EFI tuning with MS3X – light throttle surging

[billjam]

After getting some really great help from several members here, I am quite happy with my tune … power seems great and engine starts well, however I can’t eliminate light throttle surging between 2000 and 4000 rpm at 25-35% fuel load.
Actually, that’s not quite true, I can eliminate it by making it richer in the problem zone, but then I get extreme exhaust popping and banging on over-run. Leaning off the mixture in this area reduces (not eliminates) the exhaust popping but brings back the surging.
I don’t mind a bit of exhaust noise, but it really is a bit anti-social and it does attract unwanted attention around town.

Question is … is ignition timing a factor in dealing with the surging and exhaust popping?

So far, I have concentrated my efforts on the fuel table, but I suspect that there may be something to gain by fiddling with the ignition table.
Current ignition and fuel tables below. Engine is 3.6 with MS3X, ITBs, sequential COP ignition, sequential injection and DC21 cams.
Happy to share the msq file, just email me.

[JohnJL]

Hi Bill,

Ignition timing makes a big difference, and creates an iterative cycle where you will find yourself going back and forth between the fuel map and ignition map ad infinitum. Enjoy!

You might try retarding the timing a few degrees where you are getting the surging. I like to make big-ish changes to detect if that is the problem area, then refine it when I have pinpointed the area and direction of the change. So maybe retard the ignition map 5 degrees in the problem area and if the surging goes away, add back in 2 or 3 degrees and see if things are still good.

John

[cmonref]

Billjam -- great to hear you're up and running and getting on with the tune! The sage continues.

A couple of comments regarding the posted tables:

1. On the timing table, methinks your timing at idle is much too high. More like 5 degrees and maybe up to 10 degrees. Starting with a lower idle timing opens the throttles slightly at idle and enhances tuning of idle. It also allows idle control to catch RPM excursions at idle. Timing is used as the first and quickest reaction to RPM changes at idle, and works wonderfully. Throttle control at idle is much slower and coarser than timing control.

2. On the VE table, you apparently have tried to introduce fuel cut at high vacuum -- overrun -- as a means of combating the popping. You will probably find that a more gradual drop in VE table below idle will be beneficial -- but not all the way to zero. Use fuel-cut at overrun to control the highvacuums encountered during decel.

3. On the VE table, the table values around idle -- up and down and left and right one cell each -- should be the same so that the idle moves around a LITTLE bit without changing the mixture. Only <=30% and <= 1000 RPM meet this criteria -- is this where your idle is? (With your ITBs, isn't it much higher than that?) Idle is much stabilized with these values. And closed loop idle then controls timing to make it even more stabilized, if you need it.

The VE values that you highlighted -- 25-35% and 2000-4000 RPM -- don't look right. The dropoff below 35% just can't be right and is probably the cause of your popping. You have tuned, and apparently lowered, these values to get rid of the popping, but I'm thinking a more gradual reduction in those values below the 35% line would be better. Values of 20-15-10 at 30%-10% would be a starting place.

Good luck! Looking forward to hearing of your progress!

Brian

[billjam]

Managed to play around with the tables on the way to work today to try out some of your suggestions.
Managed to clear up the light throttle surging with a bit of extra fuel in the area below 35% fuel load. Been down this path before, so I expected that result.
However, exhaust popping is now much worse despite significantly retarding the spark in the same zone.

What actually causes exhaust popping - actually, they are quite loud explosions!
Obviously it has to involve unburnt fuel exploding in the exhaust system, but what ignites it? It happens whether the exhaust system is hot or cold.
Could the design of the exhaust system be a factor?

[Tippy]

This is a twin plug engine, right?

Idle seems a little too advanced and everywhere else seems pretty retarded.

Of course, don't know your fuel quality/octane nor compression ratio.

Are you speaking of fuel decel or overrun? If decel, I leave that at 100% because manual cars are severely responsive to fuel swings making for an erratic driving condition.

Mine too has light surge with nearly 50* timing (7.5:1 CR mind you) at those load areas and lean AFR's.

Doesn't really bother me because it allows great MPG's.

[billjam]

Originally Posted by cmonref

2. On the VE table, you apparently have tried to introduce fuel cut at high vacuum -- overrun -- as a means of combating the popping. You will probably find that a more gradual drop in VE table below idle will be beneficial -- but not all the way to zero. Use fuel-cut at overrun to control the highvacuums encountered during decel.

Brian, can you steer me to where the fuel cut on over-run can be set in TS? I have looked everywhere but can't find it.

[billjam]

Originally Posted by Tippy

This is a twin plug engine, right?

Idle seems a little too advanced and everywhere else seems pretty retarded.

Of course, don't know your fuel quality/octane nor compression ratio.

Are you speaking of fuel decel or overrun? If decel, I leave that at 100% because manual cars are severely responsive to fuel swings making for an erratic driving condition.

Mine too has light surge with nearly 50* timing (7.5:1 CR mind you) at those load areas and lean AFR's.

Doesn't really bother me because it allows great MPG's.

Yes, twin plug. 11:1 compression and fuel is 98 octane.

The surging problem isn't really a decel or over-run issue, it occurs on very light throttle at steady speed anywhere between 2000 and 4000 rpm, usually down a slight incline.

I don't understand what you mean by "If decel, I leave that at 100%". What is set at 100%?

[Tippy]

With MS2 (not sure with 3), there is a setting for decel that starts at 100%. If you move it to 95%, it takes whatever bin the "bubble" lands in when let off the throttle, and takes away 5% of the fuel to lean the mixture. Too much, and the slightest throttle movement makes the engine very choppy.

[cmonref]

Billjam,

Overrun fuel cut is on the Fuel Settings menu. To be honest, I have not used it yet and can not offer any advice. It is on my list to read up on and to try -- but not yet. That said, I do not have any decal/overrun popping and I do not use Overrun Fuel Cut.

As to exhaust popping, it is definitely unburned fuel. I have understood it to be the result of a too-lean mixture that is not fired in the chamber. I'm guessin' that the heat of the exhaust pipe sets it off after the exhaust valve, but the explosion is a pop rather than a backfire. A too-rich mixture produces the more-violent backfire. Take this with a serious grain of salt, please. I DO know, however, that I don't get any popping and I put a lot more fuel into low-load high-RPM decal events than you do.

Brian

[cmonref]

Billjam,

Here's my ignition curve. Note the low point at idle (950 RPM), higher at lower RPM, and then rise quickly as RPM increases. The "bucket" at idle tends to make the idle self-correcting. This is single-plug and I mirrored the stock ignition curve early in the run process and have never worked to increase the advance in the right side of the curve. The car drove with this stock curve in the original distributor for 30 years before I installed the MS3, so I KNEW it would work ... The lower advance at MAP>=90 is also seen in the OEM curve so I repeated it here.

My original OEM spec for idle ignition was 5 BTDC so that is what I have gone with. I do run closed-loop idle but the idle was very stable before CL Idle was invoked.

My only unstable region is what I call "parking lot cruise" at about 2400 RPM and load just above idle. It wants to buck rather violently. Needless to say, I avoid that region. I have played with it, 'round and 'round the parking lot, and think it probably relates to the rate of change in the advance curve. I can move the point around by varying the advance to make the curve flatter where it bucks.

[billjam]

I have made a bit more progress with a little help from some friends. Thanks guys, especially Brian.

Firstly, I tried JohnJL's suggestion to try tuning out the popping and backfiring with more retarded timing. Sorry John, I can't get any improvement with that approach.
I even retarded the timing so that it was after TDC and it still didn't make any difference.

I concentrated on getting rid of the surging at light throttle by adjusting the fuel table and got it running quite well with only an occasional hint of surge in some 1st and 2nd gear low speed conditions. I did find that the most important factor was to make sure that the table values were very close in the region where surge was likely.
The side effect of this was some extreme popping and backfiring on over-run because the low load region of the table was now richer. I guess more fuel = bigger explosions, although I still don't understand what actually provides ignition for these explosions.
I eventually found where to set the fuel cut on over-run in TunerStudio (it is at the bottom of the General Settings screen). With fuel cut activated, most of the popping and banging disappeared immediately.
I can still initiate backfiring if I lift off but keep the throttle at just over 3%. I might try and raise that threshhold setting to 4 or 5% to see what effect it has on drivability, but at present it is surprisingly smooth in the transition from trailing throttle back to light (or heavy) throttle.
I suspect that playing with some of the other settings here might help eliminate popping and backfiring completely, but I don't know enough about these fuel cut settings yet.
Tippy, I can't find any reference in TunerStudio to fuel decel setting. Sounds like it could help, but maybe the fuel cut function deals with it now?



I have looked at ignition maps from a number of sources and it is surprising that there isn't more corelation between them. Look at the OE maps on Steve Wong's website for 911, 964 and 993 for example - they are up and down all over the place and difficult to replicate.

My current ignition map now looks like this and seems to work OK, but then as I may have mentioned, I've found that you can make significant changes to timing in most of the range and it's very hard to feel any difference. I haven't yet attempted to tune the advance manually on the road 'cos I don't have any suitable locations nearby, but it seems that the main benefit to getting timing set correctly is fuel economy.

[JFairman]

I'd try overrun fuel cut when rpm is greater than 1500 rpm and see how that works. That's where bosch had it in the old L-jetronic analog EFI used in late 70's six cylinder BMW's.
Just a thought and my only experience with EFI is fooling around with the old L-jetronic.

[jpnovak]

Bill, do you have any datalogs of this area? I can help you fix them.

Here is usually what happens. When you run lean with ITBs you get a lean pop due to insufficient spark advance and incomplete combustion. When you run lean (light throttle) the fuel molecules are farther apart and need more time to burn. This means you have to ignite them earlier in the crank cycle.

The lean pop creates a surge to the MAP sensor. The ECU then sees a quick oscillation to high load and injects fuel. This fuel can not burn because it does not have enough air (remember, the throttle is closed). The leftover fuel then burns in the exhaust. Its a quick cycle that only changes when you change throttle position or rpm.

Usually you can see these transients in a datalog.

So, richen the mixture at that load point so that your lean intake pop goes away. Then advance the timing even more at the low load, closed (and nearly closed) throttle inputs. This should take care of the problem.

Also, check for exhaust leaks. If you have big backfires in the muffler this is usually a sign of an air leak.

[scarceller]

Just saw this thread so a bit late.

What looks wrong is the ignition table, generally you increase ignition as load decrease. Your low load sections don't look correct to follow this practice. For example just looking at your 4000RPM col. we see that at 73% load you are at 29deg ignition but yet at 7% load your way down at 25deg ignition that seems wrong. If the engine can tolerate 29 deg at 73% load why would it not require at least the same at 7% load?

You really need to get it on a load dyno, lock in at 4000RPMs and tune a few of those cells for peak torque and you'll find that at the lower load cells you'll need more ign to achieve peak torque.

Then the popping in the exhaust is because at these very light loads or coming off load you don't have enough timing. The reason it's popping is because the timing is way to late and results in the mixture still burning when the exhaust valve opens. You need more ignition advance not less to reduce the popping.

Your ign map looks wrong to me in the low load regions. I custom tuned my map on a load dyno in my 3.2L and it looks like this:

The rows are inj pulse width and 2.0ms is very low load while 14ms is very high loads near WOT

You have a twin plug so your ign values will be lower but the basic principles don't change, more ign adv at low load than at higher loads.

I also have some surging at light loads, parking lot crawl or slow traffic. I have not been able to totally eliminate this issue but leaning out mixture and adding ign seems to help.

I'm assuming that at cruising conditions you run 20-40% load and looking at your ign table I'm betting the values are set to low. It's important to get this section of the map correct because it has direct effect on Fuel Economy as well as exhaust temps. The basic idea is to move the car down the road at a given speed with the least amount of intake air, this means you need to achieve peak torque for the give unit of intake air.

[Lapkritis]

Decel fuel amount is in the acceleration wizard screen. When you lift the throttle quickly this rolls back the injector pw. Try 95% to start and tweak to taste; if it's 100 then you can over-fuel on decel/throttle lift and get funky behavior such as over-drive (car keeps pulling), fuel loaded exhaust, misfire/doggy response for initial return to accel.

[914efi]

I would look at the large change in fuel from the 20% to 25% and 25% to 30% in the RPM range you highlight. cmonrefs # 2 comment is same as my thought.

You have big jumps in fuel there and as you cross those load points you get a big fuel change, engine speeds up, maybe timing changes also when this happens, load drops and it goes way lean again. I remember having lots of problems like this when I built an EFI system. Even now my CIS injected car does this, but only really bad when cold. It gives you respect for carbs!

Timing probably does not matter much at low load and low RPM so you could make those values all very constant so it can't jump around.