Maximizing High Ratio Results
At this point it is pretty clear that making the most of the potential that can be had from high compression is a goal worth pursuing. But as the ratios sought get higher, counter-productive problems can begin to arise. Probably the most commonly seen of these is due to the final combustion chamber shape achieved when all the stops have been pulled out. The problem here is that as ratios much above about 10:1 are required, the only way to further minimize the volume after maximizing head milling is to have a raised crown piston. Up to a point, this is okay, but if the crown intrudes into the chamber too far, it can severely compromise the flame travel, resulting in a very ineffective combustion process. As to how much can be lost, suffice it to say I have seen a hundred horsepower disappear because of a piston crown intruding an eighth inch too much. The rule here is that unless you know what combination of chamber and crown form works or are prepared to do the necessary R&D, don't go overboard on crown intrusion into the chamber. For typical small-block V-8 from Chevy, Chrysler or Ford, a good rule of thumb is to use no more than 100 to maybe 125 thousandths crown height in your quest for a high CR.

If you are forced to stick with conventional heads patterned after the OE-style head, then big-block Chevys can be something of a law unto themselves. Compared to a regular parallel-valve engine, the chamber is somewhat less than conventional. A big-block Chevy will tolerate a substantially raised crown before the trade-off starts to cancel out potential gains. The key is to make sure the raised section of the crown does not too closely shroud the spark plug.If achieving the CR results in an overly intrusive crown, there is an alternative solution. Instead of trying to reduce the capacity of the combustion chamber, try increasing the capacity of the cylinder. Either a bore or stroke increase will do this. For instance, if you were looking to achieve say 10.5:1 with a 454, it will take a maximum head-milling job plus a piston intrusion approaching half an inch to achieve. The head-milling job is going to mean a lot of possibly expensive manifold machining to re-align the ports. An easier and only minimally more expensive way would be to install one of Scat's cast steel 4.25-inch stroker cranks. This, in conjunction with a 100-thousandths overbore will not only deliver 505 inches, but also allow a 10.5:1 ratio to be achieve with a very acceptable crown height of about 150 thousandths. The same kind of move can be beneficially applied to small-blocks. Using an inexpensive stroker crank in a 350 Chevy not only delivers extra cubes, but also allows a 10.5:1 CR to be achieved with flat-top pistons and regular un-milled 68cc heads.

Let's talk quench clearance for a moment. The quench clearance is the distance the deck of the piston is from the cylinder head face at TDC. Loose (wide) quench clearances can actually promote detonation. The worst to have for most conventional-style wedge head V-8s is about 100 to 125 thousandths. Reducing this clearance (by block milling or a taller piston) can actually stave off detonation by a substantial amount. As to how tight the quench can be made depends on how flexible the block and bottom end assembly is and how much thermal expansion has to be allowed for. With good steel rods and crank, the net clearance can usually be taken down to 30 thousandths. With a typical FelPro gasket of some 40 thousandths thickness this will mean the pistons come out the block by 10 thousandths.

If quench is so good at suppressing detonation and allowing the use of higher CRs for more power and better mileage, why doesn't the factory make it tight to start with? In a nutshell the answer is emissions. Tight quench over too large an area (such as seen in a typical small-block Chevy or Ford of the pre-1997 era) causes unburned hydrocarbon emissions to go up. However, quench is a key element toward fast burn and this in itself can lead to the successful use of a higher CR just as we see with the LS1/6 family of engines. For modern engines, the trend has been to use a more open chamber with less quench area, but to make the quench action more active by tightening it up as necessary. Although high compression benefits fuel mileage, it can bring about a dramatic increase in oxides of nitrogen, which is the primary cause of smog. Offsetting this is the fact that because a fast-burn chamber requires less ignition advance, the amount of cylinder pressure and temperature generated to develop a certain amount of output is less, so in that respect oxides of nitrogen are lowered. In all, optimizing quench clearance and quench area (as a percentage of the bore diameter) is something of a tight-wire act done at the OE level and you may ask if we should worry about this for our street machines? The answer is "no." Some high-flow cats and a well-calibrated fuel delivery system will keep emissions adequately in check.

Containing the Pressure
Having a high compression ratio brings about greater demands on cylinder sealing. The higher the pressures involved, the more attention needs to be paid to details. The first part of the equation toward sealing up the cylinder is to make sure your machine shop hones the block right. This should involve the use of a deck plate to simulate the distortion brought about by the stresses of head bolt tightening. Next, make sure your machine shop is aware of the type of piston ring material being used so they can apply an appropriate finish. Then give the bores a good rub down with a new Scotch Brite pad and plenty of Gunk engine cleaner. After that, scrub (with a stiff brush) the bores with a strong liquid detergent and hose off with hot water. After you are sure they are clean and grit-free, hose the block down and spray the machined surfaces with WD-40 to prevent rust.

Now that the bores are ready, let's look at the rings that will ride on them. With modern oils, ring wear is hardly the problem it used to be. This being the case, use the thinnest rings practical. Many older-style V-8 pistons are still in wide production. The majority of these pistons still have 5/64-inch compression rings. There is no good reason for using these wider rings. A 1/16-inch or even 43 thousandths wide rings are what you should go for. Be aware that the wider the ring gaps are, the greater the loss of cylinder pressure and consequently power. Add to this an increase in blow-by into the crankcase. This contaminates the oil faster and will necessitate more frequent oil changes. If you are going to stick with conventional rings, then gap them to the minimum recommended by the manufacture. If you can afford them, go with Total Seal rings as they really do deliver near 100-percent sealing capability and equally important, they maintain it over a substantially longer period then even the best regular type rings.

You may have heard the term "gas porting," but may not be too sure what it means. This is a technique to back up the top ring with combustion chamber pressure so that the ring is more firmly pressed against the bore. There are two types of gas ports, those that pass down through the crown of the piston and those that are located radially intersecting the top surface of the top ring groove. The radial-style gas ports are common for long-distance race engines. The current trend is to use radial gas ports as they seem to be as effective, but do not unduly accelerate ring and bore wear at TDC. With a good race blend or street synthetic, bore wear at TDC is not really an issue. I have just completed a 1,000-mile endurance test with the new Joe Gibbs Racing race oil and the rings of the gas ported JE pistons in my Cup Car engine wore less than three tenths of a thousandth off the surface. This amount of wear led to the ring gap getting bigger by only about 1 thousandth. An oil analysis at the 100- and 1,000-mile point indicated that most of the wear took place in the first 100 miles. This indicates that the ring and oil combination could be good for as much as 10,000 race miles.

COMPRESSION RATIO DEFINITION
The CR is the ratio of the volume above the piston at BDC (left) compared to the volume at TDC (right). The formula for the CR is (V+C)/C. In this formula, V is the swept volume of the cylinder (i.e. the cylinder's displacement in cubic centimeters or cc) and C the total combustion chamber volume (in cc) when the piston is at TDC.

An example would look like this: say the volume above the piston at BDC is 110cc with 100cc being the displacement volume (V) due to piston motion and 10cc the total combustion space (C) remaining at TDC. When the contents of the cylinder at BDC are squeezed into the 10cc remaining at TDC, the charge occupies 1/11th of the space so the CR is 11:1. To find out what total combustion chamber cubic centimeters are required for the CR, you want subtract 1 from that ratio and divide the result into the displacement volume of the cylinder.

CC'ing Heads
The basic essentials to cc heads (and pistons if they have a dish) are seen here. This includes a 100cc burette and a stand to hold it. Also a Plexiglas plate is required which, for most domestic V-8 heads, will require some eyebrow cutouts to clear the valves. COMP Cams has an inexpensive kit with all the parts you will need. For an easy-to-see measuring fluid, use windshield washer fluid. The alcohol in it minimizes rust and helps cut surface tension.