65 dart 225 rebuild.

Sure. Let me try to answer your first question about the factory compression ratio, since that's the root of the issue.

Chrysler was really inconsistent about certain machining tolerances. Cylinder block deck heights were taller than spec, combustion chambers were larger than spec. That 8.4:1 compression ratio was an advertised rating. See that word advertised? Yeah. Marketing. These engines usually blueprinted (true measurement) closer to 1/2 to 1 full point below their advertised rating.

Also, as I believe it was explained to you before, the 225 does not even have its own cylinder head. When the 170 was first made, all the parts were designed around the 170. When the 225 came about, Chrysler simply used the 170 head on the 225. The slant six was never a production performance engine, so Chrysler saw no need to redesign another cylinder for the larger 225. So the 198 and 225 use a cylinder head designed for the much smaller 170. Small, restrictive ports and small valves. In order to be optimal for the 225, a complete head redesign would need to be done up to and including moving the ports to optimal places and reshaping them and including bigger valves. That was never in the cards.

Yes, you can have bigger valves installed and have the head ported, but how much more do you honestly think you are going to gain? A stock 225 is "about" 145 HP. You said originally your engine was 325 HP. That's an addition of 180 HP from stock. .......AND you don't have head porting listed in any of your mods. A naturally aspirated 225 does good to produce 1 HP per cubic inch. THere are some exceptions, but that's about the "going rate".

You used stock pistons in your build and you state the block was decked .025". Well, those pistons sit over .100" in the hole at TDC. Yes, you had the head milled a lot, and that's good for compression, but might be bad for detonation. Since the piston sits in the hole at TDC, there's no quench. Quench happens when the flat part of the piston gets close to the flat part of the cylinder head. Quench is good for a couple of reasons. It helps create turbulence in the combustion chamber, aids in more complete combustion and allows more compression to be run on a given octane of fuel. The way your engine is built, you have no way to achieve quench. But that's not a deal breaker, because the slant was never a quench engine anyway. Making it a quench engine "just helps" a little.

I am building what's called a long rod 225. That's using a longer (7.005") 198 connecting rod in the 225 with a 2.2 4 cylinder Chrysler piston to achieve zero deck height. But, in order to get any quench, I am still going to have to mill the head pretty good. I may try angle milling it. Taking more off one side than the other to help remove more from the flat side of the combustion chamber. I have a long way to go yet so "we'll see".

The bottom line is, while certainly you have a snappy 225, there's just no way in the world you have 325 HP as much as you want it to be true. I am going into my build thinking that if all things work out "like I want" I might end up with 250 HP......but, "probably not". LOL