Are 4X heads a decent choice for a warmed over 400?

[JDMopar]

I'm starting into the planning stages of the rebuild/resto of my wife's 77 Trans Am SE. To be honest, I haven't even looked under the hood of that car to see what heads are on it, but I have a parts car that supposedly has a 455. I looked at it the other day and it has 4X heads on it. I want to build the original engine in her car and give it some "East bound and down" power. The goal is for it to be more powerful than it was new, but not so much a rumpity rump drag car. I had my finger on the mouse last night and about ordered a pair of Speedmaster Ram Air IV aluminum heads. I didn't buy them because I figured they would be a little overkill for what I need.
  Will the 4X heads be good enough to get 300 or so HP? The rest of the engine build is pretty easy to figure out, so any advice on these heads will help. Thanks!

[Journeyman17]

Im no head master but it wouldn't take much to get 300hp out of any v8.  But i do have a set of 6x heads on the shelf if your needed an upgrade

[JDMopar]

That's true. I'll keep the 6 X's in mind if I get too carried away. I'm in NC, and my luck you're on the west coast....and shipping would kill me...lol.

[Journeyman17]

Im up here in Michigan

[JDMopar]

Thanks. I have a buddy that is from MI, and goes home several times a year to see family. I'm sure he'd bring them back for me if I were to get them. I'll PM you to discuss once I do more research as to what would work well with this motor.

[JDMopar]

Well, I had time today to go out in the garage and look under the hood of the TA. It has 6X heads on it already! I'll just fix those with new valves and springs, and maybe try to learn about doing some port work to help em out a little bit. 

[70_71_78]

There are two versions of both 4X and 6X heads. A smaller combustion chambered and larger combustion chambered version. I know the 6x have a secondary stamp of 4 for small chambered and 8 for large chambered versions. On a 455 from the factory with 6x heads they should be the large chambered versions resulting in a compression ratio listed as 7.6 to one. I am not sure if the 4X have a secondary stamping but am fairly sure that the combustion chambers sizes are about the save as the 6X.

[scarebird]

I would run KRE D port 72cc units - modern ports and combustion chamber, lightweight.
 They made these as replacement heads for worn out iron units.

[JDMopar]

Thanks y'all. I'm learning a lot in this thread! I'm 65 years old and been a Mopar guy all my life with the exception of having a 69 Firebird 400, 4 speed car that was a rocket sled on wheels! Never heard of KRE heads, but looked em up and they look nice. They gotta be better than the Speedmasters. I've read about a lot of problems with those, so I'm glad I didn't jump on the Black Friday sale and get some. If my iron 6X's that are on the 400 in the car aren't cracked or something, I will probably just rebuild those and send em. The 77 Trans Am SE I'm fixing is gonna be my wife's car. I want it to have more power than the pig it was when new, but not an ill mannered rumpity rump motor that is too much aggravation in a cruiser type car.

[scarebird]

Pontiacs like compression - running the KRE's will bump it up to 10:1 and with them being aluminum allow the use of 91 octane without detonation.

Note the cost of rebuilding the iron units; it may be more than half of the cost of the KRE's.  Edlebrock also offers D port heads now.  Both have heart-shaped LS-style chambers that are far more efficient than the stocker's combustion chambers.

[firebirdparts]

The 4X (mostly) have the same big valves as a 6X, but they were used on 455's, so they can seriously lower compression if you don't ID what you have there.  You cannot put a 455 head on a 400.

For some reason, in the decade of the 1980's, about 50 people must have written letters to Hot Rod Magazine asking about disappointing performance "after I put 455 heads on my 400".  Darndest thing.  Either that or when the magazine was short, they printed the same letter over and over.

The chamber cc's on 4X heads aren't indicated like they are on a 6X.  You'd have to CC them or really really be sure what they came on.

There's also a 1.96" intake valve version.  I suppose this must have been the 2-barrel 400 version, but I don't have my Pete McCarthy book right in front of me, and wallace racing, whom I trust implicitly, is unclear on exactly what they're about.

I don't know anything about speed, but in 30 years of owning pontiacs, I've never heard anybody say that the 4X had any advantages over the 6X, I don't think.  Your of course, being 455 heads, are out of the question anyway.

[JDMopar]

Thanks, and yeah....I've already ruled out the 4X heads. Eventually, the 455 will go to a new home. I'm gonna leave it intact, minus the chrome factory valve covers. I'll seriously keep the KRE heads in mind. My machinist is and old school engine builder who stays up on the latest stuff. He's built winning engines for David Pearson, Jack Ingram, Dale Sr. and many others. If he told me a piss ant could pull a wagon....I'd hitch him up, lol. If he says the KRE's will do a better job at not much more cost, I'm in.

 I really do appreciate y'all chiming in to help educate me on this stuff. I'll have a lot more questions as I go along, about all kinds of stuff. I can do 90% of my own work and like to do things right the first time, so bear with me when I ask what may be dumb questions. 

[scarebird]

JD: the only caveat I know with the KRE's is if you are in a cold climate and drive this car in the winter it may not have the best warm up characteristics - they do not have exhaust ports to the intake for carb warm up.

[JDMopar]

Thanks Scarebird. We live in NC and only get a few weeks of real cold weather when a Polar Vortex comes thru. I typically block off the manifold heat to my motors anyway, so that won't matter.  It will be a warm weather car anyway, and will probably get a Holley Sniper TBI system since it will be the wife's car. I know there's something you have to do different to get a Performer RPM intake to work with the Shaker scoop, so I will have to do some measuring before it's all said and done for that.

[70_71_78]

I actually did find a list of stampings and identifying markings for 4X heads that identifies their original application. It is in the June 2005 High Performance Pontiac magazine. They did a flow bench test of several large valve smog era heads and listed identifying markings and specifications. Sorry, will not type in the list, (not that ambitious), or post scans to prevent copyright issues.
J

[tajoe]

You want more power from the smog laden 77 400 FB? Put the 455 from the parts car in it. It'll make more street power than any 400 with exotic heads. Save your cash. Do an iron 455 with any head you already got. Better replace the TH350 behind it tho. A 455 will eat it for lunch, if you actually ever use the secondaries.

[SoupMan]

You want more power from the smog laden 77 400 FB? Put the 455 from the parts car in it. It'll make more street power than any 400 with exotic heads. Save your cash. Do an iron 455 with any head you already got. Better replace the TH350 behind it tho. A 455 will eat it for lunch, if you actually ever use the secondaries.

Tahoe,

I've got a 455 with 6x-8 heads that puts out 410hp and 512tq at the crank and the TH350 lives. I did however get the tranny rebuilt for to handle the power. However, it wasn't expensive and it's got over 8000 miles of spirited driving in it.

But to your point, I would not put a 455 with smog heads in front of a TH350 that hasn't be built to handle extra power. They aren't expensive to upgrade.

[Wallington]

Sounds like the lesser power/torque option may be the best, if not wanting to upgrade everything else.

[tajoe]

In the 60s, and 70s, TH350s were as popular as the pathetic P/G. Both got dumped in a serious racer, in favor of a Muncie, or a TH400. A stock 350 behind a "drag racing" motor always lost the intermediate sprag, in short time. If you were stuck with an automatic car, and not a Muncie, (which wasn't a whole lot stronger) then the tranny of choice was a TH400. In stock form, the TH400 was the strongest and most reliable automatic out of Detroit. Even over the C-6, and 727s. Because a TH400 has a center support carrier, that no-one else has.

Supposedly the new 4L-60s/80s (?) are a TH400 with an OD, and  maybe they'ld be a good replacement for an automatic car, but I can't say, cause I've never looked at them. But unfortunately, they only come with a chevy bell. I "do" have a 93 2500 military issue stakebed truck, with a 6.2 diesel, and a 4L-80, and it shifts like a bear. Not an automatic fan, but this tranny does feel better than most. If you have-ta build an automatic behind a BOP, I guess the 2004r can be built to handle. 

But like soupman mentioned, with todays aftermarket components available, you can build anything to survive. And because class racing has all but disappeared, and the talent of drag racing has changed from building and driving a car to beat a similar one, now comes down to who can run closer to their dial-in. Which has now put the nasty P/G to the forefront. It only shifts twice, and you don't need a clutch. Makes your chances of winning that much better. So you can run a Hemi Cuda, against a 4 cylinder Pinto, and not know the out come. Sounds exciting, don't it?


As for your situation, lesser power has always been cheaper, and more reliable. It's just how much fun do you wanna have? It's all relevant.
(The yawning emoji isn't in reference to "your" situation, but drag racing with a P/G. )

[roadking77]

Im not a motor guy, but for driving around town you can build the 400 to be quite respectable. Mine is warmed over and it will get out of its own way just fine. Plenty of pep with a carb that starts fine anytime of the year including the colder winter months. I did some research and found the iron intake is just as good as the Ebrock, but the weight savings is substantial so that accounts for something. They do make a Performer that is for a Pontiac motor and supposed to be compatible with the shaker settup. IF I ever get my motor back I can check to see if its true.

If I were building a race motor I would def. go with alum heads, but for a cruiser I decided to spend that money elsewhere and have the stock units rebuilt.