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Old 05-24-2016, 06:16 PM   #1
mrsaturn7085
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Default Downsides to speed density...

So this is probably more comedy than it is technical, but I thought I'd share an interesting experience I had last night trying to obtain a stable idle setting on an 06 STI running a Motec M800:

On the M800, you must physically define a fully-closed (less a 0.5 mm shim) and fully-open position for the DBW throttle by manually moving the butterfly with the motor +/- wires disconnected.

I initially performed this calibration and found the 0.5 mm jacked throttle to be too open to allow for an idle lower than 2000-2200 RPM (mathematically equated to 8-9% open, from fully closed). Pulled the intercooler off, and took a few more TP sensor readings with no shim, a 0.12 mm shim, and a 0.25 mm shim. At this point, it was getting a little late, so I stuffed rags in the turbo outlet and throttle inlet to prevent FOD.

Next day - I've got my new tune all ready to go with the throttle recalibrated to a fully closed position, and the pedal/throttle translation table starting out around 1% open (minimum) with the pedal lifted (avoiding 0% is a must to prevent burning the motor out, or sticking the throttle closed in the cold). All intelligent idle control is disabled - at this point, I am just finding the minimum throttle positions at each engine temp. break-point to obtained the desired idle. I back the car out, plunk down in the seat, and reflash the box. Everything looks great, throttle has a full range of motion, no (unusual) diagnostic errors, etc. so I fire it up.

Immediately the car needs more throttle position, so I find a nice point around 6-8% or so where the cold idle sits at 1600 RPM (pretty stable, too!). I continue tuning as the engine warms, until I've got a nice 1600 to 900 RPM decline corresponding to a 20 to 70+ deg C engine temp.

Wideband is showing quite rich the whole time 0.70 lambda or so; I start pulling fuel (nearly 25%) until lambda numbers come up closer to 1.00. I have JDM pinks and the base map was set up to run rich with USDM blues... so I expected the rich running.

Everything is looking good at a hot idle, back to the throttle translation map - I start bringing the TP% down at the zero pedal position break-point... suddenly, the throttle starts severely hunting. Ignition killed soon after this point due to DBW-related diagnostic errors. Ignition turned back on only to find the throttle position only has a range of 25-100%, regardless of pedal position or scaling... immediately thinking I messed up calibration settings, or worse, burned up a $600 DBW throttle motor.

To make a long story short from this point on - I chased software settings for about 15 minutes before getting out of the car and realizing I had forgotten to replace the intercooler or pull the rags out of the throttle and turbo (though the latter had taken care of itself). The throttle rag got sucked into the throttle (after 30+ minutes or so of the engine running) forcing the throttle to be stuck open to ~25%. The turbo rag took me ~5 minutes to find, and was wrapped around the steering linkage on the opposite side of the engine (somehow).

I cleaned everything up, replaced the intercooler, and realized that, due to the BPV vacuum reference line being completely open (among all other boners that took place), all my work that evening had been a total waste of time. Reloaded the tune from a day prior and had a beer.

A MAF-based setup would have not allowed for this but with speed density, the ECU could care less where the air is coming from, especially when you aren't loading the engine.
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Old 05-24-2016, 10:36 PM   #2
murrdogg24
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so it's speed density's fault? I'm not following...
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Old 05-24-2016, 11:07 PM   #3
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Interesting story, I would be thanking my lucky stars that the turbo rag didn't get sucked in and bent your blades, and that you were only idling along and the TB rag didn't get sucked into your intake tract.

Id say all in all you came out pretty good!

And yes, your conclusion is correct. SD really cares nothing about the pressure and temperature of the air in the intake, so, because your temperature was showing ambient ( since the TMIC was off ) and the pressure was showing whatever ambient was ( since the TMIC was off and the tubro was effectively cut out of the loop ) you operated with no problems.

MAF functions by assuming a quantity of air moving through a known orifice, so it would have been very unhappy in your situation since that orifice would not be measuring the air actually entering the motor.
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Old 05-25-2016, 01:43 AM   #4
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Quote:
Originally Posted by murrdogg24 View Post
so it's speed density's fault? I'm not following...
No... it's mine; though I wouldn't have wasted an evening if this was a MAF-based tune. As I started out saying, it was more of a funny engine management related story than anything else. The title was a joke, more-or-less.

I realize how speed density and MAF-based tuning differ - and yes, I'm very thankful the rag didn't make it's way down one of the runners...

I revved the engine up to 3000-3500 RPM or so in the process of preventing a few stalls, but probably never made it past 25-30% throttle position... very lucky indeed; even if it made it into the manifold, I reeeeeally would have been pissed if I had to take it off to clear the obstruction from a runner.

Last edited by mrsaturn7085; 05-25-2016 at 01:48 AM.
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Old 05-25-2016, 02:12 AM   #5
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Haha I'm just busting your chops. Yeah you were lucky indeed. Interestingly enough I'm in the middle of contemplating switching over to a maf/speed density setup. Maybe I'll just stay maf for simplicity, I'm not sure I need to be boosting over 22-23psi on my new setup. Plus running alcohol injection adds more complexity.

Quote:
Originally Posted by mrsaturn7085 View Post
No... it's mine; though I wouldn't have wasted an evening if this was a MAF-based tune. As I started out saying, it was more of a funny engine management related story than anything else. The title was a joke, more-or-less.

I realize how speed density and MAF-based tuning differ - and yes, I'm very thankful the rag didn't make it's way down one of the runners...

I revved the engine up to 3000-3500 RPM or so in the process of preventing a few stalls, but probably never made it past 25-30% throttle position... very lucky indeed; even if it made it into the manifold, I reeeeeally would have been pissed if I had to take it off to clear the obstruction from a runner.
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Old 05-25-2016, 12:34 PM   #6
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I'd either go full SD or MAF - blend has never thrilled me due to the added complexity and reliance on twice as many parts to keep the engine running; yes, you can get a better idle with MAF/blend ('better' in terms of cleaner emissions rather than more stable), but I just prefer to keep it simple.

If you are racing, SD is simply the way to go - blowing an intake hose will stop you dead if you are running MAF... with SD, you just lose boost and but stay in the race with an N/A motor.

Even on street cars this can be a huge benefit - back in college I blew the lower intercooler hose off a SW20 MR-2 Turbo (not MAF, but very similar mechanical system). Rather than towing, I pushed the car to a nearby parking lot and fixed it in pouring rain on the spot. The offending hose is attached to the circled pipe shown below, but about a foot and a half lower into the engine bay; you cannot access the hose from the bottom due to a frame rail. Clearly a SD tune would have made this a much more pleasant experience...





I would retain the MAF sensor for load reference, however.
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Old 05-25-2016, 12:36 PM   #7
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There isn't any down side to full sd
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Old 05-26-2016, 02:24 PM   #8
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Quote:
Originally Posted by WhatTurboLag? View Post
There isn't any down side to full sd
Did you read the post, at all?

From an emissions perspective, there is, but I wasn't really getting to technical here.
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Old 05-30-2016, 02:05 AM   #9
WhatTurboLag?
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No because my car passes emissions fully catless and full sd
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Old 05-30-2016, 02:31 PM   #10
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Then you missed the point of the post, entirely.

Passing emissions and being the engineering designing a low-emissions vehicle are two entirely different things.
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