Re: Great info on rod ratio!!
[Re: ccdave]
#828248
10/12/10 03:56 PM
10/12/10 03:56 PM
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Joined: Feb 2007
Posts: 970 Backwater, PA
bwdst6
Bob George Racing #1 Fan
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Bob George Racing #1 Fan
Joined: Feb 2007
Posts: 970
Backwater, PA
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Yup, which is why increasing bore is the way to go for increasing power and also important, durability! I use a 4.15" stroke and also use the longest BBC rod I could find/fit... 7.1"!
This post is available in double vision where drunk.
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Re: Great info on rod ratio!!
[Re: ccdave]
#828252
10/12/10 06:21 PM
10/12/10 06:21 PM
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Joined: Oct 2003
Posts: 7,506 Az
Crizila
master
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master
Joined: Oct 2003
Posts: 7,506
Az
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Good info on rod ratios - Thanks for posting. Obviously wear is an issue, but the real issue with stroker motors is piston speed. That's what seperates the wear from the - "breakage". The formula: mean piston speed ( ft per min )= stroke X 2 X rpm divided by 12. and the rule of thimb is: Factory cast stuff = 3,750 ft/min, aftermarket cast =4,500, factory forged = 4,600, budget aftermarket forged = 4,800, Race aftermarket stuff = 5,500, high $ custom endurance race = 6,000, pro stock = 7,500. As an example, I am running a cast crank in my 408 with Eagle I-beam rods and KB Hypo pistons. Pretty much bottom feeder performance parts. I shift at 6200rpm. That gives me a piston speed of 4,133 ft/min. Theoretically, I should be safe. A lot of room for interpretation here ( I can already read the posts of those reving twice that high with stock stuff and having no problems ) and the numbers could be a little conservative. I think it's good ball park info though - and maybe it can keep you away from the " look what I broke" posts.
Fastest 300
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Re: Great info on rod ratio!!
[Re: Crizila]
#828253
10/12/10 06:36 PM
10/12/10 06:36 PM
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Joined: Jun 2009
Posts: 1,212 QLD Australia
Keith Black®
pro stock
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pro stock
Joined: Jun 2009
Posts: 1,212
QLD Australia
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piston speed will be the same at any given rpm, irrespective of rod ratio, it's the rate of acceleration that will change with shorter rods. rod ratio is relevant to stroke. a large stroke BB will struggle to obtain even stock rod ratio given deck height and piston compression height limitations.
-------------------------------- Darren Beale Keith Black Racing Engines®
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Re: Great info on rod ratio!!
[Re: ccdave]
#828254
10/12/10 09:33 PM
10/12/10 09:33 PM
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Joined: Oct 2007
Posts: 4,209 New York
polyspheric
master
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master
Joined: Oct 2007
Posts: 4,209
New York
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The difference in piston position B/ATDC with even large changes in rod ratio are very, very small. I have yet to read an analysis that explains the changes in engine function.
Example 1: 4" stroke, 8" rod = 2:1. In 10° rotation, the piston moves down .0379".
Example 2: 4" stroke, 6" rod = 1.5:1 (25% lower ratio). In 10° rotation, the piston moves down .0404".
The difference is 2.5 thousandths, or 1/16th of 1% of the stroke. BFD.
Boffin Emeritus
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Re: Great info on rod ratio!!
[Re: radar]
#828255
10/12/10 09:37 PM
10/12/10 09:37 PM
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Joined: Oct 2008
Posts: 1,015 Down South
DaKuda
super stock
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super stock
Joined: Oct 2008
Posts: 1,015
Down South
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Quote:
I have a 4" stroke with 6.123 rods in a smallblock. The pistons are shorter, not the rod.
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Re: Great info on rod ratio!!
[Re: 602heavy]
#828256
10/12/10 09:48 PM
10/12/10 09:48 PM
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Joined: Jan 2003
Posts: 5,991 Anoka County, MN
Leigh
master
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master
Joined: Jan 2003
Posts: 5,991
Anoka County, MN
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Quote:
Don't most stroked motors use a longer rod.
I'm in the camp whereby compression height rules over rod length , get ring placement right then get a rod to fit.
I'm a rube, but this makes sense. Packaging for componet optimization is key.
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Re: Great info on rod ratio!!
[Re: Keith Black®]
#828261
10/13/10 10:29 AM
10/13/10 10:29 AM
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Joined: Mar 2005
Posts: 2,172 Ohio
theclutcher
top fuel
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top fuel
Joined: Mar 2005
Posts: 2,172
Ohio
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Agree, rod just connects piston. Nascar had to institute min. deck hgt rule to keep from getting out of hand. the liter the assyembly the better. the shorter the rod, less wght, more rpms. as long as block can take it, no problem. Till they put counterweight cheeks into ringland. Heard something like that. Would like to know more about it.
Last edited by theclutcher; 10/13/10 10:30 AM.
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Re: Great info on rod ratio!!
[Re: Al_Alguire]
#828262
10/13/10 10:35 AM
10/13/10 10:35 AM
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Joined: Oct 2004
Posts: 8,165 Left Coast
BobR
master
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master
Joined: Oct 2004
Posts: 8,165
Left Coast
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Quote:
The rod's job is to connect the piston to the crank. Build the stroke you want for the cubes oyu need, find a piston CD that is liveable and order the rod accordingly. Nuff said
It still amazes me how complicated some people want to make things. This post is where this thread should end.
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Re: Great info on rod ratio!!
[Re: BobR]
#828263
10/13/10 11:41 AM
10/13/10 11:41 AM
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Joined: Oct 2007
Posts: 4,209 New York
polyspheric
master
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master
Joined: Oct 2007
Posts: 4,209
New York
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Right, those annoying auto manufacturers have no idea what they're doing! Why don't they follow your advice, and all use whatever rod they have lying around...
Hint: when designing a new engine, the deck height is frequently the last choice. Does that tell you something?
Boffin Emeritus
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Re: Great info on rod ratio!!
[Re: Al_Alguire]
#828264
10/13/10 11:47 AM
10/13/10 11:47 AM
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Joined: Oct 2007
Posts: 4,209 New York
polyspheric
master
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master
Joined: Oct 2007
Posts: 4,209
New York
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I tried that once in a desk-top program. I need a 6" stroke in an LA motor, with 1.10" minimum piston CD, so I need a 5.50" rod, right?
The piston will separate the first time you start the engine.
Boffin Emeritus
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Re: Great info on rod ratio!!
[Re: Al_Alguire]
#828266
10/13/10 01:01 PM
10/13/10 01:01 PM
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Joined: Jan 2003
Posts: 1,938 Sonora CA
Mopar_Rich
top fuel
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top fuel
Joined: Jan 2003
Posts: 1,938
Sonora CA
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This is from my HEMI book: The infamous “Rod Ratio” and why use a longer rod
What the heck is a “rod ratio” and why do I care about it? Good question. The rod ratio is the center-to-center rod length divided by the stroke.
There’s a lot of talk about this rod ratio thing, but if you think that the ratio of the connecting rod length to the crankshaft stroke is vitally important to performance - think again. The most important thing about a connecting rod is whether or not the rod bolts are torqued! You don’t design an engine around a rod ratio. For a high performance hot street engine you want cubic inches, which is the easiest to get from stroke. Maximize the stroke and bore, then make the compression height large enough to keep the wrist pin out of the oil groove and that determines your rod length. End of calculation!
Nevertheless, since curious minds want to know, so lets examine this phenomenon:
The rod ratio determines the length of time, or dwell, at TDC. As the crank rotates through TDC, a short rod will be at a greater angle than a long rod so as the crankshaft rotates, the piston will be pulled away from TDC faster. So the longer the rod, the longer the "dwell", and the longer the dwell, the longer the piston sees pressure. In other words - long rods are good. Right? Obviously the length of the rod is limited by the deck height, stroke, and the compression height of the piston.
A longer rod reduces the rod angle and less rod angle will reduce piston side loading. A longer rod also gives better average leverage on the crank for a longer period of time resulting in a flatter torque curve. A shorter rod will yield a higher peak torque but the torque won’t last as long.
Short rod ratio engines tend to be “peaky” which can be desirable in an oval track car. Long rod ratio engines have a flatter torque curve, and generally desirable for drag racing and hot street applications. So if long rod ratios are good why are so many pro-stock engines, and production engines, built with short deck blocks, which would limit the rod ratio? Simply because the shorter the deck the stiffer the block, and shorter pushrods mean less stress on the valve train. In other words, the general stability of the engine outweighs the importance of the rod ratio.
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