High Load Stumble
 

I've got this situation where at about 6500 to 7000 RPM under full throttle there is a rather violent shake/stumble. It will drive through it and on to 8000 RPM but It's un-nerving. I cannot seem to get it sorted out. Boost Control Option 2 is far worst than Option 1; Am I setting up the boost control in a bad way?? Also I can't figure out that WG duty cycle bump at 645 seconds???
The changes to the car are:
Fresh (2000 miles) streetported engine, M2 SMIC, Stock RC flowed injectors, SR Motorsports light flywheel, Mikuni OMP set up for max oil at high loads, stock twins in "like new" condition, down pipe, stock cat, Racing Beat cat back exhaust, M2 air box with K&N filters, PFC, Datalogit, PLX WBO2, hose techniques, new check valves, etc, etc.

If you could take a look at the below charts showing a second and third gear pull and comment I'd really appreciate it (note still tailoring the AFR but it's close) -
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post the map and the log, it will be a lot easier to go through. That screenshot shows everything, and I see you've gone through effort to label stuff, but it's just not the same as having the log to scroll through.

also, overboost fuel cut sounds and feels like you're hitting the PFC rev limiter. it's not exactly subtle. And you're not going to hit it most likely. With those settings you'd have to hit about 16-17psi for fuel cut to possibly occur. It's most likely some ignition breakup. It does happen on stock twins at those boost levels, I've personally seen it. Might as well get the HKS twin power and be done with it.

Here you go and thanks for taking a look!
Note that I cut much of the 2M file size down on the log; I got the nuggets of interest.

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My [internet] diagnosis, based only on what you posted (don't have the car in front of me) is still ignition breakup. I say this for a couple reasons.


1. The AFR is solid enough that the hesitation is not likely to be caused by way too much or way too little fuel.

2. Knock numbers are low, under 30 mostly. FYI, you may want to retard the trailing ignition in these higher boost areas. I prefer a minimum of 10 split for safety. On conservative tunes and/or higher horsepower setups I almost always use 12 or 15 split.

3. Manifold pressure is fluctuating. Based on your first post I think you suspected that the boost control settings (and the resulting solenoid duties) are responsible for this. I see it as an effect of the problem, rather than as a cause. The breakup could be causing pressure fluctuations and the closed loop boost control is responding, rather than the boost control is causing the fluctuations. You can still tweak the boost control but I don't see any major problems with it... later in the log the boost is steady, and I will assume that this was when you weren't getting the hesitation.

4. I've personally seen and felt (from driving the car) this kind of breakup/hesitation on an FD with stock twins and Crane HI-6 on the leading only. Removing the HI-6 and installing HKS Twin Power solved it.

5. More combustion chamber pressure means more potential for ignition breakup. That may explain why it gets worse when you select your higher boost option (option 2). 5500 is an rpm where bad things tend to occur due to the combustion pressure. For example, llots of people knock at 5500rpm. I just saw it on a turbo Rx-8 last week due to the crap timing maps that Greddy provides with their turbo kit.


https://www.rx7club.com/attachment.p...1&d=1277185466


I think you should get the Twin Power. It's about the only thing HK$ makes that's really worth it. Supra guys swear by it. Even if it doesn't solve this problem, if you don't need it now you'll probably need it later. Very rarely are you throwing money away when you buy one of those things. There are other ignition solutions but the Twin Power is the simplest, especially because you can use the plug-and-play harness.

EDIT: Is this the area that had breakup? Or am I making incorrect assumptions? There were several pulls in the log you posted, and in most of them the log shows you letting off at about 6500-7000.

I am going to say Ignition break up as well, When i was building my setup, I was trying to run in the 10-11AFR and was having a ignition breakup because there just wasn't enuff juice to fire off that extremely rich mixture.

I'm tending to think you are right. It feels like the car drives over a bunch of rail road ties when it occurs (for lack of a better description). I would have thought the AFR would go much richer during the mis-fire however after thinking about it more there is a delay, additional burning at the turbine, and mixing that occurs between the event and when the O2 sensor gets it. The strange thing is it will drive through it and smooth back out on up to 8000 (not shown in these charts).

The ignition is much more challenged at the higher RPMs (loss of coil saturation time, dwell) and the higher chamber pressures (ignition 101)..... I am running the stock plugs also. To further compound the issue I'm running much more oil injection than stock at these higher loads.

I agree with your comment on the IGN maps; in another post I was trying to get someone to cough up the M2 Stage III maps but no such luck.

I will do a couple more runs to confirm. I'll go ahead and order the twin power and adapter harness and get that behind me then start tailoring the ign maps.

Thanks for your reviews and insight.

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You can switch to four BUR9EQ plugs if you want to (two steps colder on the leading). I run those on my single turbo T04R car at about 17psi. The OEM plugs are underrated.

Stock injectors with a ported motor will limit you. You need at lease 1200 secondaries.
See the almost 100% duty cycle on your posted chart!

I needed 550/1200 to safely make 350 whp with stock ports.

You are right about that - a necessary upgrade in the future. AFR's are good and stable and rich enough so I'll get the ignition straightened out make some more runs as see where we end up. The fuel pump is also stock. I am wanting to hold on this configuration and get the best out of it for the time being.

Thanks!

Fixed!
Installed the Twin Power and the top end stumble is history. Also took some fuel out of the higher pressure ranges and got the injector duty off the ceiling. Knock is still acceptable. Below is a pull in the first three gears. Thanks for your help!

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