The terms "porting" and "milling" a cylinder head are two different things,
correct?
Yes.
Milling, in the most simplest definition is where a thin layer of metal is cut (machined) off the head gasket (aka: deck) surface of the cylinder head.
Porting, again in the most simplest definition is basically opening up, re-configuring and re-surfacing the ports (intake) of a cylinder head. There are two types, IIRC, "straight shot" and "high approach".
The straight shot is basically a "line of sight" from the inlet to the front opening of the intake valve. In most cases, this yields a lower flow value but because it is "straight" higher velocity of fuel and air entering the chamber is attained. It also creates a turbulent spin, or swirling in the chamber which provides for a more efficient and complete burn (don't confuse this with those fan-swirling things you stick in your intake...).
The high approach is where a turn/bend is necessary in the port design. I believe the term "high approach" comes from the angle of the last section of the port relative to the valve (basically it's more in line with the valve stem). I don't know much more about this approach since I've only had "straight shots" done on my past cars (which is perhaps the most common).
I'm thinking about have the cylinder head on my car rebuilt soon and was
wondering if porting/milling was worth the added expense. I don't want to
sacrifice reliability for a couple of extra ponies... :)
TIA
Beatrice L. Reynolds ([EMAIL PROTECTED])
Jackson Supercharged NOS injected 1990 Honda CRXsi
"Yes, you just got spanked by a girl..."
FWIW, the cylinder head on my CRX was milled by the previous owner. I'm not sure as to the amount, I want to say 15mm. It was done when he blew the head gasket and was having the head rebuilt at that same time. Compression is a bit higher than stock which gives me some extra pep at the wheels. It also has not presented a problem when it comes time to have it smog checked.
In your case, with the addition of NOS, I don't know if you want to be milling the head. Or if you do, a very subtle milling is what I believe would be recommended. I would seek out the advice of a knowledgeable machine shoppe.
I would, however, look into porting the head. Again, talk with a knowledgeable machine shoppe first.
If you have the bucks, I would invest in having the engine balanced. I did this with the Porsche and the last VW engine I built. If you take the crankshaft, cam and other parts out yourself, it's pretty affordable (I believe I paid around $100). Otherwise the shoppe will charge for the labor to tear down the block.
I would also take the time to check the condition of your engine and tranny mounts and replace them.
CRX Owners Group President (http://www.crx.org/southcal)
1990 Honda CRXsi (http://www.hooligan.cc)
ICQ # 3714283 (nickname: godzilla)
Alpine Drive (San Diego County) - February 8, 2003
http://www.crx.org/southcal/events.html
