Combustion Chamber Modifications

[Anonymous]

My question is Unshrouding around the intake and exhaust valves. I also understand keeping the finished combustion chamber within the head gasket cylinder outline.
How much would it hurt the head to unshroud into the flat portion or straight portion, opposite the spark plug, of the combustion chamber to keep a constant radius around the valve head?
Will it help the exhaust flow to unshroud as large a radius around the valve as possible?
I am currently cleaning up the intake and exhaust bowls and noticed there seems to be a lot of material that could be removed to allow for more flow around the exhaust valve. I realize the head will need to be milled to get the combustion chambers back to ~60 cc's.
Thanks,
Ken

 

[Lazy JW]

Some unshrouding will certainly help; there is a point of diminishing returns and then of course a point where you strike water

There are some formulas for this unshrouding called "curtain area". As you already mentioned, removing too much material will reduce compression but that can usually be compensated by milling the head. If you are installing oversize valves then it would behoove you to learn more about curtain area; you certainly would want to maintain at least as much distance from the valve as a stock size valve has, and a bit more would be desirable. Sorry I don't have more specifics, but this is a bit beyond my expertise. Lots of info can be found at www.speedtalk.com
Joe

 

[Anonymous]

Thanks Lazy JW, you pointed me in a great direction. I was looking at the LS1 combustion chambers and thinking :idea:
Yes larger valves were installed. 1.80 int and 1.50 exh in a D7 log head, now there is lot's of work left to do. I am having a ball!!!!
Ken

 

['68falconohio]

Watch taking the combustion chamber out to the head gasket line; a better approach is to lay the head on the block and scribe the cylinder bore outline onto the head's deck.

If you enlarge the chamber diameter to the head gasket's, the chamber will have a larger diameter than the cylinder and the cylinder wall will be a 'shelf' impeding flow into the cylinder.

 

[Lazy JW]

Pontiac used to notch the top of the cylinder wall to help unshroud the valve.

Be careful.
Joe

 

[Thad]

Trick was also used on FE cylinders.

 

[Anonymous]

Here is a site I like to go to for porting info:
http://www.diyporting.com/Shrouding.html
This is Joe Mondello's site.

 

[Anonymous]

I think if I was going to pay to have this head ported I would just buy Mikes Aluminum head. For a little more money I'd be done, but the iron head is such a challenge.
Ken

 

[CZLN6]

Howdy Bacck Ken and All:

Q- "How much would it hurt the head to unshroud into the flat portion or straight portion, opposite the spark plug, of the combustion chamber to keep a constant radius around the valve head?"
A- Unshrouding the intake valves on the plug side of the quench area will help some. With 1.80" intakes, I'd stay away from the cylinder wall side. I wouldn't do anymore than about .100", and that would depend on what casting I was using. Removing material from this area reduces the quench area very little, because of the non-dish lip of the pistons. Removing material will reduce CR.

Q- Will it help the exhaust flow to unshroud as large a radius around the valve as possible?
A- Unshrouding on the exhaust side is less helpful, because the exhaust gas is being exploded out of the cylinder under heat and pressure. The smaller diameter exhaust valves start with more of a perimeter clearance than on the intake side.

The Valve guide lump in the bowls is unnecessarily large. Reducing the size of the lump, reshaping and smoothing has to help- although I can't quanitfy the improvement.

Remember, the intake charge is wet and being drawn into the cylinder by vacuum. It likes to follow smooth, well radiused bends and turns. Turbulence is not good until it is being compressed in the combustion chamber. The exhaust charge is under high pressure when the exhaust valve starts to open and blasts its way out the port. The last part is drawn out in the wake of the first part.

Hope that helps Ken.

Adios, David