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Old 07-30-2013, 11:53 AM
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Pontiac 6X heads

I've been looking at my Pontiac 455 and noticed the 6x embossed on the heads. The guy I bought the engine from says the engine came out of a 1971 Pontiac Grand Prix SJ....I didnt look at the engine code and the engine is already installed. Could someone have changed the heads on the '71 or maybe this is a 1976 455.....it has great power but I would be dissapointed if its a 1976 or later.....
Also would changing the heads with some Aluminum heads be a good power adder?
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Old 07-30-2013, 12:20 PM
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Quote:
Originally Posted by billonwheels View Post
I've been looking at my Pontiac 455 and noticed the 6x embossed on the heads. The guy I bought the engine from says the engine came out of a 1971 Pontiac Grand Prix SJ....I didnt look at the engine code and the engine is already installed. Could someone have changed the heads on the '71 or maybe this is a 1976 455.....it has great power but I would be dissapointed if its a 1976 or later.....
Also would changing the heads with some Aluminum heads be a good power adder?
The 6X heads were used on some 350 and 400 Pontiacs. The resulting compression ratio depending on which chamber size you have. Look for the secondary identifier (location of code shown below), details on the codes here.



A set of aluminum heads would be a good addition. What cam are you now running? Headers? Gear ratio?
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Old 07-30-2013, 12:53 PM
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You can also check the block casting date and block code, shown in the attachments below
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Old 07-30-2013, 08:37 PM
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As for aluminum heads, what exactly do you "do" with the car? If it's not raced, I would say no to aftermarket heads. The Grand Prix is a nice "cruiser" with serious top-end muscle. The big engine and tall gears make it a freeway flyer. I had a '72 with a 400 in it, and it would stomp most cars "up high".

The most noticeable and "fun" gains for a car like yours will come from increasing low-end and mid-range torque. The 6X heads are good for that.

Once you identify the heads and block by Bill's pics, let us know what you want it "to do". We can help. There are a handful of SERIOUS Pontiac guys here.

Jim
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Old 08-01-2013, 05:37 PM
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Thanks guys. I have comp cams 278 duration. I have some headman long tube headers, comp cam roller rockers, 750 Edelbroc carb. The engine has been bored out 30 over. Electric fan with aluminum pulleys. The gears I believe are factory 278's. I want a capable street car that I can drive 60 miles and not worry about breakdown lol. It performs well but I want more torque.
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Old 08-01-2013, 08:49 PM
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billonwheels, To satisfy my curiousity, post the block date code and the two letter code from the front of the block and the secondary code of the heads. For streetability I recommend that you keep the 2.78 rear, but for more torque, I think you could change the cam and get what you want. I'm sure that Jim (Mr. P-Body) could pick a good one for you, depending on which 6X heads you have.

Bill
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Old 08-02-2013, 06:58 PM
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Yes. if 6X heads, the secondary marking is very important with 455. The -4s are just a little too small in the chamber for a 455. -8s work very nicely if resurfaced, yielding just under 9.3:1. That's 93 octane territory. 91 will "do" in a "pinch". Over 9.5 is risky, IMO. There are advocates, but they're hobbyists, not "pros", and are willing to "tinker". Most of our customers would rather not tinker with their engines. So, we make them plenty powerful, but not "finicky" like higher compression engines. If you're racing in a class where engine modifications are limited or prohibited, THEN squeezing every inch-ounce out of it IS called for.

Please measure the chamber volume to be certain. At least one in each head.

Jim
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Old 08-02-2013, 07:16 PM
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With all due respect 6X-4 heads will work just fine on a 455 if the cam matches the required compression ratio. If you want to run a short duration "RV" cam, 6X-4 aren't for you. But if you want to make some power and cam the engine for making that power, the compression ratio of the 6X-4 heads and FT pistons are a killer combo.

Don't believe me? That's fine. But you can talk to a BUNCH of guys who have built- and recommend- 6X-4 head combos for the 455.

I have no idea what type of tinkering is thought to be needed to run -4s, but I can tell you- from first hand experience- that MY engine using them ran strong (mid 12s, 89 octane for cruising) in an all steel Camaro. I could literally have welded the hood shut on that car, it was that reliable and dependable. No pinging, no detonation, no nothing. And I beat HELL outta that engine.
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Old 08-02-2013, 07:23 PM
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It should be added that the size of the combustion chamber of ANY head has to be verified and the actual size used to compute the compression ratio. DO NOT under any circumstances use any published figures for chamber size to figure the CR- and I don't care where the published numbers come from.
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Old 08-03-2013, 07:35 AM
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Relax. While I understand the use of a large cam for the application to "bleed off" cylinder pressure at low engine speed, it's not the practice we like to encourage. We've found using the correct combinations without a compromise up OR down, produces the most reliable results. Not disputing what you say AT ALL. Using 041 to make 9.6:1 "usable" falls into the same category as using fast-bleed lifters. It works, but not optimum. A more potent combination lies with 9.3:1 and XE274H, when compared. The power band is a little bit narrower, but the peaks and average are both higher, and fuel consumption is significantly lower. This has been my experience over the last few years.
For the record, I built a 455 in 1985 with 4X (400, 94 CCs) heads modified like we do today (not ported, but chamber "relieved", large exhaust valve, screw-in studs, 041, '72 factory intake, HO exhaust manifolds, Super Duty Q-Jet (actually found one!), 3.42 gears in a '73 T/A. 12.70s @ 110 on "regular gas" (I think it was 89 back then, but don't remember for sure). I was a "little ahead of the curve". I got a call a couple years ago from a man in Tucson, asking if I was the same "Jim" that had the T/A back then. He had just bought the car. It still has the engine it I did, and runs great on 89. I was a "hobbyist" at the time, working for IBM. I had 10 years already in machine shops and dealerships, though, so I had an "edge". That car surprised a few SS454s...(:- The young machinist I had grind the seats was glad to learn something new, even though it was "just a Pontiac..." If I had a dollar for every time I've heard that... Cobalt, even though you're a small block guy, you have experience and show respect for the Injun. I respect that. Just wanted you to know.

Jim
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Old 08-03-2013, 01:32 PM
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Billionwheels, you've posted on Pontiac engines before, I recall posting to your threads. If you do a search, everything has already been covered. I know that requires time/effort on your part, so I'll get you started. Here are some threads I and other Pontiag guys have posted to.


Jim, I'm not anything but relaxed. But for the record (and to trade bios), you mentioned me being a "small block" guy. Truth is I was racing Pontiacs long before I ever owned a SBC, as did my Father. He raced circle track w/Pontiac power when no one else was, and he did quite well. So you are very correct in thinking I respect Pontiacs. FWIW I stuck w/my 428 well into the time frame when BBCs ruled the roost. Spent a LOT of time/money keeping that car competitive, and back then (ca. 1970-'75) there weren't good, cheap parts available like there are today. Never minded being the underdog, and I, too, surprised more than one smug BBC guy.

But in any event, I'd prefer to think of myself as a "car guy", period. While the SBC is still mainstream and gets the majority of ink here, I have little bias, or prejudice for that matter, towards ANY particular make/model/engine. I have been through many vehicles of all types- from stem to stern- through the years. I grew up both literally and figuratively in our family owned junkyard and I learned early on they all have their faults and they all have their strong points as well. And every one of them can be improved on.

Now, as far as 6X-4 heads on a 455 goes, I suppose we'll just have to agree to disagree. But the numbers do not lie. My heads measured 95cc, my compression was 9.5:1 on the nose. This is not "on the edge" w/a performance cam like the XE278, in fact the DCR is well below 8:1 (about 7.75:1 according to one calculator). This has nada to do w/"bleeding off" anything.

Please don't take any of this the wrong way. Your contributions/input here are valued by me and others. We might have some philosophical disagreements but overall I believe we agree more than disagree.

Last edited by cobalt327; 08-03-2013 at 01:40 PM.
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