FastWoman

Thanks, Cozye Yes, I used the stock spring setting and ended up with a resistance of approx 2.6 k. I think I'm about where you are on the tuning. I'm pretty close and will be just driving it a while to get a feel for it. I'll periodically do plug readings, calculate gas mileage, etc. BCDD: Yes, I still have it. I'm pretty sure it's operational, but I suppose that needs a re-check. G'nite!

I promised pics and details, so here they are. I apologize for the washed out photos. I didn't take them to be pretty, but rather to be clear. There's a whole lot of black in these photos! I decided it would be better to put the inline potentiometer at the end of a pigtail, rather than inside the ECU. The first reason is that I can change out the ECU and still have the same mod. The second reason is that I don't have to go into the kick panel to change the mixture. I tapped into the EFI harness inside the main connector shell. There's a screw on the end of it, and the shell slides apart I clipped the small black wire in the #13 position (labeled on the inside of the connector) and connected the two pigtail leads to the ends: Here's the pigtail coming out of the EFI harness, which has been wrapped back up. The connector does not have the potentiometer attached: Here is the potentiometer assembly plugged into the end of the pigtail. I simply heat-shrinked the 20-turn trim pot to the side of the connector: The connector has a slot where it can be mounted on a prong. I bolted this prong underneath my dash, just above the hood release and vent control: ... and this is the connector mounted on the prong: Not shown here are the trim panels. The panel that goes across the bottom of the dash leaves a hole where the air/fuel mix screw remains exposed. Also not shown is the approx 5 ft extension cord I made for this connection. It allows me to make adjustments from the hood compartment area or the driver's or passenger's seat.

OK, well, I've modded my car. Results are pretty good. Construction details will be in the next posting. I think the enrichment isn't quite all that it should be when the engine is cold, but it's not far off the mark. Idle is a bit unstable; however, there's now something wrong with my air regulator. It seems to be stuck in the closed (warm) position, so my idle speed isn't elevated on startup. I'm sure when I get that fixed, my cold idle will be much more acceptable. Anyway, I've had my car out on the road now. It feels about like it did when I loosened the AFM spring. The response seems pretty well balanced. I might have a bit of missing at WOT. Also after I give the engine a good WOT rev to maybe 4000 RPM, the drop in RPM pauses/bumps at about 2500 on the way down, and when I get that little bump, there's an exhaust pop. Do you (or does anyone else) know what happens at this 2500 RPM mark and what it means and/or how to get rid of it? I know the pop means I've got raw fuel in the exhaust, but beyond that obvious point, I'm not sure why it's happening there. I'm thinking that might be where the fuel cut ends on throttle-down. Other than that, the engine seems to be running about right. The fresh plug I put in #1 reads OK -- a bit of brown, and perhaps a touch of chalky deposit on the electrode. I might be a tad lean. Tomorrow I'm going to pull all of my plugs, most of which are quite black and sooty from prior issues. I'm thinking they might be a bit fouled and could use a good cleaning. I'll also see if I can tweak my timing a bit. (I still have to fix my timing light.) All in all, this seems to be a pretty workable solution. I like this approach better than playing with the AFM spring.

BTW, I've also re-thought where to situate the series potentiometer. I think I'll put a little Molex pigtail connector on the ECU harness and make a little 20-turn potentiometer/plug doodad (incased in epoxy?) to plug into it. Then I'll make an extension cord that will allow me to make adjustments from a seated position (passenger or driver) as the car is moving. Once the correct calibrations have been determined, I'll remove the extension cord, save it in my "special tools" box, and have it available for future "while-driving" tune-ups.

I've been thinking about how to tune this system. I suppose if I were racing, I'd want peak efficiency at WOT when running the engine at the peak of the power band. However, I'm not a racer. I'm just a semi-puttery, middle-aged woman who wants a cool car and doesn't want to destroy her engine by running it at the wrong mix. I'm first going to determine which series resistance gives me the fastest cruising speed at a fixed accelerator pedal depression. Then I'm going to see how that same setting translates to WOT conditions. I suspect I'll run right much richer at WOT. I might get a tiny bit more power running a tiny bit leaner, but my preference would be to optimize for cruising and to be "safe" at WOT. I suspect that's how the Nissan/Bosch folks designed the system in the first place. Finally, I'll see how the engine idles at this setting, and I'll make any needed adjustments to the little AFM screw. BTW, you should remove your screw and examine it. It has a rubber O-ring near its head, which I think mostly serves to keep the screw from wandering. Mine had crumbled practically to dust. (I honestly thought it was dirt.) I replaced it with a tiny sliver of silicone rubber hose that I carefully cut with a razor knife. The screw now has a bit of "hold" to it, and I'm confident it will stay in place. At this moment I have it set rather arbitrarily at 5 turns CCW from fully closed.

Cozye, I've also had my reservations about the potentiometer "fix," which was why I was unwilling to consider it until now. However, after studying all the EFI info I can find, after seeing empirically just how much adjustment latitude there is, and especially after reading the short description of the temp enrichment in the link that Zed posted, I'm a lot more comfortable with the solution. I think even with roughly 2k in series with a thermistor that should level out at 240 ohms in a warm engine, I'll still have cold/warm responsiveness in the system. That's because when the resistance is infinite, gas practically pours from the tail pipe. The roughly 2-3k (??? -- from memory) from a cold engine will now become 4-5k, which will still result in enrichment. I don't know if the enrichment will be too little when the engine is cold, but I'd rather have lean running when cold (with inadequate temp compensation) than lean running when hot (from compromises to the AFM adjustment). If this becomes a problem, there is still another solution. I can insert still another thermistor in the thermostat housing (in a plugged hole), and wire it in series with the existing thermistor. That will raise the resistance right much more in a cold engine. I would still have the potentiometer in series (with roughly 240 ohms lesser resistance) to balance out the mixture when the engine warms up. What I like about this solution is that the correction to the mix is executed the same way at all RPMs and under all conditions. It seems to be an overall gain control, so to speak. I'll continue in next post, but I'll get this up on the board, since I see you're online, looking at this thread right this very moment! ;-)

Hey Zed, thanks for the link! Very useful info. He has basically the same approach to what I'll be taking, except that I've now learned the AFM pegs at 4500 RPM, and fuel delivery after that is based on RPM and other factors. Per his advice, I'll be adjusting the mix for WOT first and then tweaking the AFM for smooth running at cruising. Finally I'll tweak the almost useless little bypass screw on the AFM to get the idle right. Good to have these refinements in mind. Thanks!

No problem, Steve! Hey, if we don't bring up these things (even if we're mistaken), we can miss possible problems/solutions. While I was sick, I hit the computer a bit and satisfied myself that I indeed have the correct injectors for the '78 Z. I decided not to flow test the injectors after all. Good grief, there'd be so much stuff to take apart and put back together, and I need my life back. I'm now feeling 95%. No more virus, anyway. I thought I was courting a secondary bacterial infection, but that seems to have cleared. Just lots of gunk, but that, too, shall pass. ... so I got back to work on the Z. I was first curious as to how much "boost" I would need in fuel pressure to achieve the proper mix at idle. (I was still playing with the idea of an adjustable fuel pressure regulator.) It turns out that with the AFM recalibrated per Atlantic Z's specified spring tension, I'd probably need about 60 psi. I was getting pretty close to the right range at 50 psi relative to manifold vacuum, where I topped the system out. My pump will only put out 42, and with the vacuum added, that would put me around 50. Of course there's no way I'd be able to put out even 50 at zero manifold vacuum. Anyway, this solution is out of the question, without going to a new fuel pump and pressure regulator combo. I'm not going there. Then I said, "OK, let's figure this thing out." I cracked open the kick panel and tested out my spare ECU. Same lean idle. I then did the quick FSM check of the ECU with a miniature Xmas light bulb in the #1 and #4 injector connectors. They flashed once per rotation, as they were supposed to, and they did flash brighter when I unplugged the temp sensor. I also re-checked all of the connectivity at the main ECU connector, and everything was happy and as it should be. I also opened up one of the ECUs for a look. It honestly didn't look bad. I was rather put off by the old can-type transistors. Anything oil-filled can possibly leak its oil over time, so I was leary. More surprisingly, even the early ICs were canned in oil! I've never seen that. Anyway, I suspect there are common failure modes in these ECUs. I do know enough to know that transistors rarely fail catastrophically. They usually fade away over time, and two ECUs of the same design would be likely to fade away in the same manner. I did an exhaustive/ing search on the internet for any info about MTBF in these sorts of transistors, common failure modes, etc. I really couldn't find much at all. I take back any previous assertions that one can find ANYTHING on the internet. Apparently those can-type transistors are too prehistoric (like me). Anyway, from what I can gather, these ECUs don't really have many failure issues at this point in their lives. I'm still somewhat leary of all that oil-filled can stuff, but I'll just hang with what I've got, with the expectation that it might work, but still develop inaccuracies over the years. In other words, my current operating theory is that the ECU will be responsive to changes in sensor readings, but otherwise a bit out of whack in its overall response to those readings. That is, some adjustments may be necessary. I'll mention here that I'm rejecting the Atlantic Z approach to solving this problem at the AFM. That's because I would have to alter the spring tension so much that the vane would too easily peg out at wide-open, leaving the mix too lean at peak intake. That of course would not be good. I wouldn't mind tweaking by a tooth or two, but certainly not by 15 or so. As I see it, the most obvious place to richen the mixture is with the coolant temp circuit. I was curious to see just how much an open sensor would richen the mix and was delighted to find that my poor engine was choking and gagging on fuel, barely able to run. Plenty of adjustment room there! I'll note here that the PO or his mechanic had inserted a resistor in series with the sensor, I presume to richen the mix. I suppose this is a time-honored bandaid approach. I had removed the resistor when I rewired, refreshed, replaced, etc. Anyway, I ran upstairs and grabbed a 5k variable resistor and plugged it into the temp sensor connector. That let me tweak the mix manually. I found that the best mix was achieved in a warm engine with a resistance of 2,550 ohms, while the temp sensor's resistance had dropped to 240 ohms. Thus the mix could be richened about right by adding 2,310 ohms in series with the sensor. Interestingly, I think I remember the value of the previously added resistor being 2.2k, so apparently someone else had been down this road before me. I'm sure at one time it made the engine run very well, and then it got wonky again with age and deterioration. Anyway, that's the solution I propose: I'm going to put a resistor in series with the temp sensor to richen the mixture. I'm going to try a more refined approach than simply splicing a fixed resistor into the wiring harness. Instead I'm going to install a mixture tuning potentiometer directly inside the ECU housing, provided there's some good place to mount it. Before finalizing the mount, though, I think I'll run a couple of loose wires into the cabin of the car, so that I can play with mixture with the car in motion. I'll have my lovely assistant drive my car down the freeway, with her foot blocked against the transmission tunnel to keep a constant throttle opening, and then I'll tweak the mix to achieve the maxiumum speed. I'll note the resistance, and then set that value when I mount the pot back inside the ECU. Further tuning will be done on the basis of plug readings and mileage calculations. Oh, and when I'm done, I think I do probably need a couple more deg of advance in my timing (which is currently set to 10 deg BTDC, per factory specs. I'll probably do that too, and I expect that will tweak me to a perfectly healthy engine vacuum. I just have to fix my timing light first. So there... That's my plan! Any thoughts or comments? BTW, I'm very bad about taking all the pics I should (ironic for a photographer, eh?), but I'll make sure to document my ECU mod.

I changed all of mine out last year with Eiko A-72 bulbs. Perfect fit, slightly brighter panel, no flickering, very happy.

Phew! I was briefly seeing dollar signs.

Hmmmmm.... Are you SURE about that, Steve? I ask because mine does. Furthermore the connector to the AFM (which is almost certainly original) is wide enough for those extra pins, although nothing connects). I'm wondering whether I have the original and correct AFM. So again, are you SURE?

I agree. It's a pretty car. I just hate to see a nice car (a Z) made into something it's not. A G-nose kit would be somewhat similar and would at least be authenitic -- sort of. Of course I suppose the intent is to make everyone think you own a Ferrari.

Almost... The '78's fuel pump will run when there's oil pressure or alternator voltage (or both). The AFM's fuel pump shutoff switch is retained in the '78, but simply isn't connected. I'm feeling a bit better today, but I don't know if I'll be up to going outside and working on my car.

Ztrain, I think it might have been both the check valve and dribbly injectors, perhaps combined with heat. Even with everything new, a quick prime does help make that restart instant. Without it the engine cranks a couple of times. Cozye, the check valve Ztrain is referring to is inside the outlet nipple of the fuel pump. The nipple unscrews from the pump. The part is currently bordering on NLA, so you might not be able to find it. However, you could probably install an inline fuel check valve (from Ebay) right before your fuel filter: http://cgi.ebay.com/ebaymotors/5-16-Check-Valve-Diesel-Gas-8mm-One-Way-Fuel-Flow-_W0QQcmdZViewItemQQhashZitem3cb2622af0QQitemZ260690815728QQptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories You're correct that the FP only operates briefly when the key is first turned. My primer switch is a pushbutton that sends 12V to the relay coil, bypassing all the other circuitry. When I push it, the pump will run, even if there's no key in the ignition. BTW, it seems I've contracted some sort of bug from the college kids. Honestly, I think germ sharing in college is worse than in preschool. Anyway, I haven't yet decided whether I'm energized enough to test injector flow rates today.

As I said, I doubt it's the age on your engine that keeps it from running smoothly. It might burn a bit of oil or lack the full power of a fresh engine, but those are different issues. I don't have a hot start issue anymore. I suspect the issue you're having is from air and/or fuel vapors in the fuel rail, which have to be purged before fuel will spray. I solved my hot start problem with a fuel pump primer switch (which I operate for a few seconds before firing up the car) and insulation on my fuel rail. The primer switch simply delivers +12 to the fuel pump relay coil. I wish I had figured out that solution on my old '75. At a mere 8 years of age (when I bought it), it was a devil of a beast to start and restart.

Mazter, I'd bargain hard if it's not running. The '78 is a fine machine, but the EFI isn't exactly the easiest system to deal with. Unlike with modern cars, there's no OBD II reporting, and the system is open-loop (i.e. with no oxygen sensor). It's very primitive, and it won't run particularly well if everything isn't just right. I'm devoting quite a bit of time and patience to mine, because I'm an electronics geek and have some weird fascination with technologies in their infancy. However, most people would greatly prefer a good, old-fashioned carburetor or two/three/six. That's always an option on the '78, of course. BTW, I've also owned a '75 -- the first year with EFI. I took quite a bit of time and effort to get that engine right, but I did. I have to say that a properly functioning EFI is so much more responsive and efficient than a carburetor. However, the early L-Jetronic systems will certainly keep you busy with tinkering.

I can pull between 18.5 and 19 with the right fuel/air mix. The engine has about 160k on it, and the timing chain is almost worn enough to move the sprocket to the #2 position, so that's probably about all I could want in engine vacuum. I had a bit of time today and did a few easy things: (1) I pulled off the AFM and plugged the AFM-to-throttle boot with a yogurt cup. I pulled off the vacuum booster line to the brakes and attached a hose to blow through. I pressurized the intake with nothing but moderate lung power, held the pressure with my mouth, and got a feel for how long it took for the pressure to leak away. I then pulled the little HVAC vacuum control line (adjacent to the power brake vacuum line) and repeated the test. I found that the pressure took maybe 5 times as long to dissipate with the HVAC line in place than with it open and blowing air. Pulling that one little vacuum line is not particularly consequential to the fuel/air mix, and any vacuum leaks that might exist (but probably don't) would be far less consequential than that. The small amount of pressure dissipation I could feel was probably the result of air leaking around valves, rings, etc. -- i.e. the air leakage through the entire engine. I highly recommend this method for satisfying yourself as to whether your intake is tight. It's quite quick and easy to perform. (2) I re-confirmed my fuel pressure. I had previously only checked the pressure regulator with compressed air, so an in-situ test removed that shade of doubt as to whether I'm dealing with a normal fuel pressure. Note: I'm still using a cheap gauge, but it agrees very well with a couple of other cheap gauges I have. (3) I readjusted the spring tension on the AFM to factory'ish specs, per Atlantic Z. I had been running the engine 7 teeth richer than the original setting (which gave me a vacuum of maybe 15 in). However, when I carefully did the aluminum can and water stuff, I found the spring actually had to be wound up 8 teeth tighter (leaner) than where it was originally set. I'm going with that setting and am closing the book on all of my sensors being to spec. I'm now going to work this problem from the fuel delivery side: ECU, fuel pressure, injector flow rate. (4) I also finished flushing my coolant and refilling with fresh 50/50. Of course this has nothing to do with the fuel mix issues, but it was on my priorities list with the weather getting cooler and the coolant flush sitting in the engine/radiator. Injector flow testing tomorrow, hopefully.

I'm still shaking off the trip to see my stepson. I slept in late. Gawd, those kids can drink! Anyway, no progress to report. I'll probably start back in on the car tomorrow. Cozye, I doubt your issue could have anything to do with the major mechanicals, as your engine runs great if you give it a fuel/air mix it likes. It's got to be something that would impact the fuel/air mixture. Anyway, my stepson gave me an aluminum beer can, which I did not have , and I'll be re-calibrating the AFM per the Atlantic Z article. I'm convinced that messing with the AFM spring is not the answer. This adjustment will of course make the engine run lean again. Then I'll be double-checking the entire intake system for leaks by pressurizing it lightly through the throttle body. I'd be very surprised if I found any. Then if I don't find anything remarkable, I'll pull the fuel rail back off and flow test the injectors, which I really should have done in the first place, prior to installation. With that information I should have a better idea what my next step should be. There's a distinct possibility that I simply have the wrong type of injectors. If so, then I'll need to get a correct set or determine whether the set I have can be made to operate correctly under a different fuel pressure. Of course that would mean an adjustable FPR -- cheaper than a new set of injectors, I imagine. Anyway, I promise I'll post updates and eventually my solution, whatever it is.

Yea! Another girl! The first Z I ever rode in belonged to a college friend in the apartment upstairs. Hers was a lime 240, just like yours. I hadn't appreciated just how cool those machines were until then.

Thanks! Yes, I insulated the fuel rail. I covered it in rubber hose, slit down its length. The small length along the line of injectors has a wide gap, along which I strung the wiring. I wrapped it all up with silicone rubber tape, and I ran the wiring to a pigtail with a 12 pin waterproof connector that connects just under the throttle linkage. The reason for the insulation is that I'm fairly certain my fuel boils on hot days, just after shutting down the engine. It really doesn't take much heat for fuel to boil. I figure this is a way to minimize hard starting -- that and a primer switch. More pics early next week...

Hi Cozye, I feel certain it's not an engine wear issue, because given the right fuel/air mix, my engine DOES have a lot of pep and does pull a normal vacuum. I'm also pretty certain it's not a vacuum leak issue because there's nothing left to leak -- unless I did an awful job of mounting up the intake manifold (which I didn't). It's not valve timing, because I'm verified within specs (although I'll soon need to switch to the #2 hole to remain within specs). I think it's definitely a metering or fuel delivery issue. Regarding wear of the potentiometer: You'd be surprised just how robust a carbon trace can be. As long as the wiper successfully makes contact with the trace, all shoud be fine. Wear issues would not affect resistance or voltage, for the most part. In fact what the ECU reads is voltage, not resistance, and there is almost no current conducted through the wiper contact. I've worked with electronics long enough to STRONGLY doubt there would be a potentiometer wear problem, so long as there is good electrical contact between the wiper and the carbon trace. I've verified on my AFM that I have excellent electrical contact and that the voltage readings are correct throughout the movement range, as near as I can tell. (Note here, I did re-center the wiper on the trace, using the center glue-globbed adjustment. Mine was off less than 1/16" because of a slight bend in the flap from intake backfire. This tiny adjustment didn't really have much of an effect, though.) I always wondered about the ignition timing. Your explanation that lean mixes burn slowly explains a lot. It explains the hard-blowing exhaust, where the mix might still be burning even during the exhaust stroke. It also explains how intake backfire might occur, so at least some of this makes sense to me. I suppose advancing the timing would make the backfire less likely, so I'm more open to considering a bit more advance. Anyway, I've been a scientist and programmer in various stages of my life, and all of this involves fault tracing and problem solving. Right now, this is my thinking: (1) The air flow meter has a certain shape that is constant and not subject to wear or degradation. The flap in mine has been lightly bent, but only lightly. I might try tapping it back into shape, but I doubt it will make any difference. If anything, the direction of the bend, combined with my repositioning of the wiper, would result in a richening of the mixture -- which obviously hasn't occurred. (2) Ignoring the bend, as long as the spring tension is to specs, then the flap deflection will also be to specs. A given air flow will result in given flap deflection, which will result in a given voltage output at the wiper. Although we don't KNOW exactly what the factory specs are, we have a reasonable approximation from the Atlantic Z folks, and the adjustments that made my engine run correctly resulted in less than 1/2 of the tension at half deflection than what the Atlantic Z folks describe. I'm confident there's NO WAY this is within the margin of error. Something else is WAY OUT of specs in the system. (3) I've gone through my entire system and have verified that every sensor is within specs. Of course there could be cumulative error at play; however, Bosch/Nissan would have designed the system such that the engine would still run reasonably well even with a conspiracy of cumulative error. (4) There's a possibility that a gremlin made a solder joint go cold in the ECU. It certainly wouldn't hurt to re-flow the connections. (Ztrain is the guy who found that problem, BTW.) As I said, I've already tested my spare ECU on the prior incarnation of my engine and found it not to make any difference in the way the engine runs, so I'm doubtful I have an ECU problem. Still possible, though. (4) The only thing I have not verified is the fuel delivery -- specifically the fuel pressure (which I've checked with a rather cheap meter) and the injector flow rates (which I haven't measured at all). This is an easy thing to measure. I'm guessing that the pulse width is correct when the AFM is adjusted to specs (provided the ECU is good -- which I think it is). However, if the fuel pressure or flow rates are wrong, then all of that would be meaningless. So I'm really narrowed down to three systems to check -- ECU, injectors, fuel pressure regulator. That's not so bad. Everything else is verified good/within-specs. Unfortunately this project will need to wait for several days, as it's raining today/tomorrow, and I'll be busy with life during the weekend. So tune in later! BTW, here's a quick pic, by popular demand, that I snapped of my engine before the rain started coming down. In fact you can numerous sprinkles that hit my engine in the couple of minutes it took for me to grab my camera. :mad: I did it in Duplicolor's MetalCast blue. The "NISSAN OHC" lettering on the valve cover was done by filing flat, painting black, filing off the paint, masking, painting the blue, and then clear-coating the whole thing. More pics later, but first I want to get the thing running right!

... but alas, some of us have more time than money. When the AFM is adjusted to specs (correct spring tension, flap movement, voltage output), the engine runs way too lean. I already know that. The only thing I haven't checked out is the fuel flow rate (having assumed it would be correct from new injectors). I think my next logical step is to milk the fuel rail, so to speak.

Well, I've pulled the #1 plug. Yes, it looks lean, but not terribly so. There are still light HC deposits on the body. Insulator looks a bit whiter than I would like. However, it looks like I'm in the ballpark. In addition, I wiped down the rim of my exhaust tip before leaving, and wiped it down upon return. There is a TRACE of black soot -- not much, but enough to tell me I'm not out in the left field of lean. I suppose 36 mpg at mostly 45mph straight cruising isn't all that surprising. Fuel usage is proportional to the square of velocity, so my 36 mpg at 47 mph (best estimate) would be about the same as 19 mpg at 65 mph. If you temper that with the fact that maybe 1/4 of the driving was in-city, occasionally with moderate-heavy acceleration, I suppose I'm not too far off the mark. I'm reminded of when I drove my little '75 Celica approx 35 mph down an icy Texas highway for hundreds of miles. I got 45 mpg on that trip! Wow, was I impressed! I still think I might need more fuel pressure. I'm not comfortable with the AFM adjusted that light.

A problem area is the wiring harness connector just inside the passenger firewall. The connector is just slightly inadequate to carry the current for the fuel pump. You might find the plastic around that pin a bit "melty." (At least it was on mine.) After refreshing the connection a few times and still having that connection fail, I finally clipped the wire out of the connector on both sides and connected it via a bullet connector. Problem solved. But of course it could also be the fuel pump relay or any number of other things. The best test is to hook up a gauge and see if you have fuel pressure. Absent a gauge, see if the hoses are harder and less squeezable after you crank the ignition and supposedly pressurize the fuel rail. (It takes pressure a very long time -- even a few days, if your check valve and injectors are in good shape -- for your system to lose its pressure.) You can also break loose a hose connection and see if fuel sprays (very messy -- no smoking!!). If you have pressure, but still no fuel, check for continuity between the #1 pin on your ECU connector (harness side) and the (-) terminal on your ignition coil. If the ECU doesn't receive signal from the ignition system via this connection, the injectors won't fire. Good luck!

I would think plugs have always and will always read the same way in internal combustion engines, irrespective of gas grade, vehicle/engine type, etc. I read my lawnmower plugs the same way as I read the plugs on our new truck -- and the same way as I read them on the Z. The engine (head and block) doesn't know or care how fuel was delivered to it. There's an ideal mixture that results in maximum power and minimum emissions. The only differences I can imagine are that cars with cats are calibrated a bit leaner, and cars with EFI are able to teeter on that "just right" mark a bit better. Carbed cars typically run a bit richer because they aren't as accurate at metering, and they err on the safer side -- rich. But the plugs should read the same way.