Charlie,
fact's first. Generaly speaking "torque" is of a finite quantity and: small lobe centers mean
power comes in later and falls off earlier but is bigger when it peaks Large lobe centers.
power comes in earlier and falls off later but is smaller when its peak.
If I had to come up with an analigy: torque is like a cake. You can sit down and eat the whole thing at one sitting and get one big hit or have one piece after dinner every night for a week and savor it.
My thoughts on lobe separation angle are this for heaths Scout and any heavy car, truck or boat. Cam selection is the single most important piece of the engine building game. These are again my opinion based on the engines I have built and what I expect to get out of them. Since this is a thread for heaths build I would like to keep this blurb in that context not a “hotrod” context.
If I build an engine for a person I ask that they take a week and write down on paper what they intend to do with the vehicle. What is in there opinion the most important characteristic of the operation of there vehicle. This is what I use to gage the build.
In heaths case I have already expressed the duration window. 206-208 degrees @ .050 max. For this I recommended 110 lc based on my opinion of how much overlap tolerance the project had. Lc changes double the effect on overlap or lift at tdc. This controls scavenging and in a large part how the engine will respond to exhaust and header changes. I don’t think headers are in the cards. It also effects the dreaded exhaust reversion that causes a rough idle and low suction ( high manifold pressure) at idle.
We ask that the engine have a power band that comes in strong early and sustains it’s self over a long period of time. We need it to lug it’s way around at 1000-1200 rpm crawling over rocks. This is where I stress the point on over lap most. To much overlap and cylinder pressure won’t be a big help until it starts to clear out. The custom cam profile we talked about 212* has 4 degrees more duration than my initial recommendation of which 2* from each lobe will effect overlap or 4 more overlap. I moved my recommendation for lobe centers to really 112-110 for a compromise on power band width and overlap. Bumping his compression is more important with this cam to further bolster low end.
In this case I feel a good solid idle and nice off idle throttle response is imperative. Thusly wider lobe centers help reduce overlap and augment what he is doing.
This is not a simple question to answer. You can ask ed iskendarian, schneider, vic jr, nick arias, david crower, gayle banks, or bubby wilton etc etc and each one will give you a different answer none of which are wrong. That is why I like to see test data on stuff before I recommend it.
I will often times take weeks before I bite the bullet and commit to a grind. I honestly doubt all of my rambling will help but. What the hell.
Robert
