Valve Overlap Requirements and Calculators (2025)

In David Vizard's book "How to Build Horsepower, Volume 3, Smallblocks on a Budget" he has an overlap requirement chart that gives a range of overlap for different output requirements. What he doesn't mention, however, is what cubic inch engine these recommendations are for and at what lift point the overlap in the chart is calculated at.

Are you guys familiar with the book and section I'm referring to and do you have any comments on this? Some of the questions I have are if I want to plug a certain cam profile into an overlap calculator, what lift should I use i.e. .006, .050 etc?

If his overlap requirement chart is based on a 350 with head flow specs as per his modified head section, how much does the overlap requirement change when building a 400 with better flowing heads? Is there a general rule of thumb?

It's a really informative section but it leaves some questions on the table.

As always, any help is greatly appreciated!

Shawn

Vizard's (BTW, it's pronounced "Vye-zard", not "Vis-ard") all new edition HTBHP (SA-Design's SA24, no vol. # specified) has a greatly expanded section on overlap determination. His specs are for a typical 2-valve V-8 with ~ 22 c.i. displacement per inch of intake valve diameter and are duration at lash, which for a hydraulic cam is the usually-published 0.006" tappet lift. For mechanical cams, lash duration is (strangely) rarely published, but he says assuming 12-15° more than the usual 0.020" tappet lift duration is about right.
He speaks in detail of the effect of a bigger engine with the same valve size needing/tolerating more overlap (~1° more per 5-7 c.i.), and of the effect of canted valves, etc. and much more.

I find his cam selection process to be very novel and logical:
1. Select overlap based on relative valve size*, application, etc. *(at very low lifts, flow is related mostly to valve size, not port dimensions.)
2. Determine optimum LCA, based on a host of factors.
3. Duration is mathematically fixed by 1. & 2., so move on to lift considerations plus any factors suggesting split duration and you're done!

BTW, I correspond regularly with David and so could act as go-between for any general questions on his stuff, as his schedule is far too tight to handle enquiries individually. (Don't expect his 30+ books to be re-written on line though! They are an enormous bargain at the price and mine rapidly become dog-eared with use!)

Felix, qui potuit rerum cognscere causas.

Happy is he who can discover the cause of things.

MadBill wrote:Vizard's (BTW, it's pronounced "Vye-zard", not "Vis-ard") all new edition HTBHP (SA-Design's SA24, no vol. # specified) has a greatly expanded section on overlap determination.

Looks like I have another Vizard book to buy, lol. I'm hoping to make it to one of his lectures next year but funds are too tight at the moment to pre-register. And yes, all of my Vizard books are torn, tattered, dog-eared and highlighted to death. I sure never had a text book from school look like these do.

Thanks for offering to be go-between. I'll choose wisely!

Felix, qui potuit rerum cognscere causas.

Happy is he who can discover the cause of things.

swatson454 wrote:In David Vizard's book "How to Build Horsepower, Volume 3, Smallblocks on a Budget" he has an overlap requirement chart that gives a range of overlap for different output requirements. What he doesn't mention, however, is what cubic inch engine these recommendations are for and at what lift point the overlap in the chart is calculated at.

Are you guys familiar with the book and section I'm referring to and do you have any comments on this? Some of the questions I have are if I want to plug a certain cam profile into an overlap calculator, what lift should I use i.e. .006, .050 etc?

If his overlap requirement chart is based on a 350 with head flow specs as per his modified head section, how much does the overlap requirement change when building a 400 with better flowing heads? Is there a general rule of thumb?

It's a really informative section but it leaves some questions on the table.

As always, any help is greatly appreciated!

Shawn

Well, I don't know David as Madbill does, but David has published the following in magazine articles before:

Here's the CORRECT way to calculate your cam's ADVERTISED OVERLAP which will be needed for the chart below:
•Add your intake and exhaust advertised duration (duration at .050 will NOT give you the correct overlap)
•Divide that answer by 4
•Subtract the lobe separation angle (LSA) from that answer
•Multiply that answer by 2, and you have the CORRECT adv overlap to use below

APPROXIMATE adv overlap "operational" reference chart:

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

The thing is, his operational reference chart right above, only seems to fit Big Blocks properly. If you take percentages and scale things down for small blocks, it ends up looking like this:

9* - 35*….towing
26* - 53*….ordinary street
44* - 66*….street performance
61* - 79*….street/strip
74* - 87*….race
83* - 100*...Pro race

Felix, qui potuit rerum cognscere causas.

Happy is he who can discover the cause of things.

Bill
If you have occasion to speak to David Vizard, you might ask him for his thoughts regarding specific time area analysis. STA would specifically call out engine overlap which would be dependent on engine size, engine speed and BMEP targets.

I have been trying to develop such a design strategy but find it difficult to find sufficient data to produce useful target values. It seems that other variables keep sticking their two cents into the power production equation.

Felix, qui potuit rerum cognscere causas.

Happy is he who can discover the cause of things.

I noticed that he remarks that there is a minimum OL area needed to get value from a resonant exhaust system.
My guess is that if the window is too small (both valves at low lift) not enough pressure change occurs, and if too short (not enough shared event time) the pressure exchange will be very brief.

However: Smith tracked real-time pressure spikes in the pipe and the port for engines with what we would consider today even below mild cams circa 1940, and it appears that if there is a minimum level for useful benefit it's far smaller than we thought.

ap72 wrote:Isn't he teaching in SC now? That'd be a hell of an engine design professor to have!

Yeah, wouldn't you like to be able to crawl around inside his head for a couple of hours.

I know that he did teach at UNCC. I had contacted him there back in early '05.

Stan

Stan Weiss/World Wide Enterprises
Offering Performance Software Since 1987
http://www.magneticlynx.com/carfor/carfor.htm
David Vizard & Stan Weiss' IOP / Flow / Induction Optimization Software
http://www.magneticlynx.com/DV

Stan Weiss wrote:I know that he did teach at UNCC. I had contacted him there back in early '05.

Stan

He lost that gig.

Mike Jones
Jones Cam Designs
Denver, NC
jonescams@bellsouth.net
http://www.jonescams.com
Jones Cam Designs' HotPass Vendors Forum: viewforum.php?f=44
(704)489-2449

9* - 35*….towing
26* - 53*….ordinary street
44* - 66*….street performance
61* - 79*….street/strip
74* - 87*….race
83* - 100*...Pro race

Maybe abit conservative.... I ran 78 on my 383 sbc and I could have daily drove that car, it just was abit loud. Ran mid 11's at 118-119 without much effort. I could have gotten away with a bunch more cam for even more rpm if need be. That was an EFI car and it still had good idle characteristics and could work with closed loop systems when tuned right. It had a choppy lopey idle but was stabilized and could lug around up hills with locked converter and 3.42 gear at any low speed rpm you wanted with no bucking/surging etc.