I know that there are engineers that have for some time been working on a gapless top ring. I heard awhile back that Childs & Albert or somebody else, I can't remember, was trying to do this and had a prototype. I don't really see the urgency for that though, and I'll tell you why. If you could get a gapless top ring to work, you would still have some leakage across the face of the ring. Besides, a little leakage is necesaary for the oil ring to work properly. The only benefit of a gapless ring, top or second, is that it makes the leakdown look good. There is one exception, an alcohol or nitromethane engine that is bad about contaminating the oil with fuel can benefit. A gapless second ring will reduce oil contamination in those engines. We don't have this problem with gasoline engines though.
When we leak down an engine the leakage we see is what the ring leaks at low temperature. This is not indicative of what is really happening while under power. The problem is that we are not leak testing the standard rings at the temperature at which they operate. If we could somehow leak test with the top rings at full operating temperature we would then see the real leakage of the top ring. The top rings leak the least when hot and the gapless rings leak good at any temperature. This is the main difference between them. When an engine is under full throtle there is a lot of cylinder temperature at that time and the ring gap shrinks up considerably, yeilding a much lower leakage rate. When the end gap on a top ring is sized correctly the gap will almost close during a power run, effectively making it an "almost gapless" ring with just a few thousandths of an inch clearance for insurance against the ends of the ring "butting" and causing cylinder wall and ring face damage. The leakge past this small, almost closed gap is negligible. I know savvy engine builders that can, through careful inspection and documentation during race engine rebuilds, incrementally reduce the top ring gap a little at a time and eventually get it to its smallest allowable dimension for THAT engine. It is not something that many of them stay with though because it doesn't yeild much power per man hour invested. There are greener pastures to find power in. Why hunt for squirrel when you can get a 6X6 elk for the same effort.
The thing that affects ring gap is heat. More power, more heat. Nitrous? Blower? More heat, bigger top ring gap. Running the top ring closer to the top of the piston? More heat, more gap. More cylinder temp requires more gap. Also, some cylinders run hotter than others and therefore the required minimum ring gap can be different for each cylinder. We do what we can to make sure every port flows the same, every cylinder has the same compression ratio, same amount of fuel, ignition timing, and valve timing, etc. in an effort to make every cylinder do the same thing powerwise and heatwise. We want every cylinder to reach detonation at the same place in the tune up. The thing we can't fix completely is the cooling of each cylinder. It is very hard to get each cylinder to cool exactly the same, and you'll always have some that run a little hotter. Two of the problems with heads and cooling systems involve uneven water input and steam produced in the water jacket from water boiling off of the combustion chamber walls. Combustion chambers run very hot and it is normal for water to boil off the water jacket side of the chamber. How well a head disposes of, or IF it disposes of, this steam is very important. One of the things we do on drag engines is reverse cool them. We'll put water IN under every exhaust port, and take water OUT at each end of the head (4 corners)and through the front of the block. What this does is put cool water where it is needed the most and puts an equal of water amount to each cylinder. A cooling system like this is not really feasible on a street engine. Another thing we sometimes do is put "steam lines" on the heads. This is involves tapping into the highest part of the water jacket, where the steam builds up, usually under the valve cover area, and running small lines from there to the water neck. This allows the steam, or bubbles from the boiling water, water vapor, or whatever terminolgy you want to use, to escape from the water jacket and this makes room for fresh water to come in and cool the chambers which are running too hot, overheating and causing detonation. When you have a lot steam in the water jacket the volume in the water jacket is reduced significantly, further hurting cooling. I know one NHRA Pro Stock engine builder that uses water jacket volume as a criterion for selecting head cores. He'll cc the water jacket, and I have also seen him cut holes in the outside of the head and go in and do some judicial porting in the water jacket in critical areas that need more water flow in order to reduce detonation. The Winston Cup guys get around this cooling system problem easy, they just run a lot of pressure on the cooling system, something on the order of 65-70 lbs., to eliminate the water from boiling in the first place.
Most engine builders take the view that a blown engine does not need any particular attention given to the intake ports and instead focus mainly on the exhaust side. I disagree with this approach and feel that these guys are leaving a lot on the table. Blown and supercharged engines gain MORE horsepower from intake port work than their normally aspirated counterparts because of the higher operating pressure of the intake ports, which is boost level you have versus atmospheric pressure(14.7 psi). A 10% gain in flow equals more cfm at, say, 10 pounds of boost than it does at zero boost. If I pick up 30 cfm at 28" on a 300 cfm intake port, that 30 cfm gain goes up as I increase the test pressure, like if I were to take the test pressure up to 48", which is as high as my flow bench goes. The percentage gain stay the same but the actual cfm gain goes up.
Let me give you an example. I have a customer that ran Blown Alcohol Hydro in SDBA a number of years ago. He bought two sets of heads. One set were Veney heads and had awesome exhaust flow and decent intake flow. His other set were from Brad Anderson and had great intake flow, way more than the Veneys, and the exhaust ports while good, were far less than what the Veney heads had. We assumed the Veneys would run faster because of the higher exhaust flow and decided to use them as the primary heads, with the Anderson heads as backup. We figured if we needed more intake flow he'd just put a "wagon wheel" on the bottom and a "dime" on the top. Well, we ate crow, because the Andersons were faster every time and they became the primary heads. Another customer used to "try" to run Top Fuel, and he had heads and engines from one of the more competitive NHRA teams at the time. Those heads had 2.500" intake valves and the intake ports were big enough to throw a cat through. All of the fast, professionally done, Top Fuel or Blown Alcohol heads I have ever seen were geared towards massive intake flow. Any competitive blown head I've ever seen had really good intake flow. Basically, having a blower does not mean you can forget about the intake flow.
I like to keep the intake/exhaust flow ratio between 72-75 percent on blown street engine. In my experience this seeems to be enough. Anything more is gravy. My standard 290/220 cfm Viper heads are within this range. You have to remember that the exhaust port is somewhat self compensating in that if you get more air in (through porting or boost)and have more cylinder pressure, the resulting pressure that the exhaust port sees when the exhaust valve opens is higher, and when you up the pressure, you also up the flow. You can help out the exhaust side somewhat with the camshaft. You can run what is called a "dual pattern" cam that has some "spread", which is extra exhaust duration. A lot of people already do this on their unblown engines. You can also run an intake lobe on the exhaust. They typically have faster ramps and more "area under the curve" which helps the exhaust phase.
A lot of nitrous guys also like to focus only on the exhaust side of the head. But from what I've seen nitrous picks up an engine no matter what the heads flow. It's a no-brainer, a cure-all for lack of engine building knowledge(no offense meant to anyone). I try to avoid working on nitrous heads because I hate doing a really nice set of heads and then watch them get torched. Inevitabally, some of my heads get nitrous run on them because it's hard to beat a nitrous motor unless you also have a power-adder of some sort. It doesn't matter how good a nitrous kit is or how smart the tuner is, if you have nitrous, you're going to burn the engine down eventually. I generally view nitrous as ****** for racers anyway. It's pretty boring to me.
