[makilaak]

I have a 1973 400 engine with 4X heads (4H, late 1973, screw-in studs, small valves, 98cc, slightly ported). I believe it’s with standard bore and .068 cam - Holley 750cfm, iron manifolds, HEI ignition.
It is just working great but is leaking oil from the main seals and I just would like to open it for checking. I still believe that it has not run that much, so the cam should be ok as well as boring might not be even needed, but I am afraid that the heads might be cracked as I understood that might be an issue with 4x heads.
I would like to get some comments how to continue. It’s on my ’69 GTO and of course my wish would be to get an original, late 69, YS engine with #62 heads, but that might be over my budget…
Nevertheless I would like to get same amount of hp as originally. Also for me very critical is the original outlook of the engine – I want to keep iron intake manifold and heads, exhaust could be iron RA’s.
If the heads are ok, I probably should just machine them properly and mill a bit for higher compression ratio. We have 98 oct pump gas available locally, so 11:1 compression could be obtained, but I prefer reasonable vacuum instead. How about a mild stroker kit? Would that work with those heads and original intake manifold?
If the heads are cracked, I probably should change them. To what? Bigger valves, but what should be optimum cc for that compression? Neither I want to overbore and change pistons if not necessary.
Any low budget proposals for me?

[steve25]

Your asking a ton of questions here my friend!
First off first let me state that the more compression you gave the more vacuum you have , and with iron heads that have not been ported to flow atleast 15 to 20 % more then stock increased compression beyond 10.5 to 1 will not buy you as much added power as you would think!
Since getting the job done right in the case of replacing rear main seal requires yanking the Crank out you should decide then on what to do in freshing up the motor.

In regards to what heads to swap to you need to provide us with more detailed info on the car and what power level your looking to get out of a motor before we can help you narrow things down and get some frame work/ structure in place for you.

Just keep in mind that as you step up to better flowing heads ( bigger ports) there will come a time where your low speed throttle response and tip in feel will start to go south on you!

From what I have seen with 400 cid motors of under 10 to 1 comp ,once you stuff in enough Cam and or bigger heads get A much fatter torque band going on between 4800 rpm and 6000 rpm you will be giving up enough bottom end torque to need a 3000 stall Converter and more then 3.23 rear gears!

[Cliff R]

If the shortblock is sound replace the heads and camshaft, fix the oil leak and keep going for a while.

All that other stuff takes money, and a lot of it if done correctly.

There are PLENTY of iron heads out there with smaller combustion chambers to get some compression in that engine and the sky is the limit for cam choices to compliment the CID and compression ratio and keep things in check for pump gas........Cliff

[makilaak]

Thanks! True that not much is known before I open the engine and see the crank and bearings. I am just looking for any valuable possibilities I might have here. The car is a '69 GTO convertible, original TH400, 12-bolt 3:55 gears. I wish to get 350-400 hp, with totally street driven car and using as much current parts I have.

[steve25]

That 12 bolt is not original in a 69 Pontiac of any body style!

Yes you can attain 400 hp without much effort so first let's see how things look when you crack it open.

[makilaak]

I know..., unfortunately.

Ok, good to know that - all help appreciated.

[charlie66]

Also be careful about going higher with the compression if you only have cast pistons. I've done it before where id take a stock low compression engine and throw a set of small chamber heads on and before you know it the pistons are cracking all over the place...

[Mister Pontiac]

Originally Posted by steve25

That 12 bolt is not original in a 69 Pontiac of any body style!

Steve, I think he meant the TH400 was original, but has a 12 bolt with 3.55's...

[beertracker]

You may want to consider running a compression test to help determine the state of the current engine. bt

[makilaak]

Yes, compression test was in my plans already. TH400 PX is original, rear end not.
I would still like to learn what would be optimum heads if I need to change from 4X to something else. If I currently have 98.94cc so how low can I go with otherwise stock engine? It seems for me that newer heads like 5C, 6C, 6X have even larger cc's. Could e.g. 96 head with bigger valves and 96 cc be optimum? Would then the original heads for my car, 62 heads with 72 cc, cause too much compression (knocking issues) with current pump gas (98 oct)?

[78w72]

6x-4 heads are around 90-93cc. shaving off just .030 would take off about 6cc more getting them in the mid 80cc's.

[makilaak]

Ok, and that will not cause any issue with intake fitment?

[78w72]

Originally Posted by makilaak

Ok, and that will not cause any issue with intake fitment?

in my experience, no. but every intake/engine is different. some say to shave the same amount of the heads intake surface if needed. but at only .030 ive always been fine with factory intakes & even the edelbrock performer.

at .030 if there is a slight issue its very easy to just elongate the intake bolt holes a little too if the bolts wont start. on a stock engine .030 should not casue intake fitment issues.

[steve25]

A Mill job of .030" would knock off only 5 CCs.

The 4X/ 5c and 6X heads in stock form with the 2.11" / -.66" valve set up are fine for a 400 cid motor if your only looking for a max of some 350 hp and a rpm range of 5600.
The 4X heads are not a good pick for a full port job on the Intake side dependant on how much of a flow gain you are looking for , but someone who knows what they are doing porting wise can take them up to 245 cfm , and 245 cfm can make you well over 420 hp with the right combo of other parts and a 9.5 comp.

Your cast number 62 heads have more like 75 CCs and with a slight dish to your Pistons get you the needed 9.5 comp.

[78w72]

Originally Posted by steve25

A Mill job of .030" would knock off only 5 CCs.

The 4X/ 5c and 6X heads in stock form with the 2.11" / -.66" valve set up are fine for a 400 cid motor if your only looking for a max of some 350 hp and a rpm range of 5600.
The 4X heads are not a good pick for a full port job on the Intake side dependant on how much of a flow gain you are looking for , but someone who knows what they are doing porting wise can take them up to 245 cfm , and 245 cfm can make you well over 420 hp with the right combo of other parts and a 9.5 comp.

Your cast number 62 heads have more like 75 CCs and with a slight dish to your Pistons get you the needed 9.5 comp.

according to wallace racing, .030 milled off mid 70's heads is 6cc.

http://www.wallaceracing.com/headmill.htm

[steve25]

The ratio for 68 and up open chamber heads is 1CC per every .006" milled off.
The exception to this is the mid 1967 061 head.

[78w72]

did you look at the wallace chart? clearly shows .025 is over 5cc on all 71-76 heads. .030 is 6cc or actually a tad above 6. or are you saying the wallace chart is wrong?

[steve25]

In a word, yes!

[ho428]

If budget is a concern and it's an either or, I'd go with Aluminum heads over a stroker kit.
Head flow is everything to make hp, not cubic inches.

[78w72]

Originally Posted by steve25

In a word, yes!

yes what? you didn't look at the chart or you feel its wrong? & if it is wrong, better tell wallace about it. personally i will stick with their info which i've found to be correct when milling 6x heads.

& for the OP's situation im sure 1cc or less wont make much of a difference. my statement said "around & about" so it is still correct that .030 off 6x-4 heads will end up in the mid 80's.