- Higher output alternators require more effort to spin than a lower amperage alternator when the alternator is under a load. This additional effort places more strain on the alternator drive belt.
- If for any reason the alternator pulley is not aligned perfectly with the other pulleys on the engine, the alternator drive belt will slip. A straight edge can be used on the face of the pulleys to verify the belt is aligned properly.
- V belt applications should have no more than 3/8 inch belt deflection after the alternator has been installed. Serpentine belt applications should use a belt length that places the spring belt tensioner in the CENTER of it's travel. If the spring tensioner is too far outside of it's operating range in EITHER direction, the belt will slip.
- Vehicles with poor belt "wrap" (see photo) will be more prone to belt slip than vehicles that have better belt "wrap" around the alternator pulley. If necessary, fabricate brackets to mount an additional free spinning idler pulley to ride on the back of belt and "force" the belt to "wrap" around the alternator pulley further.
- Lower amperage alternators will not slip the belt as badly as a higher amperage unit, and can utilize a larger diameter alternator pulley than the same unit in a amperage configuration. Larger pulleys grip better, but will not charge at idle on some higher amperage alternators.
- 170 amp S series and 250 amp Elite series units are the best choice for high RPM racing applications.
- Single V belt and 4 rib serpentine belts will start to slip at about 150 amps worth of load. For minimal belt slip, 240 – 370 amp alternators should not be driven by anything narrower than a 6 rib serpentine or dual-V belt drive setup.
- Calculate pulley ratio by dividing the diameter of your crankshaft pulley by the diameter of the alternator pulley. For example, a 6 inch crank pulley with a 2 inch alternator pulley will yield a 3:1 pulley ratio. (6 divided by 2 = 3) This means that the alternator will be spinning 3 times the engine RPM- i.e. the alternator will be spinning 18,000 RPM at 6,000 RPM engine speed. G series alternators are rated to 18,000 RPM, while S and Elite series alternators are rated to 20,000 RPM.
- It is ABSOLUTELY IMPERITIVE that a properly sized positive AND ground cable be run directly from the alternator to the terminals of the battery.
- Improperly crimped, or oversized ring terminals on your charge and ground cables can easily damage the alternator or cause a fire. The size of the hole in the ring terminals you use MUST fit the diameter of the stud or bolt that they go on PERFECTLY. If the hole in the cable end is oversized, the high resistance connection will burn off amperage in the form of heat, and in some cases can melt off the stud.
Frequently Asked Questions
Q: Will the installation of a higher amperage alternator damage my vehicle or my battery(s) ?
A: No. As long as the battery(s) are in good condition, and the correctly sized alternator charge and ground cables are installed properly, nothing will be adversely effected on the vehicle
Q: My vehicle's alternator is computer controlled. Will a higher output alternator work properly in my application?
A: Yes. As long as the alternator is installed properly. (some PCM controlled vehicles require specific cable routing, instructions are provided) A MechMan high output alternator will still work fine and provide more amperage than the OEM alternator.
Q: If I install a higher output alternator, will my charging voltage be higher than stock?
A: That depends, depending on the part number, most MechMan alternators are designed to operate at the OEM voltage. This is done intentionally, so that the vehicle is not harmed by higher than stock charging voltage. A higher AMPERAGE alternator will keep the voltage in the vehicle closer to OEM voltage when drawing more current from the battery. If higher than OEM charging voltage is required, contact a sales rep about custom ordering your alternator with adjustable voltage.
Q: Will a higher amperage alternator draw more horsepower from the engine or reduce my fuel economy?
A: That depends, the more amperage the alternator makes, the more horsepower is required to turn it. All alternators are regulated, so unless there is a heavy electrical load against the alternator, a higher amperage alternator will not provide any more drag against the engine than an OEM alternator.
Wondering how to find a slightly shorter belt for your MechMan alternator?
It's pretty easy!
We recommend going to your local NAPA™ with your stock belt in hand. Ask the salesperson to look at their paper belt catalog. (don't try to have them look anything up in the computer) Most stores have a belt measuring tool on site as well. If available, you can use this tool to stretch out your original belt to determine it's length. It is also possible to look at the part number on your original belt (if it is still legible) in order to determine length that way.
Once you have determined the length of your original belt, simply go to the “belt part numbers by length” index in the paper catalog. Count the number of rubber ribs on your original belt, and locate the section in the catalog index for that number of rib belts. Locate your original belt length, then scan below it to see what the next available shorter belt sizes are. There are more available length options in 6 rib than any other belt width. If you are unable to find a suitable belt in your rib count and length, it is possible to purchase a belt with a higher rib count, and slice off the un-used ribs of the belt with a razor blade. While this does take a few minutes to do, it can open up a lot more options for belt length.
Belts can be tricky, so we usually recommend buying two or three different slightly shorter belts so that you can test fit each one. Most vehicles have a spring tensioner that puts tension on the serpentine belt. To find which shorter belt is optimum, test fit the belt onto the engine, and observe the spring tensioner arm. Most spring tensioners will have a small “arrow” or “line” cast into the pivot point of the tensioner. There is usually then a “range” area cast into the other half of the pivot point on the tensioner. Ideally, the “arrow” or “line” indicator, should be roughly in the middle of the “range” marking. Keep in mind that spring tensioners are a wear part! The spring becomes weaker with age, which puts less tension on the belt, which can cause belt slip.
One final option for fine tuning belt tension, is to change the dimeter of one or more free-spinning idler pulleys on the vehicle. Most vehicles have one or more idler pulleys that help “wrap” the belt properly through the accessory pulleys. Different diameters and widths of idler pulleys can be located in the paper idler pully catalogs. These catalogs generally have an index with photos and dimensions of all the different idler pulleys available. If you can locate a replacement idler pulley with a larger/smaller diameter, but the same mounting inside diameter, width, and offset as your original idler pulley, you can change the idler pulley to change the required length of the belt. In some cases, a larger idler pulley can be installed so that the original belt length can be used.”
Here is a link to an easy online belt length / part number calculator on Dayco's™ website:
Here is a link to the Dayco™ online belt part numbers by length guide:
Here is a link to the Dayco™ Idler pulley guide with photos and dimensions: