Originally Posted By: A_Harman
1. When I say efficiency, I mean brake thermal efficiency and BSFC, measures which include the effect of engine friction. When I say specific output, I mean HP/cubic inch. It's all clear to me.
And you know for a fact that the 6.2L LT1 has lower BSFC than the Coyote? Remember, pushrod engines have to run heavier valve springs than an OHC engine spun to the same RPM point. So while the cammer has more cams to turn, the pushrod engine has to fight against greater valvetrain mass and higher spring pressures.
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2. Oh come on, we both know that the LT1 and Coyote both have aluminum blocks and heads and plastic intake manifolds.
Right. Yet GM didn't compare the weight of the LT1 to the Coyote, they compared it to BMW's S63, which has two turbo's nestled in the valley. They mention it is 40lbs lighter than this engine, which is heavier than Coyote (I can't find the exact spec's for the weight of the S63 at this point). A fully dressed Coyote weighs 444lbs. How much does the LT1 weigh?
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3. 4-valve pent-roof and classic 2-valve Hemi combustion chambers lead to port designs and valve sizes that are best at flowing air at high engine speeds, so they are good for specific output. But they inherently have lower swirl than 2-valve wedge or bathtub chambers, and require compromises in intake port design to generate tumble to make a fast-burn combustion chamber. In the case of the modern Hemi, Chrysler has gone to dual-spark and squish pads at the sides of the chamber to generate air motion to speed up the burn. The space inside a 4-valve chamber is so dominated by the valves that there is not much ability to place an injector without getting into cylinder wall or intake valve wetting problems. At this point, it becomes necessary to limit valve sizes to put in the injector, then specific output begins to suffer.
The idea of squish pads in the HEMI isn't new. Ford did the same thing with the BOSS 429 back in the day for the same reason. The size of the bore still limits valve size in both applications, but much more so in a pushrod engine (ask any 305 owner, LOL!!)
And I honestly don't think that a pent-roof is as much of a compromise as you lead on or seem to think. VCT can be used to move the cams around to make it haul anywhere in the power band. I own an engine that behaves in this exact manner, it makes 390lb-ft of torque pretty much everywhere in the power band.... And that was the late 90's. It's torque "curve" looks like Utah. And it still manages a higher specific output on top of that.
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4. GM having the ability to optimize chamber shape is the major reason they pursued the redesign, judging by the amount of resources they put into optimizing the combustion system. Do you mean to say that they wasted their time? Remember, they have to improve CAFE if they want to sell cars, and DI is one major piece of the puzzle. Obviously, putting DI on the Corvette is only the start. The technology will spread to the rest of their OHV engines, especially trucks.
I'm just surprised that the power output gained by DI isn't as spectacular as it has been with many other V8's and particularly many of the popular V6's. Remember, Coyote is still port injected at this point.
And I'm not saying GM wasted their money, I'm saying whatever they are doing with chamber design appears to be for the benefit of fuel economy and not power output at this point.
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My point is that specific power output is only one aspect to look at when judging the "goodness" of an engine. You seem to only consider specific output.
No, I don't, but it is certainly ONE of the things I consider. Keep in mind that specific output IS what most consumers are going to consider though
