Hello list. Sorry to bomb the lists. Awhile back, there was some
discussion on SU's vs Strombergs. Here is a simple answer. The
Stromberg was used because it was a simple, already in-house carb that
would be USA emission legal. Since it is basically a sealed unit with
minimal adjustment range, it would not be capable of being "messed with"
and thus remian EPA compliant. The Stromberg has a fixed main jet
(which is also non-replaceable) with a very small selection of metering
needles. In the case of the 175CD carb, you can choose from possibly 10
different needles. The HS6 SU, on the other hand, has 4 different main
jets - .090", .100", .110" and .125" - which are replaceable. There are
also many needles to choose from for each jet size. This allows you to
tailor the carb to your cam, compression, altitude, etc without trying
to compromise. Since the Stromberg is sealed or basically
non-adjustable, the choice is fairly simple.
Compression ratio determination. There are all kinds of "cut this much
off the head and you will have this compression ratio" figures. The
problem is, unless you know the head has never been cut, you can only
hope the source of the info used an uncut head to determine the
compression numbers. Why not do it once and accurately? The ONLY way
to accurately determine compression ratio is to measure cylinder and
chamber volumes. Here is the formula. Use it and you will never have
to worry. Compression Ratio (CR) = V1 + V2 divided by V2. V1 is the
volume of the cylinder with the piston at bottom dead center (bdc). V2
is the volume of space above the piston at top dead center (tdc). By
using this formula, you don't have to worry about piston shape. Be
sure to add 3 cc's for the head gasket volume. example, V1 = 450 cc.
V2 = 50 cc. 450 + 50 = 500 divided by 50 (V2) = 10:1. Simple and
accurate. All you need is a milliliter burrette and a level. Level the
head on the bench and fill the chamber with liquid - we use acetone
because it's easy to clean up with no residue. That gives V2. Fill the
cylinder with acetone and determine V1.
How much compression is "safe"? With today's fuels, we limit CR to 9.5
on cast iron engines. It used to be the same octame - 93 - would allow
you to safely run around 11:1 with stock cast pistons. Today's fuel
burns very hot - it's a method of making sure they limit the unburned
gasses going ou the tail pipe. Because of these high burn temperatures
and the increased pressure of higher compression. the cast pistons
really get stressed. This can result in piston damage. Forged pistons
can be used but this raises the cost . Hope this information is of
benefit. Ted
--
Ted Schumacher
tedtsimx@bright.net
http://www.tsimportedautomotive.com
108 S. Jefferson St.
Pandora, Ohio, USA
Fax: 419.384.3272 (24 Hrs.)
Phone: 800.543.6648 (US & Canada)
Tech/ Gen. Information/ Worldwide: 419.384.3022
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