MFI Air Flow

[BSNMOE]

My Throttle Bodies, etc. on my 73S are all in in good shape. No air leaks and idles smooth at 950 RPM. At idle my synchrometer pulls about 5.25 KG/H. At 3000RPMs it pulls 18 KG/H.
The example in the Manual shows a 2.0 or 2.2 ltr pulling around 11.5 KG/H.
I've read a few posts where it appears my 2.4S should pull around 14 KG/H at 3000. MY car runs great and pulls air equally from off idle up to 3000.
My question: is 18 KG/H at 3000 pulling too much air?
My control rod is at 114mm. Throttle bodies off stops around 3/4 of a turn. Air screws are around 1 1/2 to 2 turns out so everything pretty equal.
Both my left and right control rod are around the 149.5 mm as the Manual states so the only way to lower the air flow would be to reduce these control rods to under 149 mm.

[Rot 911]

How do your plugs look? If they show a good burn mixture I would just leave everything alone.

[BSNMOE]

My plugs look good. I'm wondering if there is an airflow range that a 2.4 should achieve. The lengths of these rods effect airflow from start up to 7000RPM.

[Grady Clay]

BSNMOE,

Measuring the airflow is best for getting (or confirming in your case) that the flow is best-possible equal at idle, just off idle and in the ‘light-throttle’ running area.
The actual mass-airflow is irreverent; it is the resulting combustion mixture that is important.

For this, you will want a good exhaust-gas analyzer that you can take driving.

Porsche uses the protractors (P228b & P228c) to measure the progression of the angle of the throttle (butterfly) shafts compared to the MFI pump control.
This lets you confirm that the various links and arm angles are correctly tracking each other through the full range.
This becomes very important if our cars have been in many hands and had parts replaced over the years.


As I’m sure you know, your 911S MFI likes to perform well when adjusted rich.
That is usually ‘too rich’ for good long-term operation and not diluting the oil with gasoline.
The ‘art’ is to adjust the mixture in the area of “a bit too lean” when at light throttle, cruising in a lower gear in the 2500-3500 rpm range.
The criteria is how much ‘lean surge’ is acceptable to you.

When you get on the power in the 4500-7300 rpm range, the pump should provide the mixture on the rich side of any light-throttle adjustment.
You should confirm this with exhaust gas measurements and spark plug observations.

Be sure you have fresh and higher-than-minimum-necessary octane fuel. Measure your ignition advance curve and maximum advance. Confirm you have suitable (cold) sparkplugs. CMA2.

Best,
Grady

[BSNMOE]

Grady, Thanks for your response. My WOT AFR is 13.1 and idle around 12.8. The plugs and tail pipe get a little sooty at idle, especially during warm up unless I keep the hand throttle around 1500RPM. A plug checK (4000RPM for 10/15 min then shut down & coast to stop) the plugs look nice and tan.

Now if I may, I have another question. It appears that vacuum retard distributors are for emissions. But if that is so, why did the RS, which was a non US car have a vacuum retard distributor? The reason I ask is if I disconnect and plug(eliminate) the vac retard I can run the idle about 6 clicks lean which keeps the plugs a little cleaner.The timing at idle is 5deg BTDC rather than 5 deg ATDC. At 6000 it's still 35 deg. The only concern that I have when timing this way is I also have to turn in my air screws at least 1/2 turn to lower the idle.
This means that I have reduced both fuel and air in the combustion chamber. Is there a problem when doing this.
I thought perhaps vacuum retard was used to induce more air and fuel in addition to emissions at idle.
What are your thoughts on this?

[Grady Clay]

One benefit:

With the vacuum advance functioning and at-idle with the butterflies on the stops, there is high vacuum at the vacuum advance.
When you first ‘crack’ the throttle open while letting out the clutch, the rpm drop below idle and the vacuum decreases (both from the lower rpm and the opening throttles).
This decrease in vacuum advances the ignition timing via the vacuum advance mechanism.
This advanced timing helps prevent the engine from stalling when taking off from a stop.

There are some distributors (’77 USA for example) that have this advance also built into the centrifugal mechanism.

Best,
Grady

[BSNMOE]

I agree that the vacuum advances the timing, but, it only advances it to 5 deg BTDC, which is where it would be if the vac retard is eliminated. This leads me to believe that the vac retard is for emissions, but if it was for emissions, why did the RS have it. The RS's idle timing is 0deg rather than 5 ATDC, probably because of the torque curve of the 2.7.

[Eddyvdw]

Might need some help from you MFI gurus .

I rebuilt the TB and put every back together. AFR at Idle is 12.8 and somewhere around 14 when accelerating (around 13 when cruising).

The airflow at idle is a bit over 5 kg/hr and nicely correlated. Idle timing at 5 ATDC, at 6000 rpm 36-38 BTDC (no vacuum).

Still idle is too high at 1200 RPM... What am I missing?

Thanks,
Ed

[tobluforu]

Where are your idle adj screws set at and have you turned them in to see if you can bring the idle down??

[Eddyvdw]

Sure, it's closed at cyl 1 so can't go any lower on the others....

Ed

[BURN-BROS]

Originally Posted by BSNMOE

My Throttle Bodies, etc. on my 73S are all in in good shape. No air leaks and idles smooth at 950 RPM. At idle my synchrometer pulls about 5.25 KG/H. At 3000RPMs it pulls 18 KG/H.
The example in the Manual shows a 2.0 or 2.2 ltr pulling around 11.5 KG/H.
I've read a few posts where it appears my 2.4S should pull around 14 KG/H at 3000. MY car runs great and pulls air equally from off idle up to 3000.
My question: is 18 KG/H at 3000 pulling too much air?
My control rod is at 114mm. Throttle bodies off stops around 3/4 of a turn. Air screws are around 1 1/2 to 2 turns out so everything pretty equal.
Both my left and right control rod are around the 149.5 mm as the Manual states so the only way to lower the air flow would be to reduce these control rods to under 149 mm.

No, 18 kg is fine. I would not recommend chasing a "problem" when there is no real complaints. IF you were lean at 3000 rpm with that air flow number then you would have reason for concern. You description sounds like a great running engine.

[BURN-BROS]

Originally Posted by Eddyvdw

Sure, it's closed at cyl 1 so can't go any lower on the others....

Ed

If you look at the OP's first post, you will see he has performed air readings at idle, 3000 rpm and has mixture screws with reasonable adjustment between them all.

Your throttle bodies have issues and need service. Closing a mixture screw to compensate worn bushing or throttle plates is not what the screw was designed for.

Worn bushing will pull air from the shaft.....bypassing your air meter readings as well.

[Eddyvdw]

I took the plunge. Started rebuillding the throttle body.
Found a specialist with the right tools. Wish me luck!