I am wanting to build a very strong N/A 355, and I really understand almost everything very well. One thing I have never I guess really learned much at all about is increasing your compression ratio. How do you know how much you can go, before you get piston/valve collisions? Or would that ever happen? I am really confused here. Can someone please explain to me all about raising compression?

 

The pistons and valves will never collapse with higher compression. Think about it there is combustion going on in there, which raise the pressure inside many many time over the compression ratio.

Also raising compression increase engine effeciency. To a point that is where pre-detonation occurs. The the engine starts to break itself and power and effeciency go south.

9.8:1 compression is pretty much the limit when it comes to the old style bath-tub combustion chambers iron heads on pump gas. With the new fastburn chamber iron heads about 10.5:1 can be achieved. With fastburn chambers and aluminum heads 11:1 can be achieved. This is with optimized timing and a proper quench.

 

You can change compression ratio a few ways I know of - going to different pistons, smaller chamber heads, milling heads, "zero" decking the block helps too.

Piston/valve collisions happen when your valve train isn't set up correctly.

I've heard from others that 10.5 to 1 compression should be about the max for premium pump gas on a daily driver. I've always tried to keep mine at around 9.5 to 1.

With aluminum heads, I'd go 10.5 to 1 and most likely be fine.

 

valve piston interferance come from lift not compression. cam lift over .65 becomes a problem at times. the small amount of changes need to raise compression shouldn't cause any valve to piston contact, unless you're already at .65 lift, and since your asking i bet you're not.

 

And duration ede, the absolute killer is lots of lift with lots of duration. -x-

 

Holy crap i need to stop posting when i'm tired, i read piston collapse, not valve piston collision. I'm sorry for the mistake, but the info in my post still applies.

 

Here is an example of raising the compression ratio by milling the block and heads for maximum effect.

A recent motor I built had 12.65:1 compression and a long overlap cam (valves are open at TDC more than a typical street cam)
The block was decked .030". The flat top piston tops were .015" below the block deck at TDC. The heads were shaved about .070" to 52cc's. Used a .015" thin head gasket.

When checked for valve to piston clearance. the valves came within .090" of the tops of the pistons at or near TDC. This would be considered absolute, absolute minimum clearance required.