Rod/Stroke Ratio

[Taylorman]

I was checking out JE Pistons web sight yesterday and finally figured out how one can use different length rods with the same stroke crank. Im still learning about BBC parts. Is it better to go with longer or shorter rods.
6.135 rods with a 4" stroke = 1.53:1 rod stroke ratio
6.385 rods with a 4" stroke = 1.59:1 rod stroke ratio
6.135 rods with a 4.25 stroke =1.44:1
6.385 rods with a 4.25 stroke =1.5:1
Whats the deal with all the choices. What difference does it make?

[Lawler]

Depends on what compression you are looking for. Longer rods will give you more compression.

[Taylorman]

Hows that?

[revndave]

I use 4 inch stroke with.250 longer rod.Better rod angle ratio 1.6.

[Infomaniac]

Rod stroke ratio has an effect on piston speed and rod angle. How long the piston dwells at TDC and BDC and how fast it accelerates away from TDC and BDC Rod angles effect how fast the pistons or rings wear. And how much stress is put on the rods at an angle rather than straight down.
This is the age old debate that goes along with HP or Torque debates.

[Nucking futs]

So what are the pros and cons?Im building a engine right now 4.25-6.385 rod with a 4.310 bore.Any benifits to running a smaller rod on this combo?I bought some childs and alberts 6.135 rods for a 100$ from my buddy brand new.HHMMMMMM this could change everything.

[Taylorman]

So what are the pros and cons?Im building a engine right now 4.25-6.385 rod with a 4.310 bore.Any benifits to running a smaller rod on this combo?I bought some childs and alberts 6.135 rods for a 100$ from my buddy brand new.HHMMMMMM this could change everything.
That combo with 6.135 rods would require a 1.52" compression height piston. Check out JE's web sight.
http://www.jepistons.com/pdf/2002-srp-chevy.pdf
If you decide to sell one set of rods you don't use, let me know.

[IDK]

Rod ratio is an age old debate just like Informaniac said and he is correct. My experiences on dynos and in cars have showed that a longer rod will put the tourqe higher in the rpm or can wipe it out altogetherand a shorter rod brings the tourqe down the rpm ladder .It also effects the cyl head and how it flows (a shorter rod faster piston speed helps the cyl scavenge you will see this in cases were you have too much cyl or a cyl with a poor flow curve). Cyl bore size also effects the cyl and how it flows (a big bore unshrouds the valves) and depending what style(dome,dish or flattop) it will effect your qench or flame travel .I could go on for hours there are so many variables in an engine and getting them to all fall in line is what seperates the winner and the first looser. I personaly wouldn't put a rod in a BBC with a 4.00 , 4.25 stroke shorter than 6.385 in a boat with jet I would lean more to a 6.500 rod asperated or supcharged your cyl walls and bottom end will love you . I hope I helped Roger

[Infomaniac]

Pros and cons depend on the final result desired.
My personal favorite pro for long rods is the piston dwelling at TDC longer. As most know the crank rotates several degrees while the piston is up top stationary. When the piston is up top the combustion area is at it's smallest. (more pressure) If you can get the piston to dwell at TDC longer and also accelerate away from TDC slower. The pressure pushing on the crank is higher for longer. Longer in crankshaft degrees.
I use this combo mainly for blown deals. Boost only compliments the longer rods. A con for long rods is when on a N/A engine the slower piston acceleration creates less vacuum early in the intake stroke. This means the engine is more sensitive to too large of head ports, intake manifold and carb size.
Ever look at the rod/stroke ratio of a diesel? They make moster torque. How about a 12k + RPM indy engine? Huge rod stroke ratio. The crank is spinning like crazy but the piston is held at a reasonable speed. The piston can actually out run the flame front in those instances.
The pros and cons go on and on and on and on.

[steelcomp]

I find it interesting that all this techno talk has failed to address two of the major factors concerning longer rods. One is the issue of piston side loading and parasitic friction and how the higher compression (wrist pin) height effects piston performance (as much as rod angle) and how it helps resist piston rock, and lower piston weight. The other is that there is an optimum ratio, (forgot what it is...somewhere in the 1.7:1 range) which is one of the primary advantages that the 385 (429/460) Ford engine has over the BB Chev, in that the Ford is (as most Ford's are) much closer to the optimum ratio than any production GM V8 engine.
for example:
454 BBC 4.00 stroke, 6.135" rod = 1.53:1 ratio
460 BBF 3.85 stroke, 6.605" rod = 1.71:1 ratio...this makes a huge difference.
In almost every condition, the longer rod will show a benefit, since the typical BBChev is so far short of optimum. To argue that a blower motor sees or dosent see more benefit from a 1.5:1 ratio vs a 1.55:1 ratio vs a 1.59 ratio is moot. Those differences equal such minute measurable differences in the types of motors we're talking about it dosent warrant the arguement. Untill you can produce a ratio to exceed the high 1.6's, you're not going to see any disadvantage, and unfortunately, as the stroke increases, so does the difficulty of maintaining a good ratio.
BTW...longer rods do nothing to increase compression. A longer stroke will, but not a longer rod.
The 496 BBC with it's 4.31 bore, it's 4.25 stroke and a 6.385 rod is a very common and acceptable upgrade./ A better one would be to find a truck block with the extra .400" deck height and use as long a rod as possible (6.700)...you still won't get to 1.7:1, but you'll get closer. (still only at 1.57:1)

[steelcomp]

Rod ratio is an age old debate just like Informaniac said and he is correct. My experiences on dynos and in cars have showed that a longer rod will put the tourqe higher in the rpm or can wipe it out altogetherand a shorter rod brings the tourqe down the rpm ladder .It also effects the cyl head and how it flows (a shorter rod faster piston speed helps the cyl scavenge you will see this in cases were you have too much cyl or a cyl with a poor flow curve). Cyl bore size also effects the cyl and how it flows (a big bore unshrouds the valves) and depending what style(dome,dish or flattop) it will effect your qench or flame travel .I could go on for hours there are so many variables in an engine and getting them to all fall in line is what seperates the winner and the first looser. I personaly wouldn't put a rod in a BBC with a 4.00 , 4.25 stroke shorter than 6.385 in a boat with jet I would lean more to a 6.500 rod asperated or supcharged your cyl walls and bottom end will love you . I hope I helped Roger
IDK...you have me confused...you seem to contradict yourself when you say the longer rod will raise the torque curve higher in the rpm level or wipe it out altogether, (seemingly detramental) but then you suggest that a longer rod is the way to go in an application that requires peak torque to be at a relatively low rpm, ie: a jet boat.
My experience has been the opposite, in that a longer rod along with a cam designed to take advantage of it's benefits, has always offered better torque, and over a wider rpm range.
Like to hear your input.

[steelcomp]

So what are the pros and cons?Im building a engine right now 4.25-6.385 rod with a 4.310 bore.Any benifits to running a smaller rod on this combo?I bought some childs and alberts 6.135 rods for a 100$ from my buddy brand new.HHMMMMMM this could change everything.
Stick with the longer rod...it's still way short of optimum, and the shorter one will be of far less benefit. The 6.385 rod really only makes up for the stroke increase as far as ratio is concerned...not really an improvement. Next off the shelf rod is 6.535 (+.400") which would be an improvement, but would require more expensive custom piston.

[Squirtin Thunder]

Steelcomp,
Why are you saying that the 429/460 is the same rod ratio when the 429ci has a 3.59 stroke crank and the 460ci has a 3.85 stroke?
for example:
454 BBC 4.00 stroke, 6.135" rod = 1.53:1 ratio
460 BBF 3.85 stroke, 6.605" rod = 1.71:1 ratio...this makes a huge difference.
This is fun
Jim

[Squirtin Thunder]

I am wrong you only said the 460ci engine had a 1.7 ratio.
Jim
BTW- what is the rod ratio on the 429ci BBF ?????

[steelcomp]

I am wrong you only said the 460ci engine had a 1.7 ratio.
Jim
BTW- what is the rod ratio on the 429ci BBF ?????
Figure it out, Jim. Divide the length of the rod by the stroke. :idea: :messedup:

[Squirtin Thunder]

So the 429 BBF has a kick ass rod ratio 1.84
Smoken !!!!
Jim

[Nucking futs]

So would it be better to go with the .400 longer rod with the 4.25 stroke?Pistons are not a problem,i still need them and i need to get a good combo before i get them. This will be a N/A engine with a little silly unit on it as well.

[Jeanyus]

I hear those big block fords are real nice. Hey steelcomp what engine are you running inr your 100 mph jet boat. HE HE :mix: :D :boxingguy

[Squirtin Thunder]

Ronny,
Steelcomp and Calperf_19 are both ****ed up, we are the only ones that have our heads on strieght.
Jim

[steelcomp]

I hear those big block fords are real nice. Hey steelcomp what engine are you running inr your 100 mph jet boat. HE HE :mix: :D :boxingguy
Um, er, WTF jeanus! Whataya tryin to do to me here?? :hammerhea :)
Hey, ya gotta run what ya got, right!
Wanna race? :crossx:

[steelcomp]

Ronny,
Steelcomp and Calperf_19 are both ****ed up, we are the only ones that have our heads on strieght.
Jim
Careful, there, thunder. Your heads aren't on straight, your heads arent on at all, in fact they're still sittin on the bench, along with the rest of that boat project of yours that will NEVER see water as long as you're screwin around posting goofy stuff like that! :boxingguy
C'mon....put 'em up! Grrrrrr!

[steelcomp]

So would it be better to go with the .400 longer rod with the 4.25 stroke?Pistons are not a problem,i still need them and i need to get a good combo before i get them. This will be a N/A engine with a little silly unit on it as well.
All this discussion regarding rod length ratio can go on for hours, but really relates to complete packages designed to take advantage of the longer rod ie: (as others have posted) cam design, cyl head and chamber design, compression, operating range of engine, manifold and carb(s), etc. For what you're doing, I'd stick with the 6.535 rod, since it's what you've got and pistons are readily available. You won't really notice the real world difference between what you have and another .150" of rod length. Theoretically spaeking, and in general, the longer rod works better, but unless you're trying to eek every last ponie out of your motor and have a dyno and the $ for experimenting with different combinations, what you have should work just fine. Of course, that's JMO, and someone else might have another suggestion.

[Squirtin Thunder]

Careful, there, thunder. Your heads aren't on straight, your heads arent on at all, in fact they're still sittin on the bench, along with the rest of that boat project of yours that will NEVER see water as long as you're screwin around posting goofy stuff like that!
C'mon....put 'em up! Grrrrrr!
steelcomp
View Public Profile
Send a private message to steelcomp
Find More Posts by steelcomp
Add steelcomp to Your Buddy List
Today, 11:57 AM #23
steelcomp
Registered User
Join Date: Aug 2004
Location: nipomo, CA
Posts: 126
Quote:
Originally Posted by Nucking futs
So would it be better to go with the .400 longer rod with the 4.25 stroke?Pistons are not a problem,i still need them and i need to get a good combo before i get them. This will be a N/A engine with a little silly unit on it as well.
All this discussion regarding rod length ratio can go on for hours, but really relates to complete packages designed to take advantage of the longer rod ie: (as others have posted) cam design, cyl head and chamber design, compression, operating range of engine, manifold and carb(s), etc. For what you're doing, I'd stick with the 6.535 rod, since it's what you've got and pistons are readily available. You won't really notice the real world difference between what you have and another .150" of rod length. Theoretically spaeking, and in general, the longer rod works better, but unless you're trying to eek every last ponie out of your motor and have a dyno and the $ for experimenting with different combinations, what you have should work just fine. Of course, that's JMO, and someone else might have another suggestion.
steelcomp
True in every aspect !!!
Jim

[IDK]

Steelcomp, sorry if I wasn't clear wear I stood on rod length I do like longer rods but just like other things to much off a good thing can be detramental. I was tring to point out on a broad scale my OPINIONS on rod ratios. If you put a 7.000 rod in a motor like being discused in this thread the peak tourqe would be around 8500 useable in a v-drive but not practical in a jet. I do agree that a bbf bottom end geometry is ideal. If you noticed in my reply I made a little comment cyl wall would like a longer rod refering to side load and rod angle , there is another widely used practice to combat that problem and to keep the rod ratio and trouqe in a useable range , offset pistons . I also stated in my reply that I could go on for hours about my OPINIONS and that it is a big debate between engine builders in not so many words.You commented on piston weight I have built many bbc engines (550- 580 ci) with a short deck and a long rod to keep the piston weight down and had them last, only to be torn down after a season just for inspecting and freashing up .Again I was just stating my experiences and opinions of 20+ years of building engines and holding records in many racing organizations, I wasn't looking for an argument or to be slamed for my opinions, I am a nobody just like 98% of the people in this world. Thanks Roger

[steelcomp]

Sorry Roger...really didn't mean to get personal. Just was confused by what you wrote. Thought your comment about cyl wall was referring to lower (initial) cyl pressure. No one was slamming you...just expressing another OPINION.
Question: If a rod ratio of 1.7 works so well in a BBFord, why would the same ratio (little more...7"rod w/ 4" str.=1.75 vs 1.71)
I never said there was a reliability problem with light pistons...if that's what you were referring to. And I was speaking in theory, trying to avoid comparing "short deck, long rod, expensive piston" engine scenarios, and just give Taylorman some basic info he could use. I don't see anywhere in what I wrote that I wanted an arguement or even a discussion about this, but, whatever. My bad. Glad you shared all that experience and success with us. Maybe it will come in handy in the future.