All the Dyno myth solve ? DSC off Room fuse pull and ABS fuse pull
 

Took my Budget Track day Rx8 to SDS dyno
(Japanese design 4wd dyno, similar to Dynojet&Bosch)

Weather Time
2019/9/21 8:30pm-9:30pm
28C
35%hum
950baro

Car spec
2005 Mazda Rx8 Sport model
New Mazda Reman less then 5000km
New OE Mazda coil and plug and wire less then 2000km
k&N drop in filters
Twin muffler center pipe with Rmagic Catback
On 18x9.5 RE Amemiya AW7 with 265/35/18 Dunlop Z3

I know from early on I need to disable DSC complete off on Rx8 when dyno
However since owning Rx8 I learn pull Room fuse trick to rest PCM and pull ABS fuse to complete off DSC

Here are some fact number
First pull DSC complete off with button long hold 7sec +
172.8 whp
19.5kg
I was logging MAF S/G I was notice only 185 S/G so I decide to shut car down pull Room fuse for sec pull

Sec pull with car cool down for 15min , I decide to pull Room fuse rest and DSC off with button long hold off
181.2whp
19.3kg
I was logging with MAF too, this time show normal 200G/S + and dyno curve look more normal with sharper top end power

For third pull , I decide to cool down for another 15min, this. Time I pull off ABS fuse
for some reason , I notice car have harder pull
With results
182.4whp
21.4kg
With even sharper topend curve compare to previous pull


So here u go, all the dyno myth floating on forum, testing on same car, same dyno ,with in a hr with cool down

What dyno myths?

Your first run looks just like the others until 7500rpm, where for some reason it lets off..., so the peak is earlier.

Pretty certain that he's saying that pulling the room and ABS fuses results in a 10+ whp gain in power.

I don't really understand how it's possible since I doubt the room fuse is carrying 7 kW (10 HP) of power.

A proper test would have included a final run with both the room and ABS fuses reinstalled to ensure it wasn't simply changing conditions or another variable at work.

Edit: I think I see what's going on. Pulling the room fuse resets fuel trims.
http://www.racingbeat.com/mazda/perf...pcm-flash.html

In essence, by pulling the room fuse, he leaned out the mixture a bit. We've already seen that this can be done to gain power in some cases.
If he left the front O2 sensor connected, the PCM will re-learn the fuel trims and this gain will eventually go away in normal driving.

Right, but look at the curve in the 170hp run. The peak isn't lower, it's earlier. Literally looks like someone eased off the throttle. Plus he's getting lower peak airflow, apparently.

I was diagnostic my other problem on dyno for two day
And notice some gain from pull a room fuse both day , I record result for sec day of dyno only

Yes I get my room fuse info from racing beat site
For me is pretty leggit

U can also say that pcm is still a city driving mode trim

The joys of having a non-DSC base model :)

Look carefully. It's making the exact same power all the way to 7500. Something causes it to plateau on that run and not others. It's not fuel trim, it doesn't behave like that. It could be your APV reacting slowly, bur you would eventually get a code for that. It could also be the driver. The lower airflow confirms. Trims don't close the throttle.

Did you have many 170hp runs or is this the only one? Can you repeat the experiment?

Also there is no city mode trim, that's not a thing anywhere in the ECU tables.

What Loki said.

The ECU has 3 long term fuel trims that correspond to different airflow rates.

Trim 1: 0-8 g/s (idle)
Trim 2: 8-20 g/s (low-load city traffic, maybe?)
Trim 3: 20+ g/s (Highway cruising?)

At WOT, the ECU ignores the front O2 sensor and just uses the LTFT. If your LTFT for 20+ g/s is a little high from city cruising, then you'll run a little rich at WOT. We've seen from the various tuning threads that there's power to be had in leaning out the mixture a little. So, if you do a dyno run with the learned trims and then reset the trims and do another pull, I would expect a small increase in power.

It actually looks like the the 170hp run makes slightly more power midrange than the middle picture :)
If everything is working well your LTFT should be 0 or thereabouts, so even if you reset it, you still get zero. If you have vacuum leaks then yes, resetting trims will cause slight leaning until it learns to adapt.

So yeah. Dyno myths get to live another day I'm afraid, whatever they are ;)

Loki, I'm not sure I agree with your position that LTFT should be zero. I mean, in theory, yes. But, in practice, it's likely that it will be slightly positive (and anything up to +8% is NBFD) even on a perfectly healthy engine with no vacuum leaks.

Additionally, the top trim is 20+ g/s. That's a huge range since the engines top out around 200 g/s. So, the trim to get cruising on the highway at 20-100 g/s may not be the same as the one needed for 100+ g/s.
If you warm the engine up, clear the fuel trims, and then do a dyno pull or run it at a track, I can see gaining a few HP.

That said, once you drive it on the Interstate for 30 minutes, it will go away.

The better way to handle this on a permanent basis would simply be to tune the car for a slightly leaner target AFR.

Yeah I agree, I mean it should be near zero, but there is always some real world fudge. I struggle to find a good explanation for 8% more fuel, but a couple of % here and there due to injector flow, MAF sensitivity etc would make sense.

It's compensating for something, so it's like at what point is the amount of compensation a big enough problem to want to solve?

When there's another symptom, I would say.
I mean, worst case scenario, it goes over 25% and the ECU throws a lean code.

Other than that, you'll probably see other issues worth troubleshooting before then.

To put it another way, LTFT is just another clue when already troubleshooting, not a reason to start troubleshooting.