Fine Tuning a Modified Brera Engine - MAF associated.

Started by Ascari32, December 07, 2020, 02:21:37 AM

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I have detailed the work done with my "Enigmatic 159", but surprises continue to appear in areas I had little experience of, and indeed could not initially make any connection with the modifications I have made to my car. How wrong could I be!

I have detailed the issue surrounding the new valve timing of the Colombo Bariani camshafts and the error I believe to be introduced into MAF metering as a consequence of it now measuring crossflow in combination with "Cylinder Fill".

The inlet camshaft having gone from 11.5deg. ATDC opening to 0.5deg. BTDC opening has introduced an extra 11.5deg. of fill duration, which if one takes bottom dead centre as being the limit amounts to 6.38% error.

If one also considers the new exhaust timing, the engine has gone from 2.5deg. NVO to 23.5deg. PVO. Again, if one accepts BDC as the limit then this also creates and error of 13.05% in air flow metering. Combining the two amounts to 19.43% increased air flow through the MAF, which hitherto was absent. It is this increased air flow which is; I believe, the source of ECU metering issues and ultimately incorrect AFR selection. Principally because, whereas the ECU had 168.5deg. to assess the airflow and adjust the fuel metering accordingly, now it has the full 180deg. of the induction stroke. This ignores the fact that at 11.5deg ATDC the piston has to increase the velocity of intake air from a point where, as both the inlet valve and the exhaust valve are closed, inlet air has little or no velocity.

This is very different now where crossflow through the engine, which is fitted with free flow manifolds and a high flow sports cat that introduce a substantial amount of scavenging.

It is only now that I am beginning to understand the reasons for such an initially powerful tick-over and the considerable exhaust noise such fuel metering errors create.

After making temporary modifications to the original MAF and testing with both a 2.0 & 3.0 litre GTV MAFs, I purchased a MAF with a less acute response, my reasoning being the original MAF would need to respond very quickly because of the standard 3.2JTS valve timing.

However, I would have to be extremely lucky to find the new MAF element: 0 - 280 - 218 - 008 would be ideally suited. This has proved to be the case and I am continuing to investigate how I can best "Trim" its response to best suit the modifications I have made to my engine.

I may now be on the right track. Having gone from initially believing it was a voltage related output, then persuaded otherwise by detailed explanations from "Authoritative" sources that stated it was current derived, I have now returned to my initial understanding that it is voltage related, which is providing some encouraging results. As a consequence, I can now relate the MAF output signal to both the response curves supplied by Bosch and to extra information provided in a useful chart.

The output voltage change from the MAF can vary by only a few tens of millivolts yet greatly affect the AFR. But, encouragingly, by Vernier adjustment, the difference can be; not just tangible, but obvious.

Ultimately, I will need to have a gas analyser to set the optimum. But for the moment, I think I am on the right track.

I broke off from this post to go and fill up with fuel. On leaving the house I thought at the same time my latest adjustments could be tested - initial test jig only done at tick-over - whilst I also thought of returning to RON 95.

After starting the engine - very little load on the battery before she fires - I pull away. It was only then that I realised "All Code Alarms" had cleared!!!!!!!!

What became obvious too, driving the five miles to the garage, the exhaust level was down a lot more too. Almost felt cheated as it did not sound at all sporty! She feels less torque-y at low revs too but that is understandable given how rich the mixture must have been. It is early days and the components used need to be "Fixed"; more stable, as only a few millivolts of drift can change the overall performance.

To give and example. The voltage from Pin five (5) of the MAF to the ECU in the unmodified state is 1.28 volts. When modified; by Potential Divider Network, to provide 1.15 volts to the ECU, both the level of the exhaust noise at tick - over and the driving noise level are substantially lower.

And it only took a reduction of 130millivolts to effect such a dramatic change, by clearing standing codes which had been plaguing the engine since the sports boxes came off and the post - cat Lambdas were fitted with elbows. Anecdotal evidence suggested  that extra distance from the lambda probes and the sports cat sample ports was needed to get a more accurate gas sample for the Lambdas.

More to the point, when these figures are related to the Bosch Transfer Characteristics, they are closer to those of  0 - 280 - 218 - 019 & 0 - 280 - 217 - 531 which indeed do have much more acute response curves indicating to me, there is some correlation with the original MAF element which I believe to have an even steeper slope.

The element I chose however (4) was principally for the higher kg/hr range. Plus, given the change in circumstance, it seemed prudent to stick to RON 99.



Further to my last above, it is Monday and I have just returned from having the rear bank coil packs fitted. There had been an improvement after I fitted the front bank packs last week, but nothing particularly remarkable. I continued to "Tweak" the new MAF in the hope of quietening tick-over and perhaps making the car a little less raucous on acceleration; hoping to find the sweet - spot, which every now and then, the engine would fall into. A little success in that area but I inevitably triggered a engine management failure, the fuel gauge fell to zero again and ASR/VDC/Hill Start also failed.

So, I reset the MAF output voltage to the ECU after returning from town yesterday. On driving to the garage this morning I noticed all codes had cleared - yet again Without permanently driving the car in test - mode, concentrating on every aspect, ultimately one misses little clues as to what is going on.

One thing that did stick in my mind from a little while ago was how the engine was free of failures whilst she was still warming up. But when up to temperature, too much enthusiasm with the throttle, threw a code. It was  at this point, I reasoned - coil packs; more in hope than belief. But the car has 117,000 miles on the clock and I don't know how long they have been on the car. Insulation does breakdown on every coil/transformer I know of. And aging can also cause output levels to fall. From that stand point alone, it makes sense to change all six at the same time. I though £176 was a good price for all six and Borg Warner must have thought highly of Delphi to buy them.

So now both banks have new coil packs. And yes, there is a substantial improvement. There is a rasp from the rear boxes but generally the exhaust system seems "Cleaner - sounding". Just for the record - my memory - the new MAF is operating with no modification of output voltage. So clearly, it is not a bad match for the new valve timing. And it is just on the cusp. It is bordering on a wholly different level of performance.

Yet still there are things I am uncomfortable with - like the particularly noisy warm up. I believe the JTS goes through a process of mixture enrichment; on start - up, to quickly heat the manifold cats up to operating temperature. If this is true, it is possible this exacerbates my engine now, particularly given it is fitted with Autodelta Manifolds.

From that supposition, I take it there is a thermostat fitted somewhere which once up to temperature, enables the ECU to adjust the AFR for a "Normal" tick-over. Does anyone know of such a device and where it is fitted on the JTS. Or is it derived by some other means - software for instance?

I cannot for the life of me, believe that I have chosen a MAF sensor replacement which is a "Perfect Fit". And I believe, any mismatch would appear at the low end of the RPM. On larger throttle openings/higher revs, the Lambdas would have much more control over the AFR so any errors should be corrected in the closed - loop mode. Plus, the much less acute slope of the MAF sensor "Transfer Characteristic" should result in better control of the AFR/Exhaust Gas emissions.

Not so; I would have thought, at low rpm and it is this region which I am trying to adjust/trim. But I think I have been too heavy handed in my approach. meaning, the range of control I have been seeking to use makes it likely that I have been going from one extreme to the other with such coarse control of the MAF output signal.

So now with new Coil Packs and MAF sensor, I must check the point at which I reduce the output voltage and bring up code failures. Then move back from that point - half way initially, towards full MAF sensor output signal. At this point, construct a switchable potential divider network, where the steps are small enough to fine tune the MAF response. I think I will need an exhaust gas analyser to detect the changes this makes to the exhaust gases.

Ideally, I ought to be able to set it up such that there is a predictable and well defined point at which the engine management system changes over from Lean - Burn, circa 1500rpm to Stoichiometric.

It will obviously be a compromise as the error due to the new valve timing is not easily resolved. But there again, I do not know at what point on induction, the ECU chooses to read the MAF to set the AFR. If the timing occurs after the exhaust valve has closed, then the error may well be just ~ 6% (180deg. - 11.5deg.)/180deg.:168.5/180 = 6.3888%. So I am going to do what I originally said I would - adjust it on a Vernier - very fine adjustment! It will take me some time to get right, but it will come good I'm sure.

But if anyone knows about the thermostat issue on start - up, I would appreciate any info. May be Bazz



Just final up - date, in particular relating to C.B. cams, MAF changes and Modifications.

I had the rear coil packs fitted on Monday morning, so that is them all done now.

However, trying to determine flow rate through the MAF before, I had to make a guesstimate about the nominal voltage out to the ECU. In fact I was beginning to think I may be Barking up the Wrong Tree!

I still needed to set an approximate voltage on my Test – jig, as a starting point. Noise from the exhaust system had again crept up - not in a pleasant sense and from the Alfa boxes, no where else.

My theory being, air flow on idle will now be greater because of valve timing changes.

To cut to the chase, lowest flow on my New Sensor Transfer characteristic plot is 1.3395 volt – Bosch published figures.

Climate Control off, no ancillaries running it is 1.4004 volt - precisely to 3 decimal places.

This indicates the increase in flow due to the Colombo Bariani Camshafts.

Taking the Transfer Characteristic for a 3.0 Busso MAF, 1.2695 volt would correspond to 15 kg/hr at tick-over. But 1.4 volt from my new sensor; 0 – 280 – 218 – 008, relates to a kg/hr of ~ 25kg/hr. and this is at tick over where scavenging is least!!!!

Importantly, this reading is very stable, indicating the new coils packs are generating an even - burn on all cylinders, which will help maintain cylinder power balance across the range.

However, I needed to turn the flow rate down and re-adjusting my potentiometer, I was able to set 1.300 volt - accurately - which corresponds to ~ 12kg/hr Airflow. The MAF flowrate hasn't changed, just the ECU has been fooled by lowering the MAF output voltage. The intention being to add some compensation to counter the increase in crossflow and the additional fill volume, the new camshafts have introduced, which I believe is causing over rich running.

This puts it right on the cusp of the new MAF Transfer Characteristic. - brilliant - ever so pleased when theory and practice come together!!!!!!!!!!

Of course, even though my 5K ohms potentiometer is ten turns, it is still too coarse for fine tuning. So I have ordered some components to build an accurate switchable attenuator to adjust the set point by very small amounts, between 1.260 volt and 1.328 volt.

With a Lambda probe up the tail pipe, I will be able to adjust the absolute figure for best AFR.

Importantly, the engine has stepped up a notch again, with seamless progression and neck - jerking throttle response.

Post - script.

This procedure for MAF adjustment is mega bucks cheaper than any software changes, is predictable and works right across the RPM range. It is simple and is fool proof. It will even improve the performance of standard engines, which have no modifications to them.


This post is intended for Kierenc. However, anyone who has a particular interest in the Holden Derived 3.2 JTS Alfa engine may find it worth  reading - or not, as the case may be!

Hi Kierenc,

Good to see your interest has been sparked in the 3.2 JTS. The 2.2 JTS probably has problems common to the 3.2 - in particular oil pressure and timing chains. I envy you your spider, a car I have grown to like a lot. I saw a couple advertised a few months ago, one £9k and the other £11k; one being blue with special leather and the other in stunning red. They have been snapped up as they were quite low mileage. But I feared, I would end up taking on another rebuild and knowing myself would probably - yet again go "The Whole Hog".

So I passed on them as my 159 needs finishing first. and that is proving a little time consuming, not to mention costly. And time is at a premium.

I have done very little cosmetically to my Q4 as I viewed that as being the final part of the project. I have already done all wheel bearings, discs and rear differential mounting bushes, plus the trailing arms now have Polys, courteousy of Powerflex who used my trailing arms for development and graciously gave me the first production set. They work brilliantly and given there is relatively little movement under normal circumstances, do not squeak. The C.C. has a new compressor and condenser, new radiator, in fact most of the peripherals have been replaced.

The engine, from the off, was never going to be easy as there was and remains little understanding of its niceties, and its failings. But everything I have done is documented in my thread on its rebuild, on the UK Forum under Sizwell. It has been a monumental challenge to find parts, some of which were far too expensive. However, I had some considerable help from the likes of American guys, Aussie Companies and various European suppliers. For instance - a new front subframe from Germany at less than 1/4 the price Alfa wanted.

It was always my intention to get cylinder head temperatures down and from the very start I was determined the manifold cats would have to be binned. It was not primarily the fault of the Manifold Cats that was the cause of the elevated cylinder head temperatures, but the valve timing and software techniques Alfa chose to use. Most of what Alfa chose to adopt can be found in an EU funded research document, which unfortunately they overzealously incorporated. The manifold cats need to be kept at "Light - Off Temperature" to destroy the emissions, mostly the HC's, which they do very well being metal. The secondary cats, under the car being ceramic with a lower working temperature, which suits the destruction of NOx's to the extent the residual emissions were some of the lowest seen in any car of that Era.

The camshafts are key to maintaining light - off temperatures so once the manifold cats were removed, there was little point in maintaining the valve timing as was, and the timing angles do nothing for out and out performance. This is an important point as the static valve timing creates a 2.5deg. Negative Valve Overlap, where there can be "No Scavenging" - crossflow, whereby the exhaust gases draw air from the inlet manifold, across the cylinder head and out into the exhaust manifold. So changing the manifolds for free flowing Autodelta's in themselves would do very little to lift the performance of the engine. I am not interested in the odd ten bhp - my oil way mods gained me that by simply maintaining oil pressure on the timing chains and speeding up the VVT and its accuracy. The valve timing would have to be changed to generate Positive Valve Overlap, which promotes effective scavenging.

So, I am afraid equal length headers would have very little effect, without radically changing valve timing. I chose to buy a set of C.B.'s as I was aware of an unsold set in the UK and I have had several sets on previous Alfa's. However, were I to undertake another 3.2 JTS rebuild, I would not change the exhaust camshaft. The standard valve timing has the exhaust valve closing at 9deg. ATDC and the inlet valve opening at 11.5deg ATDC - hence no scavenging. My C.B.'s have the inlet opening at 0.5deg. BTDC and the exhaust closing at 23deg. ATDC, creating a PVO of 23.5deg. and generating massive scavenging.

I had hoped to avoid any software modifications as I do not have the budget for professional services and I do not trust those that see software more as a hobby from which they can make a living. But, I fear, the consequences of such a massive POV is metering errors due to the increased flow through the MAF. I believe I have identified the problem and have gone some way into resolving it. but it is not going to produce the ideal which the engine now deserves. So to reduce the error, at the price of some performance, I may have to refit the Alfa exhaust camshafts which would then reduce the valve overlap: PVO, to 9.5deg. ie, inlet C.B. opening at 0.5deg. BTDC and exhaust closing at 9.0deg. ATDC.

I am coming to think this may also help the exhaust system as it seems I cannot find a way to manage the velocity of gases, the new Supersprint Sports Cat generate. The Alfa boxes seem unable to contain them and my Ascaris, although stunning above 3000 rpm are dreadful below. I am fast forming the opinion, removing back boxes altogether and just fitting straight through pipe sections may be the only solution.

I am currently going through an exercise of changing MAF inserts. But, I don't see there being any real solution to the problem, beyond refitting the Alfa Exhaust camshafts. The car is stunningly quick - even with the 3.0 GTV MAF element and the 280 - 218 - 008 elements fitted. The Porsche 911 insert is too insensitive, although the Transfer Characteristic looks good. My last hope is probably a Bosch 281 - 002 - 421. Or to simply go back to the original MAF insert and provide my neighbours with ear plugs. Hell, she is fast with that fitted. but the police would be alerted at Martlesham, 30 miles away, as soon as I start her up.

However, until I crack this particular nut, I won't be able to get her on the Dynamometer, apart from being in lock - down again. I would however suggest, if you are that keen, you read my posts, with attached pictures of my 3.2 JTS rebuild. My understanding of this engine started at Zero, so naturally as it progressed and I learned more, I have come to appreciate the interrelationship of so many factors, hitherto not understood or indeed known of. That aside, my regard for this engine has steadily grown. That some elements of it's design are covered by patents held by Porsche, who were instrumental in the design of the Ford/Jag V6 which they had a major influence in research and development thereof - not to mention a massive budget. And when the project ended, key players took up new posts within the GM team which developed the engine which Alfa adopted for the 3.2 JTS.

One aspect I was puzzled about was how upper cylinder lubrication was performed. This too is in part a function of the Negative Valve Overlap as the negative pressure generated before the inlet valve opens, draws both HC's out of the piston - ring/cylinder - bore interface region; and oil, past the rings to lubricate the bores above the piston. The spray jets direct oil to the underside of the piston which has ports through to the oil ring groove. This explains the astonishment the Engineering (Racing) shop displayed when they pulled my 113,000 mile 3.2 JTS engine apart and inspected the bores. Their condition was as if the block had just come off the production line. Absolutely flawless. The piston rings however are seen as "Sacrificial" and considerably cheaper and easier to replace than having to re-bore the block.

I enjoy the Australian Forum for several reasons, not least the clear enthusiasm some of the guys have for their cars and the opportunity they get to drive great distances gives them a better insight into how their cars perform than one gets on congested British roads. I have picked up some interesting tips from that site along the way so I shall continue to look in to find out what is going on, despite being thousands of miles away.