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Old 11-14-2012, 02:16 PM
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Originally Posted by allansmith859 View Post
So when I bought my gaskets it says on the box/package what the compressed thickness is??? Also I have flat top pistons and around the outter edge of it its slightly higher than in the middle of it. So should I measure from the center of the piston or from the edge of the piston
These would not be flat top pistons; these are the awful circular dish that reduces the effectiveness of squish/quench by increasing the clearance dimension between the piston's crown surface and that of the squish/quench step of the head. The additional clearance while not being all the area since there is a higher rim surrounding the round dish does in fact substantially reduce the area of tight clearance resulting in a significant reduction in squish and quench activity. Some take the view that this is better for emissions than a highly active squish/quench, I haven't tested this but I rather think this is a specious argument the factory uses to deflect discussion as to why they insist on using these pistons because of cost advantages to themselves. Where compression ratios need to be contained to a suitable level with pump fuel octane ratings a D shape dish that is kept under the valve pocket and has a fully flat and close closing surface opposite the heads squish/quench deck is more effective at increasing efficiency and power this type piston needs to be made unique to left side or right side utilization. Where-as the round dish piston can be fitted to either side of the engine thus is produced in twice the quantity for the configuration than would be a D-dish piston thus allowing them greater quantity build for the design and tooling costs which lowers the unit part price. It may be only a few cents per part but across the quantity of parts they make it adds up to big buck savings for them which usually requires that the consumer either accept less power and efficiency or pay for a higher grade fuel when the engine is pushed to a greater output design.

When you are rebuilding you have an opportunity to correct these deficiencies if you realize they are there. This is where you see a true flat top piston is where the crown surface is the same surface level, except for valve reliefs, all the way across. Here again one encounters a cost savings measure taken on lower cost replacement pistons where there are 2 rows making 4 valve reliefs, again this is a means of using one set of tools to make left and right side pistons. For slightly more there are 2 relief pistons which are made in unique left and right side configurations.

Squish and quench are often referred to as "mechanical octane". This is to say that their function is such that for a given compression ratio they can add to the fuel's octane by the way they stir the mixture, concentrate the mixture before the spark plug this is the squish function as the piston closed toward Top Dead Center. The quench function is that the close closure of the piston and head on the far side of the chamber works as a heat sink to keep the mixture furthest from the spark plug from self-igniting from the temperature and pressure effects of combustion before the flame front gets to that side of the cylinder. As the bore becomes larger and where the spark plug is placed are huge players in this function's quality. To a large extent the bore is what the bore is so the place you can play is with where the cylinder head places the spark plug. Heads from the early emission days placed the spark plug way off on the exhaust side of the chamber, they were after long slow burns at low compression to keep nitrogen oxide formation low as this is hard to clean up where this also creates high levels of unburnt hydrocarbons these are easy to clean up from the exhaust. It once was very popular to put 305 heads on 350s to punch the compression up but these heads also suffer from the spark plug being a long way from the center of the cylinder so it's difficult to take full advantage of the compression improvement without hitting detonation and preignition well before all the potential power has been taken from the increased compression. Starting with the L98 and Swirl Port heads you see the spark plug being moved toward the valves to reduce the distance the flame from needs to travel which reduces the tendency to detonate or pre-ignite. This is continued even further and has the added advantage of improved porting with the LT1 and 4 heads, the Fastburn and L31 Vortecs. This idea has also become predominate with the aftermarket head industry which in many cases can also be seen in their large chamber 70 something cc heads where the spark plug is in a boss the protrudes up into the line of the valve diameter. These chamber designs that induce a lot of squish and quench while moving the spark plug as far into the cylinder as the valve location will allow are like adding 5 or more octane’s to the fuel you're using. This lets you push the compression which is where there is more power and efficiency to be had.

We've done a back to back dyno test of a 355 Chevy engine that had a common configuration except for piston crown shapes. At redline WOT and redline there isn't a lot of difference on power output between the round dish piston and the D-dish. Although the D-dish holds its top end power band a little stronger and higher. Where the big impact is seen between these dish configurations is off idle up almost to WOT, RPMs where especially the torque line is fatter sometimes as much as 30 foot pounds this of course translates into more horsepower in this region as well. The story here is where a street engine especially spends most of its time it will pull harder with less throttle which leads to less fuel consumption at cruise. It's a story that squish/quench is better with the D dish piston, the round dish starts to improve as the RPMs peak to where the torque and horsepower differences get down to only about 5 numbers torque or horsepower in favor of the D-dish. If I figure out how to get Windows 8 to make decent graphs I'll include that analysis.

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