The 2.5 is not a particularly strong engine. The early 1984s had a problem with substandard connecting rods (typical GM, they bought connecting rods from the lowest bidder and had a failure rate of around 20%) by late 1984 the problem was supposedly fixed. (many of the 1984 fires were the result of a connecting rod breaking and punching a hole in the front of the block which then splashed oil on the exhaust manifold and the main electrical connector resulting in a fire).
There are fewer stories of failed connecting rods in later years.
The SD engines were fantastic, however so is the price tag. These blocks are made from higher pressure iron castings with more internal structure to support the crank, larger bearings and forged crankshafts with larger counterweights. Together with high quality connecting rods and forged pistons we have an engine which can withstand 7000 rpm (and with the improved valvetrain can be reliably used with a 6000rpm redline) Set up with the Holly TBI, an appropriate intake, headers and a good street exhaust they are 180hp with mountains of low end torque. More radical setups can reliably produce significantly more than 200hp.
OK, enough dreaming, now for reality. There are owners who race basically stock 2.5s and, with breathing improvements, rev them to nearly 6000rpm, they are lucky. This is a 5000rpm motor (4500 preferably) so if you want more go you have to concentrate on low end torque. If you are rebuilding an engine have all of the connecting rods carefully inspected and tested (I don't know of any aftermarket replacements unfortunately so this may be your only hope). Apparently a later crankshaft, such as an 88, can still be used in the early block (check this first) if so this is a better crank. Use a high flow oil pump. Don't overbore by too much, just what is necessary to clean up the bore, any more will compromise the block (this is a very thin wall design). There are cams available for the flat tappet blocks, use one that produces power at lower RPM ( if it is designed to make torque at 4000rpm and peak power ar 5500 then stay away, this is a recipe for a broken engine). The head can be equipped with roller rockers (big block Chevy I believe) and should be ported (gasket matched) but again, don't remove too much material. The swirl port design can have the restrictive vanes reduced in size to allow better flow, the rule here is simple, if you haven't researched carefully and you don't know what your doing then don't do it. The Holly TBI can be used with the stock intake, not ideal but it does allow some gain. (the four barrel carb and manifold would work well, if it fits, but keep the carb small with vacuum secondaries so you can tune it to not bog the engine) Since this is not a high rpm engine an exhaust header may not be cost effective (not much gain for the price but still an improvement over stock).
These are just guidelines and will not produce a speed demon, however it will get you up to the 120-140hp range, it will probably not explode, and it should have a life expectancy of over half that of a stock motor (if not more) as long as you respect the rev limitations. With the lighter weight of the 4 cylinder car it will have similar performance to the V6, with a possible advantage in low end acceleration but a disadvantage at the higher rpm (the V6 has a 6000 redline)
Obviously big power will involve a swap to a V6 or V8 (or a Quad 4) but the Iron Duke can perform well enough to power a car with suspension improvements and make it a demon in the twisty bits.
Good luck
More info at my site.
Ira
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The Iron Duke Resource Site
[This message has been edited by Monza76 (edited 05-22-2003).]
