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sd4 engine

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Blip Downshift:
I know this is an old topic but the mercruiser and SD-4 are not the same.  The mercruiser is based of the old chevy 153 platform and uses a ductile iron crank, ford pistons, and a ford head.  The SD-4 is based around the pontiac iron duke (half a pontiac 302) and uses the GM corporate 60 bellhousing design so the mounting points will be different than the chevy SBC style that is found in our eagles.  With some modification, it can be made to fit but the front differential mounting may be problematic.  The SD-4 also uses a forged steel crank, SBC rods and pistons, and has a performance version of the cast iron head and two different performance variations of aluminum heads (the 431 and 801).  A mercruiser crank can be fitted into an iron duke with some extensive modifications and is much stronger than the iron duke's crank.  People have also put mercruiser engines in there eagles and spirits with some work as well.  SD-4 cranks and heads can also be fitted into iron dukes but the head bolts must be enlarged for the SD-4 head.  The iron duke has a notoriously weak bottom end which really holds the engine back.  The iron duke block in the pontiac fiero pace car with a stock iron duke block, SD-4 cranks, and SD-4 cast iron head made around 237hp.  I am not sure what else they had to do to get it up to the power and the engine probably didn't have much longevity.  The link below is from the stickman's web page which gives some good comparison info about the iron duke and SD-4:
http://thestickman.tripod.com/DUKE-HEADS
The next link is from a gentleman who goes by stroker 151, the information is top notch but the delivery is a little patronizing:
https://fiero151.wordpress.com/iron-duke-performance/strokers10s-holy-grail-of-151-performance-data-2-5l-myths-and-differences/

Blip Downshift:
I misspoke, the mercruiser crank is made of out nodular iron.

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