Dual Pattern Cams

[Purple440]

When designing a dual pattern cam, at what point is the difference between the intake and exhaust duration a problem?  For instance, if I called Engle and said I wanted a 220 intake and 230 exhaust would I still be helping to remove the exhaust gas or would I be over doing it?

Thoughts?

Thx,

- Doug

[aifilaw]

That's a really ambiguous question. The short and long answer is: "it depends on the block, rotating assembly, heads, and intake"

For example, my last 383 BB and 350 SB (chevy) builds were both meant to give a nice flat long torque curve to maximum my bandwidth, in both cases I had between 8 and 12 degree's higher exhaust duration than intake because that's what it took to make that happen.

I spoke with a guy who does LS1 cam grinds for a living, and from the dyno they've proven that to get the same result using the stock LS1 setup with or without general head porting, they keep the durations the same, and for a typical buildup (lots of ponies) the intake duration is 10 degree's higher than the exhaust and they back-cut the intake and leave the exhaust valves alone.

It really depends on the foundation, and then on where you want the engine to perform.

[idahogrumpy]

I have had very good results using dual grind cams in my big block mo par street engines. I have never spec'ed out my own cam so to speak, I have always used off the shelf cams that were engineered by the cam manufacturer. When I look for a cam I take the advice given here and actually look for a dual grind shaft that will fit those specs.    Grumpy

[firefighter3931]

Doug, the dual pattern cams work well with a stock head which has less than optimal intake to exhast flow ratios. When the heads are ported and you have a 70% in/ex ratio then the split pattern becomes a non issue. Basicly the cam (split pattern) is compensating for a weak exhaust port by hanging the valve open longer and allowing the engine to exhale.


Ron