shadango; Thinking it might be easier to just swap the heads.....wouldnt it?
I am wondering why someone would have put those heads on it to begin with....or is that a stock head in 76?
Why would someone build a 318 with J heads if it means LESS performance? Simply convenience (ie they had the heads available at the time and used what they had) ? The only thing I know for sure about the engine is that it has a deep sump oil pan on it...it also has an edelbrock performer manifold and 4 barrel carb on it....seller said it has a double row timing chain on it... He is supposed to get me some contact info for the guy who did the motor...if he does, then I can get to the bottom of what went on I guess.
318 4bbl.s came stock with 360 heads. Cost cutting measure that passed the tail pipe test.
The 360 head swap is an old MoPar suggestion to do that was in there "Tips" section in the old performance books/catolog. It works well when used right . Since this has been the old stand by to do over many other options, it becomes a staple and thought to be the best move to do since Mopar suggested it. It became a cheap upgrade.
The head has large port windows and big chambers. In order to effectively use it, the engine everywhere else must be up to snuff and often it is not. The problem arises when someone reads the info, thinks that the race tips can be used on the street and then slowly goes down in performance level from what it was intended for until most folk think it is the basic "Hop Up" item to perform.
Now the 360 head (wrongly) is used as the first or second thing to do in upgradeing a 318. With the availale size port, alotof air can be moved and the small bore and stroke of the engine can not use this amount of port well. While there can be a seat of the pants improvment, the best way of doing things is left behind and forgotten about. Useing what you have.
Sometime ago, MoPar (Slowly) got wind of what was going on and ported a 318 head, set it up on a an engine and started to dyno it. They swapped heads around and continued to play around with it.
What they found is a 318 head seriously out performed the 360 head to the tune of 55 HP on a mild engine. While there is a time and a place for a 360 head, it's not on a mild engine. The biggest problem, besides trying to tell people that the 318 head can out perform a 360 head is, tring to convince them that spending the money on the head is the right way to go.
All they see are dollor signs and prefer the cheaper (Less performing0 way of the 360 head in stock form over a ported 318 head. Cost is a factor for all of us non-rich guys and the cost of porting a head can be scarrrrrrey.
It's a catch 22 and mostpeople can't see it even when your trying to give them an explanation and the science behind it. They just see dollor signs and a quick way of increaseing performance.
The catch is the dollors spent on porting a head, ruffly a grand start to finish and in total of everything done vs. a simple bolt on they get cheap. Say, $200 form a Junk yard.
What there loosing (power wise as well) is about a point of compresion, if not more because of the bigger chamber, (and often wrong head gasket being to thick and not considered) slower response due to the larger runner and valve size. (1.8 or even worse, the 2.02 valve) which means slower air and fuel movement making the mix of air and fuel not so well atomized and that ends up in a poorer burn of the mix.
When all this occurs, alot of times a person will install a larger cam shaft. The performance cam shafts tend to have a tighter centerline about the 110 area. This and a larger overlap. (The time when both valves are open.) If you do not increase your compresion to help balance out these minus points on the low side of the RPM scale, you loose power. Velocity is slow through the intake and heads which requires more jet and a basicly richer condition in order to get it to idle and run right.
Balanced it out did we? Not allways so. It now runs rich in cruise and probably very apon accleration (Mild) The carb see a reduce signal and requires more jet. When the engine picks up speed, the velocity picks up and a more normal state, of where it should be operating in comes in and then runs richer.
(For this, a air/fuel meter that can be read while driving is needed. Something like from F.A.S.T. that can read the O2 all the time in the cabin.
The large chamber of the J head does nothing for flame front travel and complete burn. Thus the KB399 piston mention. This piston not only helps by raiseing the compresion, but also takes up room in the head to help put the squeeze and better burn back into the head.
While I'm not a fan of a domed piston on the street, it works well enuff.
I like a flat top piston and a closed chambered head. KB has a flat top piston that is said to be a zero deck height unit. This slug with a thin gasket, .040 or so, and the closed chamber head for a bit of quench, better flame travel and less of a chance on pinging with the right octane of course is a great help all around.
Careful with this combo, compresion ratio's can get high quick and out strip the pump ocatne quick if your not careful.
There is a time and place for a 360 head. A cheap upgrade is for the most part, the reason why it's used. But theres a better place for it and a cheap upgrade isn't it most times. The 360 can be effectively used, but is it the right or best hea for the job at hand?
Anyway, you asked.
