That was me. (Nerobro) "Why don't I think [valve cooling] works out in favor of larger valves."
I was on my phone at the time, and didn't have time to do any real math. I could be wrong. But it was my hunch. I'm happy to be wrong in public though, so lets see here. Assuming similar valve grinds, so you get equal contact width between the head and valve. I'm going to use some fanciful sizes, because they're easy. A 1" valve and a 2" valve. A 1" valve will have a circumference of 3.14". That 1" valve will have an area of .785". That gives you .25 sqin of face area per inch of valve to head contact. Okey, lets do a 2"valve.. 6.28" circumference, 3.14" of valve area. That gives you .5 sqin of face area per inch of valve to head contact. A 3" valve (just to set the curve.. as that needs at least three points..) has .75 square inches of valve area per inch of seat. As you get larger valves, you get less room to transfer heat to the head. Assuming that the valve stems are similar sizes, you'll get similar amounts of heat transferred to the valve guides, and oil in the heads. If the valve is larger, you will have more heat gathering area. Am I missing something? It doesn't follow that larger valves won't get hotter. The peak temps in an IC engine are around 3600degF. The cylinder head is going to be less than 460degF. Stainless steel melts at 2700degF. Exhaust gasses are high enough to melt valves. Every time that valve opens, it gets bathed by a fast flow of hot, turbulent, gasses. The perfect sort of thing to transfer heat to them. A smaller valve, will start off cooler. Because it's smaller, it's got a higher ratio of mass to places to transfer heat out of itself. Given similar valve stem diameters, the smaller valve will be able to sustain higher heats longer, as it's collecting less heat from the gas stream. This is evidenced by airplane engines have big valve stems and frequently sodium filled valves as ways to get heat out of those valves. Having a large head on a valve, increases the heat collecting area, and does unfavorable things to the heat transfer properties. Even if the valves greater mass makes it heat up less in a single cycle, it's going to get hotter in the long term. For it to get rid of the same heat load per square inch, it will need to transfer more heat across a smaller area. This means the larger valve can't cool down as fast as the smaller valve. Additionally, using a car engine at airplane rpms, makes the port speeds rather slow. I'd be apt to use the smallest valves and ports I can get my hands on. And i'm 100% on board with you on the difference between a cheap valve job, and a good one. :-) -Nerobro On Mon, Jul 6, 2015 at 2:08 AM, Mike Stirewalt via KRnet < krnet at list.krnet.org> wrote: > Somebody (unsigned) said, > > > "Bigger valves also have more area to absorb heat." > > True. > > > "I don't think that works out on favor of larger valves." > > Any particular reason you think that? > > > "Also stem area remains the same. That works out in favor of smaller > valves." > > It does? Why? > > > "You can also take care in grinding seats to increase heat transfer > area." > > I completely agree with that and in fact is why I wrote "Whether with > oversized valves or OE valves, taking great care to optimize the valve > and valve seat interface is one of several factors that differentiates > between a well-done valve job and a buggyshop valve job." > > ************* > > The heat generated within the combustion chamber doesn't vary depending > on the valve head size. That combustion chamber heat has to go > somewhere. As the poster states, a larger valve head will absorb more > heat. It won't get any hotter than a smaller valve head, it will just > absorb more heat. Because the valve and valve seat contact area of the > larger valve is significantly greater than the valve/valve seat contact > area of a smaller valve, more of the heat generated within the combustion > chamber is transferred to the head, the fins and away from the engine. > > ************* > > I've admired but not studied the R-2300 so don't know what valves are > used and whether their head diameter has anything to do with the crack we > saw between the inlet and exhaust ports. I would guess that crack was > more the result of excessive cylinder head temperature (which larger > valves mitigate) than anything to do with the distance between the two > valve seats. The seats on the GP2180 and 2100D are quite close and the > heads do not experience cracks - at least I've heard of none. If guides > were bored off center as in Jim Hill's engine or if an engine is > suffering some other defect in manufacture or in operation then anything > is possible. The only semi-chronic problem with these engines I've heard > of is cracked cases, especially the magnesium ones . . . and that is from > - as is always the case with VW's - operating the engine beyond > temperature limits for an extended period. These engines are remarkably > forgiving of temporary abuse, but not if it continues. > > Robert Gill's account of his 18 months of headaches with his R2300 > doesn't sound encouraging regarding this relatively new engine of > Revmaster's. This is the second negative report we've had on this engine > that I'm aware of. One would need to do a lot more research before > writing this engine off - Revmaster is hardly a newcomer when it comes to > building engines - but if I were engine shopping for anything larger than > a GP2180 there's no question in my mind I would be thinking Corvair with > an Ellison and all the goodies (like the one Dan Heath has for sale). > > Mike > KSEE > > ____________________________________________________________ > Buffett?s Warning for YOU > 4 in 5 Americans aren?t taking his shocking advice. Click here now. > http://thirdpartyoffers.juno.com/TGL3141/559a299d7caf4299d6d61st04vuc > > > _______________________________________________ > Search the KRnet Archives at http://tugantek.com/archmailv2-kr/search. > To UNsubscribe from KRnet, send a message to KRnet-leave at list.krnet.org > please see other KRnet info at http://www.krnet.org/info.html > see http://list.krnet.org/mailman/listinfo/krnet_list.krnet.org to change > options > >
