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Old 01-17-2009, 09:44 AM
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327 with dish piston

i have a cam with a 270 duration/449 lift i bought a327 short block with dish piston it had been rebuilt but 8 month ago spun a bearing i have replace the crank but not the piston and make this engine a daily driver in a 85 pickup as far as heads i want a set of low compression heads will work well own this engine

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Old 01-17-2009, 01:05 PM
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Quote:
Originally Posted by ellis
i have a cam with a 270 duration/449 lift i bought a327 short block with dish piston it had been rebuilt but 8 month ago spun a bearing i have replace the crank but not the piston and make this engine a daily driver in a 85 pickup as far as heads i want a set of low compression heads will work well own this engine
To start with modern Ricardo chamber heads have done away with the concept of low compression as a way to manage unleaded fuel's restricted octane level. This opens up the efficiencies and power found with higher compression ratios.

Secondly, the cam you have being a fairly long duration cam demands higher compression. With a long duration cam comes more overlap where both the exhaust and intake valves are open at the same time and intake closing point is later in the compression stroke. Both of these functions result in the loss of mixture typically at RPMs below the engine's torque peak, for this cam think about 3500 RPM. The effect is the loss of incoming mixture out the exhaust during overlap and reverse pumping by the piston of the late closing intake when mixture velocity is not high enough to overcome the effect, in other words no ram filling of the cylinder. Both of these conditions lead to low density of the mixture within the cylinder at RPMs under the torque peak. These effects reduce the engine's Dynamic Compression Ratio which is a measure of how much mixture by count of molecules thus greatly reducing efficiency and power. The answer to this condition other than going to a milder cam, is to increase the Static Compression Ratio, which is the measure of all the cylinder volumes against those volumes above piston when its at TDC.

So seeking a low static compression is the opposite direction from that required to develop efficiency and power. Modern heads such as the Vortec allow us to gain back compression to make efficiency and power while also allowing a high degree of detonation tolerance. It is possible with these type heads to run an engine with 9 to 9.5 to static compression ratio with 87 octane unleaded fuel.

To optimize your engine around this type compression with the cam you have you need to look toward a piston such as the KB156 a flat top with 7ccs of valve reliefs. Assuming the bore is still 4 inches, which it probably isn't, that the block has the spec .025 deck height above the piston, and the gasket is .025 inch as well; if you use the 64 cc chambered Vortec heads you would have a static compression ratio of 9.3:1. This is a good ratio for todays fuels with that head. I'd go with a high energy ignition, especial a multi-spark, as these are excellent at getting lower thru mid RPM mixtures to light off without having to resort to excessive advance. Big amounts of advance cause the pressures to rise too quickly before the piston comes over TDC, this loses power and efficiency trying to drive the piston backwards as well as increasing the tendency to detonate which drives you to buying higher grades of fuel.

Bogie
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