Would somebody please explain the whole rod/stroke ratio thing? Let's start with a stock Chevy 350. 5.7" rods with a 3.48" stroke gives us a 1.638 ratio, correct? Longer 6" rods with a 400crank (3.75" stroke) gives us an even 1.6 ratio. That's simple math.

Now, how does this affect engine power and torque in different RPM ranges? And WHY???

ThankSS!

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Cheston P - IM_A_0@yahoo.com AIM:Impalaitis
96 BBB Impala SS "ADIOSS"
91 white Caprice 'n0n 9c1' w/L03

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[This message has been edited by AdioSS (edited July 25, 2002).]

 

you might want to try doing a search as this subject was talked about pretty extensively a couple of months ago....good luck!

 

Agree with this sugestion. FWIW, it's interesting theory but is of very little practical importance at our level. Anything that's not too extreme will work in a street/strip application.

It's intersting that GM's ratios are consistently towards the lower end. 350 SBC = 1.64, 502 BBC = 6.13/4.00 = 1.53. I'd like to know why.

Rich Krause

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'95 Z-28 383 with Vortech, nitrous, etc.
"1FASTZ28"

 

Most builders (race engine builders) will tell you it doesn't matter that much at all.
Don't worry about it.
Your power, or lack there of, will come from your heads and cam!
The rod ratio doesn't do jack.

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If a little's good,
More is better,
And too much is just right!!!

 

It's just a matter of trying to pack as much displacement as possible into a given packaging space. With a given deck height, you can either get stroke or rod ratio, and it seems that stroke is more desirable from GM's standpoint (and for a street vehicle, I'd have to agree).



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1996 Impala SS - LT4 396, T56
1996 GMC K2500
1992 Buick Roadmaster Estate Wagon

 

I was always told that LR (rod ratio) determined how well a given engine would "rpm" (rev), and the example that was always given to me was the first gen. Camaro Z28 302, which made its best power at high rpm. Of course, some of this information also came from people who told me I couldn't run more than 9.0:1 compression with my 396 or it would detonate...

I was told that the 1.55:1 (6.0" rod, 3.875" crank) ratio of my 396 wouldn't allow it to rev well and that it would make a great torque motor, but that power would fall off after 6,500 rpm. Well, not only will the engine easily spin to 8,000 rpm, but it will make peak power (when we get the ignition issues worked out) around 6,900-7,000 rpm, and it's still making 637 horsepower out at 7,400 rpm on the base maps, so there goes that theory. The heads and the solid roller cam are the reason, obviously.

Of course the static compression ratio of my engine is also 12.5:1, so I don't believe anything any more that I'm told by people whose experience is apparently limited to iron head, carbed, "pre-tech" engines.

 

if u change the length of the rod, you have to change the stroke of the crank to gain anything. When you do this, your allowing the piston to sit at the top and bottom of the cylinder longer (this is called dwell time) thus creating more torque. Picture the piston coming down to BDC and instead of going down and coming up.. it stays at the bottom a split second longer.. and then not only does it just go UP.. but it goes up with more force! The stroke of the crank is giving you better geometry whereas your pushing the piston UP the cylinder rather than into the side of the wall... because your pushing the piston straight UP there is less friction between the rings/pistons and cylinder walls.. which in turn reduces heat.. and friction.. resulting in ..you guessed it.. more POWER. Longer strokes = more torque. Look at the difference between imports and domestics. Tiny pistons and short strokes result in high revving, low torque combinations. If it wasn't for VTEC, muscle cars would still dominate everywhere (of course.. they still DO in my eyes :-) ) If this hasn't helped ya check out http://www.chevymania.com/rod.htm there is a fantastic article about stroking and what is accomplished by it.

 

Check out this link:

http://raceseek.com/honda_specs.htm

Note that there's a lot of rod ratios in the 1.5-1.6 range (the exception being the B16A3, used in the relatively uncommon Civic del Sol VTEC). Honda car engines use relatively long strokes and short deck heights (as Honda cars have very low hoods), so their rod ratios aren't so terrific. Yet, they seem to rev well and last a while. I don't know if they do anything special (like extended piston skirts) to keep wear to a minimum, or if rod ratio isn't all that it's cracked up to be.



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1996 Impala SS - LT4 396, T56
1996 GMC K2500
1992 Buick Roadmaster Estate Wagon