Rod to stroke ratio was at one point a highly contested argument among the experts, yet very little real improvements were seen, especially when considering how much was spent in changing the rods and pistons and then the cam timing, ign timing. Even the Professor (Warren Johnson) made a statement at one point saying Pick a piston, a crank stroke, and connect the two together.

If you're class racing and looking for every thousandth of a second, perhaps you might find something. Even then, don't count on much. Speaking from experience. If its' a pushrod engine, chances are good that someone's already tried to reinvent the wheel and if there were any significant gains to be had, we'd already be reaping the benefits.

 

I don't think there's a hard cutoff number, but personally wouldn't go lower than 1.5 on anything that needs to live for a while, especially at higher rpms. The real limiting factor would probably be the lack of clearance between the rod and lower cylinder wall of the block.

Higher ratios supposedly allow for more piston dwell at tdc, which in theory helps hp wise on engines with restrictive induction/exhaust.

 

Correct rod ratio = eight rods per one crankshaft.

 

Since we are talking about V8's, I would consider 8 rods per crankshaft to be very safe too.

 

1:1 rod to piston ratio. Never put 2 pistons on the same rod.

 

long enough to keep the piston skirt from hitting the crank through. Short enough to not drive the piston through the head.

 

OP, not trying to derail just continuing the conversation with related topic. Many are saying in so many words or less "as much or as little as you want without contact". Am I interpreting that wrong? Anyhow where and how does rod angularity come into play? Reason I ask is when I mentioned a stock block 427 I got lots of pushback about bad ratio and too much rod angularity.

 

Push back from who??? Stock block, ie 351 Windsor block?? Problem is the stock bore, means the crank has to be a 4.125" stroke on a 4.060 bore block. Your going to end up with a situation with custom pistons, custom rods, and no skirt on the piston. The piston is going to rock severly... causing cylinder wear. You compound headaches with that much displacement under small heads, all the decent heads are intended for 4.125 or larger bores.

 

Pushback from the internet wizards.yes stock 351w block. I'm doing a 4.17 stroke with 4.030 bore. Rods are 6.25 and forged internal crank was available from RPM. Only thing custom was the piston but that only added a few dollars to the price. I cannot recall the compression height off the top of my head but will get back to you with that. Pistons look like they have plenty of skirt. I'll post a pic when I'm in the PC. Still think ill have too much rocking going on? The heads I bought are trick flow 205 11r.

 

there is nothing worse than the 454 chevy, and they stuck that pig in everything.......... build it, beat it, drive on.

 

What kind of revs are you planning to turn ? Piston speed will be up there.
6K = 4170 fpm

 

Me? 6500 on the dyno then never again, maybe?. It is a show car. It's a long way off from being done though. Will typically never get above 5k once it's tuned.

 

chicken feed... 3.9 stroke, 9600 RPM

 

You'll be fine, build it and enjoy it.

 

If your going to quote a "chebby" at least get your info correct.
Stock 400s are 5.56 inch rods. Making it 1.48 ratio. Its a 9 inch deck block.
I have a 3.85 stroke crank with a 6 inch rod in a dart 400 block. 9 inch deck. 1.55 ratio. Basically Im saying ratio means squat. Im not worried about ratio. Want a safe ratio keep the 302 stock.

 

Sorry if I misquoted the info. I found this chart on the net. That is where I got the info. I saw std. rod and figured that was standard.

Stroke vs. Rod Length in Common Automotive Engines Motor Stroke Rod “n” Ratio
Mopar LA 273/318/340 3.31” 6.123” 1.85-1
Mopar LA 360 3.58 6.123 1.71
Mopar LA 340 with 3.79” stroker crank 3.79 6.123 1.62
Mopar LA 340 with 4.00” stroker crank 4.00 6.123 1.53
Mopar “B” 350/361/383/400 3.375 6.358 1.88
Mopar “B” 400 with 440 crank & std. rods (451”) 3.75 6.358 1.70
Mopar “B” 400 with 4.15” crank & std. rods (498”) 4.15 6.358 1.53
Mopar “B” 400 with 4.15” crank & BBC +.400” rods (498”) 4.15 6.535 1.57
Mopar “RB” 413/426W/440; “B” 383/400 with 440 crank & rods 3.75 6.768 1.80
Mopar “RB” 413/426W/440 with 4.15” crank (494”) 4.15 6.768 1.63
Mopar 426 hemi 3.75 6.86 1.83
Small Block Chevy 302 3.00 5.70 1.90
Small Block Chevy 327 3.25 5.70 1.75
Small Block Chevy 350 3.48 5.70 1.64
Small Block Chevy 350 with 6” rod 3.48 6.00 1.72
Small Block Chevy 400 with std. rod 3.75 5.45 1.45
Small Block Chevy 400 with Chevy 350 rod 3.75 5.70 1.52
Small Block Chevy 400 with 6” rod 3.75 6.00 1.60
Big Block Chevy 396/402/427 3.76 6.135 1.63
Big Block Chevy 454 4.00 6.135 1.53
Ford 289 (Windsor) 2.875 5.156 1.79
Ford 302 (5.0, Windsor) 3.00 5.090 1.70
Ford 351W 3.50 5.954 1.70
Ford 460 3.85 6.605 1.72


dstolarc

 

I don't know the history of this, but in many mechanical and electrical things rules of thumbs get created early on with early parts and knowledge that are just not as good as later parts and knowledge.

Once the rule is created and out in public, it becomes common wisdom and is there forever.

I was up at Kaases and he showed me one of his mountain motors on a dyno. They pulled 9K RPM with something like a 5.75 inch stroke. With parts 25 years ago, that would have been impossible. It made 2K horsepower on a first pull rough tune.

In 1980, you would be lucky to see a small block short stroke stay together, but the public knowledge is still out there. This was even more true with print than with Internet.

 

Small Block Chevy 400 with std. rod 3.75 5.45 1.45

This is why you don't build an engine using Google.

Stock 400 rod is 5.565" long not 5.45" as this chart noted.

Knowledge is power. Experience is omnipotent.

:salute:

 

Mechanically speaking, a smaller rod ratio creates greater rod angularity. This results in higher piston side loading against the cylinder. It also increases the rate of piston acceleration/ deceleration for a given engine speed. Due to the increased piston acceleration/deceleration rates, piston spends less time at TDC/BDC with a smaller rod ratio for a given engine speed.

Changing rod length for a given stroke/deck height obviously changes the center of pin to top of cylinder, which will alter the design of the piston. I would be far more concerned about a good piston than the rod ratio.

Jay

 

Just ask yourself, hmm where are all the prematurely worn out small block fords at? Crickets

 

Oooh me, oh me....

Not rod ratio related, but definitely prematurely worn out.

 

I had a chance to talk to Jon Kaase about rod length back when he was building his first 351C based Engine Masters Competition winner. He said his rule-of-thuimb was rods 2 inches longer than the stroke. He said that rule has worked well on everything from short stroke 289s to IHRA mountain motors.

Dan Jones

 

Here's a quote from Reher-Morrisons site from David Reher and his thoughts on the subject:

We wanted to point out some of the common myths and misconceptions about high-performance motors. For example, I've seen dozens of magazine articles on supposedly "magic" connecting rod ratios. If you believe these stories, you would think that the ratio of the connecting rod length to the crankshaft stroke is vitally important to performance. Well, in my view, the most important thing about a connecting rod is whether or not the bolts are torqued!

If I had to make a list of the ten most important specifications in a racing engine, connecting rod length would rank about fiftieth. Back in the days when Buddy Morrison and I built dozens of small-block Modified motors, we earnestly believed that an engine needed a 1.9:1 rod/stroke ratio. Today every Pro Stock team uses blocks with super-short deck heights, and we couldn't care less about the rod ratio. A short deck height improves the alignment between the intake manifold runners and the cylinder head intake ports, and helps to stabilize the valvetrain. These are much more important considerations than the rod-to-stroke ratio. There's no magic - a rod's function is to connect the piston to the crankshaft. Period.

 

I have observed another by product of rod length. In x275 racing esp. big loads on nitrous were being dumped in to small blocks to keep up with the turbo and sc crowd. The longer rod motors get real picky about timing, (supposedly from tdc dwell) not that they won't work but you have a knife edge of a tuning window. The slightly shorter rod motors were a little more forgiving. This may not be a factor until some serious amount is being dumped in. But for the guy deciding between a stock block 408 w 6.2 rods or a 3.85/5.956 393 if you get crazy with the sauce it might help keep it together.