I've seen a number of posts and signatures stating what people are running for compression ratio's but in reality I believe that most would be surprised to learn what their actual ratios are.

I had my head (pr3) in the machine shop getting some work done on it and wanted to plane it so I could get to 11.5:1 on my build (see sig). Most of us have used the online compression calculators like http://www.c-speedracing.com/h...c.php to figure out what we're running. These tools are useful but they also do their calculations based on certain assumptions.

For instance, my head according to the calculator should have a 42.7cc combustion chamber but when I had the head cc'd before planing it to determine how much had to be removed to reach my goal, it cc'd out at 44cc. In my case I had to take .030" off the head to get to about 40cc which is where i needed to be. In discussing the issue with the machine shop, they said small differences in casting each head on the production line can be a factor, other things like the type of spark plug used (Bosch platinum +4 vs NGK Iridium) ie: amount of space it takes up in the combustion chamber can have an effect on cc's. They also said valves over time will pound themselves into their seats, sitting a little deeper in the chamber, again increasing the total volume, lowering the comp ratio. My head was in good shape with only about 35k original miles on it so an older head with higher mileage could be out a lot more than mine was cc wise.

The purpose of my post is to show what kinds of things can have a fairly large effect on your compression ratio. Simply relying on calculators to tell you what yours is, is not a good idea when you really need to be accurate. Those calculators should be used for reference when selecting different parts or engine configurations but you should always have the proper measurements taken before you start planing your head or decking your block etc.

 

A new valve job also sinks the valves in the head and makes a decent difference in CC

 

All to true. And then, people get really wrapped around the axle about static compression when places like KB silvolite have calculators that will factor in duration and closing point of cams to let you calculate the actual dynamic ratio so you can see how "pump gas friendly" a build will be when you use all the tools at hand.

 

I'm a mechanical engineer, and I don't trust any of those calculators. In addition to the points mentioned above, almost all of the programs assume a round bore on the combustion chamber when accounting for head milling. Anyone who's seen a GSR head knows that the quench pads will make a huge difference in the CC calculation, and their size is usually not accounted for.

CC your heads people!!

 

lastly.
check the compression height on the pistons. this is a huge misconception using CTR pistons.
factor in PTDH also, some people think they are running 12.5:1 compression (me included back in the day). all calculated out PTDH, CCed chambers etc etc. i was running around 11.3:1.
my current setup pistons are almost 14:1. actual calculated compression ratio everything involved 13.4:1.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by jvtec95 &raquo;</TD></TR><TR><TD CLASS="quote">

CC your heads people!! </TD></TR></TABLE>


Honestly, I thought everyone did this? That was one of the first things taught to me.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Jay_Sensing &raquo;</TD></TR><TR><TD CLASS="quote">


Honestly, I thought everyone did this? That was one of the first things taught to me. </TD></TR></TABLE> No. It costs like $50.00 and with that money you could get some Spoon socks for your master cylinders. Plus the internet never lies, just like E-bay.

 

I did it for free on a Y8 head I've had lying around. Cut a sheet of plexiglas, drilled a hole in it, used a syringe from medicene I took a few years ago. Worked well.