Hot wire AFM positioning

[Chris Wilson]

Am I correct in thinking a hot wire mass air flow metering unit will give very different outputs on the same engine if used pre or post a turbocharger (air density and all that)? I see lots of heated discussion via a Google search, but no definitive answer. I realise they can get sniffy with air flow reversal with wild cams in N/A engines and possible turbocharged ones too. I am asking more specifically how they react to simple air velocity versus reading compressed air flow. Thanks.

[Scott@FP]

Biggest issue in blow-thru apps is the induced swirl issues from bends etc. Nothing to do with mass flow, mass measurement is mass measurement as long as its post-IC temp isn't the problem. Hotwire MAF DEPENDS on laminar airflow to be accurate. Even venturi style sampling tube types are thrown WAY off by induced swirl and non-laminar flow conditions. OEM's put MAF in the places they do for very legit reasons.

Another issue with hotwires is the amount of current they put on a ground, best way to deal with it on an aftermarket ECU (factory ECU deals with this with a dedicated MAF ground) is do a voltage to freq converter measuring diff voltage at the meter and the ECU MAF cal on a digital and keep the MAF ground direct to battery. You CANNOT put a hotwire's high current ground on a sensor ground, MAJOR issues!

Been there, done that, got the Making a MAF Work Right t-shirt.