Bypass valve small points (+ a boom theory!)

[Nexus]

I've noticed something that is quite counter-intuitive that has been discussed before (smoother power delivery reverting to stock bypass valve), but now I want to drill down into it a bit more.

Modest modset : intercooler,turbo inlet, turbo manifold, race pipe, HKS bypass, hypertech tune.

I had a HKS bypass valve. I read about smoothness, wondered if the HKS being a bit more complex (two stage) might be an exception to that observation, then a week ago had the OEM bypass re-installed.

It was very smooth before so I wasn't expecting much.

Improvements with OEM replacing HKS include:
Shifting to third gear.
Shifting in general.
consistently improved torque response.
Less/No exhaust crackle/pop (usually miminal previously)

Contrary to the "smooth" observation others had when reverting to OEM, I had smooth build-to-full-spool before, and now it's coming on hard and early. Nothing wrong with the driveability there's just a big jump at the very bottom end in available torque, and it's a noticable shove - (more and sooner) sudden onset boost.

In a nutshell, I figure the OEM bypass is bypassing a lower volume than the HKS.

This explains disappearance of pop/crackle on shift-overrun : While the system is in bypass, the MAF sensor does not produce linear readings. For the period in which the bypass is venting, the MAF will return a value that is less than the actual intake into the combustion chambers.

Unknown exactly how the ECU compensates for this, but it will be either a constant correction value applied when appropriate, or it's a value calculated from observing other sensor feedback.

For the purpose of explaining the pop/crackle with HKS vs none on OEM, let's presume that the ECU uses the simplest componsation method : The OEM bypass valve's recirculation characteristics are used, defined as constants.

So during bypass the ECU essentially takes an educated stab at the volume of air entering combustion chambers, based upon sensors - but (possibly) assuming the bypass bleed rates of an OEM valve.

The HKS bypassing a greater volume of air then, leaves less air entering the combustion chambers. But the ECU assumes it's educated stab is correct, resulting in rich fueling conditions. This, in turn, is evidenced by the slight crackle/pop on overrun that doesn't seem to be present with the OEM valve.

So my questions are : How to best ensure that an aftermarket valve emulates the OEM one to best effect? To what extent is this something that can be adjusted? Was my HKS set too "soft"?

And the boom theory is this: If the ECU is encountering the above scenario, and running rich during bypass, then it's certainly running lean at some point soon afterwards while the system is equalizing. If that effect is pronounced enough.....then potential boomski!

---------- Post added at 07:20 PM ---------- Previous post was at 07:05 PM ----------

The sudden onset of bottom end boost, I suspect is due to the ECU 'picking' the OEM bypass, so to speak. I'm thinking there's a feedback loop between boost and wastegate, and that the OEM bypass bleed rate is an influence on this. So the observed behaviour fits if the ECU holds a constant -that reflects the bypass characteristics- used in calculating and compensating.

THEORY : Replace bypass valves means adjusting this ECU constant - if it exists.

[Nexus]

The simplest question is - will tightening the HKS up get me OEM bypass characteristics. Time to ask some appropriate mechanics, I think....in the meantime...happy to be told I've just had the HKS set too soft...I thought it was fine...

[TD]

Is it difficult to adjust? If not, just try it. You have more than enough logging gear to see what happens

Interesting post.

[Jrbris]

i have a turbosmart 50/50 BOV and from the stock bypass valve i noticed alot.

For example: i sniff the accell in 1st, 2nd or 3rd and i get wheel spin very easy, with the standard bypass i have to give it a boot full for the same effect.

I have a 3" Turbo back system on mine, i did notice a increase of crackle and pop, but i like that in moderation.

the oem bypass i did notice a little hole in the plastic, this is what i believe does the smoother delivery, but does not hold boost very well.

[Doug_MPS6]

I think most or all the above is complete tripe and probably over-analysis of a non-issue.

1st, A BOV, or BPV or whatever you have or wish to call it, as long as it is not "leaking" (i.e faulty), will not influence "power" or "smoothness" for it is not intended to open unless the throttle is closed, when you won't be developing any power anyway. Power on, the pressures above and below the piston are equal, so the piston won't be going anywhere.

2nd, If you are running stock boost then the OEM BOV will not leak when throttle on (unless it is faulty) and it is debatable if it does with higher levels of boost (up to the max recommended of nominally 18psi) anyway.

3rd, As to the "crackle and pop" aspects, a BOV should be set so as to (largely) eliminate backfires which are a sign of an excessively rich unburnt mixture and hence washing your cylinder wall lubricant with raw fuel, and unnecessary sooting up of your cat(s).

4th, A BOV should be tuned heavy enough to eliminate backfires but not so heavy that the turbo stalls because the BOV is not opening sufficiently to do its intended job.

5th, There is just sooooooo much crap written about BOVs but they are pretty simple devices with a pretty simple set of intended and possible functions, yet folks treat them like they are the be all and end all of tuning. I guess it is because they have a little twiddly thing on top that unwise folks can easily get their hands on and play with and get a noise out of which they invariably confuse with enhanced performance. "If it's noisy it must be going better" is a non-sequitur

[Nexus]

I think you've missed the point of 90% of what was written. re-read it please. It's written with a knowledge of high-order control systems, which is what the ECU is.

>3rd, As to the "crackle and pop" aspects, a BOV should be set so as to (largely) eliminate backfires which are a sign of an excessively rich unburnt mixture and hence washing your cylinder wall lubricant with raw fuel, and unnecessary sooting up of your cat(s).

OK, you're pretty close to the point there. Read the first post, then consider the control system and this point together.

I'm not complaining, I'm looking for constructive feedback "I feel is tripe" is not good feedback, nor is it objective. "I know it's tripe because..." is. You downplay the influence on the tune, but the whole point is that they MUST be an influence on the tune, and different characteristics will influence it in different ways.

Your over-simplification of the influence of bypass valves is OBVIOUSLY an oversimplification, as there are MULTIPLE OBSERVERS reporting changes in behaviour when aftermarket bypass is replaced with OEM. If your overly simple interpretation was correct, NOBODY would be reporting anything, because there'd be nothing to report.

Lastly, one specific observation proves you wrong : This simple device can't possibly affect low-end spool just by replacing it. UNLESS it has an influence on the tune. I can't think of anything a bypass valve can do to improve bottom-end spool and boost EXCEPT by an influence on the tune.

Appreciate the input, however, what I am looking for is greater understanding - MORE detail, not dismissing the small points because they are small points, which is what I feel you are suggesting. You seem to understand that I am wanting to discuss small points - you obviously know they are small points. That's what I want to get an understanding of - if possible. Not to ignore them - pay attention to them.

We are looking to explain observed behaviour from multiple owners. I am NOT certain, but I AM confident in the thrust of the analysis.

As I explained at the outset, the observations are counter-intuitive. Nobody's talking about noisier=better. What we are talking about is how our bypass influences the tune. Not saying it's the be all and end all of tuning, either. Just saying that there certainly seems to be an influence, and asking for input in helping to understand exactly what is happenning.

Not interested in ignoring the small points. Interested in focusing on them.

I think most or all the above is complete tripe and probably over-analysis of a non-issue.

1st, A BOV, or BPV or whatever you have or wish to call it, as long as it is not "leaking" (i.e faulty), will not influence "power" or "smoothness" for it is not intended to open unless the throttle is closed, when you won't be developing any power anyway. Power on, the pressures above and below the piston are equal, so the piston won't be going anywhere.

2nd, If you are running stock boost then the OEM BOV will not leak when throttle on (unless it is faulty) and it is debatable if it does with higher levels of boost (up to the max recommended of nominally 18psi) anyway.

3rd, As to the "crackle and pop" aspects, a BOV should be set so as to (largely) eliminate backfires which are a sign of an excessively rich unburnt mixture and hence washing your cylinder wall lubricant with raw fuel, and unnecessary sooting up of your cat(s).

4th, A BOV should be tuned heavy enough to eliminate backfires but not so heavy that the turbo stalls because the BOV is not opening sufficiently to do its intended job.

5th, There is just sooooooo much crap written about BOVs but they are pretty simple devices with a pretty simple set of intended and possible functions, yet folks treat them like they are the be all and end all of tuning. I guess it is because they have a little twiddly thing on top that unwise folks can easily get their hands on and play with and get a noise out of which they invariably confuse with enhanced performance. "If it's noisy it must be going better" is a non-sequitur

---------- Post added at 07:07 PM ---------- Previous post was at 07:04 PM ----------

There is no requirement to adjust the HKS mazdaspeed blow-off valve kit according to HKS. You can adjust it slightly, but it should not be necessary. As far as HKS is concerned the valve ships with the appropriate adjustment.

[TD]

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

[Doug_MPS6]

NEXUS, Hi. I appreciate your comments. I didn't set out to suggest that there is no improvement from OEM to aftermarket. There is. I only suggested that it is unusual for OEM valves to "leak" as often as they are reputed to, seemingly as some sort of justification for buying a new aftermarket BOV. The reason for going aftermarket is tuneability and capacity, so that you can increase and/or direct the amount of bypass air enough to prevent turbo stall but not so much as to constrain, restrict or limit a fast turbo spool up when back on the throttle.

BOVs ARE simple devices and have but one simple function, to provide a mechanism for intake air under boost pressure to have somewhere to go so it doesn't stall the turbo when the throttle plate closes and cause a back-pressure wave. The relationship with the MAF is there but only as an incidental one, in the sense that the MAF does not determine BOV function, nor the converse, although the BOV deals with the issue of backfires induced by mixture imbalance brought on by the MAF initiating a fuel loading via the ECU for air that is no longer fed to the cylinders. The only alternative is to dial in a fuel cut. Most folks don't bother if they aren't running 100% VTA.

In this context is is notable that diesels do NOT have BOVs as they do not have a throttle plate.

Incidentally, the fact that BOV-dom holds some weird magic for some people is shown by the remarkable number of diesel-driving lunatics out there who want their turbo-diesel SUV sod-buster to "have a BOV sound". One's mind can only boggle!

Cheers.

[Victa]

Incidentally, the fact that BOV-dom holds some weird magic for some people is shown by the remarkable number of diesel-driving lunatics out there who want their turbo-diesel SUV sod-buster to "have a BOV sound". One's mind can only boggle!

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

Air brakes FTW...

Chris.

[slixx]

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

Air brakes FTW...

Chris.

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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