3.2 Carrera Air Flow Meter Modification

[dkirk]

Now that my 1988 Carrera has been stored for the winter, I can concentrate on some mechanical details that I skipped during the engine’s top-end overhaul that was performed in the spring of this year. While the engine was apart, I had carefully matched all inlet ports –thermal isolators – intake manifold junctures, all in the quest for improved induction air flow. However, the sharp edges and corners present in the air flow meter were not touched. Now that the car is in hibernation for the next 5 months, the air flow meter was removed and some subtle modifications made.



The L-Jetronic air flow meter body is an aluminum die-casting which results in sharp corners, parting lines and casting flash being present in the air flow path. These are most notable in the entrance and exit areas – the actual metering area (where the flap functions) is smooth and no modifications are necessary here. This is fortunate in that the proportionality of the system could potentially be disturbed if changes were made in this location. The areas of concern are shown in the following photos; the first two showing the air entrance:





The steps and sharp corners are disruptive to smooth air flow. The exit area is equally in need of some attention:





Armed with my trusty Dremel tool and small files, the inlet side was addressed first. Sharp corners were gently radiused and the parting line step discontinuities were blended:





The air outlet side was then done, using the same technique:





During this process, rags were placed in the main air flow channel to prevent metal chips from contaminating the internal components. Frequent use of the shop vacuum to remove chips was done during the process and afterwards, a thorough blasting with compressed air removed all particles. Total time to do this modification took about 2 hours.

Now I’ll be first to admit that this modification would not result in any noticeable power increase, even on the most sensitive of engine dynamometers. But I’ve always followed the “attention to detail” philosophy – that small improvements add up to make an engine that has the “edge” over the competition. Plus, it’s rewarding just knowing you’ve done everything you can to improve induction airflow.

[dkirk]

While the air flow meter was on the bench, I discovered a neat little plug that fits the idle air adjustment port. The original plug that Bosch used here is a one-time affair, and once removed, is not useable again. I found these snap-in hole plugs at my local True Value hardware store. These are 7/16 diameter and snap in place perfectly with a snug fit. They’re easily removed with a screwdriver and can be reused.

[ALEX P]

I tell you what Dave, that really is attention to detail!

The plug from my stock AFM has always been missing, please could you post more details on the reusable ones you have found & what they are made from i.e. ally or stainless?

Thanks in advance.

[dkirk]

Alex,

I appreciate your comments. My wife thinks I'm too detailed oriented, but when it comes to engines, that philosophy always seems to have positive effects.

Regarding the metal plug, it is plated mild steel (not stainless). Measuring across the fingers, it appears to be sized for a .4375 inch diameter hole. I picked up an assortment of different sizes at the hardware store and just got lucky that this one fits the AFM adjustment hole perfectly. These were sold un-packaged so no manufacturers information is available.

Sorry that I can't be more specific.

[hcoles]

just a thought - these barn door meters could be adjusted at manufacturing time to calibrate swing as function of airflow, now you might be getting more air for the same swing therefore changing the AFR - probably not an issue but just saying, Steve Wong will know.

[dkirk]

hcoles,

I did contemplate the effect you mentioned prior to making the modifications. My logic for proceeding was that increasing air flow potential via removing restrictions would result in greater flap deflection, and therefore proportionally greater fuel flow signal, for a given throttle setting. As no changes were made in the flap or the channel in which it operates, the air-to-fuel flow relationship should be unchanged.

I won't be testing this modified AFM until Spring of next year, but I'll be sure to post any negative results, if noted.

[JFairman]

There is a much better AFM body than the Bosch L-jetronic and Motronic unit for this kind of modification. They can be found on older Torota Supra's and Toyota luxury sedans with the DOHC six cylinder motor.
Nippondenso made a much superieor cast aluminum copy of the Bosch zinc/aluminum alloy AFM that comes apart completely and looks the same as the bosch for those cars and you can really work on the inside of it.

The Bosch AFM is not aluminum, it is some kind of heavy pot metal like zinc alloy with some aluminum in it. The Nippondenso AFM is cast aluminum and weighs about 4 pounds less than the heavy Bosch casting.

The Bosch unit does not come apart other than the plastic cover over the AFM barndoor's adjustable clock spring, stylus, and circut board with carbon resistance trace the stylus swings on. The flat ribber cover on the Bosch unit is permanantly attached to the main casting and you'll destroy it if you try to remove it.

The Nippondenso flat cover looks exactly the same as the bosch but is held on with phillps head screws and a bead of clear silicone sealer. It comes off easily so you can get in there with a die grinder with carbide burs and sanding cones on arbors used for cylinder head porting and also replace the upper and lower ball bearings for the barn door if you ever had to.
Then put it back on with a bead of silicone sealer when you're all done.
You can't do any of that with the much heavier Bosch casting that does not low as muc air as the Nippondenso copy.

It's possile I've spent more time modifying several of these AFM's and swapping the circuit boards and stylus for myself and friends around 12 years ago for chain driven SOHC BMW big six engines and baby six cylinder engines running L-jetronic or Motronic injection than anyone else. One corner of the Bosch circuit bar needs to be rounded off a tiny bit to fit in place of the Nippondenso circuit noar. The sylus and plug for the harness are the same and you can swap those too.

I opened up the inlet and outlets of the AFM considerably and spread JB Weld epoxy over the seams and sharp edges and recontoured them so they are blended much smoother and flow air better through those two areas.

I also removed the conical plastic air temperature sensor and remounted it in pvc ducting for a big conical K&N air cleaner that is just before the AFM and is much bigger inside diameter so it doesn't restrict air flow or create any weird air turbulance patterns right before the barn door.

Then you can turn the big notched plastic wheel spring tension adjutment for the barn door so there is less spring preload holding it shut.
There is alot of fine adjustment range there and you want to release spring tension so the door opens farther, easier, and quicker when you open the throttle butterfly. This will make throttle response faster, let the motor breathe in air a little easier at all speeds and make the AFR richer.
After doing that the door will oepn farher and idle AFR will be too rich depending on how much aluminum you removed around the barn door at the position it is in during idle speed.
To compensate for that you open up the idle CO adjustment screw or allen head under the cover you removed in your pictures. There's no reason to put another cover in that hole, air won't leak by the threads of the CO adjustment screw.

Depending on the displacenent of the motor and how much material is removed from the AFM, and your camshaft duration the CO adjustment screw may be unscrewed alot to compensate for the lower spring tension on the barn door at idle speed.

In the end all this work doesn't make a huge difference in engine power but it's a small improvement if you do it well, and it helps.

[Tippy]

Nice work, but my turbo blows air through it quite easily.

PM coming JFairman......

[911TT33]

I can appreciate your attention to detail, however, I really don't see the point in doing all of this because the BOSCH AFM introduces significant restriction to the intake. That's why Porsche went with a MAF Hot Wire type setup in the 993+ models. The AFM is a P.O.S.

If you want to improve the intake, go to a 993 MAF type setup, or even better, a speed density MAP based solution. There are piggy-back ECU's that enable you to completely remove the AFM brick.

[dkirk]

JFairman,

Wow - thanks for the interesting AFM swap information. I'm impressed with your knowledge of this device and what can be done with it.

911TT33,

Thank you for the MAF hot-wire suggestion. I've given this lots of thought, but have been unable to locate any aftermarket supplier of the piggy-back ECU that would be necessary. Can you steer me toward a manufacturer?

I believe Porsche went to the hot wire MAF over the electro-mechanical AFM mainly due to the much lower cost of the former. And I'm sure control accuracy is gained with improved technology. Testing of the "barn door" on a flow bench indicates that very little flow restriction is actually imposed, but the hot wire would have to be somewhat better in this regard. Still, if converting to a hot wire MAF is not thousands of dollars, I'd be interested.

[911TT33]

Originally Posted by dkirk

911TT33,

Thank you for the MAF hot-wire suggestion. I've given this lots of thought, but have been unable to locate any aftermarket supplier of the piggy-back ECU that would be necessary. Can you steer me toward a manufacturer?

There is a New Zealand company who makes a piggy-back ECU designed to replace the AFM barn door style intake.

: PERFORMANCE MOTOR RESEARCH LTD :

It simply connects in parallel with your AFM wiring and you drive the car around with the AFM in place for a while as the ECU learns the signal outputs from the AFM under different driving conditions. After a week or two, it builds a comprehensive map for you, and then you can completely remove the AFM and just run this piggy-back ECU which is based on a MAP sensor.

It's used in conjunction with the OEM Motronic ECU.. not a complete replacement.

Worth looking into.

[dkirk]

Thank you for the link. Here's a California based company that looks to be making the same thing:

http://www.splitsec.com/products/mafkits/MAFkits2.htm