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| V8 vs V6 (Page 5/9) |
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Daryl M
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JAN 11, 09:15 PM
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| quote | Originally posted by pmbrunelle:
The faster an engine revs, the more the contents of the cylinder swish around, so combustion is completed in less time.
So as engine RPM increases, combustion speed also increases; the two effects seem to balance each other, and often spark advance is constant above a certain RPM. |
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The limiting factor for rpm was always said to be piston velocity. Although more modern alloys are stronger and lighter, the stresses of acceleration from a dead stop to 150 feet per second then back to a dead stop twice per revolution is not an easy thing to tolerate for pistons to handle. Smaller bore also means smaller valves., So without forced air induction, that limits flow. Lots to consider for engine designers. I do find it interesting that the 1.3 liter 3 cylinder that is in my Buick Encore GX is said to have the highest power density of any passenger vehicle except for supercars. The 1.3 ltr has a stroke significantly bigger than it's bore.
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Skybax
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JAN 11, 09:21 PM
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| quote | Originally posted by Daryl M:
I do find it interesting that the 1.3 liter 3 cylinder that is in my Buick Encore GX is said to have the highest power density of any passenger vehicle except for supercars. The 1.3 ltr has a stroke significantly bigger than it's bore. |
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Very cool, what is the MPG?
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Daryl M
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JAN 12, 12:58 AM
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| quote | Originally posted by Skybax:
Very cool, what is the MPG?
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34-36 combined. It has a CV transmission.
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Daryl M
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JAN 12, 01:02 AM
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Will
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JAN 12, 07:54 AM
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| quote | Originally posted by Daryl M:
You are correct about the time at a given rpm, but a long stroke smaller bore engine of the same displacement as a larger bore short stroke engine does not rev as high therefore there is more time for combustion.
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Tell that to Honda K24 builders.
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Will
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JAN 12, 08:01 AM
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| quote | Originally posted by Daryl M:
The limiting factor for rpm was always said to be piston velocity. Although more modern alloys are stronger and lighter, the stresses of acceleration from a dead stop to 150 feet per second then back to a dead stop twice per revolution is not an easy thing to tolerate for pistons to handle. Smaller bore also means smaller valves., So without forced air induction, that limits flow. Lots to consider for engine designers. I do find it interesting that the 1.3 liter 3 cylinder that is in my Buick Encore GX is said to have the highest power density of any passenger vehicle except for supercars. The 1.3 ltr has a stroke significantly bigger than it's bore. |
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The entire auto industry is investing huge amounts of money in combustion simulation using computational fluid dynamics. This has resulted in the ability of new engine designs to operate at extremely high BMEPs (high compression plus high boost) that would have been impossible a few years ago. One of the outcomes of this is the finding that a bore of around 84mm is "about right" to control combustion for extremely high BMEP operation, but the combustion process favors smaller bores. Once the OEM can get as much torque as they want out of an engine by cranking up BMEP, then there's no need to spin the engine very fast to make power.[This message has been edited by Will (edited 01-12-2022).]
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Daryl M
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JAN 12, 06:03 PM
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| quote | Originally posted by Will:
The entire auto industry is investing huge amounts of money in combustion simulation using computational fluid dynamics. This has resulted in the ability of new engine designs to operate at extremely high BMEPs (high compression plus high boost) that would have been impossible a few years ago. One of the outcomes of this is the finding that a bore of around 84mm is "about right" to control combustion for extremely high BMEP operation, but the combustion process favors smaller bores. Once the OEM can get as much torque as they want out of an engine by cranking up BMEP, then there's no need to spin the engine very fast to make power.
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With a lower average rpm, does it mean that the engine will last longer? Longer stroke engines have traditionally had broader power bands which made them more "drivable" . Or am I mistaken?
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ZaraSpOOk
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JAN 13, 04:36 PM
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| quote | Originally posted by Daryl M:
34-36 combined. It has a CV transmission.
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FYWIW, I had a 1996 GEO Metro (made by Suzuki) that had a 3 cyl engine, it got 50 mpg highway, 45 around town
cost me about $7500 new
it had a whopping 55 HP, funny thing is, it was so light it was not the slowest beast on the road
in fact it was so light that one time I discovered I had a flat rear tire driving home from work on the interstate, and since at the time there were metered on and off ramps, and I didn't want to have to get off the interstate, combined with the fact that tires could be bought for $15, I drove it home and the tire wasn't even ruined
return to arguing about long/short stroke, LOL
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Will
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JAN 13, 05:02 PM
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| quote | Originally posted by Daryl M:
With a lower average rpm, does it mean that the engine will last longer? Longer stroke engines have traditionally had broader power bands which made them more "drivable" . Or am I mistaken? |
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Ummm...
It's sort of a self-fulfilling prophesy...
If your engine has a small bore and small valves for its displacement, do you concentrate on top end power or low RPM torque? (In addition to port development, like BMW's S54)
If your engine has a big bore and big valves for its displacement, why concentrate on low RPM torque when you can go for top end power?
A 5 liter engine makes 5 liters of torque regardless of its bore and stroke.
Lower cruise RPM for any engine gives better mileage and slower wear. It doesn't even have to be a "low RPM" engine. LS3's and LT1's turn <2000 RPM at 80 mph. While they're biased toward the top end, they make "enough" low RPM torque to handle tall gearing. Most of the ability to cruise at low RPM comes from transmissions like CVTs and 8 speed automatics. There hasn't historically been the capability to have a first gear >4:1 AND a top gear ~0.6:1 in the same transmission.
Compare the Cadillac 4.9--square bore/stroke but done by 4500 RPM--to the S54--slightly undersquare, but makes power to 8000 RPM--to see that there's a lot more to it than just bore and stroke.[This message has been edited by Will (edited 01-13-2022).]
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pmbrunelle
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JAN 13, 06:44 PM
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| quote | Originally posted by Will:
There hasn't historically been the capability to have a first gear >4:1 AND a top gear ~0.6:1 in the same transmission.
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That reminds me...
I wish I could have the opportunity to drive something like that, but I don't think it's gonna happen.
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