"Jamie Calder" <jcalder3@cfl.rr.com> wrote:
> I have a 408 short block that's been sitting in my shed for about 4 years
> now. My local machinist built it but I got the parts from a Dodge engine
> builder (rods, pistons, cam, crank). In the last few years I've learned by
> reading on different forums that perhaps this persons setup (specifically
> pistons and cam) may not be the best combo. Before I pick up this project
> again, I'd like to know if there's a way to determine what compression ratio
> these pistons have. Perhaps how close the piston comes to the top of the
> block? I rotated it and the edge of the piston is about half the thickness
> of a dime from the top of the block, however they're dished and have valve
> reliefs...
> I did take pictures if it can help anyone identify these Ross pistons. Any
> suggested methods for figuring this out is appreciated. I may call the guy
> myself to ask but all that's on the invoice from him is "Ross Pistons"...
In order to determine your compression ratio, there are a number of
factors that you need to consider. When pistons are sold based on
compression ratio (i.e. "these are 11:1 pistons") the "advertised
compression ratio" doesn't actually mean much. There are assumptions
that must be made, i.e. that the stock heads and rods are being used,
or a particular aftermarket head, etc. However, even when these other
variables are (mostly) known, variations from engine to engine mean
that the piston manufacturer is still guesstimating.
To determine your compression ratio, you need to know the bore,
stroke, cylinder head combustion chamber volume, block deck height,
gasket bore, gasket thickness, and piston relief and/or dome volume.
If necessary, you can use manufacturer's specs for many of these, but
its really best to actually measure them yourself.
Here is a compression ratio worksheet in Microsoft Excel format
that may help you to determine your compression ratio, or at least
allow you to play around with the numbers to see what sort of
difference changing a particular variable has:
http://www.steigerperformance.com/docs/CompressionRatioWorksheet.xls
The same worksheet in PDF format in case you don't have excel or
prefer to do it manually:
http://www.steigerperformance.com/docs/CompressionRatioWorksheet.pdf
The short version is, unfortunately there is no way to determine
your compression ratio without actually measuring it. In fact, since
what you have is a short block, it doesn't even *have* a compression
ratio. The heads you install will have a huge influence on your
compression ratio, and the gaskets that you use will affect it also.
--
-Jon-
.- Jon Steiger -- jon@dakota-truck.net or jon@jonsteiger.com -.
| '96 Kolb Firefly, '96 Suzuki Intruder, Miscellaneous Mopars |
`-------------------------------- http://www.jonsteiger.com --'
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