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Dynamohum Dynamohum is offline
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Join Date: Jan 2008
Location: Midland Ontario Canada
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Originally Posted by spence88mph View Post
I think pull though would be a better setup than what I have where it pushes through, would be a cleaner boosted charge, less turbulence. If your mechanic says he performs this mod on 930s he's talking BS as they don't have an afm.
AMF Draw through VS Forced?

This question is on my mind also as Iím trying to figure out which way to go with my own 3.2 turbo conversion. It would be great to have both intake layouts compared side by side on the same motor, that would give the merits of each one and give us a real answer on which is the better way to go.
Perhaps those with experience with either system would be so kind to give their feedback.

Iíll put in my 2 cents worth on the topicÖ.

In the N/A configuration using the spring loaded barn door Motronic Air Flow Meter (AFM),,, it is placed close to the throttle plate. I think it is located there for a reason, its placement minimizes the volume of the connecting hose between the two so that this design of AFM will work efficiently.
Because Air is compressible we do not have the benefits of direct proportional transfer as we would with Fluids. This means that the total volume of the connecting ductwork will determine the efficiency of the AFM response. I suspect that increased volume of the intake air circuit (ducting length) in a Draw system would induce a proportional delay in response time, to the ECM causing lean AF flat spot in acceleration.

In my opinion the ideal intake ducting design using Motronic AFM would be one that had a cross sectional area needed for unrestricted flow, routed directly as possible from the turbo to the throttle plate body, keeping the total volume to a minimum. This ideal is soon compromised with the addition of an Intercooler. If you look at intercooler flow numbers you will see that there is a pressure drop from the input charge to the output. The trade off is a cooler charge that produces more HP and engine longevity but at the cost of lag.
Every one talks about turbo lag relating to the hot side because it is the greater factor, but what about the often-overlooked lag on the cold side. In creating boost the turbo compress the air on the cold side and the larger the total volume of the air circuit from the turbo housing to the intake valve the longer it will take to flow and develop boost and that means greater lag and slower response time.

In our low boost applications it could be argued that you could gain more by running a water/alcohol system to for our charge cooling needs than running a big IC. The bonus with running W/A is, that it has the effect of increasing the octane rating, reducing the potential for detonation while supplying additional fueling and cooling requirements brought on by boosting. The other benefit is that your motor is not sucking in all the hot air coming out of the IC for its own cooling needs.

I donít think the Motronic AFM was given design considerations to run in a boost application and perhaps thatís why they are modified by companies like Promotive. (I would like to know what is done internally?)
Personally I think itís better located as close as possible to the throttle body under boost verses long routed vacuum paths of the Draw systems.
Some will argue that the AFM is to restrictive on the boost side and you loose pressure through it, Yes you do lose a bit just like you do when you add an IC but the gains are greater than the losses of operating it from the intake side of the turbo. Think about it, would you rather have your restriction on the intake side of the turbo, plus all the additional restriction from the longer ducting path or from a shorter more direct route running under Pressure? I think the AFM will respond quicker and more efficiently mounted directly before the Throttle Body, boost on or off.

If you have a Draw system already or if this is your preferred choice, then why not consider relocating the AFM for sorter and more direct routing. Itís only a matter of lengthening the harness to it and fabricating a new mounting bracket.
Thank You for your time, Paul. We do because we can.
87 911 3.2 (Turbo conversion, build in progress, Thermal Barrier Coatings, High Pressure Dry film coatings) Modified heads, boat-tailed case, ARP hardware, OBX Header, 930 clutch disk, G50 Trans
89 5.0 Mustang convertible (For Sale)
Old 01-26-2009, 06:36 AM
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