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Discussion Starter #1 (Edited)
local guy i found is selling #48 heads for $450 and #13 heads for $250, both complete. unless the info is wrong they're both 72cc and same valve sizes, so which is the better choice and whats the difference between them that could rationalize the price difference. My 400 is in the shop 500557 code and is bored .30 over with flat top forged pistons Sealed Power. I'm trying to make 400-425HP street, stop-light car (since that is the limit with the #557 block according to Jim Hand's book) without edelbrock heads

so far i'm set on this cam if it helps
Voodoo Hyd Cam Kit - Pontiac V8 262/268 - Lunati Power

I have my 6X-8 heads still and asked the guy if he has an 6X-4 heads
I understand that i should keep below 9.5cr or so.
 

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The 48 heads were the heads originally intended for the never-produced "350 H.O." that was slated for use in the low-cost GTO "ET" (which was changed to the high-end "Judge"). The heads, intended for the 350, had 72 cc chambers. Since the 350 H.O. was not produced, these high compression heads were re-allocated and used on the Ram Air III instead.

All the other 400 heads of that same vintage, although shown as being 72 cc, are actually 75 cc's if you check them. This gets a somewhat more reasonable compression on a 400.

The two heads have the same port designs and flow the same (pretty crappy in stock format). The 48 heads are more expensive because they're "Ram Air III" heads - you're paying for a name designator which means nothing in terms of performance, since both heads have the same NHRA Minimum Chamber Volume and can be milled to the same minimum thickness.

Lars
 

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:agree with what Lars said. The #48's might represent a money-making opportunity if you sell them to someone looking to do a correct restoration on a RA III car. Be sure and check the 4 character date codes on all the heads to get a positive ID on the model year.

In terms of actual performance, they're both going to be about "the same" in stock form.

Before running any Pontiac iron head, always make sure you measure the chambers yourself. They can and do vary from factory specs sometimes due to variances and/or perhaps previous machining, and you don't want to leave this to chance.

Bear
 

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Discussion Starter #4
How does one measure the cc of the chamber?
So perhaps go with the #13s and have work done to improve them.
What would the estimated compression be?


B. Co. 1-22 Infantry 4th Infantry Division OIF 2008-09
 

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I measured mine with a large plastic syringe, a square piece of 1/4" plexiglas big enough to cover the whole chamber and then some, some thin lithium grease, and blue windshield washer fluid.

Drill a 1/4 or so hole in the plexiglas, such that when you lay it on the head over the chamber the hole is near the "top" edge of the chamber - you'll fill the chamber through this hole, and it also lets the air escape as you do so.
Put both valves in the head, and the spark plug - if the springs aren't on then use a bit of grease on the valve seats to make a seal.
Put a thin ring of grease on the head surface around the outside of the chamber so that when you lay the plexiglas on it, it makes a seal.
Orient the hole so it's "up" on the high side of the chamber, the plexiglas pressed flat against the head surface and sealed.
Use the syringe to squirt fluid through the hole until the chamber is full - carefully measureing how much fluid it takes as you go (the syringe I used wouldn't fill the chamber on one squirt, so I filled it to capacity (30 cc's) and dispensed fluid until it was down to 10 cc's, then refilled it)

I measured every chamber 3 times, and averaged the results in order to get better accuracy. Doing that will help "catch" any big mistakes and will help minimize the effects of any small ones. If you can measure 3 times and get results that are all within 1 cc of each other, then you know you're good (or you're making exactly the same mistake every time :) )
It also helps to know what the heads "should" be. If Pontiac says they're 72 cc's but you carefully measure them to 70 or 75 over 3 times --- you're probably good. If you get 60 or 80 though, time to double check your procedure.

Go slow, be careful, think about what you're doing ---- you'll be fine.

Bear
 

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I used ATF and a graduated syringe. I didn't use any plate at all....I filled the syringe with the same amount of ATF that the head chambers allegedly were, and...they were. The head needs to be dead level, though.
 

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Discussion Starter #7
Well I bought the #13 heads. Likely need milling and the shop will cc them for me.


B. Co. 1-22 Infantry 4th Infantry Division OIF 2008-09
 

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So perhaps go with the #13s and have work done to improve them. What would the estimated compression be?
Assuming a stock bore and stroke of 4.12 x 3.75, and assuming .045 head gasket compressed thickness, 75 cc chambers, 5 cc piston valve reliefs, and a .020" piston deck clearance, your comp ratio will be 9.68:1.

If you get down to 72 cc's and only have .010 deck height, your comp ratio will be 10.19:1.

Key is to measure everything - the cc process outlined by Bear above is the same one I use, it it works very accurately: You can even use it to cc the valve relief volume in your pistons. Small changes in the cc volume of any of the component parts has a pretty dramatic effect on actual comp ratio.

Lars
 

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Discussion Starter #9
It's bored .030 over and the heads more than likely will need to be milled.

Can one just add a calculated amount of octane booster for the higher compression? Are there other options besides dishing the pistons which are forged


B. Co. 1-22 Infantry 4th Infantry Division OIF 2008-09
 

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If you're .030 over and just take a .006" skim cut off the heads, and assume that you get the heads down to 72 cc's, you're only at 10.08:1 on your comp ratio. This will run just fine on premium pump gas with no additives. See my dyno test article of the 400 I built a few weeks ago on this Forum: We ran this engine, using the same cc and same overbore as you're planning, with 91 octane pump gas and a bunch of timing advance. No detonation or problems, and we had the crappy '67 closed chamber heads to make it even worse.

If your heads cc at about 75 cc's after a .006-or-so skim cut, you'll only be at 9.8:1. Your comp will be even lower than the numbers above if you have more than 5 cc's of piston valve relief volume - I assumed 5 for the reliefs, but you could have more. I also assumed .020 deck above the piston - you could have less, so you better check it.

I don't see that you have a problem.

Lars
 

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Discussion Starter #11
That's a good article, hopefully dishing doesn't cost much.
How does timing advance affect overall power, or does it just cure detonation. Would my cam choice affect advance needed


B. Co. 1-22 Infantry 4th Infantry Division OIF 2008-09
 

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Here's the thing on ignition timing: What you're trying to do is light the fire in the cylinder so that when the fuel/air burns and creates pressure, it does it in such a way that maximum pressure is created at the "right" time... i.e. - some time very near when the piston is at TDC and on it's way down. Light it too early, and you get pressure actually resisting the piston instead of pushing it down (and also creating a strong tendency towards detonation) - light it too late and the piston is already so far down that you don't get all the benefit from the pressure that you could have (you lose out on torque). It takes a certain amount of time between the instant the spark fires and the point where the burn really gets going and makes pressure. This time is pretty much a constant - it doesn't change with rpm. So, if you're going to keep getting good advantage from the pressure build, as rpm goes up you have to light the fire earlier and earlier to give it time to make pressure when the piston is at the right spot. This is why you have centrifugal advance in the distributor - to light the fire earlier and earlier as rpm increases.

The "burn time" needed is influenced by several things, including compression, how efficient the combustion chamber is, shape of the chamber and the pistion top, fuel burn rate (higher octane fuel actually burns slightly slower).

So for best power, it has to be "right" - not too early, not too late. "Retarding" the timing a little can sometimes help with an engine that's prone to detonation, but that's a band-aid, not a fix --- and it definitely costs you power when you do it.

Cam choice can have an influence, not really on ignition timing, but on the rpm where peak volumetric efficiency occurs. Lobe separation angle (which along with duration determines how much overlap period there is) and the relative timing of the intake closing event (you generally can't start compressing anything until that intake valve closes) when combined with static compression ratio both have a strong influence on peak cylinder pressure - and that definitely is a major factor in the engine's octane requirements and tendency towards detonation.

Short answer: Yeah, it's all connected to some degree - some more than others.

Bear
 
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