Rod to Stroke Ratio?
02-10-2005, 03:09 PM
#1
Rod to Stroke Ratio? There are number of varying opinions on Rod to Stroke ratio, was hoping to get some others from a few members over here.
Here's a good starter quote from another board...
I bolded the main area on which I am confused. What confuses me is what happens when a motor reaches a limit where you can't possibly attain that ratio?
Thanks for the help in advance
Floyd.
p.s.
If you're just going to say
or it doesn't matter please don't say anything at all. 
Here's a good starter quote from another board...
The piston rock and side loading become a big factor below 1.50 with piston speeds over 4500ft/min. If its a low rpm street machine that never sees the high side of 6000rpm or 4000ft/min, then the problems with blowby are much less but none the less still present. When I discuss Rod Ratio A Rod ratio of 2.2:1 or a 1.4:1 is not going to help matters any. Anywhere between 1.65 and .1.85 is fine. any less and you run into high skirt loading, blow by and frictional HP losses. Anything above 2:1 and you run into pressure lag and have to run the cross sectional area of the ports very small and maintain a higher mean velocity in order to help make up for it.The little 265 and 302 Comp eliminator engines are usually a case study in pressure lag unless they run the Aurora block with a 8.5 deck and get there ratio into the 1.8 range.
I bolded the main area on which I am confused. What confuses me is what happens when a motor reaches a limit where you can't possibly attain that ratio?
Thanks for the help in advance
Floyd.
p.s.
If you're just going to say
or it doesn't matter please don't say anything at all. 
02-10-2005, 03:17 PM
#2
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What do you mean when the "motor reaches a limit?"
FWIW- the engine in the Integra Type-R, which spins to around 8500 stock, has an R/S of 1.59.
In any case, R/S is a way to dictate where your powerband will be.
Ben
FWIW- the engine in the Integra Type-R, which spins to around 8500 stock, has an R/S of 1.59.
In any case, R/S is a way to dictate where your powerband will be.
Ben
02-10-2005, 03:40 PM
#3
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In practicality, R/S means "almost nothing" but if you are scraping after
that 1%...
In reality, too poor a rod ratio will result in too short a piston which rocks
in bore too much and wears it out fast.
Some hi mileage motors/hi power motor/hi rpm motors have R/S as "terrible"
as 1.4
To sacrifice 75 cubic inches to go from a R/S of 1.6 to 2.2 is insanity,
you might gain 10HP from the super long rods, but wouldve gained alot
more HP and proportianate TQ from the cubic inch increase.
that 1%...
In reality, too poor a rod ratio will result in too short a piston which rocks
in bore too much and wears it out fast.
Some hi mileage motors/hi power motor/hi rpm motors have R/S as "terrible"
as 1.4
To sacrifice 75 cubic inches to go from a R/S of 1.6 to 2.2 is insanity,
you might gain 10HP from the super long rods, but wouldve gained alot
more HP and proportianate TQ from the cubic inch increase.
02-10-2005, 09:35 PM
#7
Hmm a lot of top SBC engine builders replace the standard 5.7 inch 350 rod with one over 6 inches. And our 346's were designed with a 6.1 inch rod and a few other things borrowed from independant engine builders. Also note that the 5.7 rod was the design lenght in the 283; is was a good lenght for that engine, but short for the 350/400 displacement the engine grew into.
The piston spends more time at the top of the cylinder where all the presure is with a longer rod.
The piston spends more time at the top of the cylinder where all the presure is with a longer rod.
