streetsnake

Okay, on my ‘79, I feel like I’m starting to narrow down my stumbling idle. I checked all the fuel pressures today and got everything dialed in with my new fuel pressure testing set. It was very nice. Liselle. Way easier for me to hook up and acted very accurate. Anyway, 50F, SP=76, CCP = 20.5, HCP = 52. Everything within spec. I hooked up the Innovative wide and plan on testing/adjusting the AFR tomorrow. After a nice long warm-up (bungs welded in btw) I plan to set idle ratio around 14.7 and see how it reacts. The questions I have…is this correct? Do I need to test under load when checking various RPM ranges. I’m guessing I need to adjust the idle as I tweak the idle ratio, correct. What else should I be doing or if my value is wrong for idle, what should it be? WOT should be around 12.6?

hans14914

Don't forget that most gas is now E10. Stoich is not 14.7 for E10, that is only for 100% gas.

Stoich on E10 14.1

Its a common mistake that most people make as that 14.7 number was hammered into people's brain for decades.

streetsnake

Thank you very much for the reminder of that. Made note. Not to open a can of worms but unlikely because it is tucked inside this post, but are there any benefits to running E0 gas? I currently run high octacte (for me)...93 but we have somewhere that sells E0 about 20 minutes away. Thanks again for the E10 reminder.

hans14914

E0 is pretty hard to come by, especially on the road.

I would advise to just tune for E10 as its ubiquitous. These cars are made for driving, so just work around the fuel you can count on along the way to one of the national gatherings.

hacker-pschorr

The amount of ethanol in E-85 and E-10 is not consistent though. 85% and 10% are the maximum amount of ethanol you should ever see in those fuels. E-85 is typically 70-73% ethanol and E-10 can vary just as much.

Around here, the vast majority of gas stations have E-0 @ 91 Octane with a small handful of stations have E-10 @ 93 Octane (which is what I use). There's one gas station that has E-0 @ 87 Octane to cater to landscaping companies.

I keep a bottle of Boostane in my cars for any traveling if I find myself with less than 93 available. We did a lot of testing of various octane boosters years ago on the dyno, this stuff actually works as advertised.

streetsnake

interesting. I usually run 93. Everybody in agreement that afr should be 14.1?

hacker-pschorr

You're tuning CIS, it's going to bounce around no matter what you do. Shoot for mid 14's and don't stress too much over it. When I glance over at the gauge in my 79, as long as I see it's in the 14's I know everything is fine.

Where is your sensor?

streetsnake

I have 2 bungs welded in after the exhaust manifold. Doing testing now. Car is warming up

streetsnake

Going to retest tomorrow to make sure everything seems the same but got the AFR dialed into the 14.1-14.3 range at idle. Timing set at 30ish. I tried to slowly rev it but it didn’t seem like it was leaning out like I believed it would. I’m going to set up the wiring so I can take it for a spin and see how it behaves under load. Everyone that told me my pressures were off, they were correct. getting a nice set of gauges really seemed to do the trick. One of the main guys was @Rob Edwards Thanks for the persistent help. I’m hoping my first drive tomorrow will go great.

streetsnake

Took it out for a test drive. The idle at a stop is a million times better. That makes me very happy, as it was driving me crazy. Well worth taking the time to redo the pressures. Although I am by no means an expert, I'm blaming the previous gauge set. LOL I set the AFR as close as I could get to 14.1 at idle. I only noticed 2 things. 1. At very light throttle, in lower gears, there seemed to be a slight hesitation. 2. On the flip side, under WOT, between shifts (only happened 1 time), I got a pop from the exhaust. I'm sure a very slight turn of the AFR screw, one way or the other, would likely solve that issue. I'll have to research which way that might be. Thanks for the help and input as I've worked to learn the early 928 platform. It has been a learning experience.

Observer.Octo

This is only correct when defining what the stoichiometric ratio is for a given fuel type. It is incorrect when describing how to use and read a wideband AFR gauge.

All wideband gauges and sensors read the fuel ratio in lambda, not AFR. So it doesn't matter what the fuel type is, stoichiometric is always 1.0. The gauge then converts lambda into AFR for display since most people are used to the AFR numbers for pure gasoline.

Which means that if you are using a standard wideband AFR gauge, it doesn't matter what fuel type you are using, a displayed 14.7 AFR will be stoichiometric. Yes, that translates into a true 14.1 for E10, 9.77 for E83, etc. However, since a standard wideband gauge and sensor are actually measuring in lambda, what they display is just a multiplication of 14.7 * lambda.

There are some gauges that can be reprogrammed to display the true AFR for a given fuel type. However, since it's almost impossible to find fuel that is exactly 10% or 85% ethanol, the resulting AFR will never be the true AFR. You could use an ethanol content sensor to determine a multiplier to give you a true live AFR reading if you have an ECU or PLC that you can program to give you a constantly adjusting live lambda*multiplier AFR output. Or, you could just use stoichiometric or any other static scale that you're comfortable with (e.g. traditional pure gasoline 14.7 ratio)

For example, here is the manual for a standard wideband analog AFR gauge: https://www.innovatemotorsports.com/.../05/MTX-AL.pdf

Note on page 9 where it describes the calibration and it states: "By selecting the ‘use Air-Fuel-Ratio’ button you can program the curve by AFR instead of Lambda. This does not change the programming, only the unit of measure displayed. When programming by AFR the LM Programmer converts the number to Lambda before programming the MTX-AL."


Here is an example of a gauge that can be programmed to display different AFR scales: https://www.innovatemotorsports.com/...CF1_Manual.pdf

On page two it describes stoichiometric vs. lambda vs. AFR and concludes by saying: "The stoichiometric value in lambda is 1 regardless of the fuel type. It is for this reason that the ECF-1 comes set from the factory displaying lambda. This is a user definable option, meaning the ECF-1 can also be programed to display AFR for nearly any fuel type."

Those are just one manufacturer's descriptions of how to use their AFR gauges, but it's standard across all O2 sensors and AFR gauges and ECU readouts. And it's absolutely critical to understand this aspect of AFR measurement if you're going to be tuning an aftermarket ECU.



What that means is that in practice if you are using a standard AFR gauge, then 14.7 is always stoichiometric, regardless of the fuel type. So, you should be targeting 14.7, not 14.1.

A too rich fuel mixture at idle will cause bogging during rapid throttle blips.

Another approach to getting the best idle is to target maximum vacuum. Adjusting fueling until maximum vacuum is achieved at idle. This will usually result in a mixture richer than stoichiometric, but will also give a very stable idle.

streetsnake

@Observer.Octo Thanks for taking the time to write this up. However, if I'm reading 14.1 ish and seem to be running a bit rich, wouldn't tweaking to 14.7 make it that much worse? Also, how does timing affect the AFR. I was researching this and there are multiple explanations on both sides of the issue. (yes, timing affects AFR and No, timing does not have anything to do with AFR) After thinking about it, I retarded my timing when I was thinking I had advanced it. Instead of 31, I set it at 30. (3000 RPMs). I need to go back in and at least make it correct (31 degrees) and potentially tweak it to 32-33.

Observer.Octo

Higher numbers are leaner. It's a ratio of Air to Fuel (e.g. 14.7:1 is 14.7 parts air to 1 part fuel). That means a lower number, like 14.1, means there is less air per the same volume of fuel, so it is richer. Moving from 14.1 to 14.7 will make your mixture leaner.

Timing does not affect AFR at all except when you have significant cam overlap or the timing is so retarded or so super impossibly advanced that some of the burn happens when one of the valves is open. Neither of which is an issue on the stock 928 motor. Theoretically, you could effect the mixture with timing if the O2 sensor is right next to the cylinder exhaust port and the mixture hasn't finished combusting by the time the exhaust valves open. Again, not an issue on the stock 928 motor.

You can test this by leaving your mixture alone and changing the timing and watching the effect at higher RPMs.

Timing does affect idle vacuum, though.. So, if you do your test at idle, and the fuel pressure regulator is vacuum referenced, the fuel pressure will change when the vacuum changes, which will affect how much fuel is sprayed on each injection cycle, which will affect AFR. Modern ECU's monitor the manifold vacuum/pressure and account for the affect on fuel pressure when calculating fuel injector pulse width.

There are a lot of tuning "experts" on the internet. So, make up your own mind and try to filter out the opinions that aren't backed by verifiable facts and science. There are a surprising number of publicly available research papers on ICE, which cover these topics and more.