afr#S while in boost

[gorskined]

spring is here again in the northeast. i have the turbo back together and on the road with a freshly rebuilt transmission .
still making minor tweaks now that i have a better understanding of how all the adjustment effect each other .
so here are my AFR #S

I'm typically running in the high 12s afr at an idle when i adjust higher the car hunts a little.

Running high 13s when tooling around not under boost .

Off the gas after accelerating while its making popcorn ( drives my crazy getting better at feathering the gas pedal) its high 15s low 16s .

after the boost dump mid 11s the longer in boost the lower the afr drops it will drop into the low 10s.

I have the rpm solenoid boost hold out set to 3800rpms .
should i adjust my rpm hold out up or adjust my boost dump on the wur to try and get my boost a little more lean or is 10 ok?

what a good not to exceed afr# while in full boost ? what the norm?

Again don't really care about Hp and making more power looking to maximize engine life . will afr in the 10s wash down the cylinders ?
The car responds great im just concerned with the low 10s under full boost.


Thanks in advance

Regards Ned

[Alan L]

Do you have the adjustable WUR (boost dump)? If so, then you should reduce the WUR pressure drop to get your dump AFRs higher. The specs call for 0.5 bar drop at 0.5 bar boost. In reality it doesn't drop any further after 0.5 boost. Thats it. So instead of going from say 3.6 to 3.1 bar on dump, you would cut that down to say 3.3. And tweek from there. Then you can fiddle your boost switch. I run mine around 4500. Trying to get up to around 12.5 before dumping. And I am happy at 12.5 at top rpm on boost. But most will happily settle for 12+, for a bit of margin. But I have never had a problem running 12.5 at 1 bar. Timing etc has to be correct of course. 12 is definitely a safe number all day long.
If you don't have the dump adjustable WUR, then all you can do is raise the switch rpm. Set it for the point your AFrs approach 12.5.
Alan

[gorskined]

Originally Posted by Alan L

Do you have the adjustable WUR (boost dump)? If so, then you should reduce the WUR pressure drop to get your dump AFRs higher. The specs call for 0.5 bar drop at 0.5 bar boost. In reality it doesn't drop any further after 0.5 boost. Thats it. So instead of going from say 3.6 to 3.1 bar on dump, you would cut that down to say 3.3. And tweek from there. Then you can fiddle your boost switch. I run mine around 4500. Trying to get up to around 12.5 before dumping. And I am happy at 12.5 at top rpm on boost. But most will happily settle for 12+, for a bit of margin. But I have never had a problem running 12.5 at 1 bar. Timing etc has to be correct of course. 12 is definitely a safe number all day long.
If you don't have the dump adjustable WUR, then all you can do is raise the switch rpm. Set it for the point your AFrs approach 12.5.
Alan

i do have an adjustable dump on the wur. That's the one setting i have yet to play with. going to take your advice and stage the rpm to trigger at 12.5.. then adjust my dump.
as always thanks for the advice .

[Alan L]

I arrive at the trigger rpm by watching the AFRs before the trigger opens the dump. That number may be different for each setup. On boost, just before the AFR boost drop, that number is what you look for. Then work your way up the rpm range until you get to around 12.5. Somewhere north of 12. The higher you can get before the dump, then the higher the AFR after the dump.
You can tweek the boost dump settings without having to run the car. I have Teed into the trigger/WUR line so I can put a small pressure into that line - simulating the boost pressure. With my permannent gauge on the WUR, I can either idle the engine, or run the pumps, put pressure in the Tee line and watch the AFR drop, and adjust.
Alan

[stownsen914]

I've heard others prefer lower AFRs for aircooled turbos under boost and load, like 11.5.

12.5 is a good target for NA aircooled 911s. OK for turbos too?

[Black_Hat]

These are Old cars:
- no knock control
- Gas "can" have issues at times unless you run race gas
- uneven flow through the intake manifold under boost

The pancake intake manifold is notorious for some cylinders being lean as compared to the others specifically 2 and 5 ....

Tune with a safety margin to account for 2 & 5

[Black_Hat]

all that being said .....

11.5:1 to 12.0:1 is considered safe under boost

[Alan L]

I have regularly run mine to 12.5 under track conditions. Over all the years, and teardowns for various reasons, I have never damaged a P/C.
Certainly everyone (I think) would generally agree 12 AFR is safe at 0.9 bar for sure. All day long.
Alan

[David]

I keep mine close to 0.75 lambda or 11.0 air fuel ratio under full boost. I also figure I'm running at least 10% alcohol these days so better to be safe by keeping it on the rich side. Of course with EFI, I can control pretty accurately.

[Alan L]

Originally Posted by gorskined

i do have an adjustable dump on the wur. That's the one setting i have yet to play with. going to take your advice and stage the rpm to trigger at 12.5.. then adjust my dump.
as always thanks for the advice .

The first thing you should do is measure the WUR dump pressure.
You would ideally do this on a warm WUR. Disconnect the hose between the trigger switch and WUR - at the trigger end. Add your CIS gauge. Then you need to figure a way to pressurise that hose to the WUR - gently. You are pushing directly on the diaphragm membrane. I use a mighty vac tool that can suck and pressurise.
Pump that pressure up to between 0.5-1.0 bar and see what WUR pressure drop you get. You can do it with an idling engine, or just pumps running.
Alan

[chinchillawikin]

Low-mid 15s — Again, pretty normal for off-throttle decel; as long as it’s not backfiring badly or causing hesitation, this is expected.

[gorskined]

I do have a mighty vac. hope to get some time to tinker this weekend . ill figure out my wur dump pressure and adjust accordingly .. interestingly enough my car doesn't run well within the 3.0l wur warm pressure curve.
Im slightly above the cp curve.. not sure why possibly the k27 turbo or the ic or sc cams or the 200 series bosh pump . but once I started setting off the afr #s things started progressing in the right direction. ..

thanks for the info.


Regards Ned

[Alan L]

I am not sure if all Mighty Vac things can pressuruse as well. Thats the bit you want. If yours is dual function, you are good to go.
Alan

[gorskined]

Yes it's the Chinese knock off duel function with the slide....actually nicely made I was quite surprised when I got it last year.....

So I had some time to play around as mentioned idles low 13s high 12s when I tool around not in boost mid 13s when I get on it it starts to drop low 12 before boost . Once in boost I drop to low 10s even with the boost dump vacuum line disconnected I still will drop to low 10s ... I can hold .9 bar boost on the highway . the afr never comes above 10.2. any suggestions ...

[Alan L]

Your course of action will depend what you measure with the boost dump.
Are you saying you are in low 12s before the boost dump comes in? In which case the rpm switch is set about right.
Disconnecting the WUR vac line still gives you low 10s without any boost dump?
Alan

[gorskined]

Yes . With my vacuum line removed and plugged so the boost dump can not operate . When in boost low and full .9 bar it keeps getting richer eventually bottoms out at 10 . That's as low as my gauge will go .
I'm testing this by doing slow gradual pulls on the highway . With a buddy watching the boost and the afr. And system fuel pressure btw is 98 psi top of the range ... I am going to swap the fuel system pressure sending unit to the wur control pressure so I can see what that is doing under boost . Again car runs and pulls nice just doesn't make sense . And the 76 row is pretty stripped down as simple as they get.

[gorskined]

Update I brought my running temp idle afr up to high 13s low 14s .... 48 psi on my wur. I had to play with my sensor plate Allen screw and my big idle screw to get it to idle smooth at 950 with the elevated afr #s ..
My tooling around town not in boost afr is in the high 13s to mid 14s . Low boost starts at high 12s then instantly drops to mid 11s ... Full boost low 11 if I keep pulling the afr it settles out high 10s low11s.. . The car has Noticably a lot more torque and pull. I still have the boost dump disconnected.. I also noticed when I rev the engine the system fuel pressure is a 96psi at low rpm 98psi above 3000 rpm with or with out a load . Also the wur control pressure drops when I rev the motor with or without a load it drops from 48 psi at an idle to 42psi above 3500rpms .... is the fluctuating wur pressure normal when you rev the engine ?

Also I cant seam to find a boost dump pressure setting anyplace for a 3.0l ... Possibly the early turbos didn't have a boost dump? My wur is a modified later 3.3 l wur I was told it was better than the original when I sent it out to be modified . Car is running great . Am I just being over critical.obsessing over things that would have never been noticed if I didn't have all these extra gauges .

Regards Ned

[Alan L]

Yes, you will notice the power difference getting out of AFR 10s and in to high 11s - 12. It is sluggish at 10s compared to the better numbers. And not good for the engine - washing the cylinders in fuel. I hate that black sooty look on the plugs.
That WP drop just by revving the engine isnt normal - that is close to your boost dump number - 0.5 bar. Add aboost dump on top of that and you will never get sensible numbers.
I don't know why the WP is dropping with revs, but I suspect you are getting close to solving your problem.
Did you check the boost dump with your mighty vac?
The other suggestion I was going to make regarding your ever decreasing AFR numbers on boost, was whether you were certain you have no boost leaks. That will just make the situation worse. I am thinking of something possibly lifting under boost.
I would right now, leave the boost dump disconnected, and convince yourself you have no boost leaks, and spend a bit of time trying to decipher why the WUR pressure is dropping simply with revs. I don't have an answer for that right now - but that has to be a big contributor to the problem. If you can eliminate that issue you can then manage the boost dump.
I am going to be offline for a while now - heading o/seas tomorrow and limited internet and time for a while.
But hopefully the others will keep you on track.
May be worth posting a query on the 911 Tech forum about the WUR drop/rpm.
Good luck
Alan

[gorskined]

Originally Posted by Alan L

Yes, you will notice the power difference getting out of AFR 10s and in to high 11s - 12. It is sluggish at 10s compared to the better numbers. And not good for the engine - washing the cylinders in fuel. I hate that black sooty look on the plugs.
That WP drop just by revving the engine isnt normal - that is close to your boost dump number - 0.5 bar. Add aboost dump on top of that and you will never get sensible numbers.
I don't know why the WP is dropping with revs, but I suspect you are getting close to solving your problem.
Did you check the boost dump with your mighty vac?
The other suggestion I was going to make regarding your ever decreasing AFR numbers on boost, was whether you were certain you have no boost leaks. That will just make the situation worse. I am thinking of something possibly lifting under boost.
I would right now, leave the boost dump disconnected, and convince yourself you have no boost leaks, and spend a bit of time trying to decipher why the WUR pressure is dropping simply with revs. I don't have an answer for that right now - but that has to be a big contributor to the problem. If you can eliminate that issue you can then manage the boost dump.
I am going to be offline for a while now - heading o/seas tomorrow and limited internet and time for a while.
But hopefully the others will keep you on track.
May be worth posting a query on the 911 Tech forum about the WUR drop/rpm.
Good luck
Alan

Thanks Alan
enjoy your time away
going to do some digging around today with google when i get some time.
Regards Ned

[beepbeep]

Very general hint: risk of detonation is highest when torque is highest. Torque, not power. That is where cylinders are swallowing most air and cylinder pressure is high. So you want relatively rich mixture where torque peaks....around 3500-4500 RPM. Higher up, stock engine does not breathe equally well (boost is there but is not being swallowed) so you can afford going slightly leaner and actually pick up some power too.

Of you look at AFR curves for stock K-Jetronic, it is exactly what it does from the factory. It goes pig rich around 400 RPM but leans out somewhat on the top. And it is not by mistake.