[MikeyB]

Hello, I have a stock 1973 455, 4br, I plan to replace the stock 4X Heads with 6X-4 heads from a 350. The 6X’s are getting reconditioned at the Machine shop including exhaust valves upgraded
to 1.77.
If the deck surfaces need no or minimal removal what CR can I expect to obtain?
If I choose to have deck surface milled how much milling will give me the maximum CR using 93 octane pump gas?
Also what cam would give me the best street able performance?
Thanks in advance

[johnta1]

With the 6X-4 heads, I'd say you probably will be about where you want to be. (around 9.5 CR)


I'll let others answer the other questions.

[steve25]

You don’t need a bigger exh valve for what your building with the 6X heads.

If your already committed to them due to matching work then you have a choice.
Either run a single pattern cam ( if you have 3.08 gears or less ) or if your running a duel pattern cam them bowl port the intake, or run 1.65 rockers on the intakes only.

[Cliff R]

6X-4 heads typically come in around 92cc in the combustion chambers. If you deck/square the block for close to zero, .039" Felpro head gasket and take a little off the heads you'll be closer to 10 to 1 than 9.5 to 1.

The camshaft is a huge player in being ablt to manage pump gas at any given compression ratio. I ran a 455 with 6X-4 heads on it and it was fine with lower 87-89 octane fuel. I typically kept 92-93 in it but it alctial

[Dragncar]

Congrats on finding a set of those heads. Considered about the best D port heads by a lot of guys.

With these numbers Wallace comes in at 9.51 CR. That is with 93.75 ccs, 350 6X heads with 1.66 valves had those numbers.
You are close enough to the max to leave the deck alone.
Do not put a small cam in it or you might ping.

[MikeyB]

Originally Posted by steve25

You don’t need a bigger exh valve for what your building with the 6X heads.

If your already committed to them due to matching work then you have a choice.
Either run a single pattern cam ( if you have 3.08 gears or less ) or if your running a duel pattern cam them bowl port the intake, or run 1.65 rockers on the intakes only.

The heads are at the shop. From what I understand you’re saying is that I don’t have too upsize the exhaust valves to the 177s. I can just have the valves cleaned, magna fluxed, the stock valves re seated, replace the valve springs, with new pushrods and new lifters. Would I be okay with the cam they used in the test information you sent me? I have stock rear end gears, not sure what they are exactly. I will try to found out this evening.

[b-man]

Replace all the valves with new stainless valves. Might was well upgrade to 1.77 exhaust, that way you end up with fresh metal to cut the seats when you go oversize.

Don’t reuse the original factory 2-piece valves that have the heads friction welded to the stems, they do fail with devastating consequences.

Nobody can help you choose the correct cam without knowing anything about the car it’s going into as well as all the information about the drivetrain and gearing as well as rear tire size.

[steve25]

Those heads have induction hardened seats in the chamber to deal with no lead fuel.

If you move up to 1.77” valves that needed hardening will be cut away and you will need to install seats.

If you stick with the 1.66” valves and your stock seats are not too beat up then you might get away with just a light valve job which will not cut through that induction hardening.

[MikeyB]

Thank you to everyone for the help. I believe I have 3.08 gears. I’m taking notes on everyone’s input

[b-man]

We still don’t know what car you have or what transmission you’re using.

Intake, carburetor, exhaust?

How did you verify what your rear end gears are?

Any and all details would be helpful to try to recommend a cam, any cam company out there would ask for a detailed list before giving their recommendation.

[lust4speed]

I think the single pattern cam recommended above might not be the best choice along with the suggestion that the 1.66 valves be retained. Dual pattern cams are chosen to crutch a weaker exhaust side and keeping the smaller valve will hamper exhaust flow even more than a 1.77 valve (which usually benefits from the dual pattern).

Maybe I just haven't seen enough late heads with the induction hardened seats because the ones I've come across are showing pretty severe recession after 50 years and the shop usually has to install hardened seats so the valve face isn't flush with the chamber. Maybe it's a California thing and we probably were forced into no lead gas years before other states?

[steve25]

You guys are always thinking about / looking at flow and not the much needed average port velocity.

[MikeyB]

It’s a 1973 Pontiac Grand Prix SJ
Triple Black, 60k original miles
Mint Original interior
400 TH Transmission, console shift
Stock intake, but I plan on purchasing aluminum elderbrock, depending on what the thread recommends
It has MSD electronic ignition with a billet MSD mechanical advanced distributor
It’s got long tubed ceramic headers dumping into 2 and 3/4” by 3” dual exhaust
Carter high volume mechanical fuel pump
Rebuilt Factory Quadrojet but have rebuilt Holley 750 double pumper if you would advise to use it
255/60/15s T/A
Researching the internet, 3.08 gears were standard, 3.23 were an option.

So I’m swapping 4x heads to reconditioned 6X -4, I plan on replacing push rods, lifters, and upgrading to 1.5 steel roller rockers.
I’ve already purchased Billet gear timing chain and dampener

I have plans to travel Route 66 in this car one day. It has always been a dream of mine.

[steve25]

No need in the least to change that carb or intake manifold!


put that money into the heads, like new springs , full 7/16” rocker studs and a set of matching poly locks and new hardened valve locks.

And of course new valves. 2.11/ 1.66”.

Also you don’t need roller rockers if yours are not worn, as they will buy you nothing in terms of the way your building this motor and will only serve to drain your wallet more.

[MikeyB]

Thanks much Steve, what about cam leave stock one if lobes are not worn or put a more performance oriented cam to enhance the performance of the 6X heads?

[steve25]

That car is no lite weight, so the cam pick will make or break the whole deal.

What type of driving will you be doing 75% of the time?

How did you think it ran stock?

What was it lacking besides top end?

[MikeyB]

2 lane highway 55-60 mph, 50%
Cruising city blocks 50%
Occasional drag race 100% lol

[MikeyB]

Originally Posted by MikeyB

2 lane highway 55-60 mph, 50%
Cruising city blocks 50%
Occasional drag race 100% lol

Car ran fine stock, understandably it’s a 4K car but it just lacked a$$ expected in a 455.

[johnta1]

You don't know the rear gear?
Probably the 3.08?

I'd go with a little higher stall converter. (3200?)
Just enough to get the R's up to launch and will make a very good addition without a lot of money spent.