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Old 11-29-2009, 10:15 PM
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combustion chamber help

Hey guys I just picked up a set of 4V 351 cleveland closed chamber heads. Having a solution of lye from previous projects, I boiled them until they were clean.

Here is my question.

These will be going into a 10.5 compression ratio engine, and I am wanting to smooth the combustion chamber surfaces out. Right now, its cast iron so it looks pretty rough. What do I need to get them machined surface smooth?

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Old 11-30-2009, 11:10 AM
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Quote:
Originally Posted by majorownage
Hey guys I just picked up a set of 4V 351 cleveland closed chamber heads. Having a solution of lye from previous projects, I boiled them until they were clean.

Here is my question.

These will be going into a 10.5 compression ratio engine, and I am wanting to smooth the combustion chamber surfaces out. Right now, its cast iron so it looks pretty rough. What do I need to get them machined surface smooth?
For a guy at home, the process is to use mini sanding wheels and drums in a hand held tool going from course to cut to fine grits to polish. Typically this will be a high speed electric die grinder and or a Dremel tool. The die grinder is better for larger and more open spaces of the chamber while using the Dremel for more confined areas. Both tools are easier to manage with a variable speed controller installed. Don't be tempted into air tools unless you already have a thousand plus dollar, 60 gallon tanked, two stage compressor. Air tools consume a lot of air and 5 horse, oil-less, single stage compressor just doesn't have enough continuous capacity to operate these tools the way porting demands their use. So just by electric tools for this job, get some ear protectors, face shield, and breathing mask, and an adjustable worklight while you're at the tool store.

The seats need to be protected, use an old set of valves for this. damage to the seats require recutting them, this sinks the valve which poorly effects mixture flow past the seat and valve. It also is a process that makes the combustion chamber bigger which reduces compression. Keep in mind that polishing the chambers also increases their volume reducing compression. But then 10.5 on unleaded in a cast iron head is asking a lot. This is where squish/quench is really important, especially with Ford's typically oversized ports.

Ford's large ports tend to reduce mixture velocity quite a bit until the RPMs are really wound up. This leads to a stratified mixture with lines of rich and lean streamers entering the chamber. You want to get this stirred up before the spark gets applied because a homogeneous mixture burns faster and more completely reducing the tendency to detonate an gives you all the power in the gasoline you paid for. To this end getting the Ford to squish the daylights out of the mixture as the piston closes on TDC is really important. You want to have the piston crown to close within .040-.050 inch of the squish/quench deck of the head. This requires a flat top piston, possibly some deck milling, and a thin head gasket. To keep the compression in a zone that 92-94 octane unleaded will tolerate may require some amount of piston dishing beyond the volume offered by the valve reliefs. If this additional volume is required, use a D dish piston crown where all the dish in under the valve pocket. The close closure of the flat section of the piston to that of the head violently ejects the mixture from that side toward the spark plug; this stirs the mixture of fuel and air and increases its density in front of the plug for improved light off. On the back end of the burn the high surface area to volume, keeps the mixture on the far side from exploding ahead of the flame front. These are good things.

Bogie

Last edited by oldbogie; 11-30-2009 at 06:08 PM.
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