Ok, so most of what I see out there as a rule of thumb does not recommend more than a 14.final compression ratio before you go to o rings in the cylinder head and copper head gaskets. Why exactly is this? Also for an 8.5:1 static compression ratio on 93 octane the rule of thumb is about 7psi of boost(12.5 final) and 11psi on 106-110 octane race fuel(14.9 final) with a roots type supercharger. How much would you guys feel safe to run on a nailhead with just forged pistons and either steel shim or composite headgaskets?
It's not like that in real world. First, you gotta understand that the pressure reading that you refer to is just a "manifold pressure". This means that a wider cam would drop the pressure, yet increase the amount of air going inside the cylinder. Second, there is no close reference between compression ratio + boost level and compression ratio on N/A engine. With forced induction the air becomes more dense while maintaining the same volume. With 8.5 CR you can pretty much run 14 psi without a prob. I'm a little of the forged piston hater, and I'd rather stick to good cast pistons for street/strip and save forged for real racing applications. Cast pistons just don't like to be overheated. It's not that easy to understand, but if you look through many builds and check out CR and cam specs, you will understand the relations. Just an example - 4.8L V8 from Koenigsegg CCXR has 9.2:1 CR and 23 psi of boost, resulting in 1018 Bhp with 95 RON octane - 91 AKI octane in the US. That's just a properly designed forced induction engine.
I agree with everything but stay away from cast. their is no quality cast pistons for a nailhead and I wouldn't trust a piston ring lange under 15psi of boost on a cast piston. Their is a lot of great reading on turboforumns.com about guys running 10:1 comp and plaing with lsa and cylinder head chamber and seeing the effect on boosted applications. with a efi setup and electronic boost controller I am sure you could really turn the boost up and tune it effectively. I am finding my turbochaged nailhead likes more timing and less fuel at light load. too rich at light load and it will detonate. every motor is different and depending on what heads, cam, trans,gear and many other variables will all effect what your setup can handle.. also nothing like seat time and finding out what your setup likes by reading plugs and watching egt's..
okay, my point on cast pistons was true when they are cooled by oil jets underneath. As long as they are cool they could keep stable size and temp, meaning, they can be very street friendly. There are not good for forced induction without oil jets, I agree.
