Here is an interesting article I read about camshafts. I mentions backpressure in engines in respect to overlap of your cam. It seems that depending on how your engine is built will determine how much if any backpressure you will need.
Overlap: How Much Is Too Much?
Assuming we are choosing a cam for a streetable engine, how much overlap can we use before it becomes a problem? The answer here is that it depends on the valve sizes in relation to the cylinder displacement. If the heads have small valves in relation to cylinder cubes, then the amount of overlap we can use is significantly more than the same cylinder with much larger valves. For instance, a 500-inch big-block Chevy can tolerate not only more overlap, but a much bigger cam because the cylinder heads are so under-valved for the displacement. A 350 small-block with a set of decent heads has a lot more valve-per-cube, so it does not need so much overlap to get the job done.
Now that we have covered the effect overlap has on street manners, it is time to look at its effect on power output. Let's make one thing clear here: Big (but not excessive) overlap is a prime key to big power numbers, but only if your exhaust system sucks. Literally. If you have ever heard that an engine needs a little backpressure, you might want to ask yourself why an engine would want an exhaust system that literally pushes exhaust back into the combustion chamber rather than sucking it out. The simple answer is, it doesn't. If a big-overlap, big-cammed engine has an exhaust system with any measurable backpressure, the price paid is a big drop in output.
Although the foregoing might be interesting info, it doesn't actually help you make a decision as to how much overlap your engine needs. Just in case you might ask, I use a one-off computer program that was 18 years in the making to do what we are doing here, but that is no help to you. So that you have something of a guide, I have made up the nearby chart. To make the most of this chart, you will need to take into account where, in terms of valve size per cube, your engine falls. For, say, a 302-inch engine with decent-sized valves, the overlap selected needs to be toward the short end (left side) of the segment that fits your application. If it's a typical 350-inch small-block, then choose something around the middle of the relevant segment. If the engine in question is a big-block or a really big-inch small-block (both of which are typically under-valved), then select the overlap toward the larger, righthand side of the relevant segment
Here is the rest of the article.
http://www.popularhotrodding.com/tec...ics/index.html
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