454 head questions

[Rex]

Howdy, I've been lurking on this board for a while, and I finally created a username and everything. I have a few questions. First how much lift can I run on 781 casting large open chamber oval port heads? The cam I want to run is a comp xtreme 4x4 hydr. with 270 int. and 278 exh. .544 and .564 lift, will this be a good cam for street and occasional towing? Also, anyone know the compression ratio, the casting number is XXX959, its a '78 truck engine? This board is great by the way, lots of very good tech and not that much whining!!! /forums/images/graemlins/thumb.gif

 

[Sandman]

I run the same cam but the one that is one step down from it. I ran both cams through desktop dyno and found that the limiting factor was in the heads themselves. I have the same heads as you. There was almost no difference between the two. Like 1 horse or something. I went with the smaller one to avoid possible lift issues that you are looking at. I even wrote an article on the swap. Hope this helps.

Cam Swap

 

[Longhorn4x4]

That cam is a bit much, and you'll need to cut the tops of the guides to run it, you'll probably need new springs and trick retainers to get enough installed height to make the springs work right. If you have flat top pistons, with 781 heads, your compression ratio is around 7.8:1. If you want something that runs real good, get a set of old closed chamber 396 heads, and a cam with around 218 duration at 0.050" tappet lift, and about 0.520" lift at the valve. Put the 2.19 and 1.88 valves in it, with hardened seats. That combination makes solid reliable power from 2000 to 5500 RPM, with a nice fat torque curve. I've run it on the dyno, and in a truck for 200,000 miles. It ate Cummins turbo diesels and Powersmoke diesels for lunch, and it got about 12 MPG in an old truck with no overdrive.

 

[Rex]

Thanks for all the help, I thought that cam would be a liitle too much. I thought large oval ports flowed pretty well? I am going to do a three angle valve job, match intake and try to smooth them out a little too, i just don't have the money to replace the valves and everything. Can i get to 8:1 or a little higher with a thin gasket and the heads milled down a little?

 

[Z3PR]

You can have the heads milled. I'd try for 9.5 to 1 compression. MORE POWER !!!!!!! on pump gas.

 

[Rex]

Is it possible to get 9:1 just by milling the heads and not replacing the pistons?

 

[Longhorn4x4]

NO! It cannot be done with open chamber (either 781 or 049 castings), and stock flat top pistons, period. The most you can cut them is around 0.030" flat, not angled. It will BARELY yield 8:1, and then only if the piston is less than 0.025" in the hole, which ain't likely with a stock unmachined block. You have to mill the closed chamber head around 0.015" and put the big valves in it just to get 9:1.