[bober]

How well would the 1974 46 heads work on a 400 .30 over if the intake valves were increased to 2.11 and leave the exhaust at 1.66 ? I have the heads already and they are all complete but would like to know if it would be worth the larger intake valves ? thanks !!

[Cliff R]

They have a good flowing intake port even without the larger valves in them.

The chambers are pretty big, around 96cc if memory serves me correctly. They don't make much compression in a 400 built with a flat top piston, but are EXCELLENT heads for a 455 build.

Later versions come with screw in studs as well.....

[steve25]

Actually due to the 1974 # 46 casting having a 45 degree seat these heads flow better above .350” lift then for example a 1971 # 96 casting from a 400 cid motor with the 2.11” valve that it’s born with!

Peak intake air flow on a 46 casting comes all in by .550” lift as compared to the 96 casting that has to be cranked open to .620” lift to get to peak intake flow,
In regards to the exh ports they flow the same peak flow as the 1.77” valve in the 96 casting, and even more mid lift flow dispute there 1.66” valve!

My info on these heads state a 87 cc chamber for what was a 7.7 to 1 compression on a 350 motor.
On a stock 400 motor that does not have factory pistons with the chamfer around there OD I would expect a 8.2 compression.

If you go up to a 2.11” intake valve and leave the valve bowl throat stock the result of that will be to pick low lift flow below .350” lift while the high peak flow numbers will then come in at a lower valve lift, which is a very good thing,

Peak flow of a stock 46 casting is 215 cfm @28”,
Peak flow of a big valve head averages 205 cfm@28”.

The difference in low lift flow at .200” lift where the valve spends a bunch more open time due to greater cam duration then at peak lift, however is near a whopping 20 cfm more for the big valve head!!

Back to the compression talk , this is all a good thing plus you get the added benefit of reducing the chamber volume by near 2 CCs just with the valve swap.

If you want more compression and since the heads would have to be completely disassembled to cut the new intake seat you might as well mill the head .025” .

This move will get you back 4 CCs of chamber volume , so now the the bigger better flowing valve and it’s 2 CCs that’s it’s giving you back makes for a easy 1/4 point gain in compression ratio, and also a mill of only .025” will allow the intake manifold to bolt up without issue!

While installing the bigger intake valve if you have the shop doing the work open up the intake bowl throat with a cutter from its stock 1.55” size up to 1.64” you will gain you a good chunk of added flow with no added effort on your part!

If the that shop that does the work uses cutters or has what called a valve bowl Hog type cutter then I can’t see why it would cost you anything more then a added 40 bucks to get this throat enlargement work done,

With stock 46 heads and what they flow at .450” lift along with a 8.5 compression you should make 350 to close to 370 hp depending of course on the cam you run and if you at least have 3 tube headers on the car.

Hope this info helps ya!

[bober]

I have to get pistons for this rebuild so would I possibly be able to get domed pistons and a good cam to get some good hp without enlarging the intake ? or what combination would I need ? Thanks

[steve25]

What over bore size are you going with for the 400?

Just going from a stock bore size of 4.120” to ,060” over at 4.180 “ will net you a 1/4 point gain in compression.
This also takes you from 400 cid to near 413 cid.

I would have to run the numbers, but if those heads do check out at 87 CCs , then off the top of my head I would say that the pistons would need a 5 to 8 CC dome to get the motors compression into the needed low 9 to 1 range.

[bober]

I had planned 30 to 40 over and only to 60 if I have to for some reason but I'm sure it's at standard bore now . I 'll cc the heads before I order the parts . I was thinking of a stroker kit if I could swing it but I think the normal rebuild will have to do . Thanks

[steve25]

Well since it looks like your going to need to order custom pistons then that opens the door to cutting the cranks Rod journals in such a way to increase the stroke a small amount and pick up compression with less dome and to also reduce rod bearing speed, all of which is good!

[LStathas]

I should know this, Steve, but I have to ask. Are the over sized piston adjusted with the pin height to maintain CR? If so, no gain in CR unless the crank is offset ground or the deck is milled.

[steve25]

If I get the jist of what your asking then the answer is this .

99% of the time you will only find cast replacement Pistons that have there compression height adjusted to maintain a given compression ratio along with the over Bore size used.

If you add stroke to a Crank shaft during a regrind then the custom piston may need a different compression height also, it all depends on how much you offset grind the crankshaft and if your shooting for a positive or negative deck height.