Determining best power ratio without a dyno?

[E4ODnut]

How can you determine where your best power ratio is when you don't have access to a dyno?

From anything I've read, and I've read a lot for a long time, best power is usually between about 12:1 and 13:1 on gasoline, but can be richer, depending on the engine. The other thing is, how accurate is my WB over the complete range of mixtures and exhaust temperatures. Rather than use a theoretical number displayed by the WB I try find out what the engine likes, but this is difficult to do for full load at all speeds.

What I have been doing is to use my idle speed to determine this. I pick an idle speed that I want to use, say 750 RPM, then adjust mixture and timing to get the highest idle speed. It means closing the throttle stop a bit as I get closer to optimum. When I get to where I can't make any more improvements, then whatever number is displayed on the WB is my best power. This isn't my final idle setting though, because I've found that I have to take some timing out to keep the idle stable, which affects mixture.

Now, the interesting thing is that my 300 and 460 both have older TechEdge wide bands. The both seem to like about 12.9:1 for best power, give or take a couple of decimal points. My 351s, on the other hand, have LC1s and like about 12:1, give or take.

Why the difference? Is it a difference in WBs, a difference in the engines, or placement of the sensors, or is my logic flawed?

Comments anyone?

[ol boy]

cylinder head and intake design. Head gasket thickness and how far down the hole the piston is when at TDC. You should notice a difference in timing need to keep the motor happy too inbetween two different designs.

[Peter Florance]

when I dyno tune, I find a lot of engines are pretty forgiving once I've found the best ratio (+- .5 to .745 AFR)

I know that doesn't help, but it does point out that you could reach max power on the edge of the above range.

[E4ODnut]

Peter,
Yes, this is what I see too. I find it interesting that I have about full point difference between the 351s and the other two engines. I don't know if the WBs are accurate or not. It really doesn't matter in the big picture, because it's only relevant to the particular engines, but it puzzles me.

The 351s are in a boat, so tuning is a bit easier, almost like a dyno. What I do find on them is that there doesn't seem to be much difference in power between about 11.8 and about 12.5. It drops a bit to 13:1 then falls of quite dramatically. I have to be about 11:1 or less before I start to lose power on the rich side, and then it's not all that much.

[Moving_Target]

I have an old Robert Bentley book "Bosch Fuel Injection & Engine Management" by Charles O. Probst, where Chuck mentions the power curve on the rich side of stoich is much flatter than
on the lean side of stoich.

[Peter Florance]

Peter,
Yes, this is what I see too. I find it interesting that I have about full point difference between the 351s and the other two engines. I don't know if the WBs are accurate or not. It really doesn't matter in the big picture, because it's only relevant to the particular engines, but it puzzles me.

The 351s are in a boat, so tuning is a bit easier, almost like a dyno. What I do find on them is that there doesn't seem to be much difference in power between about 11.8 and about 12.5. It drops a bit to 13:1 then falls of quite dramatically. I have to be about 11:1 or less before I start to lose power on the rich side, and then it's not all that much.

Timing usually works the same for me as well. As long as I'm not octane(detonation)-limited (rare with the mild setup's I'm tuning)

[E4ODnut]

Ah yes. Timing. Perhaps this should be the start of another thread, but for now because it's all sort of related, I'll just leave it here.

My engines are all stock, low compression, low tech and fairly forgiving. At full power I just end up reverting to the old stand by of "advance 'till it pings then back off a couple of degrees" mantra. I don't like to stress my engines so I'm not comfortable with being at full power very long, but I haven't noticed any difference in power by retarding a bit to get rid of the ping. I can't be sure though. Also, it's a bit hard to detect ping with two engines running at full song in an enclosed engine room. I do have KnockSenseMS sensors fitted with inputs to MS. I have code to trigger an alarm so I can data log it, but no ignition retard feature. Because KnockSense has to be fine tuned for each application, I'm not sure if the signals I get are legitimate or not. I need to do some more field work on this.

I'm finding part throttle timing particularly hard to nail down optimum. I lean things out a lot at part throttle for fuel economy. I've found that I can move the timing a lot either side of the "centrifugal" point, with little if any difference in power output. It does seem to affect the EGO reading a bit, but not much. This is hard to determine for sure because my EGO tends to drift a bit, a few decimal points under steady state conditions. Again, more field work required.

The fun never ends.

[Peter Florance]

Ah yes. Timing. Perhaps this should be the start of another thread, but for now because it's all sort of related, I'll just leave it here.

My engines are all stock, low compression, low tech and fairly forgiving. At full power I just end up reverting to the old stand by of "advance 'till it pings then back off a couple of degrees" mantra. I don't like to stress my engines so I'm not comfortable with being at full power very long, but I haven't noticed any difference in power by retarding a bit to get rid of the ping. I can't be sure though. Also, it's a bit hard to detect ping with two engines running at full song in an enclosed engine room. I do have KnockSenseMS sensors fitted with inputs to MS. I have code to trigger an alarm so I can data log it, but no ignition retard feature. Because KnockSense has to be fine tuned for each application, I'm not sure if the signals I get are legitimate or not. I need to do some more field work on this.

I'm finding part throttle timing particularly hard to nail down optimum. I lean things out a lot at part throttle for fuel economy. I've found that I can move the timing a lot either side of the "centrifugal" point, with little if any difference in power output. It does seem to affect the EGO reading a bit, but not much. This is hard to determine for sure because my EGO tends to drift a bit, a few decimal points under steady state conditions. Again, more field work required.

The fun never ends.

I find a lot of my engines won't ping until they are way too advanced.

[Matt Cramer]

I find a lot of my engines won't ping until they are way too advanced.

Very true. Too much advance is just as bad for power as too little - even if it's not pinging.

[racingmini_mtl]

The only thing you determine by finding at what timing ping occurs is that, the timing where ping occurs. It has nothing to do with optimum timing.

Jean

[E4ODnut]

Interesting! My 300 came with factory EECIV and a knock sensor, about 8.5:1 compression ratio. With MS, about 12.5:1 AFR indicated, E10 gasoline at about 2800 RPM WOT I'm down to about 13 degrees advance to keep the pinging away.

The 351s were carburetted, 8.3:1 compression ratio. On MS, about 12:1AFR indicated, straight 87 octane gasoline about 3600 RPM and up I'm getting signals from KnockSense MS at 26 degrees advance. I don't know if they are legitimate or not.

This kind of timing doesn't seem like it should be excessive to me, but perhaps it is.

[racingmini_mtl]

No one knows if it's excessive or not and measuring ping will not tell you. The only thing it tells you is that it pings at a certain timing under those conditions. This timing could be more or less than the optimum timing (MTBT) but you can't know without doing some torque measurements and/or cylinder pressure measurements.

Jean

[Nigep]

I mapped my engine on the road. Set the timing with a set of det-cans until I could hear pinking, then backed it off a couple of degrees.

When I was finished, I took it for a final tweak on the dyno. We took 5 degrees off the timing and it made less than 1hp difference in power all the way along the curve. Any more and the drop was noticeable, but it showed me that backing it off until it is almost pinking was far too much on my engine. (VW 2.0 16V ABF on ITB`s)

[Sellig78]

Hi!

For low load, the optimal advance is before the knock limit, for high load it's after the knock limit, that's why high octane fuel are very helpful. At very high rpm in general we consider that the knock disappear because of the rpm, you burn before that the detonation has time to appear.

Determining the best power without the dyno? Maybe with an accelerator sensor, but there are still some area in your table that are impossible to run on the road.
If you want he best power on all your advance table use a dyno, you will enjoy a lot this experience.

See you!

Gilles.

[Matt Cramer]

Hi!

For low load, the optimal advance is before the knock limit, for high load it's after the knock limit, that's why high octane fuel are very helpful.

That really depends on the engine. Some engines have the optimal advance well before the knock limit even at high load. A stock Miata motor on 93 octane, for example, is pretty hard to make it ping without advancing timing to the point where it loses power.