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Old 09-18-2012, 10:00 AM
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cobalt327 cobalt327 is offline
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Good points IMHO, machine shop tom. Something that also sometimes bites first-time/novice heat porters is that just because it looks good does not mean squat about how the port may flow. Members have posted some photos of ported heads and they can look very impressive. Unfortunately that means nothing for the most part, unless the work was done by a seasoned porter and/or there was access to a flow bench- along w/the knowledge needed to digest the bench's results and turn the data into viable, meaningful, positive changes.

Doing a full-on porting job is very labor intensive and is a dirty, time consuming project- made doubly so when working w/cast iron. To bring a set of 305 heads up to what even a set of stock untouched L31 Vortec heads can do is hard to do regardless of who you are, let alone someone w/no experience. Just doing one port is a chore. By the time all 16 ports are done along w/the cost of the machine shop and parts, most guys will see that buying aftermarket or using a better production casting to start with (like the Vortec) is the better way to go.

If the decision has been made to use the 305 heads regardless, staying w/the "low hanging fruit" like removing/blending the lip that's almost always present where the bowl meets the bottom seat cut (red arrows on swirl port head below), and removing casting flash and irregularities, and careful port matching (NOT gasket matching) can result in an improvement of several percent w/only moderate time and effort. Often backcutting the valves gives an improvement for little cost. A true quality valve job can be worth a few percent more and should be considered a 'must-do' on any production head- even aftermarket heads need to be checked over carefully.

As has been said already, the difference between a properly rebuilt and prepped set of production heads and a set of aftermarket heads becomes less the more parts, time and work they consume. But if a production head is going to be used, it makes more sense to me to start out w/the best performing head available rather than using a head because it's cheap (or even free). Remember- the same machine shop time and money will be spent regardless if you're starting w/a Vortec head or a 305 head.

On the '92 heads, check the casting number to see if they're swirl port (SP) heads. If they are, proceed w/caution- while they have 64cc chambers and a relatively big intake port volume, the swirl inducing vane (seen circled below) costs flow at mid to upper rpm. If you're interested in researching the SP heads, there are several sites that have info on them.


Casting number 14102193
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