[74SDformula]

A friend of mine is about to put together a 400. He will be using some flat-top forged pistons. The camshaft will be a hydraulic flat tappet. He has 2 sets of heads at his disposal. He would like to be able to make about 350HP.


First set of heads are some 6X-8's that are completely stock. I figure they are about 100-102 cc's.

Second set of heads are some 2 bbl 11 casting heads. However they have already been machined for screw in studs. They measured at 89 cc's. If this pair of heads are used we would probably use the stock size 1.66 exhaust valves but upgrade to a 2.02 intake valve.

Thoughts on what heads would probably work best?

[John Milner]

If both sets need done, I would fix up the 11's. If it were me, I would install both 2.11 and 1.77 valves, maybe smooth up the ports and gasket match. I would also surface them. Would likely need to be in the lower 80 cc range on the chambers to get 9:1 unless you are decking the block. The smaller chambered heads will likely prove easier to get to your 350 horse goal.

[android 211]

Unmilled 6X-8s will give a compression ratio close to 8:1. You'd have to have a very mild cam to have decent throttle response.

[steve25]

Without a dought it will be far better to start off with the 89 CC heads!
This is assuming the motor will be .030".over.


Even with that he will need a total of no more then 97.78 CCs if he's shooting for a 9.5 compression.

With the average stack of tolerance's ( deck , head gasket) and parts used ( 4 valve notch Pistons and such) he will end up with like 109.53 CCs.

Going for zero deck height and and installing the common bigger 2.11" and 1.77" valves will by him back like 7 CCs of the needed reduction from 109.53 to get him down to 102.53 CCs, so even at this point he still needs a reduction of 4.75 CCs to come from somewhere and a head mill of .028" will be the way to go, that is unless he Bores the motor to .060" in the first place for a Bore of 4.18".

Such a move will get him up to 412 cid total.

At .060" over he only need a total CC of 99.2, so the heads do not have to get milled to get to a 9.5 compression.

If he is only looking for 350 hp then the factory 068 Cam will be great and is capable of some 370 hp actually!

[77 TRASHCAN]

Originally Posted by android 211

Unmilled 6X-8s will give a compression ratio close to 8:1. You'd have to have a very mild cam to have decent throttle response.

Yeah...stock bore, stock engine is rated at a whopping 7.6:1, original in my TA

[F ROCK]

#11 heads are 1970 350 heads.
They don't have the end holes for headers & I don't think they have the machined pad to drill them.

[Will]

Originally Posted by F ROCK

#11 heads are 1970 350 heads.
They don't have the end holes for headers & I don't think they have the machined pad to drill them.

The holes may not be drilled, but the pads are there.

Only heads I've seen without the pads are 1972 castings. 7K3 and 7J2. There may be others, I haven't owned every head there is, but I've owned most of the late '60s-'70 small valve castings (11, 15, 17, 46, 47) and they all had provisions for all of the manifold holes.

[F ROCK]

Originally Posted by Will

The holes may not be drilled, but the pads are there.

Only heads I've seen without the pads are 1972 castings. 7K3 and 7J2. There may be others, I haven't owned every head there is, but I've owned most of the late '60s-'70 small valve castings (11, 15, 17, 46, 47) and they all had provisions for all of the manifold holes.

You're right I just looked at mine and the pads are there.
I had a 350 with those heads that ran great.

[77 TRASHCAN]

It's not difficult to drill and tap those pads. Did it on the 1970 350 currently in my TA. Made a fixture out of a piece of a 2X4 to get the hole drilled correctly. The hole WILL intersect with the head bolt hole it is close to, no worry. You just have to make sure the end exhaust bolts don't bottom out against the head bolts when torqued down!!!

[SD455DJ]

74SDformula - Your friends 400 motor sounds like a lower compression version of the small valve ('70 #16) head 400 that we built last year and dyno'd at 380(+) hp and 450 (+) lbft torque. The #16 heads came on the 10.0 330 hp non-GTO 400's available in the Tempest/Lemans and lower line B-bodies. The #11 heads are the lower compression heads (90 cc) used on both the 350 and 400 2-bbls (8.2 compression in the 350 and 8.8 compression in the 400). They just have 10cc larger combustion chambers. The #16 heads ended up at 78 cc's after a slight cut to true them up and with .040" quench netted 9.94 to 1 compression and the cam was the Summit 2801 (214/224/112 @ .444"/.466" lift). The motor was dyno'd with stock log exhaust manifolds with Pypes 2.5" headpipes. The valves were stock size (1.96/1.66) Ferreas with no port work, or anything special, just new valve springs that could accommodate the higher lift. It revved nicely to the 5600 rpm limit and the power held on. I think a 9.0 to 1 400 with the 2801 cam and RA manifolds could make 350 hp /425 tq easy.

Cutting the 11's .030" should get you to the low to mid 9's for compression.

Dennis

[74SDformula]

Thanks for the information everyone. Looks like we will be using the "11" casting heads.

[SD455DJ]

Thinking about this a little more, 1970 #11 heads and #15 (455 - 360 hp) small valve heads are essentially the same head with 88-90 cc combustion chambers, just different casting numbers for the engine applications.

We dyno'd a '70 YH 455 with stock small valve #15 heads (9.93 compression) with a Summit 2802 cam and log exhaust manifolds and it made 390 hp and 512 lbft torque. Cliff through together a small valve 455 and it ran high 11's in the heat in a heavy 2nd gen Firebird. I have a new found respect for the stock small valve heads. On a 400 they don't give up too much on a 400 motor, and are better than their big valve brothers on a 350. I think your friend will be satisfied with the power it will make with the right cam and good exhaust.

Dennis

[389]

You want compression at 9.1 That way you can run 87 and a nice 18-20 degree initial without the vacuum advance.

[SD455DJ]

I agree with 389. Keeping the combustion chambers at 88-90 cc's and with the larger valves you're thinking about will make a fine running 400 on pump gas. Install a 2801 cam, or similar, which is a high lift version of the 068 along with new springs for the increased lift. That should keep your compression in the low 9's depending on the piston height, valve reliefs, head gasket thickness, etc. Try to keep the quench as close to .040" as possible too.

Dennis