Howdy Back Peyton:
This is what I have so far. I have not been able to locate, cheaply, a rod for direct comparison. Journal sizes are the same, but width is not listed in either case.
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Use a longer Tempo 2.5 HSC rod with a modified flattop HSC piston. This has the advantage of reducing the deck height .110" and giving a slightly better rod/stroke ratio.
Rod Length & weight;
250 =5.88" 592 grams
2.5 HSC =5.99" ??? grams
Difference = .110" grams
Rod Length to Stroke ratio;
250 = 5.88" : 3.91"= 1.5:1
2.5 HSC = 5.99" : 3.91"= 1.5319:1
I would deck the top of the block to achieve zero deck height. Use a FoMoCo or Corteco composite head gasket with a compressed thickness of .037".
I would use with an HSC flattop piston, but mill a "D" shaped dish into the top. The goals will be to maintain a high bore to quench ratio, lower CR, lighten the piston, reduce knock tendency, and maximize combustion efficiency. The "D shape would mirror the combustion chamber.
I would mill the head only enough to ensure it's flat. At this point it is critical to measure all volumes to assess CR. Reshaping the combustion chambers to reduce CR, unshroud the intake valves and match chamber volumes.
The advantages of using the 4-cyl rods are, 1. longer, for a slightly improved rod to stroke ratio, 2. 4 cylinder parts are usually tougher than I6 gear because four cylinders vibrate much more, rev to a much higher RPM, produce more power per cylinder, carry more load per cylinder than a six, and suffer more detonation than a I6 was ever designed for.
This combination will have a more ideal deck clearance, likely have a lighter reciprocating weight, and be able to tolerate CR in the 9.3:1 range, at sea level, with 91 octane gas, with the right cam.
Additionally, I would plan to use ARP rod bolts, polish the rod beams, balance the whole rotating assembly, and polish the piston tops and the chambers.
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If anyone knows a source for a couple of HSC rods for comparison I'd sure like to know about it.
Adios, David
