Want to jump in here for a second. The factory " tube like" manifold is actually a quality Toyota shorty header. Part # 17141-75080. I got mine from Bosch Toyota back east somewhere. I think a little over 2 bills including shipping. Tucks in close to the motor and is Toyotas replacement for the crack prone cast manifold. This factory headed has the stock 3 bolt two exhaust hole flange. It is a direct replacement for the cast manifold. The down pipe bolts up to it. I would have liked to have mine chromed or ceramic coated before putting it in service but time an $ did not allow. It is that nice of a product though. Any vaccum leaks after the MAF sensor is unmetered air that will not compensated for by the ECM causing a lean condition. Exhaust leaks before the first O2 sensor will cause theECM to enrichen fuel mixture due to inaccurate reading from the O2 sensor. From what I have been told the first O2 sensor handles mixture control and the second one is to monitor catalytic converter efficiency. I am not running a second O2 sensor but I deal with a CEL code for no bank 1 sensor 2 heat circuit and no bank sensor 2 voltage signal. I plan on utilizing an O2 simulator. I did not explain the exhaust leak effect very well. Some of this info I found on a MR2 forum. Obviously exhaust choice is one or the other depending on pressure or normally aspirated. Hope this helps someone. I had a fast rough idle from the vacuum hoses at the power steering idle up valve at the pump. I eliminated the hoses cause my 12:50s prevent turning to full travel anyhow.
