Compression Truth: Lipstick On a Pig?
As everyone here knows… I design engine parts. I have finished up my new line of pistons, and during this process, some truth has been shed on the compression ratio numbers game.
I use the only manufacturer that I could find that uses 3-D chamber and dome scanning. This scanning, along with “proper” math has brought me to the following conclusion: there is no such thing as a true 14:1 piston in a stock bore 700! I will explain how I got to that bold statement, and let the games begin!!
I designed 3 different domes;
11:1 uses a -.9cc effective dome
12.5 uses a 7.9cc effective dome
14? uses a 9cc effective dome, which is the largest effective dome that will fit into the chamber. When I say “effective” dome, I am referring to the actual dome size after valve reliefs have been machined.
Notice the jump from 11:1 to 12.5:1 requires a dome size increase of 8.8cc’s. That is what is required to increase the compression ratio 1.5 points. I could possibly gain 1 more cc by not using a trough style valve relief on the exhaust. Now you must realize that as the dome gets larger, valve reliefs remove more material. The dome also has to become progressively narrower at the top to fit the chamber, making it even harder to gain dome volume. Notice the piston with a claimed 14:1 only has a net increase of 1.1cc……. I do have very large valve reliefs for use with .550″+ lift cams, but the piston company I use actually has a dome shape that fits the whole chamber.
Okay, now add in the fact that I have the only off the shelf piston that sits .015 “in the hole”, while most sit in the hole .050-.065″. This gives me the ability to have the highest compression with the smallest domes. There are some custom pistons out to compensate, but the math stays the same. The valve pockets have to be machined deeper to retain clearance, and the dome only goes into the middle of the valve pockets….. Again, not happening. Now we can add in the fact that most manufacturers conveniently forget to add the 1.2 or so cc void around the piston head to the top ring…… You can see how quickly this becomes a “theoretical compression” numbers game.
Now to the defense of dealers and engine builders….. most do not have the equipment to measure actual net dome volume. This is a very difficult task. So they have to trust what the manufacturer tells them. The manufacturers give a “theoretical” number that often assumes “0″ deck, or sometimes doesn’t account for valve reliefs, and the truth gets lost in the wash.
I will be posting this on all the forums that I frequent, and will be more than happy to go “in depth” about the math I use for my conclusions. The obvious question would be; What is the highest compression I have seen in a stock displacement engine? 12.8:1. Please do not ask questions about dome shapes if you do not want an honest answer. Lipstick on a pig? (that phrase cracks me up)
