Certainly building to the best of our ability is something we all strive for. For sure checking everything during a build is worth while. Being most of us are not professional engine builders there is always the question. What do we really need? There is a budget for most. So the improvement must be weighed vs cost. Then what is perfect geometry? Mid lift at 90 degrees? Or a different theory? When max lift is gained by a change, then lift is lost elsewhere. Which is really correct? In the case above something had to be really screwed up to gain 46 hp in geometry of a mild .530" lift motor. And I'm curious. Maybe I can learn something here. I'm assuming the 600 rpm gain is where the 46 hp came from. How did a geometry change allow a usable 600 rpm increase? Before your kit was developed what were the options? Raise the shafts with shims. Or to lower you had to mill the pedestals off and relocate the shafts on blocks or stands. So until recently Mopar engines ran with the geometry they were built with depending on what rocker arm and lift it had, Correct?
Doug
Doug,
I'll try to address your questions in order, and the best I can.
First, what is needed. You spoke of budgets, and not being professional builders. It's stunning how many non-professionals want to build there own engines, presumably to save money in most cases, and then when they get to the end of the build, ie rockers and pushrods, they decide to skimp because it is costing way more than they imagined it would. At that point, the budget vs quality issue goes right out the window. They have the choice of just slapping it together, or spending an additional $150-$350 to address the geometry, and reduce failure risk. Unfortunately, too many want to save the money and end up damaging a $10,000+ motor in the process. There just isn't any logic in that. I have told a lot of people, "If you don't want to properly set up a roller rocker, don't bother putting it on the motor". That doesn't sell anything, but it's the truth.
Improvement vs cost. How much measured improvement is had with ARP bolts in a motor. They aren't cheap, yet they are put in so called budget motors all the time and don't make an ounce of power. And who's to say that motor wouldn't have been perfectly fine with stock bolts? ARP bolts/studs are used for reliability, and the same thing applies to rocker geometry. The difference is, the geometry can also improve performance, which a fastener generally will not.
What is perfect geometry? Whichever method most accurately transfers the cam lobe information to the valve. The cam gives the orders, and the rockers job is to relay the message. Any rocker motion that deviates from a straight line input is wasted motion. Because the rocker inherently has a radial motion, it can't be eliminated entirely, so the best geometry, on both the valve side, and the pushrod side, will be the least amount of non-linear movement possible. That's where the math comes in. The pushrod side is built into the rocker design, and unfortunately, we are stuck with it unless a custom rocker is built. That's what I do when I need the ultimate in valvetrain efficiency and stability. Also, anyone setting up the valvetrain for max lift is asking for serious instability. The maximum lift possible will happen when the rocker centerline is perpendicular to the valve centerline (zero sideways motion). That is also the point where velocity is the highest. The cam has deceleration designed into the lobe, but if the rocker velocity is at its max, the deceleration in the lobe is not enough to keep the valvetrain stable. This is why you hear the complaints of quick lobes not carrying rpm, and old school slow lobes would rev better.
The .530" lift motor was not any more screwed up than most other similar builds. Stock head with stock length valves and a 1.6 ratio HS copy aluminum rocker. The testing was done on the same dyno, on the same day. After the baseline pulls, the correction kit was installed, along with the necessary longer pushrods, and within 30 minutes the next pulls gave us the results. There were no other changes. The power was slightly better all the way through the pull, but the 46 hp was observed at the shift point rpm, which was 600 rpm above where the baseline pulls saw instability start. It was actually clean through 800 rpm higher, where we stopped the pull, but the torque curve was falling fast and the biggest improvement was at 600. The correction stabilized the valvetrain by moving the peak velocity away from peak lift, allowing the additional rpm. I might add, this was a
[email protected]" hydraulic flat tappet cam, which brings up another point. Hydraulic lifters are blamed for being unstable, when many times it's a geometry issue instead. They will pump up instantly when valve float occurs, where a solid will carry until float is so severe it bounces the valve off the seat.
Before the kits, the focus was on centering the pattern on the valve tip. This was copied from the old racing tech that was used for ductile iron (non-roller) rockers. Somewhere along the way it was applied to roller rockers, which couldn't be more incorrect. Even the guys who were milling of the pedestals and making new stands were mostly just trying to center the pattern. That was viable info when using stock parts and engineering, but when things are no longer stock, stuff gets messed up in a hurry. Consider this, stock being a stock head, with non-roller rockers, a stock shaft position, stock length valves, and at best, about .470" net valve lift. How many performance engines meet that criteria anymore? If they do, they will likely not have any geometry related issues, but they won't set the world on fire, performance-wise, either. You mentioned Mopars running with the geometry they were built with, but any change to the above criteria means that the geometry isn't what it was built with.
I appreciate you asking questions and being open minded. I am open to learning as well, but in all the time I've been doing this work, I haven't yet been given a good explanation of how a different way is better. I've heard all the parroting of what someone may have read in a book once, but no solid original thought. If someone were to explain the benefit of a different way, I'll be all ears. It might help me build a better motor.
