Power gain from PS pulley and M Servotronics Sport mode

[cmichaelo]

I can see how under driving the PS pump with an oversized pulley will free up a small amount of HP due to reduced mechanical drag.

But does selecting the "Sport" mode for the "Servotronics" entry in "M Drive" also achieve a reduction in the engine drag, especially as speed increases?

I suspect the answer is yes. But what you guys have to say about it?

And I know the gains are negligible...

[TboneS54]

Negligible I'd wager. WP/PS pulleys on e36/e46 were fairly popular, but I'm not even bothering this time around with the S65.

[cmichaelo]

Ok, here's my own little pocket theory about the potential HP benefits from software controlling the PS pump (via the Servotronics ECU tuning).

Thesis: Servotronics tuning to stiffen steering as speed increases improves HP!

Why: Increasingly reduces drag on engine as speed increases!

How (The theory): As speed increases, the flow control valve increasingly routes the pressured fluid back into the pump itself. Correspondingly, the pump increasingly sees less restriction from having to pump the fluid through the high pressure line to the steering rack. At very high speeds where no PS assist is needed at the steering rack, all pressured fluid from the pump is circled back into the pump itself, effectively reducing the fluid pressure to a minimum and thus presenting the least amount of drag on the pump. Correspondingly also the least amount of drag is imposed on the engine.

[TheM3Chase]

The theory makes sense but as stated before I do think it's negligible as well

[pbonsalb]

All the little stuff added up can make a difference. For example, BMW decouples the alternator and ac compressor under certain conditions to reduce drag or improve power on some later model cars.

[TboneS54]

Even the old E46 m3 decouples a/c at high rpm.

[cmichaelo]

Originally Posted by TboneS54

Even the old E46 m3 decouples a/c at high rpm.

Its my understanding this is also done on the E9x.

[cmichaelo]

Originally Posted by TheM3Chase

The theory makes sense but as stated before I do think it's negligible as well

If by negligible you mean can't be butt felt. Yes, I agree.

But if you mean can't be measured, I disagree. I think speed sensitive PS can free up multiple HPs. The PS pulley, OTOH, will give you much less because it only reduces the drag by some ~15% or so (ie, the relative enlargement of the pulley's diameter).

Have you ever sat in a car without PS and tried to turn the steering wheel at a standstill and trying to back out of a parking spot, or into one for that matter? If you have, then you will appreciate how much power it takes to get the PS to do it's job. I suspect the HP gain is into the multiple HPs.

Originally Posted by pbonsalb

All the little stuff added up can make a difference. For example, BMW decouples the alternator and ac compressor under certain conditions to reduce drag or improve power on some later model cars.

So true. Wish they also decoupled the alternator on the E9x. Anyone knows if this is programmable?

Even if we're getting a couple of HPs already via Servotronic, this is equivalent to the [indirect] HP gain one would get from the weight reduction from replacing the trunk with a CF trunk, and which is expensive as hell.

[Redd]

As I understand it, E9x M3 disconnects the alternator under heavy engine load.

Your theory is plausible but the difference is probably miniscule. Remember, while all this pressure reduction is happening, the engine is still driving the PS pulley and pump. It is not disconnecting the pump entirely (unlike the a/c compressor with its magnetic clutch). As long as the pump is still being driven, it is still moving fluid and there is still the frictional losses associated with that. So no I don't think Servotronic will result in appreciable (beyond the margin of error of a dyno 2-3hp) power gains.

[TheM3Chase]

Originally Posted by cmichaelo

If by negligible you mean can't be butt felt. Yes, I agree.

But if you mean can't be measured, I disagree. I think speed sensitive PS can free up multiple HPs. The PS pulley, OTOH, will give you much less because it only reduces the drag by some ~15% or so (ie, the relative enlargement of the pulley's diameter).

Have you ever sat in a car without PS and tried to turn the steering wheel at a standstill and trying to back out of a parking spot, or into one for that matter? If you have, then you will appreciate how much power it takes to get the PS to do it's job. I suspect the HP gain is into the multiple HPs.

Agreed!! I have driven manual power steering cars before and unfortunately even tried a Ram truck with no power steering while in a parking lot. It sucks a lot! I think it is definitely measurable, but negligible by feel. I appreciate this theory a lot