Evo X March “Mivec” Madness

April 3, 2010 in 2010 Evolution X Team, 2010 One Lap of America, Car Preperation

Just before I went on vacation I hit the dyno mid March and found out that no matter how much you prepare for a dyno session, something is always bound to go wrong. Well this time after the third run I noticed my Wideband datalogging stream wasn’t recording properly and after playing with the plug connection it went from half working to not working at all so I was flying blind in tuning my AFR’s above 6000. After hooking up STM’s tailpipe wideband sensor it was definitely confirming that my AFR’s were going from ~11.8 in the 4000-6000 range and then tapering richer to about 11.0 AFR. The power was also flat lining above that RPM at about 400 whp (~520 crank HP) all the way to 7600 rpm’s where we were letting off. I leaned things out slightly in the upper range and the power pretty much stayed the same but there was no additional knock beyond the typical phantom noise that this 4B11 engine tends to make in the upper RPM range. Not having a reliable way to pin down what AFR I was running, I decided to call it a day.

After reviewing the logs it dawned on me that the -10 exhaust Mivec that worked well to increase power with the larger runner ETS exhaust manifold may not work as well with the smaller runner Full Race exhaust manifold in the upper RPM’s. Other factors are that I’m testing a prototype intake manifold with a larger plenum so this may be effecting where the power is peaking. I plan on further testing in the next few days to test this theory using VDR (Virtual Dyno Room) which is a great spreadsheet that simply plots time versus RPM gained and lays out a nice graph. While not as accurate as a dyno, I found that the numbers usually are good enough to show trends in the curves that are greater than a 10 whp difference from run to run assuming you do the testing on the same stretch of flat, abandoned road.

I was also able to confirm that my Mivec changes were working  like a champ. On the street I’m hitting 20 psi by 3500 rpm’s in 4th, while the dyno that boost hits at about 3800. I found out very quickly that the load percent is a good 20% lower than on the street and my boost set by my MBC is about .5 psi lower once strapped to the dyno. This is more or less similar to the effect of accelerating downhill, while the car does get up to speed quicker, it also doesn’t produce as much load on the engine so boost hits a bit later. Do the same test going uphill and you will suddenly hit higher loads earlier in the RPM range. This is one reason I tend to only do 4th gear pulls on the dyno now since 3rd gear tends to see even less load/boost and the datalogs and HP curves tend to be less detailed since the run is much shorter. There are also times where a 3rd gear pull may be right on the edge of being safe, but a long run down the front stretch of a road course in 4th will expose knock issues or intercooler heat soak problems that may have not appeared otherwise.

Another issue to watch out for with larger turbos when in higher gears such as 5th is that you can get what is called compressor surge when going WOT at lower RPM’s before the turbo is in it’s normal operating range. If I tried to use the aggressive Intake and exhaust Mivec settings then 5th gear would have massive compressor surge at 3400 rpm, so much so that the boost would violently drop to zero and then rebound to ~20 psi as if it’s hitting a wall. This is extremely bad for turbo life! Essentially the compressor flow is exceeding the engine and turbo exhaust wheel’s ability to exhale and the air has no place to go. While the anti-surge compressor cover should reduce this effect, any kind of increase spool from Mivec creates too much spool for the combination to work properly at low RPM’s. So what was the answer to this annoying issue?

In comes Gear-based Mivec! Thanks to one of the resident Evo gurus best known as “Gunzo” on the forums, he was able to take the  Mivec Intake and exhaust #1 “warmup mode” maps and program them to trigger whenever you entered 5th gear instead of just kicking in until the engine reached full operating temps. While not a big deal for the smaller stock turbos this is a huge drivability aid for those running larger turbos that have surge problems trying to force air through the engine at lower RPM’s, especially when at higher loads traveling uphill in 5th gear. As you can see in the maps I’ve reduced the intake mivec advance sharply in the 3000-4000 rpm at 160+ load along with a reduction of exhaust mivec retard which after several days of testing seems to keep the car right at the surge line in cold weather. With the recent warm weather around here this seems to work out perfectly!

I plan on revisiting the dyno in the coming weeks to verify my changes in the 6000+ rpm region. Between leaning out the AFR’s safely and adjusting the exhaust Mivec retard in the upper RPM ranges I should be able to reach that 425 whp consistantly. Here is a video of the dyno run at STM, notice how the suspension has almost no movement on the dyno whatsoever!

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