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Ignition advance???
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racingenglishcars
Darth Vader
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This is always an interesting subject. The difference between the theoretical values and the hierarchical obtained practical values is rather large. not because the theory is wrong, but due to volumetric efficiency, wave fronts, resonance, combustion efficiencies etc.
So listen to the guys with the dynos.
Theory says something like this graph
So listen to the guys with the dynos.
Theory says something like this graph
JeffS
Jedi Trainee
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1 Bar is roughly 29" of mercury, a full atmosphere of vacuum, so no load/no vacuum on the left, a full atmosphere of vacuum on the right.
.2-.3 are the highest vacuum spikes you'll see in our cars upon slamming a throttle shut.
I don't agree that the chart is practical, at least not in most cases. Then again, it was probably developed on fuel in some other country. It looks similar to what would have been utilized here by the Big-3 in the 1950's, or throughout Europe around 1970. Who developed the chart and when?
You're right - dyno time is everything. I'm lucky enough to get to dyno quite a few cars, as well as share information from/with racers across the country and many engine builders. What works here deosn't work in other countries either.
.2-.3 are the highest vacuum spikes you'll see in our cars upon slamming a throttle shut.
I don't agree that the chart is practical, at least not in most cases. Then again, it was probably developed on fuel in some other country. It looks similar to what would have been utilized here by the Big-3 in the 1950's, or throughout Europe around 1970. Who developed the chart and when?
You're right - dyno time is everything. I'm lucky enough to get to dyno quite a few cars, as well as share information from/with racers across the country and many engine builders. What works here deosn't work in other countries either.
bthompson
Jedi Warrior
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I've always had good results by setting the timing with a vacuum gauge. With a 1500 engine, 1275 dizzy, Weber DGV, no vac advance, and unknown spring advance rate, I figured there're too many zany variables to go "by the book." I hook the gauge to the manifold, run the engine at 3000, advance the spark until I get the highest vacuum, and then back it off a little. If it pings, I back it off a little more. No fiddly little timing marks required. :wink:
The_architect
Jedi Trainee
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Bill, are you plugging the vacuum line to the distributor or do you just leave it? Thanx, Charlie
bthompson
Jedi Warrior
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Leave everything as-is when you're setting timing with vacuum. Since you're setting max advance at speed, you want the system to work just like it does on the road.
I have my DGV line running to the dizzy, but the diaphragm is kaput...so no air leaks, but it's not working either. One of these days I'll fix it, but for now I'm running without vacuum advance, and living with the power and economy penalty at the lower end. The neat thing about the vacuum timing method is you can find your best advance throughout the whole rpm range, and tweak your curve with heavier or lighter springs. Gotta be patient to do that, though, so I just set the max advance and let the curve fall where it may. ('Course, that's probably why I'm only getting 21 mpg, too.)
I have a little nipple threaded into the intake manifold just for the vacuum gauge hookup.
I have my DGV line running to the dizzy, but the diaphragm is kaput...so no air leaks, but it's not working either. One of these days I'll fix it, but for now I'm running without vacuum advance, and living with the power and economy penalty at the lower end. The neat thing about the vacuum timing method is you can find your best advance throughout the whole rpm range, and tweak your curve with heavier or lighter springs. Gotta be patient to do that, though, so I just set the max advance and let the curve fall where it may. ('Course, that's probably why I'm only getting 21 mpg, too.)
I have a little nipple threaded into the intake manifold just for the vacuum gauge hookup.
racingenglishcars
Darth Vader
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The chart was developed by my professor and myself based on theoretical calculations, not on the test engine (see picture of 1275 Mini 7-port on which the curve was used), or on fuel, American or otherwise... thus "theoretical". It was developed around 1990. Fuel is assumed to be "test fuel" 100.0% pure octane (C8H18).
You're correct; Since pressure is measured from an absolute point, 0 is perfect vacuum, 1 is atmospheric pressure [Bar]. Thus "Manifold pressure relative to atmosphere". Idle runs around 0.1 - 0.2, full throttle is close to 1.
I didn't imply the chart is practical (though I used it for testing). I implied that dyno testing is practical, which makes up differences that are highly complicated and sensitive to modelling of air flow and other disturbances within the air inlet system. If we could measure static pressure just inside the inlet valve the chart would be much closer.
The test engine.
You're correct; Since pressure is measured from an absolute point, 0 is perfect vacuum, 1 is atmospheric pressure [Bar]. Thus "Manifold pressure relative to atmosphere". Idle runs around 0.1 - 0.2, full throttle is close to 1.
I didn't imply the chart is practical (though I used it for testing). I implied that dyno testing is practical, which makes up differences that are highly complicated and sensitive to modelling of air flow and other disturbances within the air inlet system. If we could measure static pressure just inside the inlet valve the chart would be much closer.
The test engine.
JeffS
Jedi Trainee
Offline
Kurt, you're probably leaving som power on the table, depending on the condition and build of your current distributor. With low compression (below 150) you can probably push it to 37 degrees advance.
That's a really sweet dyno setup. You'r eright - there's nothign like a true dyno test to really find where the horsepower is hiding. You can get most of the way there by road testing and accurately measuring fuel mixture, but an engine (not chassis) dyno is the supreme test!!!
That's a really sweet dyno setup. You'r eright - there's nothign like a true dyno test to really find where the horsepower is hiding. You can get most of the way there by road testing and accurately measuring fuel mixture, but an engine (not chassis) dyno is the supreme test!!!
Pythias
Jedi Knight
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Speaking of ignition advance....
I had set mine up at about 12 degrees. I kept hearing pinging (I THINK that's what it is anyway) so kept dialing back with the dizzy adjuster knob a couple of clicks or so at a time until it stopped pinging under any condition. Let me say now that I only run CHEVRON PREMIUM fuel. The ping is all but gone, only coming when going uphill in 5th at below 2200 rpm, but when I checked with a timing light, my advance is down to about 3 degrees at idle... seems really low, but the car doesn't run any hotter, the tell tale sign of over retardation....
comments?
I had set mine up at about 12 degrees. I kept hearing pinging (I THINK that's what it is anyway) so kept dialing back with the dizzy adjuster knob a couple of clicks or so at a time until it stopped pinging under any condition. Let me say now that I only run CHEVRON PREMIUM fuel. The ping is all but gone, only coming when going uphill in 5th at below 2200 rpm, but when I checked with a timing light, my advance is down to about 3 degrees at idle... seems really low, but the car doesn't run any hotter, the tell tale sign of over retardation....
comments?
JeffS
Jedi Trainee
Offline
That's a good sign that the advance springs are worn/fatigued. Why doesn't it act retarded? Well, it may be that your distributor would have otherwise been over advancing and now its not doing that as much. It depends on how much total advance your distirbutor offers and how your advance springs are now reacting to your timing setting. This owuld be easy to answer if your distirbutor were on one of my 3 Sun machines, the Snap-On Distrib-U-Scpoe, or the Rotunda distirbutor tester. 