Bypass valve small points (+ a boom theory!)

[SarcasticOne]

"But, But, But... The kenworth beside me has BOV noises, and it's a turbo deisel!!! WAAAH"

Air brakes FTW...

Chris.

Oooooh, is that what the noise is....

[Caffeine Fiend]

I work for a company that rents out Diesel Generators. You should hear a 50ltr 16cyld Twin-turbo Marine Diesel motor putting out about 1200kW

Usually there only running at 1500rpm but they can make some noise!


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I wonder if any of them were Mitch-built (MPS2NV). That'd be too funny


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[Nexus]

Thanks. Don't mean to be testy. Specially the ltft thread.

I continue to be a little amazed. 6th gear has come alive - it's happily thumping away at lower rpm. spool has definately dropped ~300rpm? It's really noticable. I might peg spool @ 2200rpm now instead of 25.

Here's what I'm expecting to see - I probably have some logs from past that I can compare against. Lambda showed a distinctive pattern of behaviour on lift off. It got quite lean a short while after and then wobbled it's way back to equilibrium. I'm very strong on theory but don't have the hands on experience to be confident I'm interpreting appropriately. What I am expecting to see is the lamda stay within tighter boundaries, prob still stray around a bit on bypass, due to the MAF uncertainty, but I expect I'll see it holding tighter.

That MAF uncertainty during bypass is the crux of the tuning influence. The control system takes corrective actions etc based on sensor input, but the discrepency between it's hard-coded expectation of bypass conditions, and the actuality of bypass conditions gives rise to all the known symptoms - rich @ bypass (crackle-pop), etc. This effect is also going to be more pronounced the greater the volume of air you have to bypass. Very long plumbing solutions with very big frount mount intercoolers could give rise to the (still theorised) lean-after bypass condition becoming exaggerated to the point of boom. This would perfectly explain some of the legendary mystery boom behaviours - "wasn't on it had just been and then done a fairly gentle squirt...." bypass, lambda all over the shop, back on it - even just a bit - before the system has equalised again, and for no apparent reason....massive knock + boom. Because ECU thinks there's a lot less air in the system than their really is. It has over estimated the volume passing through the engine in bypass conditions.

Just putting it there as a theoretical possibility, please don't shoot it down without imparting some knowledge of why.

I expect that it might be something custom tunes handle better but with the hypertech definately I would say : Don't install a bypass.

re: 50/50 turbosmart (?) This ccould potentially mitigate the ECU's miscalculations - potentially - bleeding to atmo removes some air from the system. since the ecu is theorised to be overestimating air consumed during bypass, bleeding some of the excess air from the system (to atmo) might improve the ecu's recovery times. These were about 2 seconds or more if my lambda observations and interpretations are correct. This is where it can get very interesting - and very complicated. It is certainly VERY possible for a high order control system to become unstable in such circumstances, which might be evidenced by lambda jumping around and entering dysfunctional conditions before recovery.

---------- Post added 05-09-2011 at 12:05 AM ---------- Previous post was 04-09-2011 at 11:44 PM ----------

Adjusting the HKS is possible. There's a fine- adjustment of tension - then further adjustment of the same results in shorter throw of the valve. HKS calls this "over adjusted". However it occurs to me that restricting the throw will ultimately affect the volume of air at the second stage bypass, disabling it and restricting the valve to a single stage if you persisted, I expect. Never the less, might be all that is needed to emulate the OEM valve characteristics better.

---------- Post added at 12:40 AM ---------- Previous post was at 12:05 AM ----------

Have you seen this topic on MSF? Goes into a fair bit of the detail you might be chasing:

Discussion on how bypass valves can affect LTFTs. - Mazdaspeed Forums

Very good. I can only see one mistake. Almost the same interpretations, back to front on one aspect :

>> My hypothesis is that our ECU is programmed to expect a low release point, since the stock BPV has a relatively weak spring holding the valve shut. I think that this stock Release Point is expected to be DURING the operation/learning range of the heavy throttle LTFTs. This is the same LTFT that is used during WOT (Open Loop) operation. When you add an aftermarket BPV which raises the Release Point of the BPV such that it puts it out of the operation/learning range of the heavy throttle LTFTs, which means that the ECU will learn these LTFTs with a fully shut valve (vs the slightly leaking recirc of the stock weak spring), which means that more air would actually get into the engine, making the ECU calculate higher LTFTs, since it sees more air in the system than it's used to (making it add more fuel). This illustration shows what I mean by increasing release points putting themselves out of heavy throttle LTFT operation/learning range:

His theory is that the ECU "sees more air in the system than it's used to (making it add more fuel)". But the ECU can't possibly see that once the bypass is open as the MAF sensor is the point at which the air enters the closed system. The bypass is within the system, once the air is in the system, either all the air is going in (closed bypass under boost) or it's not (bypass open) - it might add fuel if the valve opened later, as not being in bypass, there's a touch more air entering the system.

It's the bypass open conditions that we are both theorizing are hard-coded. My point is the ECU can't use the MAF to gauge volume in the combustion chambers during bypass, specifically because the MAF will be reporting no air entering the closed system while the air already there recirculates until it's consumed, the MAF becomes near real-time again, and the ECU ceases to rely on hard coded expectations of bypass conditions.

His theory is ECU calculating bypass conditions before the system actually does etc...assuming valves open later hence more air through maf...
Mine is that the ECU is calculating bypass conditions differently to how they actually are....but I am actually suspecting the aftermarket valve to be "softer" and quicker to release and releasing more.

I haven't read the whole thread - I would disagree about the OEM valve leaking - it doesn't - and I also wonder about the valve being software than after markets. I don't think these things are necessarily true - certainly 2XS had no trouble with 25 psi and OEM bypass if I recall....

Without a custom tune, I think it is best to emulate the OEM characteristics as much as possible, so next step is to watch that lambda and see what it's doing with the oem valve etc, then inspect the HKS setup and see what might emulate OEM best.

[Doug_MPS6]

Interesting thoughts NEXUS and it's good to see someone thinking hard about the technicalities in detail.

[Nexus]

First chance to run a log today. From the glances I stole at the display, lamda does hold tighter, but that's a far from scientific observation for now. It seems wanders less and achieves equilibrium quicker than I recall last time. I've disabled the bluetooth I used to use for logging and now running with USB - so much better, so much less latency. But I don't think I'm seeing something I just expect to be there, or an artifact of faster logging - and when I find a comparable historical log, I think it'll show the variation.

Having said that, I now realize the lean condition that made me take notice of it in the first place, and start watching what it was doing, is the injector shutdown that occurs in some circumstances. Nevertheless, I felt lamda recovered quicker in this circumstance too.

So I would be wanting to log runs with HKS, OEM and another at some time in the future, taking care to be logging gentler lift-off (back-off) but not overrun, because I think this is where the differences will be measurable utilising OEM sensors.

The next approach would be to place a MAF housing in the recirc path and actually measure it - hypertech, if I recall, claim to have done exactly that during development, and also stated that the stocker holds boost and they couldn't find a reason to recommend upgrading it. This leads me to feel that the MAF characteristics that hypertech measured with a recirc MAF feeds into how they've tuned it. Agressively expecting those measured recirc flow characteristics...

The previous LTFT thread from mazdaspeed forums definately on a similar track.

Also suspect that the negative influence of inaccurate recirculation modelling at the ECU may influence the system in a similar way to badly heatsoaed TMIC with high IAT....

Accelleration from 80-110 in 4th gear logged 175Kw 320Nm. 110 in 4th gear is what - 4500rpm? Does that Nm figure stand scrutiny against the Kw? I suspect it's displaying ft-lb and displaying that as Nm, making correct figure 430Nm if so) - bug in the dashboard rather than logging. I think. Or I'm unclear on flywheel vs wheels for the torque figure. Kw I am certain is ATW. Will have to chack manuals ...

[Nexus]

175Kw 320Nm ATW it is. Factoring in 15% drivetrain loss, that's 376Nm @ 4500rpm. No dash bug. 430 - wishful thinking.
Nice to see 175Kw logged for a real-world squirt! Presuming the peak is higher in the rpm range, happy me.

But I was suppsed to be comparing logs of lambda.....