Air bypass (tickover screw).. why does it alter mid/top end

[TVR Beaver]

Have been messing about with the settings for a few days now, see how good I can get low running just of tickover… I know about setting the base idle etc but just seeing what things do if moved a tad..
One thing I’ve noticed is that screwing the air bypass screw out 1 ¼ turns (as advised by Dom @ Power) gives me a very nice slow running setting and tickover…. But a poor acceleration and top end??.. where as taking it out 2 ¼ turns makes it less nice at the bottom (a bit hesitant) but gives much better acceleration / response and top end?? Why would this be??.. I only thought it was for the idle setting?? And in comparison with a 72mm butterfly wide open, I would not have thought this would make much difference.. but it does?????

[spend]

TVR Beaver said:

..in comparison with a 72mm butterfly wide open..

Have you checked it actually opens fully? Shouldn't be enough pressure differential between the 3 points at WOT to cause what you describe AIUI.

[TVR Beaver]

No I,ve not... but guess it is.. Rob set it up so sure it will be.. but will have a look tomoro...
But thats what I thought... if it is going to have an impact it would be in the first 20 deg or so... no at full bore???..
What does the screw do?.. just take air around the butterfly at tickover... may be I also need to check where the butterfly comes to rest... .2mm gap I thought?... either way why change the top end and mid range?....
Unless... turn on car at 2 1/4 turns.. get it warm.. notice hesitance but gud top end... switch off car after run and adjust to 1 1/4. have another run and notice it's better at the bottom but not as good at mid / top. Switch off car and adjust back to 2 1/4.. notice slow running not as good but mid / top end better...
Do you think the mid / top end changes could be to do with stopping and starting the car.. and some 'other' ECU adjustments being made?... or is that cr*p??

[spend]

The 'plumbing' is fixed (same all the time) the grub screw is just a like a gate valve that shuts of the 'tunnel'.

[TVR Beaver]

Yes.. I know... I didn't mean it adjusted anything in there... but sometimes the car appears better than others when you drive it... and the stepper motor can take a few start to re-adjust etc... ?? strange... will have another play tomoro if I can get out of bl**dy Morrisons!!

[wycoller]

TVR Beaver said:

Yes.. I know... I didn't mean it adjusted anything in there... but sometimes the car appears better than others when you drive it... and the stepper motor can take a few start to re-adjust etc... ?? strange... will have another play tomoro if I can get out of bl**dy Morrisons!!

Could it be Venturi effect when throttle full open that you are drawing more air in through
idle adjustmewnt screw to mess up lean off your mix opening idle screw more etc or am i talking bull

[TVR Beaver]

I think the air screw (but don't know without taking the top of and looking) only controls the air flow around the butterfly when it's on tickover... So when it's open it should not come into play?... I assume thats how it works... You set base idle with it then the stepper motor takes over...
I assume the drilling comes from infront of the butterfly into the plenum at the back??

[wycoller]

TVR Beaver said:

I think the air screw (but don't know without taking the top of and looking) only controls the air flow around the butterfly when it's on tickover... So when it's open it should not come into play?... I assume thats how it works... You set base idle with it then the stepper motor takes over...
I assume the drilling comes from infront of the butterfly into the plenum at the back??

i do not know the construction, knowing that will verify if i am talking bull or not! the construction will determine the dynamics of the fluids relative to the idle screw setting and the throttle

[TVR Beaver]

It's only air... (not like a carb).. .so it can't be that complex... It's not involved with mixing fuel and air .. thats done by the ECU and the trickary that goes on there.......

[blitzracing]

It should have no effect on top end power, as the total airflow be it throttle or bypass is still metered by the AFM, and the fueling set accordingly. At a streach of the imagination, it could possibly alter the long term fuel trim value, as the ECU sets this at idle,(I cant really see how however) but this is ignored above 3400 rpm anyway as the ECU goes open loop, so the only input the ECU sees is TOTAL airflow, rpm, throttle pot, and temp to set the amount of fuel required.

[spend]

TVR Beaver said:

I think the air screw (but don't know without taking the top of and looking) only controls the air flow around the butterfly when it's on tickover... So when it's open it should not come into play?... I assume thats how it works... You set base idle with it then the stepper motor takes over...
I assume the drilling comes from infront of the butterfly into the plenum at the back??

It doesn't 'control' it, the path is fixed there all the time.. I just think of it like wedging the butterfly open a bit - they will all 'seat' differently the tolerances are not that accurate, but you need some air for tickover so the idle bypass screw is an easy way to calibrate idle.

Pipe wise air is bled off in front of the butterfly (the pipe stub that the stepper pipe connects to) and the idle is drilled perpendicular to that straight into the plenum IIRC.

[TVR Beaver]

blitzracing said:

It should have no effect on top end power, as the total airflow be it throttle or bypass is still metered by the AFM, and the fueling set accordingly. At a streach of the imagination, it could possibly alter the long term fuel trim value, as the ECU sets this at idle,(I cant really see how however) but this is ignored above 3400 rpm anyway as the ECU goes open loop, so the only input the ECU sees is TOTAL airflow, rpm, throttle pot, and temp to set the amount of fuel required.

Indeed... it all comes from the 72mm inlet anyway.. so why should it have an impact.. even if the stepper is re-calibrating when turned off and then on again.. it still all comes from the same inlet and MAF?.. as I said.. I'll have another play tonight... thanks!

[TVR Beaver]

spend said:

TVR Beaver said:

I think the air screw (but don't know without taking the top of and looking) only controls the air flow around the butterfly when it's on tickover... So when it's open it should not come into play?... I assume thats how it works... You set base idle with it then the stepper motor takes over...
I assume the drilling comes from infront of the butterfly into the plenum at the back??

It doesn't 'control' it, the path is fixed there all the time.. I just think of it like wedging the butterfly open a bit - they will all 'seat' differently the tolerances are not that accurate, but you need some air for tickover so the idle bypass screw is an easy way to calibrate idle.

Pipe wise air is bled off in front of the butterfly (the pipe stub that the stepper pipe connects to) and the idle is drilled perpendicular to that straight into the plenum IIRC.

There are 2 screws here.. a very small one that adjusts the butterfly stop position... I've not messed with this but think the butterfly should be set to .2mm open.. and the larger one for the Air Bypass... As I said above, It all comes from the same place so why would it alter the performance... Very strange... thanks for comments anyways... Cheers

[blitzracing]

Small brainwave, if the bypass screw itself is letting in air when it moves up or down the thread, this would effect the long term fuel trim. I guess a squirt of WD40 into the chamber above the screw would show if it was leaking if the level dropped at idle??

[TVR Beaver]

[quote=blitzracing]Small brainwave, if the bypass screw itself is letting in air when it moves up or down the thread, this would effect the long term fuel trim. I guess a squirt of WD40 into the chamber above the screw would show if it was leaking if the level dropped at idle??[/quote

Been there, done that... Have had this issue before so I always fit a rubber plug above it now... It's also set in low strength thread lock... So I can gaurantee it's not leaking... But a very valid point... because they can !!

[spend]

That is why there should be a seal on top of the idle screw

Adjusting the linkage to set the butterfly closing and fully opening is a whole symposium in itself, lets leave it at I have seem a lot of worn & butchered used ones

[TVR Beaver]

I'm sure its still doing it.. but the thin air now due to the excessive temp outside may also be contributing

Only thing I can think of is the relationship of allowed air into the system and Pot position at start up... and this ratio being held as it goes up the band until 3.5K when it goes OS?... More the screws out.. the more air at tickover and the better it fuels as it goes up... Screw in,, less air and it hold it as it goes?? Will set it to 1.1/4 and run it for a few days see if it re-callibrates or something?

How many turns out does everyone else have??

[blitzracing]

Nope, nothing that technical, it simply measures the exhaust content for the correct mixture at idle, and then sets the overall fueling to be correct at this point (long term fuel trim). It then adds the short term fuel trim to cycle around lambda 1, but its never stable, so it just keeps cycling around this mid point. This is what gives you the 0 -1 volt cycle voltage from the probes. So if you added extra air, as long as it went through the AFM, the long term fuel trim would set be correctly, whatever path it takes once past the AFM, and as long as the sensor data being fed in and the mapping is fairly good this works just fine. It can cope with the sensor data being out by some margin (bar the AFM), as it can compensate + /- 20% in mixture, which is a huge range. It goes horribly wrong if air gets in through a leak at idle however, as the mixture becomes lean, so the ECU corrects it by resetting the long term trim, but then as the plenum vaccume reduces when you open the throttle, the air leak plays less of an effect, so the long term trim is now wrong. The ECU trys to correct this error with the short term fuel trim, but it can run out of adjustment range, and this also takes a longer time to happen, so you can feel the car snatching as the ECU trys to compensate. Above 3400 rpm, none of this matters, as the ECU simply reads the preset fueling values in the map, and dumps in the coresponding amount of fuel without any correction, not even taking the long term fuel trim into account.

[TVR Beaver]

Well, its doing it for sure... just had it up and down the same road again and with it at 1 1/4 it's much slower than 2?... even noticable on power as you can spin the wheels in second on acceleration when on 2 .. but not on 1 1/4?... how strange??
I've just had my MOT this AM... CO readings are .026 @ 2883 rpm and 0.008 at Idle... is this low?..

[spend]

Clamp the vacuum advance & stepper to take them out of the equation & try your comparison again, would be my approach I guess.