For those of us that are using TWM throttle bodies, or similar configuration with individual throttle plates for each runner and MAP ports downstream; MAP sensing is a different ball game.  For both the ECU or MP referenced fuel pressure regulators, pulse filtering is required for proper operation, especially toward the low end of rpm and power.  If you don’t have this configuration, you can stop here.

 

We have already determined that some means of pulse damping was necessary with Tracy’s ECU for smooth running below about 2000 rpm.  I had initially just added some volume using a section of bigger hose, which helped considerable, but wanted to do some further work to try and decide how best to set it up.

 

This week I installed a dial type fuel pressure gauge to use to calibrate the EM2.  With engine off, running one fuel pump gave a nice steady reading of 42 psi, and both pumps gave 43.5; which is the 3-bar setting that TWM said to expect.  What surprised me was that with the engine running at 2000, the pointer was swinging very rapidly over a range of over a pretty wide range; from about 27 to 37 psi.  It is an undamped bourdon-tube gauge, responding to a lot of pulsing.  I had no pulse filtering between the manifold and the fuel pressure regulator.

 

I also was noting that at a fixed throttle setting the MAP pressure readout on EM2 was varying over about 1” with the different samplings.  Long story short; after some modeling, I started on the side of too much damping.  I now have a bit over 2 cu. in. of volume in the line from the MAP ports (the ports from each of the three runners connected together), then a 0.025” orifice restrictor, and then a T that goes to the EC2 and back to the FP regulator (about 1 cu. in. volume).  The fuel pressure gauge is now quite steady at about 32 psi for those same conditions; and the MAP readout is steady.

 

The volume downstream from the orifice (to the EC2 and the FP regulator) is now a bit too large making the response too slow.  Its fine for the normal slow power changes, but I noted that pulling the throttle rapidly back to idle results in a dead spot while the mixture is too rich (MAP decaying too slowly).  The response can be varied by varying the volume downstream from the orifice, or changing the orifice diameter.  I intend to reduce the volume I have in the line to the EC2.

 

If you want to think about it, consider the volume in the line as a spring (linear with force, more volume = weaker spring), and the restrictor as a hydraulic damper (flow varying with about the cube root of the force; I think; smaller diameter = more damping).  Best not to think about it too hard, cuz it’s not that critical.

 

Al