[pmd400]

Hi
I'm helping a mate build a 400. Short block is .030 over trw pistons, 5140 rods, zero decked. He currently has 4x heads with 1.96/1.66 valves and press in studs. Iv made a tool that makes tapping the rocker studs easy. We will get 2.77/1.77 valves installed. The question is will installing bigger valves change chamber volume much? Ideally we want it much smaller. Currently they measure at 98cc.
Thanks

[Cliff R]

Adding the larger valves will not reduce chamber volume enough to be an issue.

Depending on the power level of the build the larger valves may not bring much, if any power to the table. I've dyno'd, street and track tested the smaller valve versions of those and other Pontiac "D" port heads and there really aren't big gains adding the larger valves unless the engine is set up to make power past 5000rpm's or so.

This simply happens because the runner volume and cross section, plus the restriction under the valves at the short-turns and bowl areas are very close on all the "D" port heads. Matter of fact some of the smaller valve versions rivaled the large valve heads on the flow bench in stock form. From memory the 1974 #46 heads were one of those castings, but there are others that flow very well with the smaller valves in them........Cliff

[pmd400]

I was hoping the larger valves would decrease chamber size a few cc so the heads won't have to be milled so much. It's not easy getting Pontiac heads in oz. have been on the look out for a year to find smaller chamber heads.

[steve25]

This should help you out.
A 2.11" valve has 7.5% more area then the stock 1.96".

A 1.77" valve has 6.5% more area then the stock 1.66" valve .
Combined we now have 14% more valve in the chamber which in and of itself will bring you to some 85 CCs from your 98 CCs.

This is assuming that all of these valves we are dealing with have a flat face and not dished and that the valves will all sit up at the same height in the chamber!

Installing a 1.77" Exh valve in these Heads will not pick you up hardly any high lift flow as the restriction is down in the valve bowl!



Even with the stock 1.66" valve if you take 4 minutes to grind off the lip on the short turn left by the factory cutter you will pick up 6 to 8 cfm of flow above .350" lift.

Adding a 1.77" valve with the right valve job will however pick you up some good and always needed low lift flow.

Also note that the 4X heads still have the darn big Exh crossover openings in the center 2 Exh ports unlike the smaller ones in the 5 and 6 series heads, so if your gonna go thru the trouble of running headers on the motor I would close them off with some Ciramic Epoxy.

You can knock 8 CCs out of the chamber by milling the deck .048", and then to get things to bolt up nice you would want to Mill the Intake flange of the head atleast.035".

[Cliff R]

Depends on your goal for final compression.

4X, 5C and 6X heads were never good starting points for a 400 build IF you are trying to get enough compression in it for optimum power on pump gas. I'm not fond of whacking a bunch of material off of them to get over 9 to 1 compression considering what else is needed to get the intake to sit on the heads after all the material is removed.

With most of these engines you can get away with about .060" of material off the heads and block w/o issue, but much beyond that the intake isn't going to sit well on the heads and you'll need to remove an equal amount of material from the intake or intake face of the heads to make things happy.

Far easier to buy a head with a chamber size more suited to your needs in the first place.....IMHO......Cliff

[steve25]

It's hard to find post 73 small chamber iron heads here in the US,no less down under!

[pmd400]

The build won't be high performance. Mild hyd flat cam, 3.08 rear, stock converter. I'd like to get at least 9:1 compression.

[steve25]

So go with the bigger valves and the mill job!

In regards to the rocker studs you can buy them with a 1/2" bottom threads so that all you have to do is tap them to 1/2" -13 threads.

[Cliff R]

+2

For what you are wanting really no need to mill the heads really hard to get the chambers down another 10cc for that much compression.

I doubt if very few folks reading this have actually cut blocks for zero decking and whacked later heads pretty hard to get the compression up to 9 to 9.5 to 1. It sounds pretty easy but you'll find very quickly that the intake isn't going to sit on them very well and at a minimum you'll have to elongate the bolt holes to get the bolts started.

From what I've seen here you can do a .020-.030" cut from the heads and another similar cut from the block for decking/squaring them up without running into a lot of issues. It's not the perfect scenario but with at most a rat tail file to elongate the intake bolt holes will get you out of trouble there. You may find yourself removing some material from the bottom of the intake as well, especially if Edelbrock made it.

If you go beyond the numbers above you will have lowered things enough in the "V" created where the intake sits on the heads to start causing problems with fitment.

I've built PLENTY of strong running "lower end" 400 street engines with 7.7 to 8.5 to 1 compression that are very impressive and easily push nearly 2 tons of vehicle into the 13's w/o a lot of effort. The best part is that you can run whatever octane fuel you want to in one, and most actually do better with 87 octane that slowing down the burn rate with higher octane at those lower compression ratios.....FWIW.......Cliff

[Blued and Painted]

4x. The first of the EGR heads.
Double check for cracks thru the exhaust seats.

[gtofreek]

No way are you going to drop 13 cc's by adding bigger valves. Maybe 1-2 cc's but that's it. Like B&P said, check the center exhaust seats very closely for cracks. Every 4X head I have seen has had cracks there.

[steve25]

Paul I gotta say the site I used for the math on that this morning had me confused, but just now on a different conversion site I came up with 2.4 CCs when you round it off for the difference in the bigger valves and not having sunken seat from a second valve job on stock size valves.

Sorry for the flub all!

In terms of cracked 4X heads I have seen a bunch more of the ones off of 455 motors with cracked seats then 400 motors.

[dmac]

98 cc is a good head volume number for a stroker kit in that 400 block. even a 4" stroke will get you close to 9:1.

[Cliff R]

"Every 4X head I have seen has had cracks there."

+2

You will find that the smaller valve versions held up better in that area as there is more integrity between the valves than the versions with the larger valves in them.

I've used a good many 4X castings over the years and would rate the crack rate at close to 50 percent. It is near 100 percent for the 7M5 heads used on the 455's in 1973 so one head to certainly stay away from.

1973 was the first year that induction hardened seats will be found for all GM engines. There are however earlier versions dating back to late 1971 for GM heads. From what I've seen in engine building for 4 decades now is to pay very close attention to any head from Buick, Olds, Chevy, Pontiac, etc made in and after 1972. The process they used in the first few years was not well perfected and you will find a very high crack rate for those heads.

My late Uncle was a GM employee in that time period and worked in the foundries that poured many of those heads. He told me that they improved the "recipe" for the base material and started flipping the molds over to help concentrate material in the seat areas which greatly improved things as far as being able to induction harden them and resist cracking, etc.

Another head that is ALWAYS cracked is the SBC casting #624. The 882's made in the same time period faired a tad better, but every single 624 I've ever checked was cracked across the center exhaust seats no exceptions........IMHO......Cliff

[dmac]

Originally Posted by Cliff R

"Every 4X head I have seen has had cracks there."

+2

You will find that the smaller valve versions held up better in that area as there is more integrity between the valves than the versions with the larger valves in them.

I've used a good many 4X castings over the years and would rate the crack rate at close to 50 percent. It is near 100 percent for the 7M5 heads used on the 455's in 1973 so one head to certainly stay away from.

1973 was the first year that induction hardened seats will be found for all GM engines. There are however earlier versions dating back to late 1971 for GM heads. From what I've seen in engine building for 4 decades now is to pay very close attention to any head from Buick, Olds, Chevy, Pontiac, etc made in and after 1972. The process they used in the first few years was not well perfected and you will find a very high crack rate for those heads.

My late Uncle was a GM employee in that time period and worked in the foundries that poured many of those heads. He told me that they improved the "recipe" for the base material and started flipping the molds over to help concentrate material in the seat areas which greatly improved things as far as being able to induction harden them and resist cracking, etc.

Another head that is ALWAYS cracked is the SBC casting #624. The 882's made in the same time period faired a tad better, but every single 624 I've ever checked was cracked across the center exhaust seats no exceptions........IMHO......Cliff

I have a set of 4X heads I have been planning to use. No cracks, but now I'm worried. Do they crack solely due to overheating, or is there another cause? If it is due to overheating, if I have a good cooling system, and keep temps down, should that prevent cracking?? Wondering if it maybe the design and transfering heat from the exhaust valve that could cause a crack even if the cooling system is working and keeping coolant temps down.

I had planned to use the 4X because of chamber size, and other than checking springs, are ready to go.

[gtofreek]

Originally Posted by dmac

I have a set of 4X heads I have been planning to use. No cracks, but now I'm worried. Do they crack solely due to overheating, or is there another cause? If it is due to overheating, if I have a good cooling system, and keep temps down, should that prevent cracking?? Wondering if it maybe the design and transfering heat from the exhaust valve that could cause a crack even if the cooling system is working and keeping coolant temps down.

I had planned to use the 4X because of chamber size, and other than checking springs, are ready to go.

I blame the cracking 4X heads on the fact that the seats are induction hardened which makes them very brittle. Also the small exhaust valve I'm sure drives up exhaust temps on a big engine like this that has a lot of exhaust push. Most of the cracked 4X heads I have seen were able to be fixed by installing hard seats in them. Most of the cracks were small and cut out when installing the seats. If you have heads that are not cracked, the best thing you could do is have hard exhaust seat inserts installed. This eliminates the brittleness, and helps prevent cracking. Some of the heads I saw that were cracked did not appear to be overheated, so I don't think it's strictly an overheating problem.

[Cliff R]

Correct, it isn't an overheating issue, more due to the induction hardening process used to prepare them for unleaded fuel.

Although you can find GM heads cracked from must about any year, you will find that 1972 and 1973 (73-74 model year) castings faired very poorly for all brands until they got better at the basic process used to harden the material. My Uncle told me this WAY back in the later 1970's. He was a tool a die maker at one of the foundries and directly involved with correcting the issues imposed by induction hardening the exhaust seats on GM head castings.

One will find that by 1975 the crack rate for the 5C and in later years 6X will be much less than the earlier 4X castings for this reason.

It's almost parallel for SBC heads made in the same time period the SBC castings were already getting lightened up to save a little weight.

I was already building these engines by then and at times it would become difficult to find a pair of SBC heads where one or both were not cracked. I don't think the root cause was from overheating, most likely just the process used to harden them combined with "thinner" castings.........FWIW.......Cliff

[Blued and Painted]

The intake short turn on the as cast 6x appears wider and flatter than the 4x.

[steve25]

It's far taller also !

[Cliff R]

Going from memory Nunzi really liked the 4X castings and did well with them.

I've pretty much stayed away from them over the years and used 5C and 6X heads instead. They also have a "double pass" past the exhaust valve guide and excellent intake port.

Really not any big advantage in the scheme of things as any minor differences between all the "D" port heads disappears very quickly with correct port/valve/seat work. So the only thing we really worry about here is the size of the combustion chambers to achieve the desired static compression ratio, if and when we use iron heads for one of these engines, which hasn't been too often since the much better aluminum "D" port heads came out.......Cliff