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Old 06-24-2010, 02:14 PM
oldbogie oldbogie is offline
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Quote:
Originally Posted by ThirdDegree
I have an 85 Z28, with TPI. I have a good knowledge of these cars it just when they switched to TPI that seems to keep everybody guessing. Anyway, being that it has a flat tappet cam I would think that it has the L69 416's and not the roller 081's. After going back and fourth between 350 or LT1 or 305 my wife sealed my fate with a limit on car money to do some stuff to the house. So unfortunately I am going to have to use what I got. I have never ported a set of heads and I am very weary about doing this. It seems to me I would be better of paying someone else to do it or get some practice parts before doing so.
From the articles I have read on porting, you should do these things.
-Remove the ridge from the intake and exhaust ports
-Only remove material from 1/2 of the valve seat (into the throat)
-Scribe a line where the bore would be with a head gasket as a guide and unshroud the valves a little then smooth out the combustion chamber without removing more that 2cc's of material.
-Pocket Porting ONLY - this includes a gasket match intake and exhaust with some deep pocket action in the 1/2" before the guides.
-Last get coarse emery rolls to sand out the casting flash on the ports.
Now here's where information get a little conflicting! This article says to cut the guides down to a cone shape
http://www.popularhotrodding.com/eng...ads/index.html
This article says to leave a minimum of 0.125" of material around the guide bosses.
http://www.chevyhiperformance.com/te...ing/index.html
So which method is correct? Who has done porting? Is there anything I am missing besides a good valve job? I posted this question on thridgen.og, but it seems to me every post on that board has an argument, so.... yeah. Thanks.
Not worth the effort. Porting and larger valves do increase power but do so at the cost of moving the torque and power peaks higher up the rev range. With no other changes this will leave the engine weaker power wise in the ranges you use it most often. You'll add that experience to a stock cam that is unable to support the revs where the "improvement" takes place, on top of that the gearing will be wrong and if this has an automatic, the tranny will not have the correct shift points. All-in-all a partial approach is worse than leaving it alone and saving your money till you can have at it completely.

Simple porting is a no brainer with the Chevy. The ports of the small block really are very good as production engines of that era go, among the best of the best. The basic shapes are correct, unlike many other engines where chassis packaging compromises significantly reduced performance potential and the solutions in porting are so subtle as require a flow bench and a ton of knowledge to make any improvement. For the SBC, if you just clean up what's there, streamline the guide boss with emphasis space for flow on the cylinder wall side of the guides both intake and exhaust you've done plenty. The cuts, unless you're building a competition race motor need to be conservative. You will notice a line in the castings, this is referred to as the paste line and is where upper and lower mold pattens are joined in the cope and drag of the casting box. Grinding no further than what it takes to eliminate this line is sufficient for a street engine. Going further than this gains very little power, and that way up around 6 to 8,000 RPM, but seriously weakens the casting to where long term street reliability and durability is compromised to where the head becomes more crack prone than it already is. BUT, as I said earlier; any carving of the ports, especially the intake, moves the power peaks up the RPM band. Without a cam that can rev there and gears that allow the engine to run there, street/cruise performance will be reduced. This is because bigger ports always reduce mixture velocity. Mixture velocity provides a ram effect that allows the cylinder to continue to fill after the piston has hit bottom and is coming up on the compression stroke. Since the larger port reduces this velocity which results in lower flow inertia, the incoming flow cannot overcome the reverse pumping from the rising piston resulting in the mixture being blown back into the intake at low thru moderate RPMs. Now to get enough inertia back into the mixture to overcome the rising piston's compression forces when the intake is still open, the engine has to rev higher in the RPM band. This is what and why bigger ports and or a bigger cam allways cause the power band to move to higher RPMs. This is also why the car manufacturers tend to look at a bigger displacement when more power is needed, as the typical person doesn't want to listen to smaller engine turning higher revs as this becomes a cabin noise and engine longevity issue as the revs go up. So you'd be way ahead to save your pennies and dimes for the time when you can afford a 350 or 383 swap rather than peeing your financial resources away on the 305 which without being mated to a T5 or T56 ahead of some stiffer rear gears will always fall short of your expectations.

Bogie
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