Stronger altenator options?

[irishs2]

Jamo,at least you know your grammar.

[S2central.net]

@Jamo - if both the 90Amp and 120Amp alternator are operating with the same load of say 30Amp then it makes no difference to power at the crank.

All I'm saying is you cannot get that extra 30Amp capability for free - thats getting on for 500W difference... Unless you have been secretly working on a perpetual motion machine

[Jamo]

Ah but there is a difference between current drain and current output, current drain is associated to load being placed on the device we're talking about output of the alternator at a set state. ( which is how the alternator is rated )

drain is more or less going to be static as you we are not talking about the cars electric system just it's potential of the alternator to output more ampage to the electrical devices

Now shoot me down, i've pinned a bullseye to my head

[S2central.net]

With all respect - you are talking ******** now

Explain to me then how an alternator would be able to source its maximum rated current of say 100A at the same engine speed as when it is operating with 10A.
Where does the energy come from to create this extra capacity - if not the host engine ?

[Ross]

There is an inverse relationship there Jamo...

I am currently drained at work and therefore my current output is very low.

But surely the thing is outputting as many amps as it is rated for at a given RPM regardless of the drain? I thought it was all about flux, emf and weren't eddy currents a bad thing? Hey i got a D in physics a level gimme a break... this is too much for a Friday I'm out

[S2central.net]

The thing is only outputting as many Amps as it needs to - a 120Amp alternator does not consume that all of the time - it has a capability to deliver that capacity... In so doing it requires to sap more energy out of the host engine than when its only loaded up at say 10Amps. You canny break the laws of physics !

[Flat on my back]

.

Ross,
close but not close enough...
An alternator does not 'supply' any current until a load is placed upon it, then the current drawn will be in dependent on the 'R' of the load.

So, the same load on a 90, 120 or 200 amp alternator will draw the same current, but the higher the rated alternator you fit cos of all the lecky gubbins in your car the higher the load will be when you switch it all on!

Moral is less lecky stuff, what is fitted works proper and therefore smaller lighter alternator.

[Flat on my back]

oh i see.
so you can do that with the 120amp alternator? would the rewiring mean that there will be greater hp draw as well?

I think mine has the 90amp one.But ill have it checked and make the changes accordingly.Aiming to get the absolute perfect idle on all conditions basically and this is one of the things needed

If your having idling problems, it may well be your alternator is faultly or regulator.

Only reason to fit a higher rated one is if your fitting even more lecky options to the car or if the car should have had the higher rated one to start with.

If the 90 amp alt and reg turn out ok, the poor idling is due to something else.

[S2central.net]

He us running VEMS which IIRC has some funkiness about idle control on the I5T engine - seem to recall some specific threads on how to get VEMS idling nicely but I dunno how quickly it can adapt to changes in load at idle - say due to cabin fans or aircon kicking on/off.

[mark_s2]

Just to add to the discussion - you need to have is enough reserve capacity to keep the battery charged whilst taking the electrical load placed on the alternator into account. One thing that you have all missed is the engine speed - if the alternator doesn't maintain the charging voltage at lower engine speeds, the battery will discharge under large electrical loads.

[mcandmar]

The load an alternator places on the engine varies according to the electrical load. The more electrical load the more energy is required to turn the alternator. Thats just fact, nothing is free!

If you want to prove it to yourself sit in the car and watch the idle speed, volt gauge. Now turn everything on, lights, heater, and especially the heated rear window and you will see and hear the additional load placed on the engine. Every time the cooling fan runs the same thing happens and the ECU has to regulate the idle speed via the ISV to maintain a smooth idle.

A larger alternator (120amp) would have the capacity for more electrical load than a (90amp), but will require more energy to run at full load.

The other question is a bit of a grey area, say your consuming 10amps of current, which would produce the more load to run, a 90amp or 120amp? I did a little research into this as my Avant has a 120amp but dosent have aircon. I thought that was weird too. In theory they should require equal energy to produce 10amps however the grey area is in terms of efficiency. Its like compairing two different sized turbos producing the same airflow and boost pressure, one will be more efficient than the other. As to which is more efficient i dont know, however the 90amp is lighter and with no load attached is easier to spin up by hand. My guess is the 90amp unless you need the higher current draw.

[S2central.net]

Mark - glad to see you live in the real world

Your question about part-loading with different alternators is just a question of efficiency - you may well find that the 90A device is more efficient at delivery 10Amps and place a little less load on the engine than a less efficient alt delivering same 10A load.

The bigger rated alternators have beefier windings and componentry to give them larger current capacity so I too would summise that this increases weight and therefore parasitic load... then again we are talking about a reasonably small radius of rotating mass - nothing like a flywheel for example.

The general rule is that you only need an alternator rated high enough for the maximum load that the vehicle electrics can place upon it (Plus a bit for margin).

[Tommy200rs]

Check the latest free aspirating rs4 alternator that is water coolded. It has truly plenty of charge power !

[Ross]

I can see the next round of mods coming now... lightened alternator wheels and more efficient internals... or perhaps a bypass switch to run off battery for that little bit extra on a dyno run

[mark_s2]

Ross - you 'working' again today?