500 with 425 heads

[rtmpmm]

well im building my 500 its a 76 block and pistons with a mts cam and 425 heads should be around 10 to 1 what horse power should i expect out of this set up

[steelybill]

The power you get will depend on the cam you have in it as well as the comp ratio. I don't know the sizes of the 425 valves compared to the 500 & 472, which could also be a factor. If you have stock rocker arms, (1.65:1 ratio) it will have less power than the aftermarket 1.72 ratio arms. All these things have a direct effect on power output.

The advertised duration of your cam could help figure a ballpark idea of power. Something in the 280 degree area would give a fairly good power boost.

[rtmpmm]

the cam is a MTS #15 Series Grinds:

RPM range: 2200-5400/ Advertised duration range: 286-295* / Duration @ .050" range: 230-238* / Valve lift range: .520-.540".

[steelybill]

The cam grind looks interesting. You may not have much vacuum for power brakes.
The stock Armasteel rods in a big Caddy don't like high RPMs. About 4800 is all I want to put mine through.
These big displacement engines are not as affected by long duration cams, as say a 327 Chevy, so you may do OK with that cam.

The only thing about the 425 heads is that the ports are a bit smaller than the 472/500 variety. That may not be an issue unless high HP and torque figures are desired.

I would hazard a guess that you may get around 340/350 HP with that combo.
The torque will be there for sure, like around 500+ ft/lb
The stock 8.5 to 1 500 has around 490.
Advancing the cam timing by 4 degrees will make power a lower RPMs, and you wont have to buzz it up too high.

Get it together..... you'll have fun with it

[CadVetteStang]

The 10:1 compression is from 74- 75 blocks with a true 8.5 compression. The 76 has 8.25:1 compression so my guess is that you will have 9.5 or 9.75 compression. You may even be able to run 87 octane gas with it if you use retarded timing and cold plugs. I know that Chris from the Cad Company does NOT like the 425 head on 500 block combo because of the port size differences. I can’t remember for sure, but I think the valve size is the same or very close. You might want to make a few calls to the build shops that specialize in caddy motors like MTS or the Cad Company. It is possible that your engine would do better with 8.25:1 pistons and ground down heads to pick it up to 8.5:1 with true 500 heads than it would to be at 9.75:1 with 425 heads. And of course, with 8.5:1, you can run the cheap gas at part throttle in the summer with no octane knock. I thought about the 425 head swap wit my spare 76 engine, but what I was going to do was put early 500 heads on it. That should give 11 or 11.5:1 compression. But it was going to be a low to mid RPM race motor in a light car with a long duration cam. I would not chance 11.5:1 in a heavy daily driver- especially with a short duration cam (I think something would break).

[steelybill]

Yeah, a short duration cam with high compression will really have high cylinder pressure alright. Often wondered what would fail first

[cowboycadillac556]

according to MTS to even get a cad big block to rev that high you need valve springs so stiff that you start to have rocker tee failures

[steelybill]

The rocker set-up is one of the weak areas of the big Caddy. After-market set-ups are the ticket, but fairly expensive, like shaft rockers etc.