| [GMCnet] How to Change Cylinder Heads [message #90660] |
Thu, 01 July 2010 10:23  |
emerystora
Messages: 4442
Registered: January 2004
Karma:
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Senior Member |
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On Jul 1, 2010, at 8:54 AM, Michael Bozardt wrote:
>
> I will replace these heads with two "J" heads that I sold to a "customer" in California. He returned them saying his machinist told him they were no good. As they had obviously been rebuilt before I acquired them, I figured he was, as a minimum, mistaken. So I kept the heads and now, in my time of need, I had them checked by a machine shop that builds race engines. You guessed it, the heads are good. Explanation! He found some cheaper heads elsewhere. But, as usual, God knew I would need these heads, so he had them returned to me. You know, if we look, we will see these minor miracles happening all the time.
>
Michael
When I had my engine rebuilt while returning from the Goshen rally in October 2004 the rebuilder was able to find a brand new set of J heads. I had them pocket port them as recommended by Dick Paterson in his tech session at Goshen. This is where they use a piloted J cutter to just smooth out the casting in the area in the heads just behind the head of the intake valve (the "pocket"). Dick feels that this results is a good performance increase by letting the engine breath better without the necessity of doing any other work on the heads or any additional porting.
He did not feel that one should do any additional porting as the turbulence caused by the rest of the head air channels was desirable to mix the air and gas and, at the RPM of our engines, any more porting would really not give any noticeable results. Dick also said the exhaust passages were fine in an unported casting, again referring to the low RPM of our engines. If we were running 4 or 5000 rpm then full porting would give an advantage.
At the time, someone on the GMCnet had stated that the J heads were the worst heads to put on the 455 so I contacted Dick Paterson. He is what he said to me:
Emery the J heads are perfectly OK for our use---I have used them for 99%
of my motors---racers called them "smoggers " because of their era--and the
small a.i.r. bumps in the exh port.. (even grinding out these bumps is a
waste of time for a GMC application.---- the 2.00 inch intake is common to
most olds big block heads---some aftermarket perf valves are 2.07 the
common exh are 1.62 and the racer guys can get 1.71 ---in my opinion --
there are several parameters that balance the adequacy of the stock valve
sizes (int2.00---exh 1.62) -------- 1. the need for low rpm -max torque
(int and exh tract "flow "___velocity__) 2. the int and exh port size and
capacity in cc 's or cubic inches---the restrictions here result in the
flow being maxed out or "stalled " at around .430 valve lift--so no matter
how big a valve you put in you cannot overcome these restrictions.---- 3.
the poor design of our depressed intake mfld (relys on high velocity to
offset its poor design.)-- a larger valve will slow low rpm
velocity --the same as a thorough porting job will.-----4. the cfm demand
of our motor operating in its cruise rpm band (1500 -3500 rpm) cfm
requirements for a 455 motor at 1500 rpm is 195 cfm---at 3500rpm the
requirement is 456cfm ---a larger valve will not have any effect on that
demand- now remember my comments on this subject
are directed at our gmc application only--- --not to the theory of bigger
valves for making more power------ if I were building a 455 for competition
in a light vehicle or boat and wanted to run around 5500 rpm --then I would
definetly go to larger valves (maybe even in a set of ported J heads) to
take advantage of the other requirements -or changes made to up the rpm.
.Put in a wild-ass cam --- throw away the intake mfld and put on an open
plenum high rise .----get a good set of of headers (not the tri Y
esign ---and let the motor breathe up high in the power
band---mean-while --I,ll just make my 495-500 lbs. ft of torque in the rpm I
use with my pocket ported --stock valve sized "J" heads ------- just my
humble opinion ---- Regards Dick---
Emery Stora
77 Kingsley
Santa Fe, NM
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