Texas69

Dumb question of the day...

I have a '76 J head 455. Could I just unbolt the J heads and bolt in C or E heads to get a small bump in performance? Is it that easy, or do other things need to get changed out as well?

Also, are factory rockers interchangable between the various 455 heads?

svnt442

So long as nothing has been modified (the heads cut to raise compression or you have a different cam that would require different valve springs and/or pushrods) it should be a bolt on swap.

matt69olds

You would get more bang for the buck if you cut the heads down, and use thin head gaskets. The J and C heads both have around 80cc combustion chamber size. Depending on how far you mill the heads you might need to mill the intake to fit. Same thing with push rod length. Of course, its not much more effort to slide in hairy cam. Where do you want to stop? 🤣

joe_padavano

Unless you are running back-to-back quarter mile times, simply swapping the heads is unlikely to provide a noticeable increase in performance. This assumes, of course, that all else is the same - valve sizes, springs, etc. Obviously big valve C heads will show an improvement over small valve J heads, but likely less than you think until you get above 4,000 RPM or so.

matt69olds

Joe brings up a good point, with the stock girlie man camshaft I doubt you would see any improvement. The engine probably won’t move any move air than it would with the stock heads. I recall a article from car craft magazine where they found a bone stock low mileage 76 Pontiac 455. They did the typical poor man mods (intake, headers, etc.) they picked up next to nothing with the external bolt on (something like 20hp, with a hp peak at 4K). When they installed a semi-aggressive street cam the engine picked up something like 100, with aftermarket aluminum heads it almost made 500hp.

olds 307 and 403

I remember that 455 Pontiac article, their cam was even worse than the smog 455 cam, tiny duration an .374/.407, I believe. At least the 455 Olds smog cam is the VIN 9 307 or manual trans 350 cam, imagine a 455 with a .400 small block cam in it. As said, keeping the same low compression and cam will hardly make any difference, change those two items and the difference between big valve early heads and constipated exhaust port J heads will be very noticeable.

Texas69

Thanks for the input. My conclusion from the above is that swapping heads is pointless unless the girlie man cam and/or pistons are also changed out.

This may be a classic case where the "wants" may not align with the realities of a budget. While I may leave some HP/TQ on the table keeping the lame dished pistons, the head and cam swap would give me some tangible additional power.

matt69olds

Mill the heads, use thin head gaskets, you can always swap cams later.