piston questions

volaredon said
How inaccurate are the advertised CR on the magnums? If they're like the 70s/80s engines the CR advertised in all the manuals of the day are very ambitious. My last complete rebuild was a /6 and though advertised as 8.4 it measured out well below 8 to 1. I've seen small blocks that actually measured out as less than advertised which means less power than advertised as well.
Are these engines really 9:1 as built, stock from the factory? I want at least that much, but probably a max of around 9.5:1 so I don't have to run premium.
What do you think about running those pistons my kid has, now that I know what number they really are? The truck is an 80s which were still LA engines, basically I'd be building an LA in the confined of a magnum block ..
They said the magnum block is slightly shorter than the LA deck height wise, these 405 pistons are about 20 thou shorter compression height vs the 116s I thought they were. And the 405# are likewise flat top except they have 4 valve reliefs instead of the 2 that the hp116s have.
Even if not needed for a big cam, I would wonder what the valve reliefs did for mixture turbulence, when running especially the 2 that aren't gonna be used for valve clearance in each cylinder?

You CAN use the H405CP pistons you already have in a 5.9 Magnum block IF you get the rotating assembly balanced. There are also factory style replacement pistons available as part number H655CP and these have the factory style dish in them. If you compare the specs you will find you gain a little but of compression with the H405CP over the H655CP with everything else remaining the same. Is the goofy four valve relief piston crown on the H405CP the most efficient design? Nope. The dish in the H655CP is a copy of the factory Magnum piston and it's intention is to increase mixture motion on the compression stroke. You would probably use less fuel to make the same power with the H655CP (or a factory Magnum piston if you were not already over bored) than you will with the H405CP.

Use the compression calculators available on sites like Summit or Diamond racing to play with the numbers. But ultimately, you will have to decide if you want to run the pistons you have or buy new ones. Then, if you are ok spending the money on new ones, do you need more compression like KB107 or H116 or will the H655Cp work for you.

If it was me in this situation, I would sell the H405CP pistons and buy the H655CP for this truck engine. Then make sure you know what heads are going on it and what head gaskets you will be using and get the Magnum cam reground using the suggestion they give at Oregon Cams.

Sorry for the long post. Trying to be clear.