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What happens if you keep number of cylinders constant but increse displacement?

J

jtvang125

Diamond Member
Seems the highest most 4 cylinders go is 2.5 liters while v6s are around 3.0 to 3.7. What happens if you keep increasing displacement for a given number of cylinders? Say going up to 3 to 3.5 on a 4 cylinder or 4+ liters on a v6?
 
a big ass piston. You ever seen the engine in something like a diesel train or freighter?

rta96c_crank.jpg
 
At some point the mass of the moving parts results in them failing. Which is why there are RPM limits.

To increase displacement you have to increase bore and/or stroke. Increased bore means larger heavier pistons and rings (more mass). Increased stroke means longer rods and/or longer sweeps of the crank journals (increase mass and rotational forces). All resulting in a point of diminishing returns where failure of components will result.
 
jtvang125 said:
Seems the highest most 4 cylinders go is 2.5 liters while v6s are around 3.0 to 3.7. What happens if you keep increasing displacement for a given number of cylinders? Say going up to 3 to 3.5 on a 4 cylinder or 4+ liters on a v6?
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GMC produced an 8 liter V6 in the 1960s. ~220hp and ~440lb ft of torque IIRC.
 
BMW consider the sweet spot for cylinders to be around 500cc each. Most of their fours end at around 2 litres, 6's top out between 3 - 3.5 litre, V8 at around 4 litres and V10 at around 5 litres, etc.
 
MiataNC said:
At some point the mass of the moving parts results in them failing. Which is why there are RPM limits.

To increase displacement you have to increase bore and/or stroke. Increased bore means larger heavier pistons and rings (more mass). Increased stroke means longer rods and/or longer sweeps of the crank journals (increase mass and rotational forces). All resulting in a point of diminishing returns where failure of components will result.
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Precisely.

For practical purposes, it's mainly to do with balance. There have been a few 3.0 litre inline 4-cylinder engines, but generally once an inline 4-cylinder is 2.0 litres it needs balance shafts to quell vibration and by 3.0 litres it's starting to become an issue where the vibrations can cause ancillary parts to fail early.

Similar balance issues are in play for V6 engines.

Any time you're using balance shafts, you're adding friction and sapping at least a little power to spin the shafts. Moving up to a V8 (a cross-plane V8 can be fully balanced internally, while a flat-crank V8 can have more displacement without the heavier cylinders of a large inline 4) or to an inline 6 allows more displacement without running into the issues caused by vibration and without needing to "waste" horsepower by spinning balance shafts.

ZV
 
Demo24 said:
Porsche made a 3.0l 4cyl back around 1990, it made about 210hp.
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dammit, that was what i was going to post.

oh, and rotating mass increases, causing higher NVH, creating a "rougher" engine... torque increases, costs increase, top RPM's lower...
 
Balance, vibration, pistons rocking in the bores, flame propagation, etc. Everything mentioned already, I'm late 🙁

They have them, 4-5 liter 4 cyls but they are in slow moving diesels and H4s in planes and have 3000 rpm redlines. Used in planes for high output in a light package where they run constant and low rpm where vibration isn't as big a problem.
 
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exdeath said:
They have them, 4-5 liter 4 cyls but they are in slow moving diesels and H4s in planes and have 3000 rpm redlines. Used in planes for high output in a light package where they run constant and low rpm where vibration isn't as big a problem.
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That and an H4 with the right crank setup isn't the vibration factory than an inline 4 is.

ZV
 
My boat has a 3 liter inline 4 that produces (depending on the year and configuration), between 130 and 140 hp. It runs pretty smoothly also, but was never in an automobile. I think its basically the 2.5 liter Chevy 4 that was in the older novas that was slightly bored out.
 
Chevy produced a 4.3L V6 for many years. Had around 190hp and close to 240ft/lb torque in some of the later variants. It wasn't really anything more than a sawed of 5.7L V8. If they hacked two cylinders off the LS7 we could have a 5.3L V6 LOL
 
vshah said:
Toyota has a 4.0L v6 in asian market land cruisers, something like ~245hp iirc
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Pretty sure that was the V6 in my truck that I just sold...and the same V6 in the 4Runner and FJ Cruiser. That was my favorite part of my Tacoma....solid engine.
 
KentState said:
Chevy produced a 4.3L V6 for many years. Had around 190hp and close to 240ft/lb torque in some of the later variants. It wasn't really anything more than a sawed of 5.7L V8. If they hacked two cylinders off the LS7 we could have a 5.3L V6 LOL
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Comparing specs on a 4.3L Chevy V6 to a 3.0L Toyota V6 I own, the larger Chevy engine makes about 20 hp less, but has about 20ft-lb more torque. My Chevy is easier to move in a parking lot (torque) the Toyota is easier to move on the freeway (horsepower).

I'm not sure if more displacement always equals more torque, but it's logical that a larger combustion chamber means more PSI pushing on the piston, which means more force per ignition applied to the crank, obviously there are losses due to vibration, friction and inertia that come into play, at some point too much displacement per cylinder is worse than adding another cylinder.
 
Consider that the Chevy Small Block was sold in displacements ranging from 265 CID upto 400 CID. The LS* engines from 4.8L to 7.0L. Big blocks from 366CID all the way upto 502.
And I'm not including crate motors.

Larger reciprocating masses can't rev as high. If the application doesn't need a high revving engine, it's not as much of a problem (ie: trucks and ships). So a large piston isn't as much of a problem. The GM/Detroit diesels are often known as x-71 and x-92's where x is the cylinder count (ranging from 1 to 24), the 71 and 92 are the CID that each cylinder is. But these engines tended to redline in the 2100-2600RPM range. The large, heavy pistons are the main limiting factors.

I've always thought a better thought exercise would be "how large would engines be in various racing series if only the displacement limit were removed?"
 
exdeath said:
Balance, vibration, pistons rocking in the bores, flame propagation, etc. Everything mentioned already, I'm late 🙁

They have them, 4-5 liter 4 cyls but they are in slow moving diesels and H4s in planes and have 3000 rpm redlines. Used in planes for high output in a light package where they run constant and low rpm where vibration isn't as big a problem.
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I'm late too... you at least got flame propagation (which I was going to say).

Once the cylinder bore goes above ~106 mm you start to run into flame propagation issues (IIRC), and need things like dual spark plugs to ensure the whole charge combusts.
 
KentState said:
Chevy produced a 4.3L V6 for many years. Had around 190hp and close to 240ft/lb torque in some of the later variants. It wasn't really anything more than a sawed of 5.7L V8. If they hacked two cylinders off the LS7 we could have a 5.3L V6 LOL
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Yup. And it was a shaky bastard. I loved mine though. Never gave me any issues in the s10 I had and pulled strong till the day I traded it in.
 
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