Posted by Alan Bowes on November 11, 1997 at 20:10:53:
In Reply to: Re: SUPERCHARGING? a vaild response... posted by garrison on November 11, 1997 at 18:45:24:
Hi Garrison,
You're right on about the 230/251 being a low-speed engine, and you're correct about the increased stress and other challenges that accompany modifications that increase the operating RPM range. The 230 will always be a low-speed engine, even though some good bottom-end work might safely coax another 1K out of it. The 230 has a long rod, as you mentioned, and a long stroke to go with it, giving it that "magical" rod ratio of almost 1.7 (about the same as a typical small- or large-block American V8, just for some trivia value). Compared with a shorter-stroke engine, a longer stroke causes higher stress in the reciprocating parts at any given RPM due to the greater inertial changes. However, if the engine is cammed to KEEP the torque curve peak in the lower RPM range, then a reasonable amount of boost shouldn't hurt as long as the engine can handle some increased combustion chamber pressures. If it doesn't cause a significant increase in the operating RPM range then I don't believe that it would significantly increase the stresses resulting from inertial changes. As to harmonics, you're right about these being a concern. It's always a concern. As you know, any engine, regardless of design, has certain natural rotating and rocking couples that set up some interesting vibration patterns at different speeds, and affected to some degree also by different loading conditions. These vibrations can range from unnoticeable through damaging. Some engines incorporate a special balance shaft just to reduce the apparent vibration to the passengers, although the internal stresses may still be high. If there is a potentially damaging harmonic of these vibrations, then it is probably best to avoid loading the engine too much at that point, and better yet, some experimenting should be done to reduce it. Changes in balance, throw weight, etc. can have a significant effect on such harmonics. But again, keeping the engine's optimum operating range at a low RPM should help avoid exaggerating these problems and introducing new variables, though there's no guarantee.
You're right about too much boost killing a stock engine. If the boost were limited to sea-level pressure, there would effectively be NO boost, since all you'd be doing is fooling the engine into thinking it's at sea level all the time, regardless of altitude. A stock engine would not be adversely affected and no modifications would be necessary. Beyond that, of course, it would depend on the strength margin of the engine and how much work you wanted to do in prepping it for the increased pressures. At the minimum, you'd want to do some good rod work, balancing, possibly cross-drilling, possibly forged pistons, high-strength con rod bolts, high pressure/volume oil pump, etc.
It would be a challenge if you wanted to safely apply more than a small amount of boost.
Sounds like fun.
Alan
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