Dougs951S

I think for awhile I may have been running way too much timing on my car. I switch off betwen E85 and 93, mostly corn but I run pump when its all I can get. My corn map is pretty dialed in, my gas map, perhaps not.

I had been running ~22* at peak tq, with a pretty normal looking gas curve. Today I filled up with pump and pulled about 4* out of every cell over 3k rpm and above about 8 psi, and it definitely pulls harder. I'm sitting at 18* at peak tq, under 23* above 11.5 psi, 27* at about 8 psi, and 32* at 6 psi. Seems to like less timing better.

Thom

How can you compare two ign maps without using the same fuel?

Dougs951S

You cant. When was I comparing them? My e85 map has a completely different shaped advance curve. Like I said, the map and all the numbers I posted were with respect to a 93 pump map.

Tom M'Guinn

I thought you changed fuel too. What prompted you to reduce the timing? If you gained power doing nothing but reducing the timing, then I'd worry you were knocking with the extra advance and losing power. Do you have any way to detect knock? How are you tuning? Your best bet would be to find a load-controlled dyno and tune each cell -- adding advance until you peak power then back off a tad (to wildly oversimplify)...

67King

Coupla thoughts, Doug. First, and you know this, but a reminder. Seat of the pants can be misleading.......placebo effect and all of that. Second, given that it is so hot out, you should be more knock limited. These engines have big dang bores, which is bad for knock tolerance. I want to think you have an aftermarket PCM......do you have an active knock control?

Power usually will become flat at a certain point. "MBT" is the term used in industry, it stands for "Minimum timing for Best Torque." MST is the same, but specified (i.e. part throttle). Taking out timing, and gaining power is not typical unless one is knock limited.

Dougs951S

Tom, I usually run E85 but occasionally when I'm out of town I can't get it, so I run dual maps. I do suspect also that I was knocking with my previous over advanced pump map. I do have knock control, but its been acting a bit weird, I think I still have to dial in the filters to get it to respond properly. I get tip in knocks but nothing under WOT which is really pretty curious because 23* at peak tq is a LOT of timing to be running on pump fuel. Despite what the ecu says, I'm fairly certain it was knocking. The car desperately needs dyno time to fine tune everything.

The ECU is a megasquirt. I have a theory Harry, feel free to tell me I'm wrong if I'm way off base, but I think its possible I was over advanced past MBT in the upper rpm-boost cells. I say this with the full implication that it was probably absolutely knocking. The butt dyno can't be trusted, but is it possible that that is true? I pulled timing really just for peace of mind. On the other hand, on my ethanol tune I have still yet to find MBT, I keep adding timing slowly and it just keeps getting faster.

Dougs951S

double post

67King

To be honest, I cannot say definitively. Thinking about the combustion process, and all of that, I suppose it could be......but it usually means you break parts. 4 degrees is kind of a lot, though. But you'd "ideally" like the crank to be so far past TDC to get the best leverage to have the cylinder pressure push down on it - an analogy would be where the pedals on a bike are when you are just starting out. If they are too high, you can push hard, but you aren't really able to get it moving until the pedals are down a way, say 15-20 degrees past vertical. So I suppose that if you are too advanced, you may be the equivalent of 10 degrees, and you are losing some power in the form of heat until your crank rotates enough for the pressure to be usable.

I know a lot more about fuel. LBT (Leanest for Best Torque) is around 13.0:1, it stays flat until about 12.0:1, then you start losing power after that. But it is more common to adjust that than it is to adjust timing......like I said, bad things happen when you get too advanced. But in the ignition world, you put in 108RON fuel to make sure you aren't knock limited, then advance until you see no gain, then set your ceiling with that.

Dougs951S

Does the corrected table I put up in the first post seem ok to you? I think its probably a safe tune now, as for AFR's I'm seeing 12.5:1 high load vacuum, 11.7:1 in boost. Again this is on pump 93, my ethanol map has had a LOT more driving time and is much more dialed in.

Tom M'Guinn

If you advance the timing until the no-gain point with 108 octane, how do you know if that level of advance is within the knock limits of lesser fuels?

Dougs951S

Quantitatively you can not, but its a very good assumption that its not. E85 burns slower as well as being higher octane, these are qualities that cause MBT to vary with respect to the fuel used in the same engine. MBT on E85 would knock on pump fuel, in most engines I suspect.


Off topic edit: Harry, with respect to RBT being at ~11.8:1, I assume then its no coincidence that I get the most stable idle (lowest manifold pressure) at 11.8-12:1? Its significantly better than 12.8 or 13:1. Does this imply that assuming we had 130+ octane fuel and detonation was impossible, that an engine makes peak power tuned for rich best torque vs lean best torque?

67King

You don't. That isn't the point. The point is to determine optimal ignition timing of the ENGINE without any limits. All of those points will be determined wiht 98RON or whatever fuel. But to determine the upper end, you run it with super high octane stuff so you don't risk blowing up your several hundred thousand dollar prototype engine!

Dougs951S

thanks for the info harry, seems I misunderstood that. I was under the impression that MBT was not a hard function of engine parameters but varied with fuel, IE MBT would be higher for two motors but one on 93 and one on race gas. I understand what you are saying now. MBT is fixed, you just wont ever get there without blowing it up unless you have high octane fuel.

67King

I owuldn't expect it to be related. What size injectors do you have? Could it be that while idling the injectors are dipping into the non-linear region at stoich, but become mroe stable at richer mixtures?

RBT and LBT would be the same point.......BT being "best torque," and from stoich, you enrichen until you stop seeing gains to find LBT. Then you keep going, and it will be hte same, until you start losing power, at which point it is RBT. But RBT is not really even a term that is used, because it doesn't matter too, too much. That said, since turbos use enrichment for charge cooling to give robustness against detonation, it probably is worthy to know. I ran my old turboford at 12.0:1 (factory was just uner 10.0:1), but my EGT's were so hot that I destroyed a turbine housing in about 10,000 miles.

Dougs951S

I see. You are really giving me an education today! So basically RBT is just the "richest" and maybe "safest" ratio you can run while still making best torque. RE: the injectors, they are 830cc high impedance, and the most accurate data I could find says the dead time is 1.14 ms at 13.2 supply volts. FWIW, I'm running batch fire injection, 1 squirt providing all of the cylinders fueling needs. IIRC my pulsewidth at idle is ~2.68ms @ 2.1% duty cycle. FWIW I pull 38 kpa when warmed, and like 45 kpa at a cold start, which is better than most guys here see I think. I dont think my 75k miles motor is going to pull much more than 19" no how well the idle is tuned.