amps is more than enough unless your running a lot of accessories Wonder what the strain on a single belt is at 145 AMP on a dead battery at 5000 engine RPM. Many guys I see with high output alternators on hot rods often don't have heavy enough wiring if it's charging full out very long . Had a amp clamp on a few of these recently and I seldom see them make what there advertised at even on a discharged battery and a load tester on. Seems the higher output ones come on at a higher idle . On the last street strip car I had switch to shut off the alternator off 4 speed car 8000 RPM. Work truck with all the cool stuff running a factory inverter charging computer and cordless batteries only has a 95 amp from the factory .
High performance solid lifter Fords came with larger alternator pulleys as part of the engineered package. Pretty sure, but not positive, that the z28 did the same.
Alternators can tolerate much higher RPMs compared to a generator. The 'big' pulleys were used on generators to make them live, when alternators came into use, they were then used to reduce parasitic power losses. Generators will explode the commutator or throw windings if RPM gets too high. Alternators have one-piece 'slip rings' and the windings are 'wrapped' by the iron core so that's much less of an issue. Then it becomes more of a bearing speed issue.
Do you have a hand held tachometer? One that you put against a shaft to measure r.p.m. Put it against something that uses that alternator at idle. Then you would have a base to work off of to measure the speed. Then you would have a base to work off of.
Modern off topic stuff that I have tested at work are all about 3.00:1 I would consider the duration of time you will spend at high rpm compared to low rpm.
Crank pulley in inches divided by Alternator pulley in inches will give you the ratio, 3 to 1 is pretty common. I have seen stock pretty ****py very high mileage alternators go for hours in the 24 hours of lemons race at high rpm. Don't really think it is an issue unless you are running NASCAR at 9500 rpm for 500 miles or something like that. JMHO, Mark
Nobody ever thinks about alternator rpm. Same goes for the water pump. They do indeed matter, both too low and too high gives you problems.
I'm just needing to know how many accessories do you have on that rig that you need a 140 amp alternator or need to pay the extra price for it? Any alternator is only going to put out the amps that the rig's system asks for to keep the battery charged and work the accessories. 140 amps is what my old 2000 Cad DTS with every bell and whistle available including heated seats had and it did keep up with it fine. I'm just saying that unless you have stuff on the car that really draws the amps such as electric fans and maybe electric power steering and a big sound system it is probably overkill.
I’m speaking from experience on this one, if you’re not going to be sending your motor to the outer limits then a stock size pulley will be just fine. On my 51 I had a set of March pulleys on a 347 with the under drive alt pulley and driving around normally with the air on at night and headlights beaming it would kill the battery about every other week. Before it did it for the 3rd time I swapped the pulley out to a factory sized one for the year of motor I was running and never had a problem with it.
Factory engines shift automatically at WOT at 4500/5000 or so, imports spin faster. I don’t think I’d even be concerned out it myself. The most I’ve seen on a SBC, ‘70? Z/28 my sisters HS BF had, 4 speed and he’d routinely run to 6500 just to get a burger. Maybe the alternator manufacturer could tell what maximum RPM for their unit is and a duration it can be sustained at.
Power Master is just one of the Companies that can answer all your questions in their catalog, Starters and Alternators, ,Speed , what wire size to use /recommend Ext .
Use these calculator to figure the rpm. https://www.blocklayer.com/pulley-belteng The crank determines alternator rpm unless the alternator is driven off the water pump only. 6" crank pulley to 3" alternator is a 2:1 ratio for example.
Thanks Jim..... My car will have some non-Hamb friendly things which require a consistant voltage/ampherage , so if your Cad had no problems, I hope mine won't either. The cost of my alternator from RockAuto was pretty reasonable, and anyone cobbling up an electrical system might want to consider using these readily available units in their build. These days quite a few people run electric fans and A/C and electric fuel pumps, so this should cover my needs. Since the engine will be 500 Cu In and have a BDS 8:71 blower on it, I purchased a PowerMaster gear reduction starter for it. Looks really well made. I was looking into one of their mini-alternators because I hope to be able to place it down low on the drivers side. They had some that put out even more amps than what I bought, and were apparently set-up to provide a LOT more amps at idle. Not sure how they accomplish that. Seems like the best choice for my needs..........but it was pretty pricey . The other consideration was that if I did have a failure with it, I could not get a replacement locally (if on the road). As I get further along on the project, I'll find out if this alternator will physically fit where I want to put it. If not, I'll bite the bullet and pay for the PowerMaster unit. Then this one will be a candidate for another project I have awaiting in the wings............
[QUOTE="ekimneirbo, post: Since the engine will be 500 Cu In and have a BDS 8:71 blower on it, ....[/QUOTE] Message sent
This thread might be helpful to me regarding a problem I'm having with my HA/GR. It's a small Holden six cylinder that only has around 100 r.w.h.p. but revs freely to 6500 r.p.m. Lately it's been throwing belts or at least the belts are turning inside out each run. The alternator is the only thing driven off the balancer pulley as it has an electric water pump so I have a very short fanbelt. I've been told to get a larger diameter alternator pulley but I'm wondering if the alternator will still charge ok at reduced revs.
The bearings wil need looking at, no name chinese wont stand it , you need a quality bearing rated for the RPM suggestions, bin any charging circuit, if that cant be done look at a stronger belt , I may be wrong but I bet the belt issue is when you back off causing a bit of a whip in the belt, try tighter? try double belts? try deep pulleys. if you want to change puley diam. look up a calculator to see what revs the change will make, do no generation at idle matter?
800 RPM idle x 3:1 pulley ratio = 2400 alternator RPM at idle. 6000 RPM redline x 3:1 pulley ratio = 18,000 RPM max alternator speed. If your redline is higher, you would need to do the math to figure out a pulley ratio that results in 18,000 RPM max alternator speed. That shouldn't be a problem at idle, engines with high redlines often idle higher than 800 too. Mike
First I would check that the alternator really is parallel to the crank and in line with the crank pulley. Maybe try adjusting the alternator fore or aft with a shim/washer or two and see if it helps or gets worse. Also, I would measure both pulleys if you adapted an alternator or changed from the original one. I have two 145 amp alternators that look very similar. To the naked eye (or the well dressed one), they look virtually the same. I measured them and one is slightly smaller than the other and the mounting ears are clocked the same...........but there is a half inch difference in the space between the mounting holes. Also, the pulley on one is 2 1/4 while the other is 2 3/4 diameter. So changing an alternator may be causing too much rpm....... If none of that provides an answer, I'd see if you can get a deeper pulley and/or add a tensioner pulley to steady the belt.
The distance between pulleys can be a factor, too. On the drive side the belt will be taut under load. But if you have a very long run on the slack side, you may need an idler to prevent the belt from bouncing. It's amazing to see how much a belt can stretch and bounce under load.
Most hipo Fords came with a larger diameter pulley but I thought it was mostly used so you won’t throw a belt at high rpm, not for any charging issue. The belt also sits a little deeper in the pulley also.
That was the first thing I checked. I used a straight edge and then a laser pointer set up in the groove. Adjusted fore and aft till I was happy. I've changed from a grooved internal to a grooved external belt but haven't done any runs to check it out.
Worst case scenario, you could go to a Gilmer belt. These guys will likely have what you need. Had to do that when we built a centrifugal supercharger setup using double v-belts. Things were fine up to a certain rpm, then stuff got weird. Check it out...
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