I actually had some Supersprint V2 stepped headers on order when I heard from Asbjorn @ ESS:
"You can not run SuperSprint stepped headers on the Z4M. There is a major primary O2 feedback issue with these and we have nothing but problems with people trying to do so. If you install these, then I am afraid we can not support you with any problems that may arise. We specifically list this in the product description on
www.esstuning.com also. Use OEM Euro headers, these are the best IMO."
In a later reply:
"No, the Supersprint issue is not the secondary sensors but the primary. They do not get a good read of the exh flow due to the stepped design and you have all sorts of fuel trim deviation codes."
OEM Euro headers run $2,250 new and I've found them difficult to find used...and don't flow as well, either. Plus they require relocation of the secondary O2 and EGT sensors. ESS' website for the Z4M also broadly forbids Supersprint headers, without even singling out the stepped ones.
With my plans to install the VT2-500 soon, I'm eager to get the immense heat source out of the engine compartment. Any help on finding some good catless headers that ESS won't refuse to support?