HO Alternator Idle Conundrum

[bananapehl777]

Recently rewired my subwoofers to 1 ohm. Allows my DC Audio 1.2 k to output 1200 W (was only getting 750 W with them at 4 ohms). My mids and highs amp is outputting about 400 W. On the safe side total system power is 1700 W. I have big 3, Interstate MTP-35 in front, and XS Power 3400d next to sub amp. The issue is with my alt, which is a DC Power 270 XP. At idle, my voltage drops into the 12.6 - 12.9 range when system is on full tilt. It shocks me that it gets this low after doing all these electrical system upgrades. After a lot of research I found that the alt is only outputting about 60 - 70 A max with an engine speed of 600 RPM (about what my idle speed is). I measured crank and alt pulley diameters and got a ratio of about 2.21 (seems pretty small to me) to calculate all this and looked up the amperage vs. alt rpm chart from DC Power's website. This problem goes away obviously when I am cruising on the highway with engine spinning at 2500 RPM yielding a 260 A output from the alt and the voltage only drops into the high 13's when system is playing at full tilt.

How can I go about increasing idle output from the alt? I know it boils down to increasing engine RPM, decreasing alt pulley size or simply not playing my system on full volume. What's the best option here?

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[Popwarhomie 5,000+ posts]

Overdrive pulley would be your cheapest option.

 

[Slo_Ride 5,000+ posts]

What happened to Miss Iraggi 2021?

 

[bananapehl777]

Overdrive pulley would be your cheapest option.

What size would you recommend? I worry about belt contact and if overdrive pulleys are compatible with my DC Power alt.

 

[Popwarhomie 5,000+ posts]

What size would you recommend? I worry about belt contact and if overdrive pulleys are compatible with my DC Power alt.

Couldn't tell you. I would contact DC Power and ask them. One of the reasons I bought my DC 270 because it did the highest amperage at idle out of my choices. Going on 10 years now with no problems.

 

[metalheadjoe 10+ year member]

What size would you recommend? I worry about belt contact and if overdrive pulleys are compatible with my DC Power alt.

I suppose you could go with a larger crank pulley and enjoy the added benefits of super power steering and colder a/c (at the expense of power and efficiency). If the shaft diameter is the same as oem, the pulleys are interchangeable.

 

[Slo_Ride 5,000+ posts]

I suppose you could go with a larger crank pulley and enjoy the added benefits of super power steering and colder a/c (at the expense of power and efficiency). If the shaft diameter is the same as oem, the pulleys are interchangeable.

I do not recommend that at all

 

[metalheadjoe 10+ year member]

I do not recommend that at all

Me neither. Just listing options.

 

[bananapehl777]

Thank you for all the info. I agree the larger crank pulley is not the option. I may try a combination of a slightly smaller alt pulley (if i can find one) and a slight raise in the rpms at idle (I have a program that allows me to do things like set the idle speed).

I also noticed just tonight after poking around that the grounds of both batteries could be better. The front battery is grounded at the factory location but no paint is stripped off (also grounded to alt). The rear is grounded underneath rear seat with paint stripped, but resistance measures at .4 ohm. Seems high to me. Something else to fix.

 

[metalheadjoe 10+ year member]

Thank you for all the info. I agree the larger crank pulley is not the option. I may try a combination of a slightly smaller alt pulley (if i can find one) and a slight raise in the rpms at idle (I have a program that allows me to do things like set the idle speed).

I also noticed just tonight after poking around that the grounds of both batteries could be better. The front battery is grounded at the factory location but no paint is stripped off (also grounded to alt). The rear is grounded underneath rear seat with paint stripped, but resistance measures at .4 ohm. Seems high to me. Something else to fix.

I wouldn't adjust my idle speed to suit my stereo unless the alternator was bogging the motor down.

Where is the factory ground location? Where do you see .4 ohms? This is in your Subaru? I'm not familiar with them; do they have a frame or a unibody?

 

[bananapehl777]

I wouldn't adjust my idle speed to suit my stereo unless the alternator was bogging the motor down.

Where is the factory ground location? Where do you see .4 ohms? This is in your Subaru? I'm not familiar with them; do they have a frame or a unibody?

I think the alternator does seem to bog down the motor a bit. When at idle at a light, when the bass hits the rpms do drop a bit and the car shakes a little like the motor is struggling, but they raise back up to correct for it.

The factory ground location for the front battery is on the driver side strut housing. I'm seeing 0.4 ohms from the rear battery (XS Power d3400) negative to where it is grounded underneath the rear seat. Yes this is in my 05 Outback XT. After some research it seems to be a unibody design.

 

[metalheadjoe 10+ year member]

I think the alternator does seem to bog down the motor a bit. When at idle at a light, when the bass hits the rpms do drop a bit and the car shakes a little like the motor is struggling, but they raise back up to correct for it.

That's to be expected, as aftermarket electronics aren't designed to feedforward to the alternator.

The factory ground location for the front battery is on the driver side strut housing. I'm seeing 0.4 ohms from the rear battery (XS Power d3400) negative to where it is grounded underneath the rear seat. Yes this is in my 05 Outback XT. After some research it seems to be a unibody design.

By "strut housing" I assume you mean upper strut mount (where the struts bolt to the body on either side of the engine bay)? That sounds like a poor oem grounding system all around. Personally, I would run ground wires for everything rather than rely on the factory system. It will be more money, more work, and more reliable.

Add this to the list of reasons I dislike Subaru.

 

[bananapehl777]

That's to be expected, as aftermarket electronics aren't designed to feedforward to the alternator.

By "strut housing" I assume you mean upper strut mount (where the struts bolt to the body on either side of the engine bay)? That sounds like a poor oem grounding system all around. Personally, I would run ground wires for everything rather than rely on the factory system. It will be more money, more work, and more reliable.

Add this to the list of reasons I dislike Subaru.

The ground is not on the strut mounting bolts, rather it is on the side of the housing wall. Like this.

(This is not my car, just an example). On mine, when the big 3 was done by a shop they slapped the 0 gauge negative ground wire on top of the factory ground wire, but didn't strip any additional paint off. Seems to me like that could cause a poor connection and cause a bottleneck.

I think the plan right now is to buy a paint stripping kit and redo the ground for this front battery to make sure there is a good connection. As for the second battery, I am not sure how to get that resistance reading down from 0.4 ohm as the grounding connection looks good with paint stripped. Maybe I'll try to find another location back there. If nothing else, overdrive pulley for the alt might be in my future.

 

[Buck 5,000+ posts]

Recently rewired my subwoofers to 1 ohm. Allows my DC Audio 1.2 k to output 1200 W (was only getting 750 W with them at 4 ohms). My mids and highs amp is outputting about 400 W. On the safe side total system power is 1700 W. I have big 3, Interstate MTP-35 in front, and XS Power 3400d next to sub amp. The issue is with my alt, which is a DC Power 270 XP. At idle, my voltage drops into the 12.6 - 12.9 range when system is on full tilt. It shocks me that it gets this low after doing all these electrical system upgrades. After a lot of research I found that the alt is only outputting about 60 - 70 A max with an engine speed of 600 RPM (about what my idle speed is). I measured crank and alt pulley diameters and got a ratio of about 2.21 (seems pretty small to me) to calculate all this and looked up the amperage vs. alt rpm chart from DC Power's website. This problem goes away obviously when I am cruising on the highway with engine spinning at 2500 RPM yielding a 260 A output from the alt and the voltage only drops into the high 13's when system is playing at full tilt.

How can I go about increasing idle output from the alt? I know it boils down to increasing engine RPM, decreasing alt pulley size or simply not playing my system on full volume. What's the best option here?

What type of vehicle and motor do you have?

 

[Buck 5,000+ posts]

Thank you for all the info. I agree the larger crank pulley is not the option. I may try a combination of a slightly smaller alt pulley (if i can find one) and a slight raise in the rpms at idle (I have a program that allows me to do things like set the idle speed).

I also noticed just tonight after poking around that the grounds of both batteries could be better. The front battery is grounded at the factory location but no paint is stripped off (also grounded to alt). The rear is grounded underneath rear seat with paint stripped, but resistance measures at .4 ohm. Seems high to me. Something else to fix.

I wouldn't remove any of your factory grounds. You could check for corrosion on contact points and maybe clean them. If you want, you could add another ground wherever you think that the factory grounds are bad. I've seen and heard many times from long-time installers that sometimes people doing an extra motor to frame ground actually made a car run better, because the factory ground was so old or used up that it just wasn't flowing properly. Corrosion creates resistance, because the corrosion does not conduct electricity like when the metal is in good shape. Dirt and grime and things like that are the same, if it's on a contact surface.

Something you could look into is a real capacitor setup. There's some high quality capacitors that actually have a bit of storage, and that might help your idle voltage, if you get an actual proper setup. There's different types of batteries and power storage that work better than just AGM batteries, and they do cost more money (from what I've seen).

I ran all of my batteries on a closed loop system, basically, in multiple vehicles. I ran + and - directly from alt to directly from battery in the exact same sized wiring. Then from front/factory battery I ran both + and - in the same exact size to my battery bank. I did not ground to the frame with anything except the front batter and alt casing. The entire battery bank in the back went to the front battery.