Calculating Overlap

[6667fan]

Can advertised @.020 numbers be used when employing the divide, subtract and multiply formula? I believe @.050. numbers don’t tell the story. How about @.006?
Even Comp does not seem to always use the same advertised starting point from one card to the next.

Thanks

[blykins]

The advertised duration is usually based on the type of camshaft.   It's typically .006" for hydraulic cams or "hybrid" cams and .020" for solid cams. 

With that being said, the overlap can be calculated by any duration that you want.  I will often calculate overlap at .050" and compare it to overlap at seat/advertised duration. 

[6667fan]

So on my hydro roller I have 310/318 @.006 with a LSA of 112. This maths to 85 degrees of overlap.
A buddy just ordered a Comp solid roller stick for his SBC, (strip car), 296/304 @.020 with a LSA of 106.
I get 88 degrees overlap on that one.

Thanks much for responding.

[blykins]

Pretty typical.  Tight LSA adds a lot of overlap. 

If I were to do your cam over again, I’d go 114-115 LSA.  I’ve done lots of testing since then and have learned a few things.

[6667fan]

How ‘bout if the next iteration of my engine had a solid roller? Including having no concerns from the standpoint of idle quality/vacuum. More strip than street.   Would you advocate for still spreading the centers more?

Thanks

[blykins]

Maybe.  You make the overlap what it needs to be for the application and how well the heads work.   LSA is just a product of all of that, not a driving factor. 

One of the biggest mistakes I see made with cams is that guys think 106 LSA cams are "circle track cams", 108 LSA cams are "drag race cams", 110 LSA cams are "street", and 112-114 are "EFI, N20, etc." cams.  Some of my drag race cams have 112 LSAs and some of my pulling truck engines have 119-121 lobe separations.  You build the cam to do what you want.

The biggest challenge in all of that is finding what amount of overlap works with what engine combination and what application.   Takes a lot of dyno'ing and cam testing. 

[My427stang]

Just to add on, .020 on a solid lifter and .006 on a hyd are pretty close in most cases except tight lash solids.  You have to think where you are on the lobe when you take up the lash, for a hyd you are there minus a little compressibility/bleed.

With the wide range of lashes, efficency in design, varying rocker ratios, etc, and unless you are trying to balance on a knife edge, .020 for most solids and .006 for hyd works pretty well.  You could get closer measuring your own effective ratio and desired lash if you wanted to, might even find some crazy deflection or ratio changes with the combo