underdrive pullies
#1
04-13-2007, 12:22 AM
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#2
04-13-2007, 02:07 PM
RE: underdrive pullies
cut from the FAQ Engine mods section:
---
Underdrive Pulleys
Undersized crank pulleys can increase MPG slightly by driving the power
steering, air conditioning compressor, and water pump at lower rpm where
there will be less friction.
Some who have tried undersized pulley sets report an additional
alternator pulley in a pulley set is generally too slow for the street truck
that may
have to idle a long time without enough rpm to charge the battery.
A pulley that does not slow the alternator down as much as the aftermarket
ones is the factory installed one used on the aluminum block V10 Viper.
This is DC Part Number 5037204AB for a 2003-2007 Ram SRT10 crank
pulley, which usually runs about $60 retail at most Dodge dealerships and
less at discount dealers like Koller.
There have been some warnings posted that the underdrive pulleys for
5.7 Hemi's do not have a torsional damper of correct design. Beware.
Breaking a crankshaft out of warranty is not fun or cheap.
The horsepower that an alternator consumes is
mostly set not by the rpm it turns, but by how much the 'voltage regulator'
inside the Ram pickups PCM computer increases the 'excitation field current'
inside the alternator's rotor. Because of this I see no benefit in slowing an
alternator down - no significant horsepower will be saved or fuel economy
gained. The AC compressor can be simply turned off when you want either
more power or better MPG. NASCAR cooling system guru Howard Stewart
points out that on modern engines like the Chrysler 4.7/5.7 with knock
sensors if you slow down the water pump you might have more horsepower
for the first few seconds at Wide Open Throttle, but then the cylinder head
will begin overheating at its worst spot, some pinging will start, the knock
sensors will trigger, and the PCM computer will start pulling back up to
16 degrees of ignition timing advance - then the horsepower declines
much more than you "saved" with the water pump pulley reduction.
If you must change pulleys it is best to just slow down the power steering
pump alone.
---
Underdrive Pulleys
Undersized crank pulleys can increase MPG slightly by driving the power
steering, air conditioning compressor, and water pump at lower rpm where
there will be less friction.
Some who have tried undersized pulley sets report an additional
alternator pulley in a pulley set is generally too slow for the street truck
that may
have to idle a long time without enough rpm to charge the battery.
A pulley that does not slow the alternator down as much as the aftermarket
ones is the factory installed one used on the aluminum block V10 Viper.
This is DC Part Number 5037204AB for a 2003-2007 Ram SRT10 crank
pulley, which usually runs about $60 retail at most Dodge dealerships and
less at discount dealers like Koller.
There have been some warnings posted that the underdrive pulleys for
5.7 Hemi's do not have a torsional damper of correct design. Beware.
Breaking a crankshaft out of warranty is not fun or cheap.
The horsepower that an alternator consumes is
mostly set not by the rpm it turns, but by how much the 'voltage regulator'
inside the Ram pickups PCM computer increases the 'excitation field current'
inside the alternator's rotor. Because of this I see no benefit in slowing an
alternator down - no significant horsepower will be saved or fuel economy
gained. The AC compressor can be simply turned off when you want either
more power or better MPG. NASCAR cooling system guru Howard Stewart
points out that on modern engines like the Chrysler 4.7/5.7 with knock
sensors if you slow down the water pump you might have more horsepower
for the first few seconds at Wide Open Throttle, but then the cylinder head
will begin overheating at its worst spot, some pinging will start, the knock
sensors will trigger, and the PCM computer will start pulling back up to
16 degrees of ignition timing advance - then the horsepower declines
much more than you "saved" with the water pump pulley reduction.
If you must change pulleys it is best to just slow down the power steering
pump alone.