MarkRobinson

Sometimes you have to know when you're beaten: this one is for the pro's:

'85-86 cars were programmed with a lean torque curve to ensure the car's pass rolling dyno EPA emissions. Under normal chips & exhaust, you will never notice this, but with chips & a good exhaust, the hole is very noticable (I've personally seen this on my cars 3x now).

I'm countering this with the HKS Super AFR that will allow me to lie to the computers about the air flow reading from the MAF, by graphing my RPM to my newly designated percentage points of AF correction over the stock map.

Problem: HKS requests a 5v tachometer signal for it's tach reference, and the reference has to match the number of cylinders your motor has.
- LH Jetronic appears to send a 4 cylinder reading to the tach for which the tach multipies by 2. The coil ignitor wires (green & white) under the dash give me a 4cyl readout (as it should), but so does the LH harness PIN 1 (which receives the PIN 16 wire from the EZK box as well as having the tach sending lead as well. If these two leads are indeed the only referneces available, then I'm screwed as I cannot lie to this box as HKS has stated to me: the computer chip will adjust for 4cyl & make the car run poorly.

With the box set to 4 cyl, the tach readout matches my tach, but at 4,000rpm, the unit shuts down my fuel curve as if the MAS has reached it's 5v limit in metering. Likewise, if the unit is set to 8 cyl, the tach on the unit displays 1/2 the rpm. Even if I set it up this way, the same thing occurs: out of metering at 4k.

Here's my options: find a 5v 8cylinder RPM reference (if I can find a 12v 8cyl, that will work as a resistor & diode can be added to reduce to 5v)

Determine that the MAS is working correctly & not having an issue at 4k: Just swapped in my spare earlier, but have not had time to check.

Ditch it & try another piggy-back device. Any clues???

Mark

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John Speake

The MAF will go way over 5v in its metering cpability, to far more flow than you will ever see.

If you think the 12v tach signal from the EZK is too high, you can simply use a resistive attenuator to drop it down to 5v max.

Have you measured the MAF voltage you have a 4k rpm ? It sounds that the problem is the MAF input circuits on the piggy back rather than the rpm signal.

Does it do the same thing at 4k rpm, any load, any gear ?.

MarkRobinson

Where did you learn that the MAF will go way over 5v if the manual states it's a general 0-5v MAF? The 12 signal can be bumped down to 5v, but still appears to be just a 4cyl signal..which will not do me any good with this unit. 4k issue exists any gear, 4k, WOT.

Mark

John Speake

I rebuild MAFs, I have measured them on flow jigs, I have measured them on 928 engines. You suggested that the MAF might be the problem with the piggy back system.

But the 928 MAF will only get above 5v at max load. (With s/c 928's the MAF volts go well over 6v and the air mass is still being accurately measured)

So under the varying load conditions you state at 4k rpm, the MAF volts will be much less than 5v.

So it does appear that the rpm feed is the issue.

It would be possible to build a pulse multiplier for rpm if that is what you need.

Jack '84 928s

Call hks or find a dealer of them near you that installs them.

Fastest928

Approx 4.5-5v is the switch point for running closed loop vs open loop. Thus, you cannot control any fuel after the maf reaches that limit using typical piggyback technology.

Hope this helps.

Marc

MarkRobinson

The HKS probably isn't compatible since there's not dialing in the MAF like the Apexi, so ordering one in the am as the Apexi is fully dialable...you even get to tell it the volt range on your MAF so it doesn't run out..plus, it'll have maps for both low & high throttle as you can set it per your TPS as well. We'll see. Worth a try, tired of this typical hole in my torque curve, and the S4's dont press my button like the 928S' do. So I"m stuck with this issue until I resolve it.

Mark

mspiegle

I don't believe your problem is in the RPM signal unless the HKS device has some lame design flaw in it.

If your unit is only modifying the MAF voltage, it doesn't matter what RPM signal you send to it (either 4 or 8 cyl), just as long as the RPM is correct. The 928 computer system thinks in 2 banks of 4 cylinders, not a set of 8. On my SMT6, I have it set to 4-cyl mode and am capable of advancing/retarding ignition.

John Speake is correct, he knows what he's talking about. If that isn't enough, I have hundreds of SMT6 log runs of the MAF voltage going well over 6v. If the HKS unit can't work with voltages over 5v, i'd sell it and buy an SMT6. The SMT6 works amazingly well for MAF voltage modification. It does ignition too... but its a little flaky.

Marc is also correct that you can't gain fuel by modifying the voltage when it reaches above 5v or so (4500RPMs @ WOT). I have verified this on a dyno with him. Its the only region where I wasn't able to richen my fuel curve.

MarkRobinson

Hi Michael,

The MAF is capable of reading well into the 600hp range. If more air is passing through cooling the wire, the voltage increases & so does your injector pulse: I believe you guys when you state it goes over, no biggie there: I have no experience in that field.

FYI: The SAFC-R has a low-throttle range map-correction as well as a high-throttle range (WOT), that you set by inputing your TPS switch points as well as your custom MAF Voltage range. At WOT, LH Jetronic does not simply follow the WOT fuel map since the MAF can still regulate overall fuel (as with the supercharger or displacement) as we see when supercharging & such. So, since the voltage is still changing at WOT to what air we're tossing at it, I don't understand why you state we cannot modify that signal

"Marc is also correct that you can't gain fuel by modifying the voltage when it reaches above 5v or so (4500RPMs @ WOT). I have verified this on a dyno with him. Its the only region where I wasn't able to richen my fuel curve." Also, my car lost it's fuel map (full lean-out) at 4k on WOT, not under normal conditions, if that's worth anything.

Make no mistake, I'm not trying to challenge your knowledge, but I've managed quite a bit of power out of 928's as well & I'm just trying to see where your perspective comes from:

http://www.apexi-usa.com/productdocu...-AFC2Final.pdf

Here's the manual on the S-AFCR, in the 40's & 50's (page), you can see how you set it up "custom" for your car. The HKS's main issue is that the HKS techs stated that it HAS to be set to 8cyl if you have an 8cyl: tricking it with 4cl mode will not work..and since it doesn't, and the MAF is only 0-5v on that unit, I'm done messing with the HKS unit.

Tell me what you think.

Mark

mspiegle

Yes, the MAF will continue reading higher voltages as more air flows, however the LH will not do anything about it once the voltages are over 6 (to be more accurate, its around 5.7). That is why I am saying there is a tuning limit becuase you'll hit this voltage/max-injector-duty-cycle limit at around 4500rpms.

It doesn't matter how good of a piggyback you have, they're always limited by the stock management system, so they can only do so much.

I said around 4500 is where you stop being able to make adjustments because in my setup, that's about where the voltage begins to max out the tables in the LH. I assume it is very similar for other applications.

Now, i've been strictly speaking of pulse width on injectors since the LH doesn't control fuel pressure. You can always add more fuel pressure, and that's what us supercharged guys end up doing because the LH can only flow so much at stock pressures. On my particular setup, I do an interesting little trick to eek the last bit of tunability out of my setup. I can run my car with my SMT6 maps zeroed out. So, I bump the fuel pressure up a little, then take AWAY some fuel on the LH side of things. This way, I can keep the MAF voltage going to the LH lower while still getting enough fuel.

If you are able to set your unit to 8cyl, will it give you more voltage to play with on the MAF side of things?

Jack '84 928s

you could try megasquirt. http://www.megasquirt.info/ I will go with one of these if i ever turbo my 928, wich i hope to try and do this summer if i have the extra money.

MarkRobinson

Hi Michael,

I've witness differently on a number of occasions: We put our last supercharger kit on an '86 car (5psi) last October, making 410rwhp/370rwtq at 5psi w/o any adjustments to the fuel pressure at all, nor with a FMU.

The mass air (even being blown-through) completely compensated for the extra air w/o any extra changes.

See graph: That contradicts your phylosophy does it not? (still just trying to get things straight) Mark.

MarkRobinson

PS: the richness was on purpose...over 100 degrees in some parts of Texas.

John Speake

Mike's comments are certainly true for the LH in the 87 and later cars. I've done no research into the LH 2.2 used on the 85/86 cars.

Was the car in question fitted with a RRFPR ? Did you measure the fuel pressure at WOT ?

MarkRobinson

Y, we actually lowered FP by 2psi to compensate for the 30# injectors (stock are 24# on the '85-86 cars), '87 on up are 19#, so it's not as large of a difference. FP at WOT was about 41psi or so.