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Originally Posted by OhioStatePilot  Man, you guys are throwing around different efficiencies here. Volumetric Efficiency is a very very different story than Thermal Efficiency. Now I could be completely wrong in this, but I'm not exactly sure that thermal efficiency is pertinent to the over square question at hand. The reason I say that is because the real concern about operating at a high MP isn't necessarily the pressure in the manifold, but instead the cylinder pressure.
The quick and dirty, in my view, is that the argument revolves around operating a high MP with low RPM which results in a higher cylinder pressure working against a greater resistance. The reason I say volumetric efficiency is pertinent is because with higher VE, more air can be introduced to the cylinder. With more air, more fuel can be added to create a larger burn, and therefore more cylinder pressure. Thermal efficiency doesn't really come much into play for building cylinder pressure, or at least in my understanding. Oh, and by the way, automotive VEs greater than 100 can be reached N/A.  |
Yes it can. But it is through such a narrow operating range, that it is negligable. You may have an engine that has a 4500rpm power band, but will only achieve better than 100% through 50-100 rpm. We are talking about extream max performance engines (think NHRA or NASCAR), and not in an everyday street driven vehicle. Most of those engines are not even streetable.
As for the greater cyl pressure debate, I have to just shake my head at that one. It takes so much pressure to bend metal in an engine, that running oversquare values of 1900rpm and 50" of MP still will not bend metal. The problem more or less lies in the direct corilation of pressure=heat. Your typical light GA engine will not be hurt by running oversquare within resonable limits. Also, as RPM changes, so does VE(lower RPM's = lower VE). The engines are desgined to be at their optimum VE at max RPM. So running an engine that has a redline of 2700rpm at 2200rpm, the BE would decrease by a large enough amount to reduce VE low enough to not hurt things. It is eaiser to bend a con rod by over speeding the engine than by running lowRPM/highMP. But it is eaiser to overheat an engine with lowRPM/highMP's. I would be more worried about detonating the thing than bending something. And properly set up, and monitered closely, you will have nothing to worry about.