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From: <jon@dakota-truck.net>
Sent: Friday, October 09, 2009 1:08 AM
To: <dakota-truck-moderator@bent.twistedbits.net>
Subject: Re: DML: Compression
>
> "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 --'
>
It's the advertised compression ratio I'm looking for.
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