Vizard's adv overlap chart corrected for displacement - Chevelle Tech
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post #1 of 3 (permalink) Old Nov 2nd, 07, 3:58 PM
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Vizard's adv overlap chart corrected for displacement

The original thread on this topic was getting long in the tooth, so I went for a fresh one.

There were comments from some who said Vizard's chart should be setup for displacement, since a general chart makes no sense. I agree, so I did some simple math to do just that. Check it out:

Comparing the chart I made from real world engines, for the beginning of the truly bad mofo lope/chop (middle chart), to Vizard's original chart (immediately below), you can see that his chart matches up nicely for a solid roller 540ci motor at the top of the street/strip range, which makes sense:

Vizard's original chart:

10* - 40* towing
30* - 60* ordinary street
50* - 75* street performance
70* - 90* street/strip
85* - 100* race
95* - 115* Pro race

Min. solid roller adv overlap for that bad mofo lope/chop:

350ci………..77*
383ci………..78.5*
400ci………..80*
454ci………..84*
496ci………..87*
540ci………..90*
572ci………..92*
632ci………..96.5*

So then, matching up the lope/chop chart, with the top of the street/strip range for other displacements, and adjusting all other numbers by the same percentage as the original chart, you get the original chart adjusted for displacement, so………...

For a 383ci small block roller motor, you get:

9* - 35* towing
26* - 52.5* ordinary street
44* - 65.5* street performance
61* - 78.5* street/strip
74* - 87* race
83* - 100* Pro race

The same idea could be carried out for any displacement. Of course, the real world has a lot of gray areas, but the range overlaps try to take that into account. Though these charts may not be perfect, they do give you a general idea of what to shoot for, as opposed to only guessing.
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post #2 of 3 (permalink) Old Nov 2nd, 07, 5:29 PM
pdq67
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Re: Vizard's adv overlap chart corrected for displacement

May I add...

It's the point where the intake valve closes w/ respect to where the piston is in the cylinder that set's all this, imho

So therefore rod length play's a slight role in this too!

This is why short rod, long stroke engines can use a slightly bigger cam than a longer rod engine of like displacement. B/c the piston will be deeper in the cylinder at intake valve closing! (Or do I have this back-as-wards?)....

pdq67

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post #3 of 3 (permalink) Old Nov 2nd, 07, 6:07 PM
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Re: Vizard's adv overlap chart corrected for displacement

Quote:
Originally Posted by pdq67 View Post
May I add...

It's the point where the intake valve closes w/ respect to where the piston is in the cylinder that set's all this, imho

So therefore rod length play's a slight role in this too!

This is why short rod, long stroke engines can use a slightly bigger cam than a longer rod engine of like displacement. B/c the piston will be deeper in the cylinder at intake valve closing! (Or do I have this back-as-wards?)....

pdq67
Overlap is far away from the intake valve closing point. It's at the end of the power stroke, while the intake closes at the beginning of the compression stroke.
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