I have read that if the exhaust side of your cylinder heads flows less than 70% of the intake side, you should use a dual pattern cam. If the exhaust flows 80% or better of the intake, you should use a single pattern. In between 70 and 80 percent is considered a grey area for trial and error with both dual and single pattern cams. I know these are probably guidelines made up by C***y builders, so they may not apply to the Ford FE. Still, is there a reason why everyone on this board seems to consistently recommend a dual pattern cam for all FE builds, including builds using aluminum Edelbrock heads which flow about 73% (exhaust/intake)? I know some of the Mustang guys are using single pattern cams with Ebrock heads. Is this just becuase they have a lighter vehicle and an get away with it easier? Are dual pattern cams better at maintaining low end torque which makes them more favorable for truck usage?

 

Actually, a single pattern cam is more suitable for low end torque. Torque = cylinder pressure. The longer your valves are open, the more overlap they have, and this bleeds off cylinder pressure....

The basic power curve of your engine is determined by the intake duration and lift. This controls how much air and fuel is sucked in, which, in turn, regulates how much power can be made... but, on engines with comparatively poor flowing exhaust ports (ie most fords), ultimate power is limited by the inability to evacuate the spent charge from the cylinder. Hence, longer exhaust duration.

So, assuming you have two cams, with the same intake duration and lobe seperation, using similar lobe profiles.... the single pattern will make more torque down low, and the dual pattern will allow it to breath up high.

All of the above is a generality, but it's true. Now, there's more to it than that:

A lot of times, when cam makers design a dual pattern cam, they use a wider lobe seperation (ie, 112 vs 110). This negates the extra overlap and gives you a better all around cam. So basically, the only way you really know what works is to ask people who have tried it. But hey, you asked, so I thought I'd explain it to you.

 

I respectfully disagree Ben. Overlap has absolutely nothing to do with cylinder pressure. The point in time at which the intake valve closes has everything to do with it. Wider Lobe separations will push the intake closing event further into the compression stroke and bleed off cylinder pressure. Increasing the duration also pushes the closing event further into the compression stroke so it is a combination of both.


Like stated above adding exhaust duration does not reduce cylinder pressure. You are right though, for some reason they like to use wider LCAs on dual pattern cams most of the time. Crane probably stands out the most on that point. Laser Cams and Comp cams still like to stay in the 108 - 110LCA range.

Overlap does not reduce low end power but it will make the engine idle rougher due to gasses being pushed into the intake during the overlap period. Above idle though scavenging takes over and actually pulls air through the intake valve during overlap.

 

So which LCA do you prefer? 108,110,112? I know the cranes like the 110 i think.. but i was looking at the single patterns from Isky and they had lots of lift and good duration with the 108 LCA. I was thinking they looked like a hotter cam then some of the comprable cranes.

 

Welcome back, Sean! You are correct (again) on the cam stuff.

When I asked around about what cam I should use in my project, dual pattern was universally recommended. The cam grinder said it is common to do it even if the exhaust side flows real well, so go figure. Most off the shelf cams are going to be dual, so I wouldn't get to concerned about it unless you have a good reason to be. The small amount of overlap we are talking about with most street cams is virtually irrelevant.

-Scouder

 

Hey Sean nice too hear from you.