mini7boy wrote:
I'm not happy with the way I worded this last sentence in my second posting(above) on this thread.
I said: "There may be reasons why a 940 head is not a good choice of head for a given 998 engine, but the need to pocket the block isn't one of those reasons."
I could have better conveyed the thought I intended by instead saying:
"There may be reasons why a 940 head is not a good choice of head for a given 998 engine. It may be that a given 12G940 head has ports and/or valves that are too large to support good port velocity on a 998 engine that is of a fairly mild build spec. The head may also have been skimmed too much to allow a suitable compression ratio or room for an adequate amount of cam lift. But, a 12G940 can be used successfully on some 998 engines without the need to pocket the block."
I found a 940 head bolted to a stock 850 engine . it worked but a waste of a good head.
As for port velocity you can have low velocity and high volume flow, or high velocity and low volume flow. So what is better. What ever the head is "flowed at" does not mean much if you have a variable flow restrictor in the system.
On the subject of valves consider the shape of the combustion chamber and the amount of valve shrouding it has. This also has a BIG influence on air directional flow into the cylinder.
I have used cylinder heads on my performance 351 V8's that were "U beaut" flow tested and they would not pull the skin of a rice pudding, yet other heads had less "maximum flow' and left them for dead.
With any performance engine it's a balancing act to get everything to work.
