better yet is a dash-mounted, dial-back, timing retard box, so you can adjust your timing on the fly, searching for the highest mpH at a specific throttle opening. This goes along with what
@66fs is saying.
To do this, you will need an adjustable, repeatably consistent, throttle stop. You set the stop to achieve the ~mph you want to cruise at, and go. Once the combo has warmed up, I retard the timing 3* and watch the mph drop, to prove the last test is still valid, then I add the 3 back in, and let it stabilize, then add 3 more, and so on until the MPH no longer picks up. When I get home, I put the timing light on it and record it, together with the mph/rpm.
Next time out, I will cruise at plus 3mph or minus 3mph, and again record the timing and rpm, for the new MPH. Eventually I have enough data points to plot a cruise timing curve, and then go make it happen.
To get it right, I use the same flat level hard stretch of hiway.
Then I change a different parameter; like tire pressure, engine temperature, or rake; and start all over. I might even change the rear gear.
Before I start, I make sure the cruise rpm is not too low, for the cam.
What I do is, with a decent PowerTiming curve already worked out; in Neutral, rev it up to the lowest rpm that achieves the highest vacuum, indicating the lowest rpm that all the air that is going into the engine is finally all going in the same direction, and not being pushed back up into the intake, past the late-closing intake valve. Usually, the bigger the cam is, the later the Ica and the higher the
minimum cruise rpm will have to be.
Then I advance/retard the timing at that rpm to achieve the highest rpm. Now she's ready for road-testing.
My engine has had three cams;
a 223/230/110, a 230/237/110, and a 292/292/108, which IITC is about
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The smallest liked~1900, The current likes 2200, and the 292 liked 2500. Your results may/most likely will, vary.
The cranking cylinder pressure was adjusted for each, to maintain about 177 to 185 psi.
