3G PULLEY IDLE SPEED

[SleepingStang]

Ok so I have my 3G conversion complete. It came out quite nice. I used a 3G from a 94 Mustang 5.0. I have checked the forums about pulley size as well as did some research on-line. I thought that the 3G pulley would be too big at 2.375 inches (2 3/8), so I plugged some numbers into PA Performance's pulley RPM calculator and came up with the following.

3G Pulley

Crank Pulley size 6 inches divided by 2.375 inches = 2.52

Max Engine Speed (Redline) 5800 RPM

5800 RPM X 2.52 = 14616 Alt Shaft RPM at Max Engine RPM

Seems to be within limits set forth in this calculator.

Idle RPM I believe to be somewhere between 800 and 900 RPM in my Manual SSP Coupe.

Idle RPM (800) x Ratio (2.52) = 2016 Alt Shaft RPM at Idle
Idle RPM (900) x Ratio (2.52) = 2268 Alt Shaft RPM at Idle

So it seems to me that if this calculator and my figures are correct that the alt should be putting out the required volts at idle. I ran the same numbers with my stock alt pulley size (1 3/4 inches) and it seems that I would be blowing up the alt if I run to redline, somewhere close to 20000 alt shaft RPM at redline. The crank pulley that I measured was an extra I had lying around from an 89 5.0 automatic Mustang, so I would assume that I have the same size crank pulley on my 92 5.0 5 speed. This is the only reason that I could think of that these estimates would not be correct.

Just wondering if this seems correct to everyone. Maybe it will also help someone else out if they are trying to figure out why their alt is not charging enough at idle. The only thing I will have to change if this is correct is the belt length by about 1/2 inch.

Also I have a 150 AMP wafer style fuse (see photo link mine is 150 AMP) for the alt, do you think this is too high. I have the 4 gauge power and ground wires with everything soldered and heat shinked. I did not connect the stock power wires at all. I will get some pics attached to this to let everyone see as well. Thanks in advance.

 

[87GT Drop Top]

I have a 6G alt on my car and I'm running a 1 7/8" pulley on the alt with no problems. I have the same fuse, but it's 175amp. I took off of a 1998 F150 that runs a 3G 130amp alt.

 

[67coupe]

I don't have the pulley sizes in front of me....but I'm using ASP underdrives, including the U/D pulley on my 3g (sill larger than stock 2g pulley). Voltage at idle with all accessories (electric fan) on is 13.3 volts. I've been using this setup for 6 years with no charging issues.

A good friend of mine is using the stock 3g pulley on his 3g with ASP underdrives (crank & water pump). His voltage with all accessories (electric fan as well) on is right at 13 volts. He's had this setup for 2 years with no charging issues. Both cars are daily drivers. BTW, we are both using stock belt length.

You should be fine with the stock 3g pulley. Be sure to add a 4g ground wire from the engine block to chassis.

 

[SleepingStang]

Ok so today I got everything buttoned up. Car running I am showing 14.6 volts at battery and at alternator with the stock 3G pulley. So with this information it appears as though I have plenty of voltage at idle. I think the only thing I do not have a definte answer on yet is if the 150 AMP fuse is too large. Should I step down. Thanks. BTW my SSP already has a 4 gauge ground cable from the block to the area near the solenoid, plus countless other grounding straps so I assume that I am good in the grounding.

 

[87GT Drop Top]

The 150 amp fuse is fine. On a stock application they use a 175 amp fuse.