Poor mileage caused by carputer?

silencery · 02-08-2005, 02:08 PM

As the subject says, I've been seeing 3-4 miles less per gallon ever since I installed the computer. I admit, I don't know much about the details of how alternators work, but I'm assuming that since I'm drawing more electricity from the car, the alternator now has to work harder, causing strain or resistance on the motor.

Is this mode of thought correct? If so, is there anything that can be done to alleviate the problem?

Any tips and ideas appreciated!

jcdillin · 02-08-2005, 02:17 PM

this has been discussed before, but if the car computer causes a mpg drop, i've never noticed it. I get exactly the same with or without the computer.

Maybe it's time for new battery because your alt is having to work harder to keep it charged?

TimmyM · 02-08-2005, 02:18 PM

Have you tried turning off the computer for a tank full and see if it goes back up?

I dont think an alternator works that way, like an air conditioner. It spins with the same amount of resistance no matter how much amps you use. I guess it could give you less spark which would leave un burnt fuel there for requiring more fuel. But 3-4 mph is pretty hefty.

Vchat20 · 02-08-2005, 02:20 PM

actually, it may be true. i dont remember exactly where, but i do remember hearing someting about more load on the power system will cause the alternator to work harder which in turn could cause low gas mileage.

also, while the car is running, the electronics dont run straight off the battery, they run mainly off the alternator, and the alternator just makes sure the battery stays at a full charge.

p.s. check scouse monkeys post below. it explains it a bit better.

Scouse Monkey · 02-08-2005, 02:21 PM

Quote: Originally Posted by TimmyM

Have you tried turning off the computer for a tank full and see if it goes back up?

I dont think an alternator works that way, like an air conditioner. It spins with the same amount of resistance no matter how much amps you use. I guess it could give you less spark which would leave un burnt fuel there for requiring more fuel. But 3-4 mph is pretty hefty.

Ever heard of Lens law?

Your alternator WILL be harder to turn the more load you put on it. That is why ou use more fuel if you have your heater and lights on. My main reason for going dc-dc is that i only have a 1.6 engine so want to keep the system as efficent as possible to stop it sapping my power and reducing my mpg.

silencery · 02-08-2005, 02:28 PM

Quote:

I dont think an alternator works that way, like an air conditioner. It spins with the same amount of resistance no matter how much amps you use. I guess it could give you less spark which would leave un burnt fuel there for requiring more fuel. But 3-4 mph is pretty hefty.

Yeah, my average range before was about 310 miles. Now, my average range comes out to roughly 270 which is a difference of 40 miles on a 12 gallon tank! I didn't believe the numbers myself.

I tried a search, but couldn't find any relevant topics. If someone could possibly point me to the original discussion, i'd appreciate it. From the sounds of things though, it seems like there was never a conclusive result?

What I was wondering specifically is if there was indeed significant load from running the carputer, and if there was something I could replace (i.e. battery or alternator) to help improve my mileage.

Oh yeah, I'm running an Opus 150W and the motor in the spyder is a 1.8.

silencery · 02-08-2005, 02:30 PM

Thanks for all the responses btw. I haven't actually gotten around to running on a full tank w/o the carputer yet... haha.

I don't think i could bring myself to do it.

stevieg · 02-08-2005, 02:31 PM

If you're going DC-DC on a Mini-ITX the MPG drop will be around the same as using your sidelights which is practically nil. I tried in the UK for a whole tank to see if there was a difference and saw nothing noticable, MPG figure was basically the same. I'm using an Opus 150W (not heaviliy loaded) too.

n8scstm · 02-08-2005, 02:34 PM

Alternators do not have a clutch like an a/c unit. The resistance never changes. A gas mileage decrease from your carputer would only come from the added weight which is so little realative to the car that it is neglagable. Therefore it's only a theoretical loss which you will never notice. If the loss is indeed real, you need to look for another cause. And by the way, it's called Lenz's Law. Not lens. It will also cause a theroetical loss, so small that it wouldnt be noticed on a lawn mower let alone a 1.6 liter vehicle.

silencery · 02-08-2005, 02:34 PM

That's the thing... I AM seeing a big mileage drop... weird huh?

I'm off to google Lens Law and a few other things; thanks scouse et al.

Vacheron · 02-08-2005, 02:36 PM

As an engineer, I would say it's extremely unlikely.

If your car has a pessimistic 100BHP, (though if it ths same as the UK's MR2 I'd assume it's nearer 150, but we'll stick with 100), then that equates to around 75,000 watts of engine power, or less than 1/1000th of an average carputers power consumption.

The only thing that would come close to affecting your fuel consumption would be some extreme amplification, but even that would be pushing belief a little.

Even assuming you had 1000W of amplifiers (true RMS not PMPO etc) in the car and that they were running a 0dB sinusoid at full gain, 100% of the time, (deafening to the point of hearing damage) then that would still only be a 1.00% power drain. (For reference, a true 1000W amp would be one that would require in the region of a 150 amp fuse!!!)

I would say it's far more likely that your new carputer is causing you to adapt your driving style a little as you are driving along kicking out the tunes from your new toy.

mpattonm · 02-08-2005, 02:38 PM

Thats correct.

TimmyM · 02-08-2005, 02:40 PM

Quote: Originally Posted by Scouse Monkey

Ever heard of Lens law?

Your alternator WILL be harder to turn the more load you put on it. That is why ou use more fuel if you have your heater and lights on. My main reason for going dc-dc is that i only have a 1.6 engine so want to keep the system as efficent as possible to stop it sapping my power and reducing my mpg.

Nope, never have. I don't know that much about autos, motors, or electiricity.

What actually causes it to turn harder? There really isn't anything that would make more contract like in a air conditioner. Theres a bearing front and back that would be the only actual resistance. The rotor spins but only in a field.

Vacheron · 02-08-2005, 02:44 PM

Quote: Originally Posted by TimmyM

Nope, never have. I don't know that much about autos, motors, or electiricity.

What actually causes it to turn harder? There really isn't anything that would make more contract like in a air conditioner. Theres a bearing front and back that would be the only actual resistance. The rotor spins but only in a field.

As the current through the alternator is increased it creates an increasing and opposing magnetic field to that of the stator magnets, making the rotor harder to turn.

If this was not the case then power stations could generate the whole countrys electricity using a hamster in a cage and we could kiss goodbye to global warming
(Unless that was, someone forgot to feed the hamster!)

silencery · 02-08-2005, 02:45 PM

haha, good response Vacheron. My amp is certainly not pushing anything close to those numbers. I'm running a Zapco iforce 450. If that's the case though, that means that something else would be causing the poor mileage.

Last night, i did a cursory check and looked over the following:
EDIT: left out a few other things

- air filter.
- tire pressure.
- new oil.
- fairly new spark plugs (10k miles).
- drivetrain is clean.

Everything seems to be ok...

I'm wondering if there's an off chance that replacing the battery would help?