Power Steering Fluid - CHF11S to ATF

[prostcfc]

Just bought a track prepped E46 M3. I noticed the PS fluid in the reservoir is low. Cap says CHF11S, but the reservoir was replaced a couple years back and they all say that now (I think). There defiantly is not ATF in there.

Since this is a track car, I've read ATF is better for that sort of application. Has anyone switched from CHF11S to ATF with a thorough flush, without issue? Have seen some folks talk about potential seal compatibility in the rack, though I'm pretty sure my rack is factory, thus was originally filled with ATF. I did notice that the remanufactured racks that Bimmerworld sells say CHF11S only very specifically, but since I have the factory rack, I'm assuming that doesn't apply and there shouldn't be an issue with switching to ATF.

Any advice is appreciated.

[Tbonem3]

I think the chf11s is better, hence why bmw went that way with PS fluid in e9x era. Member Obioban here cleaned out al th atf and moved to chf11s for specific (track related) reasons iirc.

That said, for anything besides state of the art race machines and huge factory teams, I don't think lowly ol' ATF is gonna keep you from realizing new PBs.

[bigjae46]

I think the chf11s is better, hence why bmw went that way with PS fluid in e9x era. Member Obioban here cleaned out al th atf and moved to chf11s for specific (track related) reasons iirc.

That said, for anything besides state of the art race machines and huge factory teams, I don't think lowly ol' ATF is gonna keep you from realizing new PBs.

I've seen some say it is the opposite? Bimmerworld says you can use ATF in the E9X M3 instead of the CHF11S which is more suited to track use. IIRC, BW says CHF11S tends to overheat, not sure I agree with that. I need someone to test some pump shims for the S65 to verify.

But...I'd rather run ATF if for no other reason it is easier to find in a pinch. Been there...wanting CHF11S but not having any. Then O'Reilly's wants $35 for a can.

[jayjaya29]

When I got my AAE Z3 rack, the instructions said CHF had to be used. So I filled it up with CHF. First track weekend, the CHF boiled over and made one hell of a mess.

I switched back to ATF and havent had any issues since.

FWIW, I prefer the ATF over the CHF for one reason. If there is burps or spillage, the CHF turns into an almost impossible to remove glue/cement if it gets onto anything hot (like the rack itself or the engine block). Whereas ATF cleans up easily with brake clean or any degreaser.


After taking apart the power steering pump to throw in a shim from bigjae46, there is a lot of fluid trapped behind the pressure regulating valve. I just kept pouring in atf and bumping the starter (with the hoses pointing down into oil drain) until it turns from CHF green to cherry red. If I had to do it again, I would remove the pressure regulating valve after doing the drain/fill and wash that out with fresh atf as well.

[bmwfnatic]

I also remember reading somewhere that ATF was the better option when running very hot.

Some BMW’s had a cold climate option which gave them CHF over ATF, so presumably the CHF flows better when extremely cold, but that might mean that when very hot it performs less, at the end it’s just hydraulic fluid though, so I don’t know…

Pentosin writes that CHF is for use from -40C until 130C. With the flash point being at 160C.

If you look at some of the data sheets for ATF, many have a flash point of over 200C, so perhaps they can be pushed a bit further.
One would have to install a temperature sensor to see if those temperatures are ever reached.

Also something interesting to note is that the original BMW spec for ATF is Dexron 2, which is a mineral based ATF. CHF 11 is synthetic, might have something to do with mixing them is recommended against.

[bigjae46]

After taking apart the power steering pump to throw in a shim from bigjae46, there is a lot of fluid trapped behind the pressure regulating valve. I just kept pouring in atf and bumping the starter (with the hoses pointing down into oil drain) until it turns from CHF green to cherry red.

Sweet! Let me know what you think.

[Tbonem3]

I've seen some say it is the opposite? Bimmerworld says you can use ATF in the E9X M3 instead of the CHF11S which is more suited to track use. IIRC, BW says CHF11S tends to overheat, not sure I agree with that. I need someone to test some pump shims for the S65 to verify.

But...I'd rather run ATF if for no other reason it is easier to find in a pinch. Been there...wanting CHF11S but not having any. Then O'Reilly's wants $35 for a can.

Maybe Ian can give his reasoning then

[Obioban]

Maybe Ian can give his reasoning then

I went back :P

I did it originally because it's thinner, and has stable viscosity over the temperature range-- so should let the system work less, provide better feel, and be more consistent.

But, it boiled on track, which was an issue I never suffered with ATF. So, back to ATF I went.

[Tbonem3]

Got it. But there has to be some reason bmw moved from atf to chf11s on e9x m3. And it's not cost, not much is cheaper than atf.

[samthejam]

Does chf11s have longer service intervals?

[Obioban]

Got it. But there has to be some reason bmw moved from atf to chf11s on e9x m3. And it's not cost, not much is cheaper than atf.

I don't know what the reason was, but the e9X M's suffer from power steering feeling boiling on track even in noob group.

[Slideways]

I don't know what the reason was, but the e9X M's suffer from power steering feeling boiling on track even in noob group.

Quick search comes up with "better cold flow characteristics" for street cars that are used in colder climates, so the temp range could be different, which might explain why it does not do well on track.

[Tbonem3]

I don't know what the reason was, but the e9X M's suffer from power steering feeling boiling on track even in noob group.

Are guys switching to ATF or installing some sort of cooler?

[bigjae46]

Are guys switching to ATF or installing some sort of cooler?

I think many are going with an EPSS. I don't have many details on it.

I'd say PS fluid weeping is more of a well known issue on the E9X M3. Not sure if its because its right behind the headlight and very easy to see vs the S54 where the reservoir is somewhat hidden. I cooked the fluid during auto-x. Never had a problem on track but the car was much younger.