Vizard's secret '128' cam spec # revealed

[Racedad]

He wrote that piece 10 years ago,maybe he has changed his thinking.

But 128 minus(CID/Valve Diameter) comes close to fitting that old graph and for a standing out in the shop type of calculation would be easy to calculate. In the 8th grade when up to date on Algebra I could have given the formula to fit it exactly but it would seem to be a bit too much figurirn' for a "rule of thumb".

It also lines up with my personal experience in that we run better on the track with far tighter LSA than either Dynomation or 99% of the Comp Cams stuff. Years ago I tried designing circle track cam timing using Dynomation and Jerry at Schneider Cams talked me out of it.I'm glad he did we always run strong with his cam recommendation.

[Johnwesley]

I just wrote an .xls worksheet using that concept:
128 - (cubic inches per cylinder / intake valve diameter x.91)
and the numbers don't line up very well with the line on the chart.

Did you leave that part out?

Thats puts out a very readable LSA of 107 for my app. Which is 1* more that the old chart.

[David Vizard]

598ci
2.400in valve

98* LSA?

If this is a BBC it needs a correction to cater for the canted/incline valves.
Wait for the article and you will get the whole story instead of fragments that could be misleading.
DV

[piston guy]

With ALL due respect to David's experience, something doesn't add up with the formula. Some of it is spot on for a specific combination yet keeping the same "valve" constant and reducing cubic inches suggests a wider lsa which is not right by my 50+ years of experience. The cylinder head flow capability (or lack of) has to be a factor here.

[novadude]

With ALL due respect to David's experience, something doesn't add up with the formula. Some of it is spot on for a specific combination yet keeping the same "valve" constant and reducing cubic inches suggests a wider lsa which is not right by my 50+ years of experience. The cylinder head flow capability (or lack of) has to be a factor here.

Is it really max cyl head flow that matters, or low (<0.100") lift flow that is really important? Since the formula uses valve size and not peak port CFM, I guess I was assuming that what happens a low valve lifts has more influence on LSA selection than peak flow capacity. I'd love to hear more on this.

After reading and re-reading David's various writings on cam selection, I picked a 108 LSA for my recent 355 ci mild street build and chose overlap per David's charts. I was nervous about the 108 LSA, as I feared poor mpg, stinky rough idle, narrow powerband, lousy driveability, etc. You know - all the bad stuff magazines / cam companies typically preach about tight LSA's in street cars.

Now that it is running, I am 100% a believer in David's methods. My 355 drives great, gets good mpg, makes plenty of vacuum, has a broad powerband, and pulls like a freight train compared to engines I've run with similar duration on a 112-114.

[bmcdaniel]

Maybe I missed it, is the formula only intended for carbed engines?

[roadrunner]

Maybe I missed it, is the formula only intended for carbed engines?

No, it would work for injected fuel systems too. I purchased Davids book "how to build horsepower", selecting a cam is covered in 2 chapters, and now I know why half of my "performance builds" didn't perform (wrong LSA). Best money I ever spent.

[David Vizard]

An aside: IIRC the Mini book contains a calculation and chart for the LSA, but uses the area of the intake valve (not the diameter) vs. displacement, all in metric.

I revised my .xls to round the LSA to the nearest 1/2 degree (.5), it uses sliders to input intake valve OD, engine displacement in inches, and number of cylinders, and provides LSA (as 128 - ratio as discussed) without correction factor. E-mail me for a free copy.

panic,
Due to the Siamese ports on a Mini engine (2 intakes and 3 exhausts) the 4 cylinders concerned need different timing for every cylinder. That means the 128 deal just won't work here.
DV

[David Vizard]

OK guys,
Some bad news but only sort of because it's fixable.

The publisher won't put up the article for you all to read and I don't think any further persuasive efforts on my part will convince him otherwise. That's the bad news.
Offsetting that is I have decided, due to the big interest here, to basically re-write the article and post it here.

This is going to take a while and it looks like time constraints (building my 427 SB Ford and doing book work) will mean I have to do it a little at a time so it may take about 4 days to do the whole lot.
How does that all sound?
DV

[falcongeorge]

Sounds good, Thanks for doing that for us.

[Johnwesley]

I think it's great, it will probably take me more than 4 days to digest it and get some understanding. That for the work to get it to us.

[modok]

beyond and above.
Best automotive tech author there ever was.

[pdq67]

I sure wish that UDHarold was still with us! He would fit right in here, imho.

pdq67

[falcongeorge]

I used to buy a lot from Harold when he was Ultradyne. First time I called for a cam for my FE, I was on the phone with him for over an hour. He was really good to talk to.

[F1Fever]

I should probably wait for Mr. Vizard to publish his revisedd article but I'm curious if the 128 only calculates th suggested LSA or if it's also calculating open/close points or any other specs as it seems LSA by itself is rather lacking.
A 240/244 on a 106 is a completely different cam than a 239/254 on a 106