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Old 12-21-2009, 12:39 PM
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Originally Posted by Zeeb
I bumped into some on-line debate about L-31 CR with 64cc heads and stock pistons and would appreciate some input from the forum. Someone was saying that Chevrolet used two different dish cc's depending on the application - the heavy duty trucks being set up for less CR for some reason. I'm asking because I have a stock L-31 short block salvaged from a 2001 CK2500 (yes they did put some L-31's into 2001 vehicles, although the computer at Auto Zone won't agree). I'm considering the Edlebrock top end kit (E-Tec heads, RPM "Gap" intake, and a cam). Edlebrock claims 400 plus HP on a 9.5:1 350ci. What HP can I expect out of the stock-piston , stock bore and stroke Vortec with this top end kit installed? What will my compression ratio be? Finally, if someone can settle this one - are the stock GM dished pistons used in these late model Vortec engins hypereutetic??
With aluminum heads you can up the compression by at least a full point many be a point and a half. But the factory L-31 piston follows GM keep it cheap as possible formate of a circular dish. These things are the bane of performance as they lead to a less tan ideal compression ratio for the fuel being used because of their lack of adequate squish and quench. As you can see from the picture that COBALT was so kind to include, the dish is .080 inch deep. Add to the dish's depth the typical production head gasket of about .020 inch crushed and the typical Chevy having another .025 inch between the top most part of the piston and the block's head deck. All told that adds up to .125 inch between the bottom of the piston cup and the head's squish, quench deck. The optimum squish/quench is achieved at .040 inch from the pistons head surface to the heads squish/quench deck. That's a long way from the .125 (at best) the factory lets you live with. You make up the difference in squish/quench function with the octane's you buy at the pump. Now some part of the OEM piston, that being a rim around the outer edge does get close to the head but it is too small to be of much value.

Now you're thinking of an aluminum head. An aluminum head gives off heat at a rate much higher than cast iron, so if you don't bump up the overall compression ratio to something like 10 or 10.5, you will end up with less power than a cast iron head at 9 or 9.5 to 1. And even if you stick with an iron head the squish/quench needs to be fixed. So I'd surly put the fancy Edlebrock fuel injection in the can and start worrying about basics first. Wiz bang toys can always be bolted on later, but fixing incorrect fundamentals is a lot more work later on.

So the pistons have to go. They need to be replaced with flat tops or with a D dish piston. The selection will be that which dials in the SCR to what the fuel octane of what you want to use is. Go to the Keith black site, they have a ton of pistons and some compression ratio calculators.

Squish and quench are functions that build what's called mechanical octane into the engine, they also improve off idle and high axle ratio cruise performance as well as optimizing performance against the available octane fuels. The same parts perform both functions, which are merely separated in cycle time. Squish happens first, as the piston closes to TDC the mixture on the far side of the chamber is ejected by the close closing of the piston and head decks toward the spark plug. This stirs the mixture and increases its density before the spark-plug making it easier to light off and faster to burn.

As the burn proceeds from the plug, the temperature and pressures go up very quickly. The so called end burn on the far side of the chamber wants to spontaneously ignite creating and explosion, colliding flame fronts, and high pressure waves. The mechanical way of reducing this is to have an area on the far side of the chamber that has a lot of surface area to its volume to quench the explosion by being a heat sink. These two functions work together to improve the burn giving more power and economy and to delay the onset of detonation, especially under high loads and part throttle operation. Certainly not as sexy as EFI, the basics seldom are sexy, but the foundation is what keeps it together.

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