Z_WAAAAAZ

I'm apologizing in advance for the title. I didn't know how to state this question better.

So I was down at my buddy's shop yesterday and they had a mobile dyno trailer on site. I figured I might as well take advantage of it and get my car dyno'd. Last time I did it was in December. The car made 212whp peak then but peaked at 202whp yesterday. Interestingly, peak torque was almost the same, 185 in December vs 183 yesterday. Comparing the graphs below, you can see they're very similar until peak torque was reached, with yesterday's hosepower curve flattening out more and the torque curve falling off more sharply after 5,200rpm. Only things changed on the setup between the two runs were that I upgraded to LS2 coils and a Fluidampr. It was probably 25 degrees hotter yesterday than when I dyno'd the car in December. I'm also now running 225 tires on 15x9 wheels as opposed to 205s on 15x7s previously, although I don't believe that'd make a difference?

I haven't dove into diagnosing the potential issue very deep yet but I checked the spark plugs (BK7RE) today and found them to be gapped at .040" which seems high, even with the LS2 coils. The plugs have 7k miles on them and were brand new when I dyno'd the car in December. I've never played with the gap so they would have been at the stock gap back in December (.032 or .035"?). I've regapped them to .031" for the time being and see if I can feel a difference via the ol' butt dyno (I'm guessing I won't be able to).

Anybody have any additional insight into what might've caused the changes in the graphs?


First dyno graph

Yesterday's graph

LeoNA

Spark blowout is not a real phenomenon. The actual issue is as the cylinder pressure increases it acts as an insulator which prevents the spark from bridging the gap. .040 (1mm) is too large for an FI application. I would start at .030" (.75mm), but it does depend on output/boost and at that level you should not be having an issue.

Z_WAAAAAZ

Yeah, that’s about what I figured as far as plug gap goes. Never really bothered to check because I never had an issue with it after initially installing the plugs. Never had a misfire, no drivability concerns, etc. I’m aware that the phenomenon is caused by increasing cylinder pressure adding resistance to the “circuit” but figured people still called it spark blowout. Oops, lol

That’s also what I thought though regarding the boost/power level. I’m running 10psi and couldn’t imagine the LS2 coils were THAT stressed even with the .04” gap. Could the ambient temps account for that much of a variation in power output? I’m spitballing here but temps were about 50*F during the first dyno run and maybe 75*F yesterday. Also yesterday, we did the runs out in the sun whereas the first run was done indoors.

Now that I’ve typed all this out, it’s occurring to me that I’m probably just being neurotic about this.

SpartanSV

If you didn't have a misfire then insufficient spark energy for your gap and cylinder pressure wasn't the issue. Should sound very obvious like a spark cut limiter, and your O2 readings would swing lean.

A Ambient temp difference causing increased IATs is probably enough. A very small difference in boost pressure would do it too. Do you have logs showing identical manifold pressure for both pulls?

codrus

BTW, next time you go to the dyno bring a flash drive with you and ask the dyno guy to copy the run files onto it for you. You can then download the run view software from dynojet and generate a PDF that overlays the two different runs to make it easier to compare them.

That said, I wouldn't read too much into this, the two charts are not comparable for a few reasons:

1) the correction factor is not the same. See where it says "CF" on the charts? One is using the SAE correction factor, the other is using "STD". The correction factor is used to try to correct a dyno chart based on ambient temperature and pressure, and the two graphs used different formulas for it. You could fix this if you had the original files by regenerating them with the same CF.

2) the temperature is significantly different. Even if the correction factor above worked (which for turbos it doesn't), all that does is correct for air density. It does not correct for the fact that your ECU will sense the higher intake temperatures and back off the timing, costing you power to try to prevent the engine from pinging.

3) The graphs were made on different dynos at different locations. If the altitude at which they sit is different then that's going to throw everything off. The correction factor mentioned above works OK for fixing this for NA cars, but it is completely useless when the car is turbocharged. There just is no way to compare two dyno charts for a turbo car that were generated at different altitudes.

--Ian

Z_WAAAAAZ

Ok, that's about what I thought. So as far as insufficient spark energy goes, it's more in the direction of an all or nothing issue? Either you have a misfire or you don't. I was slightly skeptical of my original hypothesis because my AFRs stayed consistent during the entire second set of pulls.

Unfortunately, I didn't datalog either session. However, I didn't change anything in my boost setup. Currently just running wastegate pressure. 8psi wastegate spring with a couple washers pushing it up to ~9psi and then boost creep causes it to sweep up to 10psi near redline.

I did some quick math last night and applying the ideal gas law to the ambient temps on both days, it seems like a ~25*F difference in temps would account for the change in power. I know there's a slew of other factors I'm probably not considering, but at this point I think that is probably enough to account for most of the recorded power difference.

Z_WAAAAAZ

Ian, thanks for the insight. These are the factors I said I'm probably not considering haha.

I don't have much to reply with or add to your post but I appreciate the thoroughness of your comments. Learned a lot in those few paragraphs!