by Jim Hand
We regularly hear admonitions in print about the dangers and hazards of running ideal compression ratios. Unfortunately, most of the comments about adequate or optimum CR are negative, and don't inform us of how to actually build an engine to live with the desirable and performance enhancing higher ratios. The following information was prepared to provide an insight on all the factors that must be considered when designing and assembling an engine for best performance on the selected and available gas that each plans touse. No one can tell you what is the "safe" CR, and this series is no exception. However, it will provide a detailed look into the factors that must be considered.
When preparing and engine for high performance output, selection of the optimum compression ratio (CR) is one of the important factors in the process. However, CR by itself is somewhat meaningless. What we are really interested in is compression pressure, because that is what the engine sees. High compression pressure increases the tendency towards detonation, while low compression pressure reduces performance and economy. Maximizing cylinder pressure benefits power, and one sure way of increasing cylinder pressure is to increase the compression ratio. The cam selection and intake system can also have a major affect. In my opinion, the most important factors in selecting the optimum CR is the deck height, the cam timing, the shape and finish of the combustion chamber and pistons, the operating rpm range, and of course, the fuel available. All of these factors can be controlled to some extent, especially during initial build and assembly of an engine.
Deck Height or Deck Clearance:
This most important step consists of measuring the distance from the assembled piston tops to the surface of the block deck (deck clearance), and milling as necessary. The general feeling is that the total quench or squish distance should be about .040". The quench distance is the compressed thickness of the head gasket plus the deck clearance. As most of our Pontiac head gaskets compress to about .042", that means we want about 0 deck clearance. The quench area is the flat part of the piston that would contact a similar part of the head if you had .000 assembled quench height. In a running engine, the .040 quench height decreases to a close collision between the piston and the cylinder head. The shock wave from the near collision drives air at high velocity through the combustion chamber. This movement tends to cool hot spots, averages the chamber temperature, reduces detonation and increases power. The shock wave also provides better fuel/air mixing, and this allows the fuel to ignite better and burn faster. A faster burning fuel charge means less timing is required for optimum power output. An example of this--after running my 462 for years with a factory deck height of about .020, we set the deck to 0. There were no other significant changes to the engine (new rings and bearings, but same cam, heads, intake and exhaust systems). The optimum timing setting prior to the change was 34 degrees - that provided the fastest MPH and quickest ET. After the change to 0 deck, the optimum timing using the same Amoco gas changed to only 30 degrees total mechanical. Not only did the lowering the deck raise the CR by several tenths of a point, but by retarding the timing 4 degrees, we were later able to increase the CR even higher due to the optimum lower timing setting.
Most cam companies recommend increased CR to be used with their "higher performance" cams. Why is this necessary or recommended?
Comp Cams P-306R (Intake 275 duration, 102 LC installed,, Exhaust 278, 106 LC
What do these numbers mean in regards to practical CR? What they show is that a shorter duration/ advanced intake lobe cam, will have much higher real compression then a long duration high performance cam in the same engine. Note that the 268H provides higher real CR with 8-1 rated CR then does the 275/278 at 12 - 1 rated (static) CR. How can we use this information? First, if we have a factory high-CR engine, we sure as heck don't want to install a cam like the 268H. We want to look for a cam that will provide the desired power range while keeping the dynamic CR as low as possible/practical. Note the intake closing points of the two cams above - the 268H closes at 35, and the race cam closes at 59.5. If we can find a cam that has the intake closing later then 35 but before 59, we should have a better chance to live with the resulting CR. How abut the Pontiac cam grinds?
How about the cam that I use - Wolverine 234/244? (Intake 234, LC 107, Exhaust 244, LC 117). Intake valve closes at 44 degrees ABDC.
Again, a review of these numbers shows us that a RA III or RA IV cam will provide the actual CR of the CC 268H while running a full point higher rated CR. None of this is to recommend or discount the use of any cam, but is only to show the relationship of the intake closing points on true engine CR. Another way to use this relationship is to see what cam might work best if we have low static CR and want improved performance in the driving rpm range. The CC 268H provides almost a point higher real CR then does the RA III or RA IV, so on an 8 to 1 engine, we would have much better throttle response and engine power within the operating range of the CC 268. Do be aware that this type of cam has a much shorter rpm range then either of the Pontiac cams.