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Just Some Dude in Jersey
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Discussion Starter · #1 ·
I'm going to install an Air Fuel Ratio gauge and I would like to know what the best A/F number is under varying condition like idle and cruising. I am NOT seeking max power at 6000rpm. I want to know what AFR is best for a car that is casually driven like a passenger car.

14.7 is supposed to be the optimal ratio. But I heard RallyBob say that you shouldn't try for more than 12.5 on a carbed GT because you will lose power.

I have Motronic FI on a 2.4. I drive mildly with only occasional bouts of sportiness. After so much recent engine trouble, I just want the healthiest for the engine AFR I can reasonably manage.

I don't have a lot of adjustments I can do to my first generation spark and fuel control FI system, but fuel pressure is one of them and messing around with the pressure, especially lowering it, seems to make a big difference in engine sound, economy, and power. I'm actually seeing more power at lower pressure.


So my question is: As I monkey with the fuel pressure, what AFR am I hoping to achieve at idle and steady speed cruising at various rpms?
 

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I put a narrow band AFR gauge in my wagon (Motronic 1.5).
I welded the bung downstream of the factory one. The narrow band is less expensive but it takes a bit to warm up. A wideband O2 sensor will give a quicker response.
45psi gives a range in the "normal zone" (12-14) when cruising.
 

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Can Opeler
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The CIH suffers from overscavenging. Which as I understand means too much fuel air mix in the combustion chamber gets stolen by the exhaust scavenging.

CIH engines need to be pretty rich to counter this. I would aim for 13.5:1 or richer at idle and see how lean you can get it. I’ll bet you’ll end up at about 12.8:1.

At cruise I would start no leaner than 14:1 and expect to be close to that. My GT can cruise between 14.5 and 16:1 but it causes drivability issues when it gets hotter. I’m most happy when I get my cruise between 11.8:1 and 12.8:1 as it provides excellent throttle response quickly for transition.
Fuel injection should be able to handle a lot leaner though.

All always aim for xx.8 numbers for everything. I swear the car likes them better:p
 

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12.8-13.2 gives the best power in a naturally aspirated CIH engine.

You can still strive for those numbers on a street engine, at least at WOT.

For idle and cruising you definitely want it leaner, but keep in mind a CIH will start to lean-surge when it gets around stoich, at least under load.

I’ve found 14.5 is good for idle, while 13.8-14 works well at cruising speeds.

If you get too lean at WOT, not only do you lose power, you risk damaging the engine under sustained loads. These engines run very high EGT’s.
 

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About time! Congrats on FINALLY utilizing proper modern tuning technology....
 
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Opel Rallier since 1977
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I would not hesitate to go to 15 for light cruise but at heavy cruise, I would shoot lower and 13-ish maximum at WOT.

One important thing to know: Do NOT try to get the AFR on a certain number while the engine is warming up. The AFR meter will always read leaner than actual when things are cold, even if the actual AFR is rich (like it should be). The reason is that when cold, there is incomplete combustion which leaves excess, unused oxygen in the exhaust. The AFR meter interprets the excess oxygen as a lean condition. (It has no ability to see the unburned fuel so as to make any correction.)
 

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Why mess around when the Motronic automatically takes care of the A/F ratio ? If you had a hotter cam it would make sense to increase the fuel pressure to richen the mixture in open loop.
 

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A monumental moment indeed! Congrats!
I’m happy for you. I try keeping it in the mid 14’s as a general rule of thumb for my kind of around town driving. I drive there 80% of the time, light acceleration 13.5:1-very light accel 15.7:1, drops down to 12.5:1 quickly when under harder acceleration. Medium acceleration is 13.5:1, those feel like good numbers to me, plenty of power, good economy🙂🙂. I’m anywhere from 11.5:1-15.7:1 with the way it’s tuned now. Mostly in the 12-14 range.

Surprisingly my GT makes smooth power up to 16.7:1 during highway speeds I saw that when I had it tuned a bit leaner. 405/256 cam, close to stock.
 

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Just Some Dude in Jersey
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Discussion Starter · #10 ·
My reason for all this is because I realized that I had forgotten to back down the elevated fuel pressure of 50psi that I had resorted to in order to get the big aftermarket Nissan throttle body to work. Recently I returned to the 40psi that we had previously calculated, but then Charlie said that he likes 42psi, so tried that. When or how to set the pressure is now an issue of vagueness. Is it the same thing to set pressure with the fuel pump on, but engine off, vac modulator hose attached vs. engine running/vac modulator hose disconnected? It seems that both conditions would be the same: No vacuum applied to the pressure regulator. Various fuel pressures between 35 and 50 had their own unique driving and idling characteristics.

But which one is best for the things I care about?

My main concern is that I have the cleanest running engine I can manage. I DO NOT WANT TO HAVE TO PULL ANOTHER ENGINE OUT OF THIS CAR. I don't give 2 schitts about power, I just want the engine to run forever and not give me any trouble down the road.

Without a gauge I have no idea if any chosen fuel pressure is good or bad for my engine. The car pretty much acts and drives the same no matter what pressure I choose, but there are unique subtle differences at 35, 40, 45,+50psi.
 

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Opel Rallier since 1977
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Totally agree with the thoughts above. KNOW what you have. Yes, the O2 sensor on your Motronic will enable closed loop operation and allow the ECU to adjust the fuel. But I bet there will be modes, like WOT, where the computer may just set the fuel to 'x' and then pressure will make a difference in the fueling and AFR.

Just don't drive yourself nuts with reading the AFR, and trying to make it perfect everywhere.

BTW, the difference between 40 and 42 psi is 5%, which will make an approximately 2.5% difference in fuel amount, if the computer does not correct it in closed loop mode.
 

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Just Some Dude in Jersey
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Discussion Starter · #12 ·
Gordo Tunes His Fuel Pressure
This morning I performed a series of tests at varying amounts of fuel pressure to determine which fuel pressure gave me the best air/fuel ratio and overall driving performance. I performed 5 test ranges at 35, 37.5, 40, 42.5, and 45psi. I didn't actually bother doing the last one for reasons that I'll reveal later.

To refresh your memory, I have a basically stock 2.4 with slightly bigger valves and the stock 2.4 Motronic Bosch 1.5 fuel injection system. I have modified it to use the Edelbrock returnless high pressure fuel pump system mounted in the engine compartment, with an Edelbrock low pressure pump to bring the fuel forward from the tank. I'm using an automatic on a separate tranny cooler. I have a 3-core copper radiator with an engine fan, plus an auxiliary electric fan on a thermostat set at 200* that turns on both radiator and tranny cooler fans. Both fans turned on while I was doing fuel pressure changes in various parking lots along the way. I have a heavily ventilated hood and lots of chrome. I use 20W50 oil and premium gasoline and the fuel selector chip for the computer is also set for premium.
Weather conditions were a real torture test for a GT, although not maximum bad. I went out at 10am and the temperature was 80 degrees with 80% humidity. 90+degrees is maximum bad around here with that level of humidity. A carbed normal GT would surely overheat in backed up traffic under these conditions. My temp gauge was reading 160-180, although I think it is under-reporting the temp by 10-20 degrees. I was well warmed up and the excursion took about an hour and a half, with all the stops to adjust pressure and jot down notes. I dealt with 5 miles of stop and go 20-25 mph traffic until I got to a country-ish road where I could get up to 50mph and try flooring it a bit. There were a few very gradual hills that most of you wouldn't even notice, otherwise the road was dead flat and straight.

Here's What I Discovered:
I thought I would be tuning for air/fuel ratio, seeking the safest A/F to assure engine health and longevity. I'm used to my Opel engines lasting forever(except for my recent 2). I don't give a fig about max power at 6000rpm, I drive at passenger car speeds and rpms, with only occasional bouts of spiritedness. My driving is about 25/75% highway and lower speeds.

Well, guess what? The computer seems to want to maintain an A/F of 13 at various 50 and below cruising speeds, no matter what the fuel pressure was. Idle was basically at 12.5 no matter what. Floored, the ratio went up to 14-14.5. It was interesting to observe the ratio wildly fluctuate for a few seconds, as speeds and load changed, while the computer compensated to try to get A/F to the vicinity of 13.
Since none of that stuff really changed, my results boiled down to how the driving performance changed at various pressures. Here are the results of that:

35psi - vacuum compensation at idle at 28 - Smooth economical driving, idle a bit faint, but steady, hesitation when floored.
37.5 - vacuum compensation at 30 - Nice driving, a bit more power, idle good, maybe a teensy bit of hesitation when floored
40psi - vacuum compensation at 32 - Nice driving, a tiny bit more power, no hesitation when floored, idle starting to get unstable when limping around at idle in Drive, computer trying to compensate, possibly on the verge of stalling.
42.5psi - vacuum compensation at 35 - Exhaust starting to sound loud and maybe rough with nice driving, power about the same as previous, maybe more, stalled twice while limping around, lots of idle compensation, A/F meter went off scale at 20(engine was pretty hot and weather temps were starting to get pretty unpleasant
45psi - No point in trying this, I could see a negative trend developing, didn't want to stall in traffic.


My Conclusion:
The computer keeps me at a safe, economical, A/F of 13.....my primary concern. I dialed down the fuel pressure to 38psi and liked the driving characteristics on this hot steamy day. I have to set my system to work harmoniously with my car's set up and the driving conditions in my area and my preferences. I could probably bump up the psi to 40 in cooler weather, but why bother, the difference in power would be negligable and I would probably unnecessarily waste a bit of gas. I don't normally drive my GT's in weather as hot and humid as today, but today's conditions were a good test of the upper limit of what my car can handle.
 

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Can Opeler
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That sounds perfect. 12.5:1 idle is the optimal best lean on all CIH engines I’ve tuned that had bigger than a 32/36 carb. 13:1 cruising is fantastic but you won’t win any mpg contests and 14:1 WOT is fine for your driving style. I bet it richens up above 5000rpm.

Congrats Gordo! Go enjoy your FI Opel
 

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A bit rich overall, for me 13.5 straight across is about as rich as I’d like except for WOT, speaking of that, when you say it jumps into the 14’s does that mean it comes right back down to 13:1? 14:1 seems lean for WOT but I’m assuming that it was just for a brief second. I would think that FI would correct the lean spot within a second.

I’ve got a question for your readers, since you seem to have similar driving habits, it hopefully will complement your thread. I like to goose the pedal in short spurts for fun once in a while but don’t do any sustained driving at 6,000 rpms either. Is 12.8-13.4:1 too lean for 10 seconds or so at WOT? The engine seems to have more get up & go there as opposed to lowering it into the mid 12’s.
 

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Opel Rallier since 1977
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That is a very good report, Gordo. Thanks for the info. Did your new AFR meter have a calibration sequence to do?

Interesting that the system would put leave the AFR at 14-ish at WOT. Was this the case for all pressure settings?

Realize that these numbers are what this particular system is doing in response to different loads and inputs. The optimum overall may be different. With a fixed system and programming, you are really stuck with what it wants to do, with the inputs it sees, for better or worse.
 

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Just Some Dude in Jersey
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Discussion Starter · #16 · (Edited)
A bit rich overall, for me 13.5 straight across is about as rich as I’d like except for WOT, speaking of that, when you say it jumps into the 14’s does that mean it comes right back down to 13:1? 14:1 seems lean for WOT but I’m assuming that it was just for a brief second. I would think that FI would correct the lean spot within a second.
It jumps to 14+ when under fairly hard acceleration. It might hit 16 for a split second before settling in the vicinity of 14 while under aggressive acceleration. The numbers aren't linear, as you would expect to see using a carb, it's got a computer with instructions to shoot for good fuel economy and low emissions. There's a time delay factor as it tries to understand what's going on. First it sees the throttle position signal indicate that I'm flooring it, then there's the sensing of the air flow from the airflow meter, all the while it's monitoring the rpms and the oxy sensor and making adjustments to spark and fuel, then it starts to react to the WOT and other signals to keep things in reasonable bounds. It's computer programming is a mystery, no one knows what's written in it or what it's writers intended in various situations. All you can do is speculate. Some guys start messing around with the guts of the air flow meter to try to "trick" the computer into giving more power than it was programmed to give under various circumstances.

Carbs do the same thing, but they do it mechanically, hydraulicly, and aerodynamically via jets, emulsion tubes, progression holes, weights, springs, etc.

Modern FI computers let you almost write the computer code yourself. Certainly they let you set the parameters differently if you want to. If you want 13.5 as your baseline AFR and 16 at WOT, I presume you can program them to do that.

I want the engine to last forever, I'm not that concerned about fuel economy or emissions or power. Is Opel's selection of 12.5 at idle, 13 at cruise, and around 14 at WOT the best for engine longevity? I don't know. I'm going to presume that they know a lot more about this stuff than I do.

Clearly, I can't do much about the AFR. My reason for running this test was to dial in my fuel pump pressure optimally. I wasn't expecting no change to the AFR at various pressures. I DID observe different engine behavior when I had inadvertantly left the pressure at 50psi and there was a lot of disagreement when I brought up the subject of what the stock Opel fuel pressure was set up at. The only things I could test for were AFR and engine performance, so that's what I did. The results showed no AFR change, but noticable changes to the engine performance.

Yeah, I'm looking into the possibility of installing Microsquirt or some other system, but that's mainly to achieve the elimination of the air flow meter in favor of air density sensing, which would vastly declutter my engine compartment. I now know that Opel/Bosch like 12.5, 13, and 14 AFR's under certain conditions. With that knowledge now in hand, I could presumably set up a Microsquirt controller with those numbers as the baseline. The power guys will always be suggesting tweaks for more power. They are the out-sized voice on these forums, they are always seeking more and trying experiments to get it. Guys who just want a nice long lasting Opel engine experience never speak up. I'm that dude.

:)
 

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Just Some Dude in Jersey
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Discussion Starter · #17 ·
That is a very good report, Gordo. Thanks for the info. Did your new AFR meter have a calibration sequence to do?

Interesting that the system would put leave the AFR at 14-ish at WOT. Was this the case for all pressure settings?
Uhhh...y'know......I didn't check that. There's a button on the front that I just assumed was there to change the color of the display. I'll get the instructions out and take a look.

As far as 14 at WOT, that doesn't concern me because I'm a casual driver and I almost never floor it. I figure that the short time I'm flooring it doesn't amount to a hill of beans in the long run. You're a racer, you're always flooring it, so I guess you would be very concerned about that sort of thing.
 

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Just Some Dude in Jersey
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Discussion Starter · #18 ·
The meter had no instructions other than wiring. It does seem to have a self calibration at start up and the needle swings and numbers change for a second or two. The button does seem to only change the color. I have chosen red to match my instrument cluster.

:)
 

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1970 Opel Gt - Purchased July 1972 - Chartreuse - restored - 3000 miles as of 02-16, 2021 -
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Thanks Gordon, a really good read that mostly went right over my head, but still enjoyable and I always learn something that will help me later down the road. I guess the more knowledge we acquire and the more we understand, the more we can achieve, that is, bringing the car to a level of performance never dreamed of in earlier times, and level of performance can mean many things to different people. My restoration has been a slow process for many reasons but after 25 years of work I finally have the car running the way I remember it when I was 19, albeit with a bit more power that in 1972. I still have a lot to learn but my goal may be some what different - I want a safe reliable car that looks great and runs great. And considering the fact that I will never put a lot of miles on it, or drive it in the rain, my expectations are that it will last a life time for me and it's next owner my daughter. Time for that second beer. Later, Carl
 

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Can Opeler
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A bit rich overall, for me 13.5 straight across is about as rich as I’d like except for WOT, speaking of that, when you say it jumps into the 14’s does that mean it comes right back down to 13:1? 14:1 seems lean for WOT but I’m assuming that it was just for a brief second. I would think that FI would correct the lean spot within a second.

I’ve got a question for your readers, since you seem to have similar driving habits, it hopefully will complement your thread. I like to goose the pedal in short spurts for fun once in a while but don’t do any sustained driving at 6,000 rpms either. Is 12.8-13.4:1 too lean for 10 seconds or so at WOT? The engine seems to have more get up & go there as opposed to lowering it into the mid 12’s.
You are fine as long as you don’t have pinging.
You could spend all day at 6000rpms at 14:1 if your cooling system could keep up and you aren’t pinging. If have a lot of advance, a milled head, etc, go richer to avoid pinging from hot spots, but a stock high comp 2.0L doesn’t give a crap.

I would be willing to bet that over 50% of Opel owners are either running WOT afrs above 14:1 or below 11:1 Because you can’t tell without either an AFR gauge or a stop watch with most engines. And spark plug will still be pretty tan even at those relative extremes.

Now if you are on the race track. really pushing the car I wouldn’t expect the motor to last long anywhere above 14:1 AFR. The EGTs and cylinder temps would be crazy I bet.
 
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