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Re: Never Assume You Know Too Much
Terry,
You are correct on the instruments used.
1) I start with the distributor and test it to ensure that the default curve is at least correct. I also ensure that I am using good fuel (92 octane)
2) I then install the distributor and check the timing. I start with a factory setting at idle and bump it advanced a few degrees if I put in a hotter cam. I am careful to keep total mechanical advance to less than 36 degrees (obviously, the initial advance plus the mechanical advance will equal this. I do this with the vacuum line plugged. After the test, I connect the vacuum line to the manifold vacuum source (not ported source, typically). Most all the time, I will be under 36 degrees with the factory initial advance setting.
3) I then check the fuel ratio at different cruise speeds and under power. I especially look for changes in the vacuum and the fuel ratio (i.e. the relationship between the 2).
4) I adjust the carb for the fuel air ratio. I try to avoid lean mixtures under power. I've adjusted quadrajets (many options), edelbrock's (fewer options) and finally holley's (minimal options for part-throttle adjustments)
5) Then I use the knock sensor to check the advance under load (high and low engine operating temps). This will change with fuel quality and engine temp, so it is a bit of a dynamic issue. I typically adjust it for worst case temp, and I am careful to use 92 octane or better fuel.
I advance the ignition as far as I can go without knock under load. I am also careful to determine the smoothest engine idle advance setting (temperature dependent - so do this with a hot engine if that is your concern.
This is where it gets interesting. I may find that the idle setting and power setting are such that the mechanical weights provide too much advance and thus I need to put the distributor back on the SUN machine and have the mechanical come in a bit sooner, so I can have more initial advance.
The vacuum advance is the more difficult issue, because total advance (vacuum and mechanical) can be larger than 36 degrees.(I did not know this until last year, and this caused me much confusion) This is where I have found the knock sensor to be key. I'm still new to the knock sensor and I am trying to tweak these settings (starting with mechanical and moving to vacuum).
I have noticed that ported vacuum sources retard the ignition timing at idle in order to heat up the heads and provide more fuel burn for better emissions. This can also be a source for idle-based overheating. As such, I'm mainly using manifold vacuum on my hot engines and ported on my street cars without heating problems.
Terry,
You are correct on the instruments used.
1) I start with the distributor and test it to ensure that the default curve is at least correct. I also ensure that I am using good fuel (92 octane)
2) I then install the distributor and check the timing. I start with a factory setting at idle and bump it advanced a few degrees if I put in a hotter cam. I am careful to keep total mechanical advance to less than 36 degrees (obviously, the initial advance plus the mechanical advance will equal this. I do this with the vacuum line plugged. After the test, I connect the vacuum line to the manifold vacuum source (not ported source, typically). Most all the time, I will be under 36 degrees with the factory initial advance setting.
3) I then check the fuel ratio at different cruise speeds and under power. I especially look for changes in the vacuum and the fuel ratio (i.e. the relationship between the 2).
4) I adjust the carb for the fuel air ratio. I try to avoid lean mixtures under power. I've adjusted quadrajets (many options), edelbrock's (fewer options) and finally holley's (minimal options for part-throttle adjustments)
5) Then I use the knock sensor to check the advance under load (high and low engine operating temps). This will change with fuel quality and engine temp, so it is a bit of a dynamic issue. I typically adjust it for worst case temp, and I am careful to use 92 octane or better fuel.
I advance the ignition as far as I can go without knock under load. I am also careful to determine the smoothest engine idle advance setting (temperature dependent - so do this with a hot engine if that is your concern.
This is where it gets interesting. I may find that the idle setting and power setting are such that the mechanical weights provide too much advance and thus I need to put the distributor back on the SUN machine and have the mechanical come in a bit sooner, so I can have more initial advance.
The vacuum advance is the more difficult issue, because total advance (vacuum and mechanical) can be larger than 36 degrees.(I did not know this until last year, and this caused me much confusion) This is where I have found the knock sensor to be key. I'm still new to the knock sensor and I am trying to tweak these settings (starting with mechanical and moving to vacuum).
I have noticed that ported vacuum sources retard the ignition timing at idle in order to heat up the heads and provide more fuel burn for better emissions. This can also be a source for idle-based overheating. As such, I'm mainly using manifold vacuum on my hot engines and ported on my street cars without heating problems.
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