YOUR THOUGHTS ON THESE ADVANCE CURVES PLEASE

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Actually, The MP distributor dropped from 14 to 12 at 3500 rpm. I thought something was wrong with the machine LOL.
Sorry. Had these conversations before and most of the time they go nowhere, and I've been too busy at work to even log on to FABO.
First thing you probably don't want to hear is that engine should not need or want 20 deg initial.
In fact discussed this with on a thread a few a months ago, it might have been with you, where the engine was on a test stand.
As stated above in this thread. No load on the engine doesn't tell us much about what an engine will need when loaded.
An engine load is based on the percentage of power being used versus the max it could make at that rpm.
If it takes 10 hp to turn the transmission pump and the engine with accessories is making 20 hp net at 600 rpm, then 10 hp is causing 50% load, which is fairly high.
I was under the impression that to get idle rpm, you keep advancing until additional timing does not increase rpm, then drop it back a couple of degrees.

Initially, I found this occurred at 25* for this engine, but I dropped it back to 20* to maintain, what I thought, was a good t slot/idle mixture screw relationship.

The OEM recurve will be a little bit more sluggish off idle unless you've set idle too high to begin with. Otherwise it will be pretty good and probably won't retard until 5500 rpm or so.
So, if I reduced the primary spring tension (perch adjustment) and had it coming in a little quicker (steepen the first little section) and leave the rest as is, what would your opinion of that be?


BTW, idle is set at 850 Rpm.
 
I was under the impression that to get idle rpm, you keep advancing until additional timing does not increase rpm, then drop it back a couple of degrees.
Well you're not the first person who has heard that. I've seen it posted here before. No idea where it came from, but our goal isn't maximizing timing with the engine freewheeling so it makes no sense. Our goal is to provide the right amount of time in degrees to provide maximum leverage on the crank without wasting a ton of fuel or washing down the cylinders. Also we all prefer the exhaust not to burn our eyes even if we're not trying to meet current emmision standards.

How can we judge maximum power at idle?
The least drop in rpm and/or manifold vacuum when going from neutral into gear will be the strongest. This is a good foundation and will also be immediately useful for at least driving around town.

Initial can be ballparked from here:
Distributor starting point for a curve
The burn is slower with lots of overlap and exhaust dilution at idle. Also not getting the compression of a stock engine at idle. So thats why hot rodded engines often need a bit more initial than the factory cam.
Initial for factory 273 4 bbl (non smog) were something like 10-12 * BTC around 650 rpm. So that's another reference point.

Since you ahve it running at 20* 850 rpm I would make a step by step approach. Reduce the timing 2* to 18* at 850. Then see if the idle mix screws are responsive. If not, reduce the throttle positions. With a 4150 type Holley I would have suggested setting he throttle position first based on the transfer slot exposure. I don't have a rule of thumb for other carbs but they all have a limited range where the t-slot exposure provides good off-idle as well as idle. In either case if the idle mix screws don't change things, the throttle blades are open too far.

For setting the idle mix screws, turn in a 1/8 turn at a time until the engine starts rpm to fall off, then turn back (richer) 1/8 to 1/4 turn. We make it richer because when the engine goes into gear it will need richer mix than when there is no external loads.

If its an automatic you can put it in gear and observe rpm and vacuum drop. if its less than 50 rpm excellent. If its 100 rpm, OK.
Then if you want to try 17* degrees at 850 rpm, or a lower initial rpm, go for it. Keep testing until you are satisfied. Different people have different levels of interest in pursuing tuning.

I think the timing curve your engine will like will be similar to Rick's. Except I doubt your engine could easily be made to run with 12* initial. I expect 14* minimum, and 16-18 more likely. Depends how radical the cam's idle characteristics are.
The timing in gray is basically a factory/MP advance for 273/340 with more initial and the assumption you will be running high octane street fuel.

and here's the approach I suggest for tuning any non-stock build
Tuning for Performance.
 
Well you're not the first person who has heard that. I've seen it posted here before. No idea where it came from, but our goal isn't maximizing timing with the engine freewheeling so it makes no sense. Our goal is to provide the right amount of time in degrees to provide maximum leverage on the crank without wasting a ton of fuel or washing down the cylinders. Also we all prefer the exhaust not to burn our eyes even if we're not trying to meet current emmision standards.

How can we judge maximum power at idle?
The least drop in rpm and/or manifold vacuum when going from neutral into gear will be the strongest. This is a good foundation and will also be immediately useful for at least driving around town.

Initial can be ballparked from here:
Distributor starting point for a curve
The burn is slower with lots of overlap and exhaust dilution at idle. Also not getting the compression of a stock engine at idle. So thats why hot rodded engines often need a bit more initial than the factory cam.
Initial for factory 273 4 bbl (non smog) were something like 10-12 * BTC around 650 rpm. So that's another reference point.

Since you ahve it running at 20* 850 rpm I would make a step by step approach. Reduce the timing 2* to 18* at 850. Then see if the idle mix screws are responsive. If not, reduce the throttle positions. With a 4150 type Holley I would have suggested setting he throttle position first based on the transfer slot exposure. I don't have a rule of thumb for other carbs but they all have a limited range where the t-slot exposure provides good off-idle as well as idle. In either case if the idle mix screws don't change things, the throttle blades are open too far.

For setting the idle mix screws, turn in a 1/8 turn at a time until the engine starts rpm to fall off, then turn back (richer) 1/8 to 1/4 turn. We make it richer because when the engine goes into gear it will need richer mix than when there is no external loads.

If its an automatic you can put it in gear and observe rpm and vacuum drop. if its less than 50 rpm excellent. If its 100 rpm, OK.
Then if you want to try 17* degrees at 850 rpm, or a lower initial rpm, go for it. Keep testing until you are satisfied. Different people have different levels of interest in pursuing tuning.

I think the timing curve your engine will like will be similar to Rick's. Except I doubt your engine could easily be made to run with 12* initial. I expect 14* minimum, and 16-18 more likely. Depends how radical the cam's idle characteristics are.
The timing in gray is basically a factory/MP advance for 273/340 with more initial and the assumption you will be running high octane street fuel.

and here's the approach I suggest for tuning any non-stock build
Tuning for Performance.
It's a REAL old school way of setting timing. I 100% agree it ain't the most accurate by far, but if you happen to not have a timing light, it normally works till you can get one and map the curve out.
 
Thanks for taking the time @Mattax I'll work on getting the curve similar to Ricks, then test it out on the street. I will revisit my initial base timing as well. :thumbsup:
 
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