Hearing that certainly makes the day better lol. Thanks, Mixxer.
I thoroughly believe that anyone coming from porting 2 strokes to porting 4 strokes will do well. I may have a KX85 cylinder that I'll be posting in a different thread, probably in off topic or other bikes. This is of a higher priority than the 450R head because it's a paying job. Furthermore, if he likes it, he has a shed full of 2 stroke bikes that he said he'd send my way. I couldn't help but smile out of hopeful anticipation.
I may also be looking for someone to collaborate on with this KX cylinder. Are you interested, Mixxer?
Back to 4 strokes for a bit. I'm sure that the non-2-stroke crowd is getting bored with this stuff lol.
I'm a firm believer that the less timing needed for maximum power across the board, the better. MacDizzy said it best when he said that we should always strive for 0° of ignition advance. A lot of guys are probably wondering why I brought it up (and those will also google MacDizzy, The Enthusiast), but I think it pertains since those porting a head will, or REALLY SHOULD, have the head off, which is the best time to modify the combustion chamber. Altering the combustion chamber shape should improve on fluid flow, heat absorption, and flame travel. Since porting touches on fluid flow and since those 3 topics overlap more than anyone would like, I believe that it is highly relevant.
First, any time I say "timing requirements", I mean the amount of timing required to make the most power at a given RPM. Second, everyone should be aware that the type of engine at hand (450R) derives it's power from the expansion of gasses due to heat and not by an explosion. That being said, we want to create as much expansion possible by heating the gasses as much as possible. This is why an increase in power by 50% will result in an increase in heat production by almost exactly 50%. This is also why thermal coatings on pistons, sleeves, and chambers will produce more power. The common idea is that it reduces friction between the piston and sleeve, which is true, but it only accounts for a small portion of why it works better. What really happens is that the coating insulates the materials and retains the heat within the pressing gasses, which would otherwise soak into the piston, sleeve, and head, reducing pressure and ultimately reducing power. Short version: If we contain the heat, we retain the pressure and increase power.
If you've ever worked with a coated engine, you'll notice that you can reduce timing requirements by a few degrees and make more power because of the reduction in pumping losses. The cheapest option is to mirror polish all of the chamber and piston crown, but take heed that this will affect the last bit of atomization that the AFM will see before ignition. For leaner engines, I like to keep everything smooth, but those that run a little rich can benefit from a rougher finis. I mirror polish every 2-stroke that I'm given free reign to build, including those of mine. It also helps prevent detonation because everything is cooler AND because we can sometimes reduce timing requirements.
Now, before we can retain the heat, we have to produce it. Hopefully, we can do it as quickly as possible for the sake of power production. This is where flame travel comes into play. Since the largest factors to flame travel are combustion chamber geometry, contents, and RPM, we can be assured that any and all improvements to the combustion chamber will alter at least one of these three things. We generally don't run exotic fuels and aren't running forced induction here, so contents aren't so much a factor. We also run in a wide variety of RPM ranges, so target RPM doesn't come into play as much as with other engines. That leaves us with geometry.
Since this thread is for porting, I'd say to start with that, but Mixxer basically nailed that. Next, we have flame travel. This is where areas begin to overlap. A head that's been engineered to flow the very most with nothing else in mind may not necessarily have a design that is conductive of a fast burn, also called maximum squish velocity (MSV). I tend to come from an area that can easily separate flow and MSV (2-strokes) where the flow is down low and the burn is up top.
With that being said, I tend to lean more towards optimizing flow, but have more of a concern for MSV with 4-strokes because the subjects overlap. Truth be told, I'm more focused on MSV than flow in the chamber because of my background. Too much, almost. Think of some F1 engines where it was told that the piston came close enough to the head that the spark plug's ground was the piston. That implies 2 things: one, the squish clearence is very tight, and two, that there can't be too much advance if that actually is the case. The mind boggles...
I'll get on to the practical side now. Much like the ports should be free of major obstructions, so should the chamber. This includes the head, valves, sleeve, and piston since they all see the action. If anyone comes up with a way to improve on the sleeve for MSV, I'm all ears. Until then, let's focus on the rest. Up next, valves can't be improved upon too much because the rounded lip absolutely has to shed heat. If you shave them down flat to make the chamber as free flowing as piossible, you're going to have some very sharp edges just hanging out there for hot gasses to soak into AKA not good. So we have the head and piston crown.
Be aware that taking too much material away will reduce compression, which will result in a slower burn. Higher compression configurations place all of the gasses closer together, resulting in each molecule seeing radiant heat quicker. Remember, heat is what we're after, so keeping everything together is a must. However, a faster burn with lower compression is roughly equivalent to a slower burn with high compression. We aren't after compression, really. We're after faster burn, which compression aids. As a side note, the higher compression also places all of the expanding gasses in a smaller area, therefore increasing pressure to aid the downward travel of the piston. See just how encompassing heat and pressure production are? Eh?
Now if you take the 450R head for example, the spark plug is recessed. This is a design that I don't like. However, it does shield the plug from direct exposure to cold, wet gas, which I can tolerate. But in the end, I'm going to make the transition from the spark plug area to the rest of the chamber more streamlined. This will aid flame travel, I think. See, if the flame travels downward, it creates turbulence in the chamber when it slams into the piston, which will ensure that each molecule has a higher chance to burn and radiate heat to even more molecules. On the other hand, a direct path will allow the burning molecules the same opportunity to affect other molecules because they're traveling outward because from the spike in pressure at the plug at ignition. As such, I like to think that a straight path will allow more radiant heat to be dispersed evenly, increasing MSV.
So it's up to you there. You can either keep the plug shrouded or open it up a bit. Unfortunately, there isn't much room to play with.
Now the edges of the valve shrouds play a huge factor here, but I haven't quite put my finger on what's happening. If I could, I could design a better shape. However, I do know that from my 2-stroke days that obstructions = bad. The almost 90° bend fromt he shrouds to the flat surface of the bottom of the head can't possibly help MSV, but I may be wrong. The only thing keeping me from smoothing it out altogether is that I haven't tampered with deck height and squish clearance with very many 4-strokes. I think that the squish clearance will be a hand-in-hand endeavor with altering those edges, but I have other things on my mind.
Keeping in line with the above (fluid flow, flame travel, heat absorption), we haven't touched on heat absorption in a way that matters to those with a carbide spinning. To be quick, sharp edges are bad for all three areas in equal proportions. Sharp edges will absorb heat and create hot spots. In severe cases, this will induce pre-ignition = 5 minutes till rebuild time. Some people can come up with reasons as to why everything I mentioned above is complete BS, and they should for sake of intellectual discussion, but no one can argue there. High spots will absolutely absorb more heat, no exceptions.
Keep in mind to balance the 3: fluid flow, flame travel, heat absorption. A few hours of critical thinking will answer most of your concerns before you grind. Alternatively, share your questions/findings/concerns here and keep this thread alive.
