Originally Posted by mikebaker87
I am looking to build a streetable 400 for my camaro, looking to use closed alum heads (64 cc), old bow tie victor jr single place manifold, 650 holley double pump carb, short hooker comp headers, those are the parts I have so far. I'm looking to buy dish top pistons in 20 cc range to keep compression down, use the stock rods, aftermarket crank and spin it up to around 6000 rpms give or take with a rebuilt 700R4 trans and 3.23 gears in the rear. This is my first engine build so any help anyone can offer in areas of setup combs, cam, carb selection or anything I am forgeting will be greatly appreciated, thanks.
OK here's the deal with combustion chambers. It's all about squish and quench. That is as there's flat spots in the chamber, what older presmog heads and modern heads have these behind the spark plug and opposite the valves. These are very effective in squeezing the mixture into the space by the spark plug, this is called "squish". When the burn is ignited at the spark plug it spreads out in all directions increasing in temperature and pressure as it goes. But an event can occur like you see in those house fire film clips where a flashover occurs and the fire goes from a burning sofa and curtains to a sudden explosion. The same thing happens in the cylinder, we all know this as detonation and its close friend pre-ignition. The far flat spot that squished the mixture now serves a new function of using it's high and cool surface area to suck the excess heat out of the mixture so it doesn't suddenly explode, this is "quench".
The features of squish and quench build what's called mechanical octane, that's to say the shape of the combustion chamber is a very important consideration in determining how much octane has to come from the fuel to eliminate detonation and pre-ignition. This process is most effective when the piston crown's flat surface comes within .040 to .060 inch of the squish/quench deck(s) of the combustion chamber. Most stock type pistons include a circular dish to reduce compression. These can be .1 to .2 inch deep which really cripples the effectiveness of the squish/quench. While this reduces hydro-carbon emissions, it really makes the engine sensitive to detonation and pre-ignition and causes the needed octane rating of the fuel to go way up for control of these effects.
Now with a 400 you really need to pay attention to compression ratio, it's really easy to build one with way too much or way too little depending on parts selection. Open chambered heads and flat top pistons get around too much compression, but the burn tends to be slow requiring a lot of spark lead which puts a lot of reverse rotation force on the crank resulting in reduced power output and increased fuel consumption.
Now a nice tight chamber like the Vortec head or the older 64 cc heads do a much better job of controlling the incoming mixture for a nice swirl which really packs mixture density into the cylinder. Plus they have great squish and quench features that burn fast but manage excessive temps and pressures, so you can back down the advance and reduce the amount of reverse forces on the crank, power and mileage go up. The down side is the compression ratio can get away from you. So with these heads you need a so called "D" dish piston where all the extra volume is captured under the valve pocket so the squish and quench can function tightly together as works best for octane management.
Phew, it's late and I'm gettin out here.