Ok the results are in, and the dyno vindicates the M370 actually, so that whole theory about the intake manifold potentially being a problem was bunk. This is the comparative HP+Torque. The red lines are the M370 yesterday, the blue lines are the stock manifold today. Both are on the stock ECU map, roughly similar weather conditions, etc.
You can clearly see the M370 makes decent torque and power gains virtually everywhere, but comes in a little low beyond 7K RPM. This is what I was originally expecting based on the M370 thread, so there's nothing wrong with the M370 at all. The gains are actually quite impressive to me. Those yellow grid lines are worth 8 hp/torque each. Pretty fat torque gains in the low end, and the slight loss at the top is totally worth it.
We also played with that canned UpRev tune a bit as well. The bottom line is that on the stock manifold, it makes virtually no difference in power, but considerably leans out the engine. On the M370, the UpRev tune hurts a bit on power. Either way, the whole setup obviously could use some real dyno tuning love, and I really don't fault UpRev for this canned tune, seeing as how it's based on zero real data from the car.
The M370 will be going back on the car this weekend, obviously. The remaining issue is explaining the loss of ~20HP vs my older dyno graphs.
15K miles of hard driving could introduce a number of variables, perhaps some deeper maintenance is in order (some kind of fuel injector cleaning treatment? Spark plugs? Are there some fittings loosening somewhere that I haven't found?). I had a peek at my air filters this morning too, and the grooves are full of road debris (bits of grass and dirt, etc). They're really in bad need of a cleaning, so that could be a contributing factor if they're starting to restrict.
Another potential change between my ancient run and these recent ones is that I changed a leaky and broken Stillen catback for a working Fast Intentions one. It's possible the Stillen's leak was actually helping my power output before
.
Another factor is when we went back over all the dyno's data log for the old and new runs, there was a substantial ambient temp difference. The old runs were logged at 74F ambient, and the new runs are at 95F. Ignoring any difference in atmospheric pressure or humidity (no data log on any difference there), just the 20 degree temp rise could account for about 6HP according to normal SAE correction formulas.
So I don't really know at this point. I'm chalking it up to 6 from the temp change and the other 14 I'm guessing is from dirty air filters, lack of an exhaust leak possibly, and general wear and tear? I was already planning to swap filters when I put in the 7AT cooler, hopefully soon. Maybe after that I'll see if I can get another run in on a cool day in the morning just to see what difference temps+filters makes.
Thanks to Brian at Engine Logics for doing the dyno runs and going over the results with me. I think I'll be using them to do my tuning once the long tube headers are on.