Hi all I know in this forum are very smart people. So I would like to discuss and learn more about the correlation between the cam overlap vs the AFR specially at idle. I have a moderate cam 248* / 267* 0.05" 110 LSA that give me a 38* overlap in theory. But that cam is a solid roller, so the lash of 0.022" will make the cam smaller and give me less overlap. Car idles fine with 14" vacuum at 900 RPM has a Holley HP EFI and the injection starts after the exhaust valve is closed, but my car still smells like it had a carb. My WBO2 shows 14.5 AFR, the plugs looks a little rich. I tried to go leaner, but the engine doesn't like it, when I press the clutch sometimes the RPM drops to 500 and after a second it raises to the desired 900 RPM. So its this normal that the spark plugs looks richer because the reversion? Whats our experience with bigger performance cams?
Can I ask where your timing is set, at idle, and then when does it come fully in. If your cam has 38* of ol at .050 then it will be more seat to seat. It's possible depending what your timing is your setup might like more initial timing timing and less swung in. This might help with the rpm a little, and will help burn the fuel more in the clyinder, will also gain you some vaccuum. Over the years I have become more and more a fan of locked out distributors and using the ignition box for cranking retard. On my car I gave egt not af and when we locked out the timing we saw 600 to 700 degree drop in egt, plus a drop in coolant temp, plus a very smooth idle on cams with much more ol. Our smallest cam has about twice the ol yours does @.050 we only pull 5 or so inches of vac, but can idle at 800 when warm and even lower when cold. If your timing is say 10 or 12 at idle, you might be able to do a simple test to see if the more timing at idle with less swing might help, just start the car bump the timing up at idle to 20 or 25, you find should bring the rpm back down to its desired rpm, see if your smell goes away, You won't be able to drive this way cause your swing will go to far, but it's a simple easy test to see, just make sure to mark the base of the distributor to the timing cover so you can get exactly back to where you were before
Skip to end for simple answer... Random narrative and thoughts; Need to look to the plug for the heat and timing indications and you could try a hotter set, although I suspect not enough timing and support the info in post above this. Ignore the AFR and plug color for now, fuel might be dropping out and/or not enough timing. You should keep a few heat ranges around as testers anyways. If you were to go further in the cold direction and things got worse... When you are too cold, the ignition randomly acts like it cuts out and you have to use excessive timing with more throttle opening to keep a stable idle, kind of like a weed whip cold started without choke and with massive fuel primered in, it pops more and randomly dies... I have noticed more of a correlation between too big of port and/or losing vaporization (affecting combustion) and having to go up a heat range than simply overlap-heat range issues, but it's only a quick fix and can lead to other troubles, like detonation obviously. Examples I've seen this on have much more overlap and less cubes than yours in situations where top end swaps were done either for more power yet or to make a wild combo more driveable using a faster port. Deposits in the chamber as well as heat and timing marks on the plug were fairly chaotic. The bigger, slower port needed a hotter plug to stay lit, timing beyond what would normally be considered max timing for that chamber. Really what overcame that was running the idle speed higher. I was attempting to run idle rpm near 900-1100. I didn't dyno the hotter plug in order to prove any suspicions of detonation. What makes me think yours might not be too cold is that you have a decent amount of vacuum and a fairly low idle speed yet. The gear drop amount also supports the condition of too little timing, you should be close to 20 initial anyways. If it died more frequently going into gear, that would also suggest plug too cold. **You should be able to steer it the same you would a carb...more timing and vacuum goes up, then less throttle opening for curb speed...less rpm drop going into drive. You should still see an improvement irrespective of the minimum plug temp needed to keep the fire lit. A temp gun at the exhaust is quite helpful.
Do you have any exhaust leaks BEFORE the sensor? If not...then with that much overlap, AFR can be misleading at idle. Have you tried adjusting your idle fueling to the greatest idle vacuum? I also agree with the timing statements above. I run my wideband on my buddies car that has a flat tappet hydraulic cam with 20° of overlap at 0.050". At WOT the readings aren't trustworthy until high gear. That fresh intake charge dilutes the reading. I'm assuming once there is significant exhaust flow that it is trustworthy.
Thank you for your advices. Timing is 27* idle and all in @2500 with 32* . (See timing below) I have two WBO2 Sensor, one is driver behind the collector flange ( approx 3")in the exhaust itself this one reads leaner than the other at Idle. ( This one is used for the Holley ECU) The second is a Innovate LC2 in the passenger header, close before the header collector flange. I will try to make few degrees more advance until i reach the highest vacuum.
I would think 27 at idle should be enough timing, I just did not want to hear anything like 10 or 12. Your vacation at 14" is decent for a carb, it could be the holly system is seeing this as actually a load and adding more fuel than needed. I don't know how close the system is to other systems used by gm and ford.....but on those system 14" of vacation would be looking like a load on the engine adding more fuel...... What does the map read with engine not running. On most gm early systems this number would show something like 28" once started it might read say 10" this would show the engine making 18" at idle. If you closed the throttle at 3000 running down the road you might get a reading in the 6" indicating 22" in the intake. As the reading gets closer to the key on reading this wold had been seen as more load and give more gas. For an easy test you could take a handheld vaccum pump and hook it to the map sensor and plug your hose, pump up to 18 or 20" and see if your smell goes down or away. If so your getting the computer to comand the injectors to the stay open too long. Also look at the ms the injectors are open now as compared to when the extra vaccum is apied to map sensor. You might have to remap the table and shorten the ms up at the vaccum level your car is idling at. Keep in mind here a little change is alot........the difference from 2.3ms to 2.1ms is almost 10%, that's a big change. Your fuel pressure will also greatly effect the amount of fuel deliveried. 45psi at 2.3ms will deliver more fuel than 35psi at 2.3 ms. As will injector size the lbs listed makes a large change in outif comparing like a 21 to a 30, the size of your injectors can't be change without spending more money. But things like fuel pressure can. Is your regulator a vacuum sensing style, meaning that the psi at idle drops with the addition of vacuum if so verify it's actually dropping when the vacuum is applied. I would assume you should see a 10psi or so drop in pressure at idle as compared to just a full pressure reading I would assume thesee are all things you have already look into, but it's info not posted in your first post and might help us find you an answer
The Holley system has a 4 hole throttle body like a carb. My intake has 8 injectors on the ports. The fuel map is fully adjustable, I just put the desired AFR in the cells vs. MAP & RPM that table gives values to the fuel map and correct's them to the right values. The fuel pressure regulator runs with the vacuum = 31 PSI idle and 42 PSI at WOT. Holley recommends a pressure of 43 PSI, so when the idle is just 31 PSI can the spray pattern from the injector be to bad? Spark plugs were NGK resistor 6 and they were getting to hot so I recently replaced it with non resistor NGK 7.
Spray pattern could be bad, but since you can't see that without basic taking the injector someplace and having them flow balanced. A lot of machine shops can do this and ultra sonic cleaning them. I don't think this is your issue. So your only inputs is the afr wanted @ rpm and the map reading? Does it let's you see what ms of open time the injectors are firing at? If so maybe try adjusting the table inputs till you get a lower ms of opening time. When I worked years ago on an accel mpfi system, I could select a group of cells I wanted and there was a way to change the fuel deliver to just those blocks by percentages. Meaning I could select them and change the levels by adding or subtracting 10 or 15 or 20% and not change anything else and not have to do it cell by cell Afr reading at idle can be very unstable. When first started an O2 sensor reading are junk till the sensor get in in the area of at least 600*f plus. Does your sensor run a heater it for the holley? I would assume it would, being that far down stream it would take awhile to get warm and be hard to stay warm. But afr reading on a O2 sensor not up to temp are worthless and not accurate. You said you have a second sensor in the other side and it read's richer than the holley one.......what does it read for afr at idle? Most efi systems also use a coolant temp signal...this basically acts like a choke on a carb. When it reads lows it gets more fuel added till warmed up. If for some reason yours is not reading correct or missing that can cause major over rich at idle. Seen several go bad and read like -40*f causing similar symtoms Efi that manly rely on the map sensor are tougher to get adusted than ones that use maf. Especially if your idle vaccum is choppy.......meaning if you put a gauge on it you see the needge vibrating or sway in up and down. All those movements you see in the needle with be the same as the computer trying to take and add fuel to the verify load inputs it's seeing .......as where with maf your actually reading the volume of air coming into the motor as your main change for fueling.
