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Old 05-17-2020, 07:37 PM   #29
Irace86.2.0
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Originally Posted by NoHaveMSG View Post
Yeah there are exceptions. I was mostly just generalizing since it is typically cheaper to design an over square high RPM engine so they tend to be more common. Even the F22 in the AP2 is under square with an 8200rpm rev limit which is pretty good. Something, something you take the engineered load capability of a rotating assembly and determine what speeds would exceed that load capability and either set a limit that doesn't exceed that, or increase the load capability of the assembly. Or something like that.
The 7 millimeters difference in stroke between the AP1 and AP2 might have a significant difference to lower rpm, but in general, the high rpm might have to do with more than piston speeds.

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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Old 05-17-2020, 07:41 PM   #30
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Originally Posted by Irace86.2.0 View Post
The 7 millimeters difference in stroke between the AP1 and AP2 might have a significant difference to lower rpm, but in general, the high rpm might have to do with more than piston speeds.

Facepalm*

My first post in this thread.

Quote:
Oversquare engines are a design favored for high RPM's since it lowers piston speeds. The same size engine that is undersquare, would have a higher piston speed at the same RPM since it has to cover a longer stroke in the same amount of time.

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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Old 05-17-2020, 08:09 PM   #31
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Originally Posted by NoHaveMSG View Post
Facepalm*

My first post in this thread.

Quote:
Oversquare engines are a design favored for high RPM's since it lowers piston speeds. The same size engine that is undersquare, would have a higher piston speed at the same RPM since it has to cover a longer stroke in the same amount of time.
EDIT: Missread your post.

Yes I know there is more to it than piston speed.
Fixed it.

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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Old 05-17-2020, 08:16 PM   #32
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Originally Posted by Irace86.2.0 View Post
Fixed it.

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.
I guess I should say same configuration and size. Does what I say make more sense when looking at it that way? Again, this goes back to the OP's video saying it is oversquare for throttle response.
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Old 05-17-2020, 08:52 PM   #33
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I guess I should say same configuration and size. Does what I say make more sense when looking at it that way? Again, this goes back to the OP's video saying it is oversquare for throttle response.
I got what you meant before because I made assumptions. It is best for anyone reading who can't make assumptions to read those qualifiers, lest they don't spread misinformation.

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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Old 05-17-2020, 09:10 PM   #34
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I got what you meant before because I made assumptions. It is best for anyone reading who can't make assumptions to read those qualifiers, lest they don't spread misinformation.
I figured you did. You had me going WTF
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Old 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.

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Old 05-22-2020, 01:22 AM   #36
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You said in the video you're 6'1", is that more torso or leg? I'm a hair shorter but it's mostly in the legs so I don't feel cramped in most cars. I am planning on buying an AP1 and waiting to see how the next gen Lotus cars turn out.

For Miatas, the aftermarket support is so awesome and the cars are really well thought out, so I really really wanted to accept a lot of the shortcomings. After spending a few days in one, I just couldn't get over the windshield being right up next to my forehead. I really loved that car and would've been willing to dump money into titanium valves and custom pistons.
I had an NB at one point but ducking under or over to see the traffic light gets old quick haha. I don't necessarily feel cramped in the S2000, but it's not the most comfortable place, and I definitely do not pass the broom test for track events which is something I never worried about when I has the BRZ.
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Old 05-22-2020, 01:31 AM   #37
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I guess I should say same configuration and size. Does what I say make more sense when looking at it that way? Again, this goes back to the OP's video saying it is oversquare for throttle response.

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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