You really need a buddy for this, but adjusting the timing on the fly is almost as good as a dyno. I have read many times that the hidden horsepower is in idealizing timing. On the way to Mason Dixon I set it for auto tune of fuel. I was working on the principal that I did not want to adjust timimg later on a lean mix.

We took the back roads both ways to give varied load and rpm conditions. But on the way back, my copilot Caroll Ripley manned the laptop and played with timing all the way home. Timing needed to be removed from the high load areas, above 85 kpa, but below that, in the 1500 and 2000 rpm range, 10 degrees or more were added without creating preignition in those areas of the map. If it was already 40, it is now 50. If it was 45, it is now 55. Inthe rpm's above that, the timing was already 55, but 1500 to 2000 is where I like to cruise.

This kind of adjustment is so easy with EFI. There is a small chance a cell or two may be a tad too far advanced, but there is a chance it can still go further. This kind of makes all the earlier struggles worthwhile. If you have EFI and haven't played with timing this way, I encourage you to do so.

Sam

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Sam,

Was Carroll watching the injector pulse width to make sure you didn't go too far? It's possible on lower compression engines to be too far advanced and not have detonation.

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Joshua

Josh has a good point, I've run about 55-56 degrees for highway in previous distributor recurve experiments and there's a point I had to upjet the Holley 1945 to combat pinging in my stocker duster at the time. I also had to limit my curve on the Hpak duster at 9.2:1 to no more than 54 if I was easy on it, or have to upjet the Holley 390.

-D.Idiot

What should we be looking for? How will the injector pulse width behave when you have gone to far? I know, slap you in the face. Sorry! Seriously, what will I see at that point. Thanks for the input. I think it was cruising along at slightly under 2 and slightly over 2 the entire time, until we came to hills, and then it went up. This was actually the area where I took timing out, as it absolutely went into detonation then. Thoughts? Your answer will be extremely useful information. I think everyone would like to know how to tell when you have advanced timing too much. I have been led to believe 55 is kind of the magic number for that.

Sam

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If the timing is over advanced there is a loss of power. Of course if you have a loss of power with a fixed load the manifold pressure and injector pulse width will both need to increase.

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Joshua

So without a dyno you are still in the dark. I was hoping there would be some observable feed back that would tell me when that loss of power happened. Seat of the pants tells me it ran better when finished. But, I cannot guarantee that of course. I suppose if you could control the conditions absolutely, you could watch the vacuum gauge, but there is just too much going on, and too many variables to see that.

Sam

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If you light the fire too soon the pressure will try to keep the piston from moving up the bore on the compression stroke. David Vizard calls this negative work. You'll see a loss of fuel economy and probably a hotter engine same as with late timing. Doing this kind of tuning while driving would be difficult at best. A dyno with a brake would be the best by far.

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Joshua

Because the 225 has a small bore-stroke ratio and usually likes 4-10 deg less timing advance than big bore motors (most US V8s) under WOT, I am of the opinion that anything over about 50 deg will not help you, and could hurt. For this reason, I run mine in the mid-high 40s at part throttle cruise, but have not experimented much above that.

Lou

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Home of Slant6-powered fun machines since 1988

I think I get what you are saying now. You can tell by the injector pulse width if the MAP has increased. Excellent idea. I will go play with it again with that idea in mind. I will have to find an area where I can hold the engine at steady map and RPM for a distance. I do not think I made any of the changed cells go over 50. But I suppose they could still be advanced too far. We worked on the cells that are 1500-2000rpm between 50 and 85 MAP. That was driving through rolling hills of central MD.

Thanks for the thoughts.

Sam

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The optimum spark timing at any given load and speed is what maximizes the torque on the crankshaft. It is possible to over-advance the timing so that the pressure peak occurs too early.

If you are driving at a constant speed, your optimum timing will have a minimum injector pulse width and a maximum manifold vacuum. There is no benefit to advancing the timing beyond this point. The limitation is engine knock, which can be minimized with higher octane fuel. If you are knock-limited, you would have set the timing to just before the onset of knock.

If you have a working EGR system, the recirculated exhaust gases will allow you to run more advance. Also, because EGR dilutes the fuel mixture, you would have to open the throttle further which would in turn reduce pumping loss, thereby further improving fuel economy.

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1965 Plymouth Barracuda,
225 engine, Quadrajet, HEI, Dutra Duals, 904 Torqueflite, 2.76:1 axle, Addco front bar
Rods & Relics - Fort Erie, ON / Collector Car Tech

Slant six testing. Lots of charts on timing etc. Timing charts start on page 146. http://www.ntrs.nasa.gov/archive/nasa/c ... 024483.pdf

another slant six test http://nepis.epa.gov/Exe/ZyNET.exe/2000 ... &ZyEntry=1

I read both those files. Thanks for posting them. It is interesting that they were choosing to test slant six engines then. I am not sure what I was able to learn from it though. I did note that the timing was maxed out around 40 in the first test. EFI has pretty much made the experimental carbs obsolete. I was not even sure what they were talking about there.

Sam

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