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REPLY FROM: Kempinski, Robert M.
Roland asked
>>I'm currently putting together the HiPo 289 for my Cobra and I've come
>>to a question I'm going to have to decide the answer to within the next
>>week or so: Do I want to have oversized valves installed in the heads.
Frank added
>Do you want to make more HP? Dou you mind some loss of low RPM torque?
>
>If you answered Yes and No then larger valves may be right for you. You
>should consider additional porting work as well. Your cam selection should
>work fine. Generally the bigger the valves the less cam timming required
>for a given peak output.
>
>The popular aftermarket heads sport larger intake, and sometimes exhaust,
>valves. The ports are usually much larger than stock Ford units. Most
>people report very satisfactory gains with these heads. It is worth
>considering purchasing a set rather than sinking a lot of money into a stock
>set. A cast iron set of good aftermarket haeds will run you about $1000.
Ramon suggested sticking with the original heads which I believe is agood
idea - you can save some money. There are a few things to do to the heads
to improve them without major mods. One thing is to equalize the combustion
chamber volume. This is good for a 2 or 3 HP improvement. My combustion
chambers varied by about 10 cc (or about 20 percent of the overall
volume). Another good thing about CCing heads deals with combustion
ratio. Since you had the cylinder bores opened 30 thou, this will increase
the compression ratio a little, about 0.5. Increasing the volume of the
combustion chamber will help bring the compression ratio back down to the
stock number, which I think is good considering the current state of
gasoline.
After CCing the chambers, you should polish them. This gets rid of the
rough casting surface that could contribute to predetonation.
Another thing to consider is looking inside the intake and exhaust ports.
There may a real rough surface and liitle spikes of casting material. I
would knock these down with a Dremel motor tool. If you are real
adventurous, you can do a minor port job with a die grinder. This is risky
without a flow bench and a certain skill level.
Another thing a do-it-yourselfer can do to a head to help it flow is to
match the ports of the manifolds to the ports of the head. I actually opened
up my exhaust manifold openings to match the head. I also smoothed the
exhaust passages. This took about 6 hours with my die grinder.
Replacing the valve springs is a good idea too.
There are lots of suggestions for head work in the hot rod books and
magazines. I used one book called something like "How to add horsepower to
small block fords".
Tootles,
Rob
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