Megasquirt tuning

[jdbubb at ix.netcom.com (Dan Bubb)]

Just spent Saturday tuning a 2.0L 8V Rabbit GTI with Eurospec head, 280 cam, TT race headers, 50mm ITB's and Megasquirt.

I guess the best way to put this is, it was alot more difficult to tune this fairly radical engine than it was to tune an ABA with cam and exhaust.

We started off using a 486 laptop. A 486 can run the Megasquirt software, but it's so slow it's hard to see where you are when you're tuning on the road and everything is bouncing around. Also, the screen needs to be at least 800 X 600 to see the tuning screens and an active (TFT) screen is going to help alot. We switched over to my relatively new laptop after one short run. You can pick up a killer laptop (for this application) on ebay for less than $200. It's worth it to save the aggravation.

Check for ignition trigger stability by watching the RPM display on the Realtime display before attempting to drive with or tune MS. If you don't have an accurate trigger MS doesn't know if it's fueling for 900 RPM or 5000 RPM due to spikes. Some people have been able to pull the signal straight off the coil, but it appears on VW's that this will give problems. If you intend to use one of the MS ignition derivatives the best bet is to pick up the hall signal directly in conjunction with a pullup resistor (1K ohm between the hall signal line and switched 12V). If you're using stock or KS ignition you can pick up the signal off of pin 6 or 7 on the ignition module. This is one of those items you need to get right before you can proceed.

If you're running ITBs or perhaps if you just have a really wild cam, you're going to see the MAP signal jump around quite a bit even at high engine speeds. It appears easy to fix this by putting a volume (old style inline fuel filter) in the vacuum line to the MS. You may also need a restriction in the line. A 0.031" mig welder tip has been suggested. I would highly recommend adding these if you need to before starting tuning. With a jumpy signal you're tuning more VE map points at once and since some positions on the VE map have pretty steep gradients it makes for less effective or accurate tuning. 

When I was tuning the ABA I felt pretty good about tuning using a narrow band O2 sensor. With the wild ass 2.0 w/ITBs it seemed less effective. Maybe it was that the VE map was so far off to start with. In any event the WOT tuning looked good at .8-.9V, but after installing the WB-O2 it looked too lean. So, here I guess that most of the tuning can be done with a NB-O2, but when you get to WOT you should have a WB-O2 or plan on hitting the dyno early in your MS installation if you have a fairly radical engine.

One other thing to look out for is if you should change to a different TB or ITBs make sure your KS ignition switches are operating correctly. Without the idle switch closing your base timing will be way off and the ignition response coming off idle will be to retard the ignition instead of advance it. This caused a really bad stumble moving off from a stop. So, make sure all the other systems are operating correctly before you try to tune MS.

Last, once you get some tuning under your belt and are getting into the WOT tuning make sure you keep an eye on your injector duty cycle. No sense tuning WOT VE entries when the injectors are maxed out!

Overall this particular tuning experience was only mildly satisfying considering the expectations which were awfully damned high. The KS switch issue (which was not even thought of until the next morning!) hurt power and caused some really dramatic drivability issues!

None of this dampens my enthusiasm for Megasquirt. It just points out there are things that need to be taken care of before you get serious about tuning.

Dan