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05-17-2020, 07:37 PM | #29 | |
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For reference, the Coyote 5.0L engine has a redline of 7k with virtually a square setup, yet the larger Voodoo 5.2L engine has a 8250 redline with a larger stroke and 1mm oversquare setup. The LT1 in the Corvette is very much an oversquare design with a bore that is 11mm larger than the stroke, yet the redline is 6600rpms. Probably more going on than bore/stroke. It isn't even about absolute length in stroke necessarily because the 86 has an 86mm stroke, the AP2 has a 91mm stroke, the 5.0 has a 93mm stroke, the Voodoo has a 93mm stroke and the LT1 has a 92mm stroke. None of those lengths predict max rpm of the whole engine.
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05-17-2020, 07:41 PM | #30 | ||
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Facepalm* My first post in this thread. Quote:
EDIT: Sorry, miss-read your post. Yes I know there is more to it then piston speed. I was just saying in general, if you are trying to design an engine with "X" displacement, 2 liter in this case, and it is for high RPM, chances are it will be oversquare. Especially if you are building it to a price point. Yes you can make a longer stroke motor spin fast if you want to balance the rotating assembly to a higher tolerance and build the rotating assembly out of higher quality materials, ect, ect.
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Last edited by NoHaveMSG; 05-17-2020 at 08:03 PM. |
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05-17-2020, 08:09 PM | #31 | ||
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When I think of size, I think of displacement only. Are we talking dimensions? The LT1 is smaller at 6.2L than a 5.0L Coyote. Are we talking about configuration? A 2.0 Synergy V8 has more cylinders than an I4 K24, but less displacement. My point, there are many ways to build an engine. An undersquare engine could have a shorter stroke than an oversquare engine with the same displacement given a difference in cylinder count, which means the undersquare engine could have a higher redline, if valvetrain, piston weight, and all other factors were a constant.
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05-17-2020, 08:16 PM | #32 | |
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05-17-2020, 08:52 PM | #33 | |
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I didn't really understand a few things he said in that video. I'm assuming by throttle response he means fast revving (to get into the power faster), which can go hand-in-hand with high revving (because of piston speeds and a short stroke, which is not the same as being oversquare), but only when certain assumptions are made like the ones you were making about engine configuration and displacement. In general, they weren't the best generalities for him to make.
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05-17-2020, 09:10 PM | #34 |
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I figured you did. You had me going WTF
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05-19-2020, 01:58 PM | #35 |
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The primary limiting factor is rod strength and valve spring stiffness of course.
Next, stroke alone (aka mean piston speed) limits rpm practically speaking from an intake velocity and flow perspective. Of course, rod length changes this by a little. If you have less valve area like on 2 valve/cylinder Chevy engines, you run into the limit at a lower speed. Car engines usually have pretty long strokes (except Porsche, Lambo, Ferrari) and stroke is difficult to modify, so it's easy to run into this limit. Bore limits rpm from a flame front speed perspective (for a traditional single spark non-SPCCI, non-prechamber design), but you don't really run into this problem on car engines, since >105mm is pretty much unheard of and they're all comfortably under 10000rpm. Rod/stroke also has a small effect on this. The bore/stroke ratio is most important for low speed cooling system losses, though as the cylinder increases in displacement the surface area/volume ratio improves too, so a Chevy engine might be undersquare but it still runs like a champ at low speed. Last edited by serialk11r; 05-19-2020 at 02:23 PM. |
05-22-2020, 01:22 AM | #36 | |
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05-22-2020, 01:31 AM | #37 | |
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Sorry I meant to say willingness to rev, hence why I made the Ferrari and F1 reference. I talk about throttle by cable vs drive by wire and I got it mixed up a bit. Thanks for checking out the video!
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