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Questions about Timing and AFR's

anwat

anwat

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Looking for a little help understanding AFR's. I'm setting up the tune on my FiTech, and am reading pretty consistently, that "big blocks like to run a little rich". That doesn't make sense. Other places, almost all of them, are saying you need to set the AFR up to your individual engine, some run better rich, some better a little lean. Here's my question: From a math and chemistry standpoint, 14.67 to one is ideal for gasoline. Now, I understand that there are a ton of variables here, such as air density, gas additives, even humidity. But generally speaking, when you lower the ratio, making the mix more rich, does that produce a more powerful explosion in the cylinder? Is that why they say you want to run rich during acceleration? Seems to me ideal would be ideal, meaning you're using all the gas and all the air, meaning you should set it to 14.7:1 no matter the rpm or map load. But obvioulsy this isn't the case. At some point, on the rich scale, though, you run into a mixture that won't ignite. Same going on the lean side. How much of this is science and how much is hot rod legend? What type of ranges do the OEM's set up in thier tables, and is there any way to find out, or is that a closely guarded secret? If anyone knows a good article, point me to it..I understand this is a lot to explain in a post, but I'm just not buying that there is that much of a variation in "ideal'. If you can burn more fuel, and more air, then that's ideal, but either exiting the cylinder is going to be less than ideal, right?
 
Stoichiometry is chemically balanced, but that's not necessarily ideal for the operating conditions. Peak torque is generally richer than that (maybe 13.5) and peak economy can be somewhat leaner. You also have to consider operating temp and air temp. Some engines can do light cruise up near 16.0 before starting to misfire. Generally, on decel, a modern car cuts fuel so hard that the O2 signals go away entirely. Also, maximum resistance to detonation happens at even richer mixtures, like 12.5. As you tune a boosted engine, the WOT tuning gets really rich for safety, even down to 12.0. As you lean it from there, you gain torque, but put the engine at more risk.

Generally, cars are set for 14.7 in the majority of the fuel map. It's at/near WOT they will push richer and at very light loads let it go leaner. Idle is kind of a wild card. A lot of factory tunes are 14.7 at idle for the sake of emissions but idle much smoother a point richer. There's nothing wrong with starting with 14.7 everywhere except WOT and play with it in different regions to see how it responds. Factory maps are probably the best place to start, but I don't know what the best example would be unless your FiTech is a throttle body. If MPFI, maybe the 8.1L map, but that has much different heads and intake. Actually, I don't know anything about your engine.

Keep in mind 14.7 is correct for gasoline, but not E10, E15, etc. which have richer stoichiometry since Ethanol is 9:1. Unless it's a flex-fuel car, the ECM is just letting the LTFT drift a bit to account for the seasonal changes in blend.

Will you have WBO2?
 
If you sign up on the HPTuners website, there is a repository of fuel and spark maps there. Keep in mind those are the very simple things and tons of things affect the actual AFR besides the number in that map. Every time the load or throttle changes, AFR is swinging all over and there is a lot going on in the ECU to predict and manage that. It's not just a matter of waiting for O2 signals and adjusting in closed loop like some people think. That's way too slow for real-time control. It's for longer-term adjustments.
 
Which FITech do you have? Are you using it to control spark also?

You can fool it into thinking it needs more fuel and more importantly more power enrichment by telling it that it has a bigger motor. Deceleration tuning is pretty important on a FITech TBI.

Depending on timing, the most you'll probably ever need is about 12.8:1. If you have a lot of base timing and aren't using the FITech to control the timing, then maybe 12.2:1. This is because of having to tune the weights and springs on a HEI distributor is a PITA so you don't end up with too much timing on a WOT roll from say 2nd gear.

Air fuel ratio is a target function based on a ton of things. More fuel means a cooler charge which means you can have more advanced timing. However, the leaner the more power you can make.
 
What is interesting is on a V8 each cylinder sees a different air fuel ratio normally. There are some Engine Masters episodes where they show this.
Then the deal with the fitech (tbi version) is it has one O2 sensor which is reading off one Bank so you have to hope that it's compensating for both sides of the engine.
Just run it in self learning mode for a while and it'll do fine.
 
OK, so good info there. Starting with the newest first, let me address what was said. Problem with the self-learning mode is that it isn't actually learning to run most efficiently. It's only learning how to keep as close to the parameters you have set up. So the setup is what is important, the ECU will then "learn" how to keep it close to that. And yes, one 02 sensor, so I'm just hoping the other four cylinders are behaving similarly. Looking at the plugs from the other side, it would appear so.

I have the Go EFI 600 (30002), and yes, I'm using it to control the spark through a MSD 6al box. I'm a bit different from most, though, as I'll probably never see a WOT run. The closest I'd get would be to gas it up a hill or through some sand. I think the only time the RPMs have ever been above 4000 is when I had it on the dyno. And that made me nervous. I'm looking for the best tune for second gear, maybe 15-20 mph, but I need to be able to drive on the freeway without holding up traffic, too. So I'm trying to tune for the middle of the road, not doing any type of racing in my 3.5 ton blazer with 37" tires. I'm running 18 degrees of base timing, up to a total of 35 degrees.

I do my best to avoid ethanol, but sometimes I can't. The spot I go won't put more than 10% in there, but during the summer, you can't really avoid it entirely. On a side note, for those of you who think we're kidding about how expensive everything is in CA, Flex Fuel out here is 3.40 a gallon. Like the price of corn went up with the price of oil. Couple weeks ago it was 2.55.

What's a LTFT? And thanks for the HPTuners info...that helps. What I'm starting to realize is that you're setting this all up to predict what's going to happen. By the time you see the AFR, it's done, you can't go back and change it. So the reason you may want to set something up rich is because you're anticipating needing it richer based on what's happening. As MAP increases, and your TPS is on the way up, you anticipate needing more fuel, so you set things to go rich so you're not starving the engine. Which is why it's hard to get right...you never know what's going to happen in the next few seconds.
 
2.55...? There hasn't been anything here cheaper than $3.25 in a LONG time.
E10, E15, Or even E85...
Which it would surprise some people. I live in Iowa, and I only know of 1 gas station that I've seen that sells E85. And that one is a farmers Co-op place way out in the country.

I wanted to run high compression and E85 in my LS build, but the lack of availability killed that quick.

I've been wondering these same questions myself .
I'm running 12 at WOT, 14.6 at cruise, and it holds like 13.2 -13.6 at idle .
 
You might be overthinking it?

The only place to really make efficiency gains that generally aren't tapped into is "lean cruise". GM attempted it early on, but it was seen by the EPA as an attempt to cheat emissions (higher cylinder temps increases harmful emissions), so it was never used in production vehicles. But people have activated it via programming, and can pick up a fair bit. But that has inherent risk as well. Running lean can lead to knock, and in worst case scenarios melt pistons. GM had it setup to run lean cruise for x seconds when the right conditions were met, then back to "normal" for x seconds, and if conditions were still right, back to lean cruise, and so on. But again...it's lean cruise, not lean in general.

Dont think you should be limiting fuel where the engine needs it. An engine has a very narrow range where it's happy with 14.7:1 (and as mentioned, that's not right for the current fuel blends anyway). Even light acceleration needs more fuel.

As I didn't have a wideband when tuning, I could only watch the O2 under heavy throttle to ensure it didn't cross over to lean and make sure I wasn't hitting knock with a bit of margin. Then play with fuel and timing until I couldn't extract any more power out of it under heavy load. Lack of fuelling and timing is very clearly felt when driving. I recall a fairly steep section of highway that I couldn't maintain 60MPH without downshifting, until I got fuel and timing right.

Slowing down and driving smart is going to save you fuel. Tune the engine so it makes as much power as it can except under very light load conditions (steady state cruise on flat ground, where ~14.7:1 is the target) and call it good. A wide band O2 that can be data logged would probably make things far easier than it was for me. But still fun to be able to change how the engine performs with some button clicks lol.
 
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CyberSniper said:
Air fuel ratio is a target function based on a ton of things.
Click to expand...
I don't know anything about FiTech. It may not be as complicated as a factory ECU. Those have like 8 modifiers added (or subtracted) from the base spark table and likewise with fuel. Sometimes it's hard to figure out what the actual target is and which modifiers are pushing it under different conditions. Some tuners give up and zero out lots of tables. A factory ECU has to work in a huge range of conditions and across all build variation that will occur. A custom tuned aftermarket setup only has to work on that one car. The truth is few of them are actually tuned right everywhere, like from -50 to +140 degrees, all possible elevations, all load conditions, various fuel grades, etc.

anwat said:
What's a LTFT?
Click to expand...
Long Term Fuel Trim. These are the tables in a factory ECU that store the "learned" difference from the VE (fuel) table to where it actually hits the target AFR. They change by having Short Term Fuel Trim pushed a certain distance for a long time. I don't know what terminology FiTech uses, but it must have a target AFR table that it uses for learning, working the fuel table until it's mostly hitting those numbers open loop.
 
So like @Bent77 mentioned if you generally find yourself in an E10 area running 14.2 can be good. With E15 becoming super common now its a good compromise.

Power Enrichment AFR will be much different. This is what the ecm uses when the engine is under load and is generally anywhere from 12.1 to 13.0. 12.5 to 12.7 is common here.

The AFR the engine uses for 'cruise' or light throttle loads is the leaner value, and then the PE will kick in at certain throttle percents, cylinder fill percentages, or rpm etc... Not sure how fitech does it but there will atleast be the cruise AFR and the PE AFR. Running stoich under heavy load will cause detonation.
 
I hesitated to use the word efficient...and guess I shouldn't have. By efficient, I mean running as well as it can, not saving fuel. If I was worried about that, I would buy a toyota. I just want to have it running as close to ideal as I can manage...and from looking at this mornings datalogs, I think the fuel is about right...I have a few different options, idle, two cruise settings, and three wot settings. I set both cruise settings to 14.7, both WOT to 12.5. The computer uses a VE table to fill in the rest of the settings between. In driving around, my actual AFR only went below 14.7 a handful of times, and only for a second or two. most of the time it spent between 1Never passed 15, so I'd say I'm about right. Meaning the lack of power must be coming from the timing. So I'll start adding that in. The base is set to 18 degrees, and the max is at 33, I think It's running about 22 degrees at 3000 rpms. I'm thinking I need more advance, so I'll start adding that and see how my vacuum numbers look and how it runs. And you are right, it is nice to sit in the cab and just type in the changes rather than twisting the distributor and checking, twisting an checking.
 
Here's the stock map from a 1998 7.4L:

1713097916513.png

Of course, there are tons of modifiers in there for temp, baro, knock, etc, etc. To me, it's pretty lazy. I would expect a lot more timing in the middle of the table where you're cruising down the highway, but it would take a deeper dive to figure out what the actual timing would be after all of the modifiers.

There's also a table called "max torque timing", but I don't know if it's ever trying to push another dozen degrees or not :dunno:

1713098075583.png
 
The FITech controller doesn't let you change much. I had about 20 hours and a pair of Michelin LTXs tuning the FITech on a very healthy Cadillac 500. The part I liked about the FITech is you can store all the spare parts you need in in a lunch pail. That was my personal goal, as trying to get parts for a Cadillac 500 in a GMC Motorhome, or any help whatsoever, when on the road is pretty much impossible. I've only used a FITech "go4" on two SBCs and the one Cadillac 500.

I think I found that to get the best throttle response and cold/warm start characteristics I went with a 5% larger engine than advertised and either a cam profile of "2" if it idled at 700rpm or less or "3" if it idled above that.


Remember that the FItech is slow in the number of samples per second, so simply tuning off the narrowband is going to be tough. You can do it though.

What you are doing in the FITech is building a base map to operate off, and giving it parameters on what to do based on temperature and pressure. There isn't a lot you can do. It's not like in HPTuners. The spark table is pretty lame.

Do you have a fuel pressure gauge right at the throttle body? The FItech is insanely sensitive to fuel pressure deviating from 4bar.

Are you using vacuum advance to manifold vacuum? If so, how much total?

Lack of "felt power" is difficult to diagnose without being there. I assume you feel as if it has sluggish throttle response, or is it a dog still after about 4 seconds from tip-in?
 
Yeah, whether you're manually tuning or using self-learn, you want the speed and load of the engine to change very slowly or even hold it at operating points that aren't very well fleshed out. I like to use this steep, long hill near my house as a poor man's dyno. Getting all the way to WOT with slowly changing RPM is hard with a BBC, but it helps to be in 3rd gear. Of course you can't get too high in RPM without going crazy fast, but in the end there is a lot of interpolation going on between data that's well learned and less well and with a N/A engine you can estimate/predict past the range you have good data for.
 
Blue85 said:
With this system, how do you confirm/set the timing so the ECU knows where it's at?

I found somebody posting their maps here: https://www.chevelles.com/threads/timing-on-gen-vi-l29-454.324354/
Click to expand...
Tons of good info on that site...thanks. Also thanks for the stock map. The tune I had done on dyno is still working good for starting and idling...but once I get on the gas, it just doesn't go, and after leaning it out a little, I"m getting legitimate backfires through the exhaust, before it was more like a popping. I"m going to check my spark map compared to the posted info. I notice the one on the chevelle site has a lot more timing than the stock maps. I may be a little slow on this, project today is to id my heads and check the valves. I"m assuming some stuff I need to verify. The cam card I'm going off of, that was given to me by the seller is for a flat tappet cam. But a gen vi should have a roller cam. So something is off. The block definitely is a gen vi block, so I'm going to pull the covers and check for a casting mark, and while I'm there, I'll do a leakdown test and see if maybe this isn't a fuel problem. The oil on the threads of the last set of plugs kind of worries me.

I do have a good hill to use right down the street, but I think I'd better get the backfiring under control before I use it, there's usually a bunch of people walking up there to exercise, someone might shoot back.

As far as the poor power, the tip in is OK, it just never goes. It's sluggish at any rpm any load up after about 30 mph. Even when I downshift, it just keeps chugging along with the same acceleration rate, no change except in the rpm. My timing isn't being controlled by the vacuum at all, the dist. is locked out. Computer is controlling the advance. Only vacuum line I've got is the one to the PCV valve. Yes, fuel goes from the pressure gauge directly into the throttle body. No load pressures are good, don't know about the loaded pressure. Haven't been too worried, since most of my issues seem to be rich problems.

Magnetic pickup in the distributor is how the computer knows where everything is at.

One more day of work this week, then on Thursday I've got a buddy who knows a lot more than me coming over to help me figure this out...Thanks for the help and advice, there's no lack of knowledge here, except for my part!
 
So if you look at the timing on the computer and put a timing light on the engine at the same time, they match?
 
anwat said:
Tons of good info on that site...thanks. Also thanks for the stock map. The tune I had done on dyno is still working good for starting and idling...but once I get on the gas, it just doesn't go, and after leaning it out a little, I"m getting legitimate backfires through the exhaust, before it was more like a popping. I"m going to check my spark map compared to the posted info. I notice the one on the chevelle site has a lot more timing than the stock maps. I may be a little slow on this, project today is to id my heads and check the valves. I"m assuming some stuff I need to verify. The cam card I'm going off of, that was given to me by the seller is for a flat tappet cam. But a gen vi should have a roller cam. So something is off. The block definitely is a gen vi block, so I'm going to pull the covers and check for a casting mark, and while I'm there, I'll do a leakdown test and see if maybe this isn't a fuel problem. The oil on the threads of the last set of plugs kind of worries me.

I do have a good hill to use right down the street, but I think I'd better get the backfiring under control before I use it, there's usually a bunch of people walking up there to exercise, someone might shoot back.

As far as the poor power, the tip in is OK, it just never goes. It's sluggish at any rpm any load up after about 30 mph. Even when I downshift, it just keeps chugging along with the same acceleration rate, no change except in the rpm. My timing isn't being controlled by the vacuum at all, the dist. is locked out. Computer is controlling the advance. Only vacuum line I've got is the one to the PCV valve. Yes, fuel goes from the pressure gauge directly into the throttle body. No load pressures are good, don't know about the loaded pressure. Haven't been too worried, since most of my issues seem to be rich problems.

Magnetic pickup in the distributor is how the computer knows where everything is at.

One more day of work this week, then on Thursday I've got a buddy who knows a lot more than me coming over to help me figure this out...Thanks for the help and advice, there's no lack of knowledge here, except for my part!
Click to expand...
If you start another thread, I think all of us can help you get where you're trying to go. What are you using for software?

The reason for wanting to know what the fuel pressure is at its destination in realtime is because on a longer pull the pump can begin to not deliver. It's not usually noticeable until 6+ seconds in to a pull because the fuel trims will take care of any loss... but most of the time STFT is limited to 30%. A lot more fuel flows at 4 bar than 3.5 bar in a pulse width. It's even more wild on the DI engines. I've driven my wife's Equinox home a hundred miles on the lift pump (80psi) instead of the 450-1800psi high pressure pump.

This is the base timing I pretty much always start with, assuming if it has a distributor it is at TDC, if I have nothing to start with, assuming it is a modern camshaft not terribly far advanced, and I left the PE adder alone.

basetiming.jpg
 
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