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More about a 3.0L crank in a 2.5L block...

At work today I was thinking about the recent discussions about putting a 3L crank in the 2.5 block, which increases the displacement to 2.8L. But what about using the 3.0L crank, sleeving the cylinders, and getting custom pistons/rods made to keep the displacement at 2.5L? That way, you'd have a (some-what) long stroke, small displacement, (somewhat) high revving 2.5L. Similar to the Ferraris and BMWs with 4L V12 (Ferrari) and the 5L 12 that was in the 850 BMW.

Obvioulsy it would cost a small fortune (crank ~$1200+, rods ~$2500, and pistons ~$120/ea) but it'd be a kick ass engine. Does anyone have any good idea on how it would change the torque/hp?

Someday I'll build it...Supercharged..:-D One day when I win the lottery, perhaps.

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Old 10-09-2004, 08:43 PM
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Personally, I'd think that you would gain more with the displacement increase =)
But, if you're looking for a high-revving 4, you will also have to look at the valves. The lifters and springs in our cars don't last much past 7000rpm.
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Old 10-09-2004, 09:13 PM
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Why on earth would you want to decrease the bore after increasing the stroke? That's just silly. Small bore, long-stroke is a recipe for a _low_ revving engine. High-revving engines have are typically over-square, not under-square to keep the mean piston velocity down.

Plus, as has already been said our engines are valvetrain limited, increading the stroke and decreasing the bore wouldn't have any real effect on redline.

As long as you're putting in a new crank, you're better off to take the displacement increase as well, you'll get more power that way.

Aaron
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Old 10-09-2004, 09:41 PM
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actually..wouldnt a longer stroke make a motor dwell around top dead center longer, and put less stress on the sides of the piston? And since it moves slower at top dead center, you dont have to increase timing...maybe even have to retard it some. My boss was mentioning to me something about our mitsu motors...instead of using a 2.4L block we use a 2.0L block witha 2.4 crank, and we can still keep the revs high if not higher because a 2.4L engine has a slightly larger bore. It also depends if your wrist pins can handle that kind of abuse. Ive pulled apart motors where the rod pulls the wristpin out of the piston, or bends it, or connecting rod end cap bolts shear off and put the rod through the side of the block from too high rpms..
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Old 10-10-2004, 03:23 AM
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ive put a rod through the side of of a block! but that was because i started and reved a motor that had been hydrolocked.

i do see where slowtoday is coming from though, i would think the one with the lower displacement would be able to stand up to supercharging better than the one with the larger displacement... but im definately not a expert on the subject.
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Old 10-10-2004, 03:33 AM
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Umm, unfortunately, between 2 engines with equal displacement, one with a longer stroke, one with a larger bore, the one with the longer bore will put MUCH more stress on the rods and wrist pins. Don't forget about the rod bearings being under more stress as well.

The one with the longer stroke will actually dwell around top dead center for a smaller percentage of the total stroke time.
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Old 10-10-2004, 11:05 AM
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Ferrari uses short strokes big displacments thats what makes a High RPM engine, Stroking a engine increase the piston speed which in turn puts lots more strees on TDC and BDC. There really would be noe reason the sroke the engine and make the bore smaller, it would put you back where you started.

The best thing to do would be Bump the CR to 11.1 to 1 make the stroke shorter and the bore bigger, Better valve springs and Titanuim retainers and a better profile cam, and have the bottom end Balanced and lightened, Then you would have ONE hell of a engine
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Old 10-10-2004, 11:20 AM
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I wonder if a 924 crank would fit in a 2.5? and bring it down to like 2.2?
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Old 10-10-2004, 01:38 PM
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Quote:
Originally posted by Zero10
The one with the longer stroke will actually dwell around top dead center for a smaller percentage of the total stroke time.
Uh, sort of.

Time from TDC to BDC at a given RPM is exactly the same regardless of stroke. So while the piston will technically pass TDC faster with a longer stroke, it is passing TDC faster because the piston velocity is greater. The greater piston velocity causes its own set of problems.

And again, look at the really high-reving engines, they are essentially invariably large bore, short stroke engines. Diesel engines, which are notoriously low-revving engines typically have small bores and short strokes.

Add to that the fact that a smaller bore means less room for valves, which means less valve area, which means restricted breathing at high RPM compared to a larger bore.

Not only that, but you'll get much more power from the displacement increase than you will from simply keeping the same displacement while increasing the stroke.

Aaron
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Old 10-10-2004, 10:07 PM
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Quote:
Originally posted by AaronM
Uh, sort of.

Time from TDC to BDC at a given RPM is exactly the same regardless of stroke. So while the piston will technically pass TDC faster with a longer stroke, it is passing TDC faster because the piston velocity is greater. The greater piston velocity causes its own set of problems.
I wasn't arguing that the piston would travel from TDC to BDC in less time or something like that.

What I was saying is that due to the longer stroke, it will spend a smaller percentage of total stroke time AT TDC. Which was what I thought bluebullet was saying.

Quote:
actually..wouldnt a longer stroke make a motor dwell around top dead center longer, and put less stress on the sides of the piston?
It isn't passing TDC faster because the piston velocity is greater, but rather because the stroke is lengthened. It will pass the mid-point in it's stroke much faster though.

I would really rather not get into the physics of the piston's movement at the moment though.
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Old 10-11-2004, 10:31 AM
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Old 10-11-2004, 08:13 PM
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Quote:
Originally posted by Zero10
I wasn't arguing that the piston would travel from TDC to BDC in less time or something like that.

What I was saying is that due to the longer stroke, it will spend a smaller percentage of total stroke time AT TDC. Which was what I thought bluebullet was saying.



It isn't passing TDC faster because the piston velocity is greater, but rather because the stroke is lengthened. It will pass the mid-point in it's stroke much faster though.

I would really rather not get into the physics of the piston's movement at the moment though.
It will spend exactly the same amount of time at TDC as it does in any other stroke length for a given RPM. The total time at TDC (which is infinitesimal since TDC is a single point) is no different than the total time at TDC for an engine of shorter stroke if the RPM's are equal.

The total time for the stroke is the same. The time at TDC is the same. It will _not_ spend any smaller percentage of time at TDC because of having a longer stroke.

However, what _will_ happen is that it will come up on TDC at a higher velocity and it will move away from TDC at a higher velocity. This means that the time spent near but _not_ at TDC is shorter.

Aaron
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Old 10-11-2004, 10:09 PM
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You guys are both correct. It's the stroke-to-rod ratio that determines dwell time at TDC. Ideally, you want the longest rods possible for any given stroke (place piston pin as high as possible).

Unfortunately with a stroker crank, you have to shorten the rods which makes the stroke-to-rod ratio head in the unfavorable direction. So the way to a high-revving engine is to increase the bore and decrease the stroke. F1 engines are around a 1.5:1 bore-to-stroke ratio. And the stroke-to-rod ratio is best around 0.75:1 or so.

In building a 2.8L, it's also better to bore rather than stroke because you fit larger valves into the head with the bigger bore. With a stroker, you have to suck in 10% more air on each stroke through the same small valves. As you can guess, this only works at low-RPM and as revs increase, the air has a much harder time getting through. So you may not gain that much in high-end HP.
Old 10-12-2004, 01:59 AM
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Unless you change the air's velocity, then you can crame the same amount of air threw a small hole

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Old 10-12-2004, 01:48 PM
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