I like it when you do that...usually tells me that a light bulb went off and you are going to have sooner vs later... (fingers crossed :-)

 

Autozone here in the USA sells the 2-bar Delphi MAP sensor for around $58.00 plus tax. I seem to remember the Electromotive packaged sensor was a bit more (70-ish if I remember correctly).

Cheers!
Carl

 

One thing I like about the MPX4250AP is that there are very detailed specifications for it available, making it easy to calibrate/configure:

http://www.freescale.com/files/senso...t/MPX4250A.pdf

And at <$14, it's hard to beat on price.

Dan
'91 928GT S/C 475hp/460lb.ft

 

Thanks again Carl and Dan. I think we have a way forward :-) I've ordered a couple of MPX2450....

Yes Dave, a eureka moment !

 

Bringing this back up-

For some reason I popped over to JDSPorsche.com this morning and saw that the manual for the Alpha-N is up on John's site. I noticed on the last page of the manual a revised target AFR map that is set very lean in low load cells for the idle loop, for use with the stock intake. (Meant to post a pic of the map but my file server is down. Placeholder for later




So the dumb question: John (Jim?), should I just cut n paste this target AFR map into my ST2 for re-tuning purposes? Seems to me that I'd only visit those cells after lifting off at the end of an excursion into the high load+ high rpm corner of the map, and my recollection is that those cells are always pretty lean anyway (I'd guess because I have the 'cutoff fuel under deceleration' box checked on the 'Fuel Parameters' tab, even though the butterfly is closed?) Even so, many of those cells I don't think you can get to. So are the leaned out cells there to help with the the surge issue just off idle, or ????


Dumb question #2: Can you envision any scenario where adding the ST-Alpha is going to provide additional optimization to a car still running the stock intake and MAF? I have a warm/hot starting issue where the car will crank and crank but not fire. It isn't battery/starter/coils/relays/fuel pressure, and I'm beginning to wonder whether it's tuning related. (Though if so it's likely a lack of optimizing my 'tune' due to my own lack of expertise ) I'd be interested to hear the rationale for adding in an ST2-Alpha to a stock intake/MAF stroker. Any excuse to pull the intake again!

 

Hello Rob. That map should only be used the ST-Alpha with stock intake, or any other engine with the ST-Alpha system and an ISV valve. (The race car on ITBs doesn't have an ISV fitted but could be so modified if required). You are quite correct that those cells aren't visited during normal driving, just on the over-run.

Those cells were optimised by Jim when he fitted the ST-ALpha to his GT. The idle control loop is VERY complicated, and it was a real problem to adapt the software for the ST-ALpha and its different way of calculating engine loads. I wouldn't recommend using them with a MAF based system.

At present my Avatar shows what happens if you don't enable fuel cutoff on the overrun with ST-ALpha - (since been fixed)

You shouldn't have a hot start problem, the usual reason for a reluctant start is too weak a mixture. Try increasing the cranking pulse width on the LH Paramters screen.

I couldn't say there would be any particular benefit in fitting ST-ALpha with a stock intake (stroker or otherwise) except for removing the MAF which can prove troublesome sometimes.

Of course you wouldn't have to fully remove the intake to convert to ST-Alpha. Jim managed to fit the throttle pot by unbolting the intake at the heads and propping it up while he fitted the pot with adaptor to replace the stock TP switch.

Other than that, installation is plug and play.

 

I should expand my answer to say that once you start modifying the engine, then the Sharktuner Alpha allows modifications that are impossible with a MAF setup.

Like more radical camshafts. As you know, if you fit cams with more overlap than stock, then at low rpm you get intake reversion (pulses of air back out of the intake ports). These pulses totally confuse a MAF as it can't tell the difference between a forward or a reverse pulse. So the idle fuelling gets messed up and it's not possible to correct it. The only solution is to raise the idle speed, which is not acceptable for an auto 'box.

The ST-ALpha has no problems with intake reversion. The race car has a very steady idle.

So this means you can use radical cams even with a stock intake.

 

If you have a plenum intake and a MAF feeding it, how is it possible that there is reversion at the MAF? There are eight pulses canceling each other in the plenum. If you were to run eight MAFs right after the ITBs, then there would be reversion, sure. But reversion at the single MAF if there's a plenum in between? I wouldn't have thought that to be possible, just thinking out loud.

 

Silly question m8...

Any advantage to this on a MAF based system under boost?

 

It's primary an idle issue, at low RPMs I think the plenum is too small to be effective.

Dunno, we need John to weigh in here. My (very limited) understanding is that forced-induction doesn't respond as well to valve overlap as well as NA, so it might be a non-issue.

The Alpha-N system uses throttle-position as the primary load sensor, so fueling as a function of throttle and RPM (plus intake temp and pressure) needs to be predictable. Which is true for NA motors but not so much for boosted motors. JDS & associates are working on a MAP-based system for boosted motors.

 

Jim,

The old school thought was that bigger cams and slight overlap didn't work with boost.
This has been disproven time and time again though. If you have a turbocharged motor and put in big cams you will pick up power and more of it.
This includes cams with big overlap too.

 

I posted two or three threads over on Speed Talk, which has some pretty savvy eng techs, and never got a totally satisfying answer as to exactly what is going on with reversion. The basics of what occurs or why seem clear, intake manifold pressure is much less than exhaust manifold pressure, but it almost seems the lope in idle is due to the opposite condition and its flow from intake to exhaust during overlap.

After a month or so of scratching my head, reading, and asking more questions doing my taxes appeared entertaining.

 

This is a difficult phenomena for most to get their heads around. Basically it is the difference between steady state flow and transient flow. The steady state flow is what you see/feel and to all intents and purposes it is constant for a given engine speed at tickover. For sure you can get your head around the concept of air being sucked in and making more noise as the throttle is opened and the air mass flow accelerates.

So, generally speaking, at tickover air flow is constant and the computer tries to keep a constant speed [670 rpm on the S4] by applying pulses to the stepper motor in the ISV. This clever little device thus modulates air flow to the engine when the throttle is in the closed position.

Now, this works fne until the computer receives a "false" signal. If the computer receives a signal from the MAF suggesting that air flow is too high it immediately tries to shut off air flow until the system responds.

Based on what John said in his post, the MAF must have a very quick response time and thus can fool the computer into believing there has been a suddden increase in mass air flow. By relative comparison, the output response is very slow and thus if the MAF picks up a reversion pulse it thus sends a signal to the ISV that effectively shuts off air, the tickover goes unstable and the engine thus cuts out.

So why does this happen? Perhaps a simple way to visualise what is happening is to think of a salmon swimming upstream through a rapids. Compared to the water flow, the mass of the salmon is irrelevant, however it is an element of mass flow that is travelling [in vectorial terms] backwards at a faster rate than the mass flow of the water. Thus the MAF can detect a mass flow but it cannot differentiate between the "salmon and the water".

We measure mass air flow in terms of kg/sec or whatever unit of flow we require. At the MAF under tickover conditions the air flow is quite slow and is a tiny fraction of that at full throttle. A shock wave propogates at sonic velocity [about 1000 ft/sec] but dissipates quickly. Bigger cams propogate bigger reversion pulses and a higher tickover speed helps overcome the erroneous signals the MAF is fooled into transmitting. Thus the GT has a higher tickover set point than the S4. A manual transmission can handle this without problem but with an auto the torque converter reacts. If you watch the ST2 output graphic at tickover you can see the cross hairs bouncing a round a little. Tuning reduces this but it is not eliminated entirely and I put this fluctuation down to reversion pulses.

Thus John's system MAP system, having no elements sensitive to the transient conditions, can handle tickover conditions on hotter cams with no disruption.

This is my perception of what happens and apologies if there is anything erroneous in my thinking. If anyone knows/understands this phenomenon better please correct.

Regards

 

Well, my practical experience is as follows - when Marc Thomas at Devek started fitting his more radical B2 cams to his strokers some years ago, then he found he had to raise the idle speed 200rpm or more to get a good idle.

Louie Ott uses the identical cam in his stroker with an excellent idle.

Adrian Clark's stroker racer (in my avatar) runs a cam equivant to the B2 and when he first ran the stroker with a stock intake the idle was very uneven, and it was properly Sharktuned.

When we modified the car to run ITBs and ST-Alpha the idle is very smooth and relaible at a low idle speed with the same cams.

Alex (Cheburator on RL) recently showed a video on here of his race engine running on BMW ITBs and ST-Alpha. This has some pretty radical cams, I am told. He remarked when I spoke to him yesterday how smooth the idle was. He is still using the stock ISV loop.

 

John and others --

As you all know, I have between little and no experience with these cars and absolutely no experience with tuning cars with ITBs. Just trying to understand what's happening, all your help is greatly appreciated!

With those caveats, my opinion is that one should not confuse the control systems with the physical set up. Some points here:

- I think that ISVs and other such closed loop idle control systems run closed loop based on the deviation of RPM from the set point. They don't target the MAF signal. The MAF signal usually only influences the amount of fueling and timing of spark (and if it gets those wrong the car idles poorly or stalls). I may be oversimplifying. FredR wrote something that implies that the idle control algorithm in the 928 uses MAF signal as an additional (predictive) input. I didn't think this was the case, can FredR or someone else please verify this?

- When big cams are used, individual throttle bodies usually idle better than plenum manifolds irrespective of how the engine is controlled. I believe (but do not know) that this is because the almost completely closed throttle plate near the port reflects the pressure wave back to the cylinder in a certain way. Alpha-N, MAP, MAF, anything will all get a better idle with hot cams from ITBs than from a single plenum with a single throttle body.

- I don't know of any mechanism that would make an Alpha-N system idle better than MAP or MAF based system if an ISV system is retained. The ISV passes air past the closed throttle plate.

- If there's no ISV circuit and one wants to closed loop the idle, the main throttle plate(s) will have to be used to control the idle speed in the closed loop. I can't think of a way to do this without a fly-by-wire throttle.

- I don't see how a V8 with a single plenum, a single MAF, and ITBs could possibly confuse the MAF because the V8 pulses cancel so well in the plenum.

- MAFs are really wonderful devices and almost all modern, normally aspirated street cars have them, whether they have ITBs or not.

I am not writing the above as firm facts, just opinions and conjectures. Please correct me where I am wrong.

Best, Tuomo