Removing WGFV for good? Mechanical vs N75 boosting

[S2central.net]

I'm not dissing this company here on the thread - just giving my opinion based on the knowledge I have built up.

Purely mechanical boost control is fine up to a point but its an 'old school' method from the 70's and 80's.

The advantage of the WGFV system is two-fold. Firstly it allows the ECU and its software to define the boost profile over a wide range of throttle and load conditions and no mechanical tweaks are required to alter the boost profile - just software. The 2nd benefit is that the ECU can choose to reduce boost (gently or firmly) in the event that it detects fault conditions which could be harmful to the engine, such as prolonged knock with the use of poor fuel or excessive temperatures etc etc

If you remove the WGFV then you are removing some of the ECUs ability to failsafe when faults are detected. You will be reliant on the ECUs ability to limit fuelling and ignition in these scenarios which is something useful, but not as good as being able to falback to a safe level of mechanical boost.

In stock trim the mechanical boost ceiling is 0.4bar on the S2 - which you cannot achieve with a thumping great aftermarket WG spring that sets (say) 1.4bar - you could be mashing all that boosted air into the engine regardless of any fault detected by the ECU.

Paul

[Jamo]

Surely Aelred wouldn't send his cars to Automark if Mark wasn't a capable tuner, keeping all Motronic safety features intact??

I'm watching this thread closely, as I'm already planning my next set of mods (it's not even back yet) to hit 400+bhp, obviously requiring another remap. I've had a custom MRC remap in the past and am currently on a custom Automark remap. It flies, considering I'm still on a stock K24. I know that limp mode works as it cut in when I blew a hose off, although that was before the MAF sensor was moved to the pressure side (so now I'm guessing it would "know" about any leaks upto the intake pipe and limp mode wouldn't be needed).

@Jamo, what makes mechanical boost perform better? Is it because turbo longevity is sacrificed? Is there somewhere I can read up on this, without having to get a degree first to understand it?

It would be great for some forum comments on this from Aelred (or maybe Corey, as he uses Automark too).

It performs better as it is directly acting on boost pressure from the tubo.

With the ECU controlling boost via the N75 it is subject to ECU signal duty on the valve causing a slower build up of boost if there is no duty on the N75 valve.

The main reason though is that maximum boost / overboost is only available when the ECU sees the WOT ( wide open throttle ) where as mechanical boost maximum boost is available at part open throttle.

Have a read of this article.
http://autospeed.drive.com.au/cms/A_...popularArticle


Lets get one thing down though. I'm not commenting on Automarks work. I can't be subjective as I haven't used them but I know of other cars that have.

I'm merely commenting on the method derived to control boost which works but is retro.

[S2central.net]

Jamo - those points are really only valid when talking about stock software... The ECU could be programmed to provide full boost much earlier than WOT by altering the WGFV duty cycle, but only in so far as the turbo internals will produce.

That great article you link to shows some nice pneumatic tricks that were done to provide increased boost without fitting a big old spring (and minimize WG creep) without getting into ECU software.

This is why the old school method is often used today, as its a sure fire way to set the maximum boost without any ECU knowledge.

In fairness, there is some inherent delay (fractions of a second) with the ECU and WGFV approach as it has to sample the demands of the engine (and driver) in real time and modulate the WGFV to provide increased boost. It is inherently less direct than the mechanical WG spring. For that reason, I know the likes of MRC can add some additional mechanical boost if required to reduce spool times on the bigger turbos.

[Jamo]

Jamo - those points are really only valid when talking about stock software... The ECU could be programmed to provide full boost much earlier than WOT by altering the WGFV duty cycle, but only in so far as the turbo internals will produce.

That great article you link to shows some nice pneumatic tricks that were done to provide increased boost without fitting a big old spring (and minimize WG creep) without getting into ECU software.

This is why the old school method is often used today, as its a sure fire way to set the maximum boost without any ECU knowledge.

In fairness, there is some inherent delay (fractions of a second) with the ECU and WGFV approach as it has to sample the demands of the engine (and driver) in real time and modulate the WGFV to provide increased boost. It is inherently less direct than the mechanical WG spring. For that reason, I know the likes of MRC can add some additional mechanical boost if required to reduce spool times on the bigger turbos.

Completely agree Paul, it was aimed more at people with stock cars really as in all likely hood most remaps will be done from stock ( is there such thing as a stock S2 anymore? )

[S2central.net]

Oh and the other thing is - why would you want full boost at half throttle ?

The boost profile is heavily influenced by the position of the drivers right foot, so if you want full boost then nail the throttle! Modified ECU software can provide higher boost values, with improved spool times to provide lots more mid range torque and driveability. Much of that gain is done thru ignition timing which cannot be altered with a pure mechanical boost upgrade.

[Jamo]

Oh and the other thing is - why would you want full boost at half throttle ?

The boost profile is heavily influenced by the position of the drivers right foot, so if you want full boost then nail the throttle! Modified ECU software can provide higher boost values, with improved spool times to provide lots more mid range torque and driveability. Much of that gain is done thru ignition timing which cannot be altered with a pure mechanical boost upgrade.

You'd be surprised how different the car feels with boost avaiable at part throttle.

It's the only thing I miss from the electronic boost controller I had connected.

[S2central.net]

Bet you also miss the fuel bills though

All that extra boost at half throttle is pointless IMO for a daily driver. I can't see how that is nice to drive. Surely max boost is only required at full throttle when you are telling the engine that you want to go fast.

I don't get it !

I can see the need for more torque across the rev range - especially on a car like the S2 which left the factory with very conservative programming that leaves you wanting a bit more go from a car that weighs more then 1500kg.

What you are describing is like having your favourite guitar amplifier that plays full volume when you only have it turned to 5. Now granted, I like mine to go to eleven - but only when I want to

Good lord - wondering when I last saw the guitars, never mind actually trying to play one

[S2_Dave]

Have a read of this article.
http://autospeed.drive.com.au/cms/A_...popularArticle

That's a great read! Thanks Jamo. Part 2 also. Infact, the whole site's really interesting and is now in my favourites.

Unless I'm missing something, with mechanical controlled boost there's a very marginal reaction speed increase because the ECU isn't needed to make any calculations. The other (beneficial) characteristics of mechanical controlled boost, like full boost at part throttle, could be replicated by ECU software (if you so wished, but only with the right knowledge). Is wastegate creep not a problem with ECU controlled boost because of the N75 valve? I'm not exactly clear about the N75 valve and what signal duty is, but I'll have a bash with the search button tomorrow.

From the recent posts I have a clearer understanding of ECU controlled boost benefits. I'd love to read posts from advocates of mechanical controlled boost (or failing that, someone that was on the debate team at school who wants to make a case). At the bottom of Part 2 it compares the two approaches, but mechanical seems to win there mainly on cost grounds (which I don't think holds for our cars given the good value of a Homefry and the similar cost between custom remaps). Is there anyone out there that believes mechanical controlled boost on a Motronic S2/RS2 is better overall (taking into consideration performance increase, safety, longevity, driveability etc but not cost), assuming that it's done properly?

[Corey]

Why would myself, Aelred and countless other's chose someone (Automark) if he isn't capable at what he does? Mark choses to map that way. He certainly has the knowledge and skills to map via the WGFV but choses the mechanical route as that is the more reliable way for him.
If in doubt or you would prefer his opinion (rather than second guessing) feel free to talk to him on 01642714200.

[Jamo]

Bet you also miss the fuel bills though

All that extra boost at half throttle is pointless IMO for a daily driver. I can't see how that is nice to drive. Surely max boost is only required at full throttle when you are telling the engine that you want to go fast.

I don't get it !

I can see the need for more torque across the rev range - especially on a car like the S2 which left the factory with very conservative programming that leaves you wanting a bit more go from a car that weighs more then 1500kg.

What you are describing is like having your favourite guitar amplifier that plays full volume when you only have it turned to 5. Now granted, I like mine to go to eleven - but only when I want to

Good lord - wondering when I last saw the guitars, never mind actually trying to play one

What I found is that at part throttle I would only get wastegate boost limit, where as with the electronic I could get a bar of boost at half throttle.

In traffic or modulating speeds it would be a god send for that quick overtake without having to floor it. It's just gives a differnt boost characteristic and I quite liked it.

[Jamo]

That's a great read! Thanks Jamo. Part 2 also. Infact, the whole site's really interesting and is now in my favourites.

Unless I'm missing something, with mechanical controlled boost there's a very marginal reaction speed increase because the ECU isn't needed to make any calculations. The other (beneficial) characteristics of mechanical controlled boost, like full boost at part throttle, could be replicated by ECU software (if you so wished, but only with the right knowledge). Is wastegate creep not a problem with ECU controlled boost because of the N75 valve? I'm not exactly clear about the N75 valve and what signal duty is, but I'll have a bash with the search button tomorrow.

From the recent posts I have a clearer understanding of ECU controlled boost benefits. I'd love to read posts from advocates of mechanical controlled boost (or failing that, someone that was on the debate team at school who wants to make a case). At the bottom of Part 2 it compares the two approaches, but mechanical seems to win there mainly on cost grounds (which I don't think holds for our cars given the good value of a Homefry and the similar cost between custom remaps). Is there anyone out there that believes mechanical controlled boost on a Motronic S2/RS2 is better overall (taking into consideration performance increase, safety, longevity, driveability etc but not cost), assuming that it's done properly?

The thing with mech boost, you'll get the same boost all the time no matter if there is an engine problem.

With the ECU if you've got poor fuel, if the intake temp of the air is high, if the engine begins to ping and it detects knock, if the coolant temp is too low or too high, if the cam timing is incorrect, it will reduce boost to a safe level to safeguard the engine.

My problem with tuners mapping cars to mech boost is they can do it to hide problems on the car without resolving the underlying issue.

Take AAH! for instance, his RS2 wouldn't make boost via the ecu after a service, so they remove the N75 valve and wired in mech boost to get the car boosting correctly in their terms.

It turns out that when they realligned the timing on the car they retimed the cams incorrectly and the ecu detected this so wouldn't allow it to boost.

When the timing was corrected and the n75 put back in place the car ran wonderfully.

It's these kind of failsafes that are taken away from the ecu via mech boost.


http://www.s2forum.com/forum/showthread.php?t=6757

About 4 months ago, after having some boost problems, (similar to many others I have seen posted on various forums around the same time), It appears that a "quick fix" was performed on my RS2 to resolve the problem. As far as I can see this involved disconecting the pipework from the WGFV/N75 but leaving it wired up. The WGFV control hose were joined together and the WGFV connector in the MAF to turbo hose stopped up. A 1.5bar wastegate spring was then fitted along with a bolt on the cap of the WG to adjust the spring. I was told that this was the most reliable and easiest way of solving the boost problem as finding the actual cause can be a time consuming and expencive nightmare, which from the various "boost problem" posts seems to ring true.

It has since been ponted out to me that that this is not the "best" way to do it and although the "quick fix" got rid of the boost problem fairly effectively, giving me a constant 1.5bar of boost,(as noted on a newly fitted boost gauge), it could in fact result in some major problems.

As I learn more about this type of fix, it is encouraging me to try and put it back to how it should be!

I have found the relevant info to correctly reconect the hoses to the WGFV

http://www.s2forum.com/forum/showthread.php?t=5107
http://forums.audiworld.com/s4/msgs/1046677.phtml

The stock spring and a new WGFV seem fairly straight forward to refit and I assume that now I have a WG cap that allows adjustments to be made to the WG springs tension, that when I refit the stock spring (unless advised otherwise) I should adjust the spring to give me a maximum boost of 1.5 bar, (this is where I believe the boost was set when the car was remapped some years ago, although I had no boost gauge then to confirm) and hope that the new WGFV cures the original problem.

But am I missing anything. Might something else have been done with this kind of quick fix that I haven't spotted?

http://www.s2forum.com/forum/showthr...?t=6853&page=5

Mihnea, or should i say MRC Tuning mapped my car today and it has been transformed into a brand new ride and a better one to boot!

Yesterday Lee, Tom C and Mihnea prepped the car with a new Bosch motorsport fuel pump, a new fuel filter and finally got to the bottom of my long term boost problem which was the result of the cam pulley being set half a tooth to the wrong side of the mark by another garage who should have known better, which threw the car into limp home mode.

My car now drives as it has never driven before; faster with better throttle response, better bottom end, better top end, less lag, smoother and more progressive through out the range, more torque, more power with boost peeking at about 1.85bar dropping to 1.2 at red line.

I left the boys as they headed off to map Pete-RS2's car and had a very, very enjoyable drive/thrash back to London in my brand new car.

I am a very happy man.

Regarding the N75

The control on our engines is the N75 valve. It's purpose is to bleed off the pressure signal to the waste gate. If we completely bleed off the signal (100% duty cycle from the ECU) then the turbine gets all the exhaust to generate boost on the compressor side. At the other end (0% duty cycle) the system operates like a direct feedback system and boost is limited to the waste gate spring value.

The bleed off rate of the N75 is what makes boost smooth or not (i.e. other versions of the N75 change the oriface size or effect of duty cycle on how much of the signal is bled off) and is designed to work with a particular engine/turbo combination. This is genberally controlled by the ECU program. In general reducing the oriface size will delay boost but hold it longer.

The ECU attempts to compensate but the feedback loop has a pretty long time delay as well as having the ECU predict N75 duty cycle based on other inputs. In the end, the system has to learn driving habits and make best guesses at how to control the duty cycle for the N75. This is the primary reason for spikes and surges.

source = http://www.modshack.info/bm.htm

Q: What is it?

A: The N75 is an electronic air pressure regulating valve. It sees ambient air pressure (1.0Bar, our atmosphere is pressurized) and the pressure of your charge air after it leaves the turbo. Then based upon the duty cycle signal that the valve receives from the ECU, it will allow more or less of that charge air pressure to flow to the wastegate actuator on your turbo. This in turn will allow the turbo to either make higher boost pressure or drop pressure by letting the exahust gasses bypass the turbine wheel on the turbo.

Q: What does the N75 look like?
A: The N75:



Q: How do the connections on the N75 relate to the other parts in my engine bay?

A: In this diagram you will see the N75 as it would be plumbed in your 1.8T or 2.7T (the S4 has two turbos though...):



Now, as we now know, the N75 regulates boost pressure based on the ECU's signal. The only problem is that the N75 is pre-designed to react a certain way to this duty cycle signal. Since there are various turbos out there and various engines to pair them with, there are several different revisions of the N75 that you can buy from any VW/Audi parts dealer.

The main change between these various N75 valves is the way that they react to the duty cycle signal. Some valves have a small threshold for the need to lower or level out boost pressure. Some valves are intended to allow much higher boost levels and hold them even throughout the entire RPM range. In the end, the ECU still controls the valve's open/closed state. The valve just reacts faster or slower to these signals.

The main part number for the N75 valve is: 034906283.

Each revision of the N75 has a different letter added to the end of it's part number.

Here is a current list of the various N75 valves that you can find at your local VW/Audi dealership and what motors they were made for:

034906283C - Oldest valve, A4 1.8T original
034906283F - Newer revision for 1.8T (maybe 170HP or TT 225)
034906283H - 1991/1992 ABY engine and AAN Engines in S4/S6
034906283J - 3B engine in early S2
034906283K - ADU and ABY engines in late S2 and RS2

There are alot of places on the web that offer "upgraded" N75 valves for your 1.8T/2.7T.

ECS-Tuning sells a "race" valve. THis valve is infact the N75 version "H". It will cause boost to spike very high and in some situations cause your ECU to cut power in fear of detonation (also called "limp mode").

The N75 version "J" is a very popular model. It makes the K03 react very linear throughout the RPM range and doesn't cause the "limp mode" problem with most 1.0bar chip programs.

The N75 version "K" is an uncommon version to see in use but, it is very similar to the "J" version and many people see no need to bother with it.

**NOTE**: As with any upgraded item for your car, check to see if other people have tried this part with the exact same program you are using on your ECU to see what the effects are.

On a side note, most ECUs will have a preset limit for boost pressure. The ECU can determine the boost pressure by reading the air mass as it enters the air filter end of the intake pipe. By pairing an upgraded N75 with a high boost ECU program (ie. GIAC "X" chip) you are very likely to hit "limp mode".

Source = http://www.seatcupra.net/forums/show...ght=n75+signal

[newsh]

The main reason though is that maximum boost / overboost is only available when the ECU sees the WOT ( wide open throttle ) where as mechanical boost maximum boost is available at part open throttle..

This is my only complaint about the WGFV control, you have to use the whole pedal movement to get an increase in boost above the amount available from the spring, this makes the throttle feel like its doing nothing.. till suddenly you get that extra (in my case) 12psi. thats a lot of extra performance for a 5mm extra pedal pressure and results for me in the car being driven mostly at 1/2 throttle using the 1Bar available from the spring, the extra is just too violent in normal driving, and you certainly wouldn't like to hit that extra power in a bend in the wet..
I think the ECU could be mapped to work like a spring though, I've spoken to Mihnea in the past about this, you'd need to map the last say 1/3 of the throttle (via TPS and WGFV) to give increasing amounts of extra boost instead of getting it all at WOT. IMHO this would make the car feel more responsive and drivable. Often in road driving you want to modulate the power with small adjustments of the pedal not an extra 100BHP or so, on and off like a switch.

[Jamo]

This is my only complaint about the WGFV control, you have to use the whole pedal movement to get an increase in boost above the amount available from the spring, this makes the throttle feel like its doing nothing.. till suddenly you get that extra (in my case) 12psi. thats a lot of extra performance for a 5mm extra pedal pressure and results for me in the car being driven mostly at 1/2 throttle using the 1Bar available from the spring, the extra is just too violent in normal driving, and you certainly wouldn't like to hit that extra power in a bend in the wet..
I think the ECU could be mapped to work like a spring though, I've spoken to Mihnea in the past about this, you'd need to map the last say 1/3 of the throttle (via TPS and WGFV) to give increasing amounts of extra boost instead of getting it all at WOT. IMHO this would make the car feel more responsive and drivable. Often in road driving you want to modulate the power with small adjustments of the pedal not an extra 100BHP or so, on and off like a switch.

Thanks for explaning it better than myself

[S2central.net]

the extra is just too violent in normal driving, and you certainly wouldn't like to hit that extra power in a bend in the wet...

Which is why you wouldn't floor it - on MOtronic, the boost 'demand' is controlled by the throttle pedal. Custom ECU mapping could indeed create any form of boost profile (within limits of what the turbo can actually achieve).

Again - I'm not totally dissing the approach here, or named individuals that do it this way. Some extra mechanical boost is advantageous so as to reduce lag and spool times, but NOT (in my opinion) at the potential expense of preventing failsafe routines in ECU to come into effect. Adjusting the WG spring to 0.6 or 0.8 bar could provide a nice benefit - especially on larger turbos which are inherently slower to spool than little uns. In normal conditions, the ECU would permit higher boost levels - AFAIK this is what MRC does.. So that you retain the ECU safety routines - at a higher level of mechanical boost provision.

What I do on the generic ABY software is run with factory WG spring at 0.4bar and let the ECU boost to 1.4bar when all is well. This is inherently safer than with a stiffer spring, but if someone wants to run with stiffer WG springs they can - although I've never tested to see if the software would hit an adaptation limit on the WGFV map on such a setup... I might have a play with that one day - I have a spare WG that I can play with to see where the limit is.

[S2_Dave]

@Jamo, thanks for all the reading. I've had a good couple of hours today.

Something newsh said has got me thinking that my preference might be to have the majority of boost controlled mechanically, with a bit extra controlled by the ECU at WOT, e.g. with Paul's example of 0.4bar mechanical and 1.0bar ECU totalling 1.4bar, I'd want to switch it around for 1.0bar mechanical and 0.4bar ECU - I'll get a more torquey, high boost part throttle response and quicker spool up from the spring, but if the ECU finds something wrong I should be able to notice not getting the extra 0.4bar. It would also give my 3B a similar overboost feature to ABYs, but without the time- and gear-limits.