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Old 01-03-2013, 11:27 AM
oldbogie oldbogie is offline
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Originally Posted by thefox View Post
thanks for your info that helped alot i did forget to say that i did install a comp cam 268ex cam if that makes a difference. but if its not going to make a difference i might not bother. thanks again.
An XE268H likes more compression than what this engine probably has. Compression for the best performance you're paying for in terms of parts and fuel needs to be matched to the cam, specifically the closing point of the intake valve in crankshaft degrees. This gets to define a thing called the Dynamic Compression Ratio or DCR. The DCR is a computation starting with the Static Compression Ratio (SCR) which is simply the ratio of the piston sweep volume plus the volumes of gaskets, clearances, domes or dishes, and the combustion chamber divided by the volumes of the gaskets, clearances, domes or dishes, and the combustion chamber. The DCR adjusts the SCR for the loss of stroke in linear measure (inches or metric) due to the position of the crankshaft at the degrees where the cam closes the intake valve. To that point, the rising piston pumps out the charge it took in which reduces the charge density trapped in the cylinder thus the power output falls off and fuel consumption increases. This is an RPM bottom end condition which improves and reverses as the engine RPMs approach then exceed the cam's torque peak. This is the result of competing effects where a long duration cam that develops a lot of top end horsepower closes the intake late to take advantage of a phenomenon where at high RPMs the velocity of the intake charge can overcome the reverse pumping of the rising piston to force more charge into the cylinder and extend the RPM range. The down side is that below the torque peak charge is reverse pumped back into the intake by the piston. This greatly reduces power below the torque peak. To recover this power the SCR is significantly raised to force a higher thermal efficiency out of the charge that is there. To some extent the added compression is self adjusting for higher engine RPMs as above the torque peak time runs out to fill the cylinder so again the charge density begins to drop but the compression ratio again holds the power up by having greater thermal efficiency. Compression ratio relates to the physics of how engines operate in terms of the study of Thermodynamics, where actual pressures are more an engineering function of material strength and fuel characteristics. These are very different things that are often confused (my wife would use the word "conflated" here) in the popular knowledge base.

There are many DCR calculators on the web, you need the timing card data and rod length to use them, Keith Black has an accurate one. Generally, for the street you want to keep the DCR at 8 to maybe 8.5 to 1 with cast iron heads. Large chamber heads need to be on the lower side while Vortecs can be on the higher this is due to the difference in swirl and turbulence activity and spark plug location. Smaller chambers tend to have higher values of swirl, turbulence (squish) and quench (far side heat sinking) along with the spark plug located closer to the cylinder center to reduce burn time across the cylinder and aimed to favor the exhaust side as the initial burn point. This latter function reduces the gas temperature over the valve as it opens which allows greater detonation resistance. The exhaust valve is a much larger contributor to detonation than most people are aware of a good example of this is going back to high performance sleeve valve aircraft engines of World War II where it was found that eliminating poppet valves for sleeve valves allowed at minimum an increase of a full ratio for all other things being equal. But converting you engine to sleeve valves isn't going to happen so aiming the spark plug as best as can be done at the exhaust valve is as much improvement as we'll see short of rotary valves in the head, which has been done for the SBC but is extremely costly and outlawed by all racing sanctioning organizations that I know of.

Gearing also comes into play with how much DCR the engine will tolerate without detonation. Stiffer gears and manual transmissions make it possible to use more compression by increasing the engine’s leverage against weight and drag while the manual gear box makes it easier to select gears that match the engine’s operating characteristics against the vehicle’s weight and drag.

Largely this comes down to how much edgy performance you want. If you want all your paying for in parts and at the pump then these things need to be tightly matched.

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