Can we ask 30% or 50% of what parameter or indicator that we are measuring?
There is a point where what you get can be lived with in lower performance uses and cruisers, etc. There is always a cost-benefit to all of this.
Every engine is different and has different requirements, so there is no set parameter. Those numbers are from of the corrections I have done, based on where the geometry was, compared to where it should be. As the Trick Flow heads become more prevalent, that average number will go down because they have raised the stands, which requires less correction.
If everyone is so anxious to "just get by" without fixing this, why does anyone modify anything? After all, it would be "good enough" with a stock motor. Why mess it up with aftermarket parts? It's not a race engine, and you can lose just as easily with a stock motor, as with a modified motor that isn't done right. And, if it's all about the sound, you can get that with a lazy cam that uses mild spring pressures that won't kill the stock rockers.
I'll say it again, if it isn't worth setting up a roller rocker correctly, it isn't worth the roller rocker to begin with. You're better off leaving it stock. The cost vs benefit argument means you either can't afford roller rockers, or you aren't willing to spend the money to install them properly. The correction should be considered part of the rocker budget, just like boring and honing is part of the cost of installing oversize pistons.
In all seriousness, what other aspect of an engine build would guys readily accept even a 30% error? Balancing? An 1800 gram bobweight is now either 1200, or 2400 grams, depending on whether it is heavy or light. Bearing clearance? A targeted .0025" oil clearance is now .00175", or .00325", again depending on whether it is big, or small that percentage. Ring gap? A typical .020" gap would now be either .014", or .026". Oh, here's a good one. Guys are always stressing over net valve lift. Let's take a simple .500" lift. At 30%, you could be at .350" or .650". Some would love to see the .650", but imagine getting the .350" when most guys complain about losing .010". There would be a national day of mourning.
For everyone who says "it's not that bad", when it comes to rocker geometry, I challenge them to tell me exactly how bad it is. Not with relative terms, but with actual measured numbers. If you can't do that, then you can't truthfully say "it isn't that bad", because you don't know. I don't say that to come across as a jerk (I don't try to belittle anyone, ever), but to say that if everybody knew and understood this subject, they wouldn't put a motor together without making it right. I'm confident of that!
Forgive me for the lengthy post, and again, I'm not trying to talk down to anyone, and I hope it isn't taken that way. I'm just trying to help folks understand, using pure logic, why it's so important.
