[C-Rad]

I bought a 1968 Tempest convertible that has a late 1976 Pontiac 400 block (casting number 500557)
It has heads (casting number 94 (1971?)). The power feels a lot like my ‘66 Tempest with an stock 326 w/ 2-barrel, so sufficient but not impressive power (250HP??). I would like to increase the HP on the 400 up to 350-375Hp (I know the 500557 blocks don’t like more than that). I just want a fun street cruiser/daily driver.. It already has an Edelbrock performer aluminum intake and 4bbl carb. I don’t know anything about the cam, or if the cylinders have been bored.
I have an opportunity to buy a set of set of rebuilt heads (casting number 13) for $550, which have the big valves and smaller volume.

What kind of power increase can I expect if I swap out the #94 heads for the #13?

[J.C.you]

I just did same thing with 13s and the later beveled lower compression factory pistons, good runner. I softened the eyebrows in the chambers a little to avoid a hot spot detonation area

[Formulas]

Buy the heads even if you dont need them, great price for cores let alone rebuilt

They will work fine on your engine as well

[steve25]

It’s difficult to nail down a answer without known what cam is in the motor now..
Essentially you would be going from a 8 to 1 compression to about 9.4 to 1, that would gain you around 3.4 % more HP.
Figurine that at best now your making 240 hp so the gain in compression buys you about 10 more hp.

[Joe's Garage]

We did this several times on 1977/78 Trans Ams back in the 1990s and it REALLY woke up the engine.

With stock camshafts, we'd switch out to 1.65 rockers are the same time.

Then they felt a LOT like the 1969 and 1970 GTOs we owned at the time.

You not only pick up compression, you also pick up the increased flow from the larger valves.

Plus the increase in power having a fresh valve job and new springs, etc.

Go for it!

[steve25]

While the HP gain might not sound like much the difference in throttle responce is and the increase in fuel mileage is nothing to sneeze at either these days.

[J.C.you]

Originally Posted by steve25

It’s difficult to nail down a answer without known what cam is in the motor now..
Essentially you would be going from a 8 to 1 compression to about 9.4 to 1, that would gain you around 3.4 % more HP.
Figurine that at best now your making 240 hp so the gain in compression buys you about 10 more hp.

Just saying only 10 hp is deceiving to someone that may think, why bother? I hear this frequently on the boards by keyboard warriors armed with simulation hardware.

That’s across the entire useable rpm range of the engine. Carry on

[Kenth]

Fresh #13 heads + 068 cam equals fun 400 engine, even with the beveled pistons.

JMHO

[steve25]

Great point!
I forgot that motor has the chamfered pistons .
Go right ahead and bolt the 13s on!

[steve25]

Info.

Change in Hp.