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Long rod and offset grinding crank?
https://www.slantsix.org/forum/viewtopic.php?t=18672		 Page 2 of 3
Author:  panic [ Sat Jul 22, 2006 6:53 am ]
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"If time near the top is more so is time near the bottom"

No, it's not.

"The crank rotates in a perfect circle to the best of my knowledge. "

Sory, I have no idea what that means but it has nothing to do with the piston position.
Author:  panic [ Sat Jul 22, 2006 8:32 am ]
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Let me see if I understand:
You're going to have a crank ground off-center by .030" (and presumably use +.030" bearings)?
This increases your stroke by 1/3 of 1% and adds less than 1" to engine displacement. This has about the same effect on performance as leaving 1 shoe off when you race: almost none.
This will move the piston up @ TDC by .015". This is a helpful change in your deck height, except it will cost many, many times more than simply having the block decked. The possibility of error (rendering all the expense wasted, the crank is junk, and you have to assemble the engine to find it out) is much, much higher than an error in decking the block.
By the way: off-set grinding is usually done to only .005" of the maximum size (they have to reduce the journal on all sides by .005"), because even with a brand-new crank only NASA will get the wheel exactly to the last molecule before grinding.
BTW2: 1mm = .03937", not .080".

To me, the question you pose is not "what change can I make that will have a beneficial effect on the engine?" as much as "I'm going to do this regardless, but I still want you to tell me what a good idea it is".

The people on this board have a cumulative total of hundreds of years of experience in doing this stuff, and there is very little that hasn't been tried.
Author:  SlantSixDan [ Sat Jul 22, 2006 8:49 am ]
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Quote:
To me, the question you pose is not "what change can I make that will have a beneficial effect on the engine?" as much as "I'm going to do this regardless, but I still want you to tell me what a good idea it is".
That is indeed a critical distinction you've spotted. It's easy to get enchanted with some particular idea for how to put something together or set something up. And when that happens, it can be really difficult for anyone to change your mind. Usually it turns out that all the people who tried to tell ya not to do it that way...

...were right.
Author:  Bren67Cuda904 [ Sat Jul 22, 2006 12:01 pm ]
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I reread the articles about this and mix two different articles. The gain is not 30 in3 but 8 in3.
Don't bit my head off :shock:
Author:  panic [ Sat Jul 22, 2006 4:48 pm ]
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The extra displacement is the overbore. The offset is worth 1".

There are certainly moments where you hit on an idea that might have merit, even though it's a bit unusual. I like to encourage these ideas, provided that:
1. they may actually do what you think they will (i.e., don't violate laws of physics, not perpetual motion, etc.)
2. there isn't a safer or less expensive method of doing the same thing

The whole 198 rod thing is a good idea which serves several purposes, but not if you can't find the parts and you need a different deck height anyway. At this point, using the stock rod and a custom (taller and lighter) piston begins to look more practical.
A longer rod is, IMHO, an advantage in this application but the change here is too small to make an improvement by itself. The general rule is anything over 5% can be "seen" in testing. For example. I made my own rods (cut and welded stock rods from another engine) to change my ratio from 1.81-1 to 2-1, about +11%.
Author:  panic [ Sat Jul 22, 2006 5:09 pm ]
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Re: the stock bearing diameter is 2.187". going to a 2.000" diameter bearing size would allow increasing the stroke +\- .093" to get a 4.218".

Reducing the rod journal moves the centerline by 1/2 the diametric change. The stroke increase is the same as the offset. Going from 2.1875 to 2" adds .1875".
2" is the 1955-67 small block Chevy size which is why it's popular, but those rods are much narrower. You need to search a motor with a long crank (i.e., wide bore spacing) but old enough to use small journals. I can't find anything - perhaps something metric?

It's not on this list: http://victorylibrary.com/mopar/m-table.htm
Author:  zedpapa [ Sun Jul 23, 2006 9:51 am ]
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so if i understand you right, the stroke would then be 4.3125. so with a 3.480 bore the dispalcement would then be 246.1. is this correct?

i wouldn't bother going to the trouble of doing this unless i had the right parts. in this case, a set of custom rods are all that is needed as you would hopefully still be able to use the 2.2 piston. that is if the piston is still in the bore.

another question. if the piston spends more time at one end of the stroke, how can it not spend the same amount of time at the opposite end? as was stated, the crank rotates in a circle.

zedpapa
Author:  sandy in BC [ Sun Jul 23, 2006 10:02 am ]
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"another question. if the piston spends more time at one end of the stroke, how can it not spend the same amount of time at the opposite end? as was stated, the crank rotates in a circle. "


I cant explain this verbally.....but if you draw yerself a picture of the crank throw at the top of the stroke and another at the bottom of the stroke you will see that the difference between the 2 is the distance the piston is from the crank centreline. The further it is from the crankcentreline....the longer the time it spends at the stroke apex.

What a connecting rod does and what different speeds it does it at is very facinating. I spent time on a deepsea ship with a 30ft 2 piece connecting rod and a 10 ft stroke.....fun to watch at an idle.( Crank and rods are not in a case.) Piston travel per minute is the same as your Honda.....rpm....WAY diffent.
Author:  argentina-slantsixer [ Sun Jul 23, 2006 10:38 am ]
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Quote:
I cant explain this verbally.....but if you draw yerself a picture of the crank throw at the top of the stroke and another at the bottom of the stroke you will see that the difference between the 2 is the distance the piston is from the crank centreline. The further it is from the crankcentreline....the longer the time it spends at the stroke apex.

What a connecting rod does and what different speeds it does it at is very facinating. I spent time on a deepsea ship with a 30ft 2 piece connecting rod and a 10 ft stroke.....fun to watch at an idle.( Crank and rods are not in a case.) Piston travel per minute is the same as your Honda.....rpm....WAY diffent.
good. So if the rod's .3 longer, the piston would we .3 above the original relative position to the center of the crank at BDC, making it stay longer (.3 worth of time wich might be insignificantly more but more time indeed)
Author:  argentina-slantsixer [ Sun Jul 23, 2006 10:45 am ]
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that said, I agree with panic, I wouldn't do the crank thing, just the longer rods (if longer than 7" and custom pistons with the smallest amount of head to pin distance) and the overbore. Some folks use AMC connecting rods from their inline 6's (7.3") and the crank journal of those are really small, don't have the data in hand but I'll check out) and renault 18 pistons (88 mm and really close to head pin placement) those work great but you have to stay away from decking the block or milling the head more than a cleanup pass because you go 11:1 CR in a heartbeat.

Wich makes me wonder: jeep 4.0l IL 6... isn't a direct relative of the AMC 6? what about those conn rods?
Author:  SlantSixDan [ Sun Jul 23, 2006 11:32 am ]
Post subject: 
Quote:
makes me wonder: jeep 4.0l IL 6... isn't a direct relative of the AMC 6? what about those conn rods?
Length is 6.125".
Author:  panic [ Sun Jul 23, 2006 3:05 pm ]
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"Stay" isn't really the best term. The piston doesn't stay anywhere while the engine is running; the time spent @ TDC & BDC is infinitesimal - there is no "dwell".
However, since all the clearances reverse when the piston changes direction (several thousandths) the piston appears to remain motionless during the first few degrees especially around TDC while these clearances transfer to the other side since the piston motion per degree of rotation is wayyy below .001".

Again, the difference between the 198 rod and 225 rod changes the piston motion by almost nothing for the first few degrees, at 10 degrees it's .0002" (1/5th of 1 thousandth) and at 30 degrees ATDC the difference is less than .004".

The explanation for the apparent "it's a circle" is unfortunately long and complex. The crank journal follows an circular path, but the piston motion is absolutely not regular but asymmetrical re TDC vs. BDC, with the shapes becoming mirror images only with infinite rod length. The piston does not move 1/2 of the stroke in 90 degrees, for example. Depending on the rod ratio, 1/2 the stroke occurs between 71 and 78 degrees for most engines.
Author:  zedpapa [ Sun Jul 23, 2006 4:07 pm ]
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i understand now. i see how the "dwell" is different from tdc to bdc. i learn sonething everyday.

zedpapa
Author:  argentina-slantsixer [ Sun Jul 23, 2006 4:25 pm ]
Post subject: 
Quote:
"Stay" isn't really the best term. The piston doesn't stay anywhere while the engine is running; the time spent @ TDC & BDC is infinitesimal - there is no "dwell".
However, since all the clearances reverse when the piston changes direction (several thousandths) the piston appears to remain motionless during the first few degrees especially around TDC while these clearances transfer to the other side since the piston motion per degree of rotation is wayyy below .001".

Again, the difference between the 198 rod and 225 rod changes the piston motion by almost nothing for the first few degrees, at 10 degrees it's .0002" (1/5th of 1 thousandth) and at 30 degrees ATDC the difference is less than .004".

The explanation for the apparent "it's a circle" is unfortunately long and complex. The crank journal follows an circular path, but the piston motion is absolutely not regular but asymmetrical re TDC vs. BDC, with the shapes becoming mirror images only with infinite rod length. The piston does not move 1/2 of the stroke in 90 degrees, for example. Depending on the rod ratio, 1/2 the stroke occurs between 71 and 78 degrees for most engines.
yup, it's far from being one circle spinning and a connecting rod making another circle spin (say an old steam train). However, the difference might be a decimals story but's there. Nothing that would make a significant change, perhaps, but still there.
Author:  panic [ Sun Jul 23, 2006 4:36 pm ]
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"AMC connecting rods from their inline 6's (7.3") and the crank journal of those are really small"

More about this engine? The US 199, 4.0, 232 and 258" engine use either 5.875" or 6.125" rods.
Do you mean the older pre-1964 engines?
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