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Not sure how many folks tend to favor closed heads

May 11 2012 at 11:01 AM

Fordmech  (Login F0rdmech)
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Response to Forged Piston for supercharged Cleveland?

over open when specifically talking about naturally aspirated higher compresssion builds over say 10.00 to 1. I'm thinking it's probably a majority
but some like to write them off as just not properly informed. To me, if you just consider the two chamber designs, they are of radically different geometry, not? All else equal, simply due to differing geometry, one of these heads will have less tendency to detonate than the other, however small. Consider two billiard balls randomly chosen from a set of billiard balls. One will be heavier than the other if weighed accurately enough. One will be larger in diameter than the other if measured accurately enough. This difference exists in two balls of the same general geometry (round ball). Although not so easy to measure as with the billiard balls, of the two different heads, all else equal, one is going to be more detonation prone than the other. I don't think this can be refuted but I also can't offer a definitive, reasonable, easy way to quantify or measure it. Several things about the closed head give me more confidence at higher compression than the open head. The quench area peaks the curiosity of my brain cell in several ways to help reduce detonation tendency. It adds additional turbulence at a critical point (near or at TDC) that the open head cannot. Most all literature on detonation talks to turbulence as a good thing to help avoid detonation. The word itself, QUENCH means to cool rapidly. This quench area squeezes the volatile end gas into close proximity with the cooler surfaces of the head and piston. If you subscribe to theory that hot end gas is where the spontaneous explosion (detonation) may start (some distance away from the flame front) then there seems to be some advantage here for the quench head as it cools the end gas. If the end gas is cooler it is less likely to detonate. Along the same lines, if you can burn the charge faster, burn the end gas before it takes on excessive heat, ie; in a smaller chamber with shorter distance to burn (and less time to heat the end gas,) you have probably reduced detonation tendency. If you don't subscribe to the theory that the closed head offers a shorter distance to burn (i choose to ignoring the quench area as a detonation prone area) you should at least recognize that the quench area reduces the volume of the end gas significantly by squeazing much of it out into the chamber proper. If the detonation explosion occurs in the quench space it has much less volume of gas to burn and cause harm. Of course this all assumes the flame has not reached the quench area before TDC. All of this I admit is a very loose discussion of detonation. I'm not saying you cannot build a successful engine combo with either head. There are many factors that figure in to detonation. Detonation is not a universally well understood phenomenon. Folks have been working on it since the advent of internal combustion engines and continue to do so. My point is only that when looking at the heads by themselves, the closed head seems to have some features one might explot to help reduce detonation tendency. I tend to give tthe benefit of doubt to the closed head as the better head with regard to detonation tendency.

 
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