[cobalt327]

Originally Posted by 377MONTE

cobalt,

why am i under the impression that dish pistons are second best? maybe I am mistaken, but i figure lower compression by a whole ratio, maybe even more, would equal less power? Are my 450+ hp/tq goals still realistic with dish pistons?

The piston dish or dome volume is a means to an end. You need to lower the CR. Your choices are a larger combustion chamber, a larger dish volume, or both. There are a few heads available w/larger chambers than 64cc. But in the beginning you had specified cast iron heads having 64cc chambers, so the recommendations reflect that. That said, I hope you have the newer Platinum castings.

Using an inverted dome or D-cup piston places the dish over the deep part of the chamber, which maximizes the quench effect (there's an echo in here! lol). This is an important consideration because good quench action is the same thing as raising the octane of the fuel you burn in that the propensity for detonation is lowered in either case.

Good quench is sometimes referred to as "mechanical octane". This is why we do not recommend the factory-type round dish pistons- they do not offer as good a quench action. This is also why using thick head gaskets or lower compression height pistons is not such a good idea, unless they allow a quench in the area of 0.040" (distance from the block deck down to the toop of the piston at TDC, more on quench here).

Now you are correct in thinking that compression IS worth power. The generally accepted rule of thumb says there's about a 4% change in hp per number. So going from 8:1 to 9:1 on a 400 hp engine will add 16 hp. This is a relatively small gain to chance having problems w/detonation because the compression ratio is too high. If you have to retard the timing to prevent detonation, you will lose FAR more power and economy than you will by simply building the engine w/a lower CR to begin with. Since you have your choice of pistons, now is the time to build it using parts optimized for the application. You can thank us later.

Oh, and 450 HP is totally doable w/dished pistons and the right heads.

[hcompton]

16% is very low. I cant see you only make 16 more hp going from 8:1 to 9:1 its worth almost a 100 hp on a big v8 as long as everything is tuned for the upgrade. Heads, cam and carb. You can see this very clearly on chevy 454 engines over the years. When the compression goes down the hp drops off in gobs. Lower compression engines use less gas so its common for mfg. to lower cr to gain mpg in the 70, and 80's it made the cars slow as dirt and still didnt solve the issue. Smaller high compression engines are used now cause of its drastic effect on power output and effency(VE).

[cobalt327]

You also have to remember that during the same era that CR was being lowered, the factories went to net power ratings.

As far as the validity of 4%, YMMV and that's fine w/me.

[1Gary]

Originally Posted by cobalt327

You also have to remember that during the same era that CR was being lowered, the factories went to net power ratings.

As far as the validity of 4%, YMMV and that's fine w/me.

YMMV??.