Reply With QuoteDepends on what compression you are looking for. Longer rods will give you more compression.
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?
Depends on what compression you are looking for. Longer rods will give you more compression.
Hows that?
I use 4 inch stroke with.250 longer rod.Better rod angle ratio 1.6.
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.
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.
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.
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
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.
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)
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