I'm working on a 451 now and I picked the stock 440 R/S ratio because I thought Chrysler knew something when they decided the rod length and stroke way back in 1958. The 451 combo has a light piston on a 440 crank and that will relieve some stress off the crank, which is nothing to sneeze at. Lighter reciprocating assy should save on some HP too.
So let’s break this down by the numbers. A 440 rod is 6.76” C-C. A 400 rod is 6.36” C-C. That’s a difference of .400”. The stroke length difference is 3.38” vs. 3.75” for the 440. That’s .375” difference in stroke. To keep all things equal at the 1.80 R/S ratio you would shorten the 400 rod ~.300” - as it is the 400 has a 1.88 R/S ratio so shortening the rod will make it like a 440. Or you can lengthen the 440 rod on the 3.75” stroke and get 1.88:1 R/S ratio like the 400. Some have argued that changing the ratio is meaningless but others try to split hairs and will do anything to get their desired number. I understand NASCAR guys go upwards 2.0. You can uncover a whole can of worms when dealing with R/S ratios, which is why I’m just sticking with the MoPar plan, but in reality is a R/S ratio difference of .1 even worth the time it took me to type this? The 451 / 440 rod combo parts were off the shelf so it’s not like hundreds of dollars were spent on custom orders.
Something else to think about. The ratio is irrelevant when manufacturers are creating bigger engines under the direction of marketing. Let’s take the 400 SBC. It probably has the worst R/S ratio ever created but they built it anyway. Bottom line: It was a 400 CI engine when people were into “bigger is better”. It had to pass the warranty period. And GM was not going to make a special tall block SBC just to satisfy the marketers.
Another observation: If you look at all the American V8 engines around the big design change in the 50’s you will note that they all were originally designed for around 1.80:1. Ford, Chevy, MoPar, AMC, Buick, Olds, Pontiac, Cadillac, etc... So, following this logic, when the cubic inch wars started to heat up, the most economical way to increase displacement was to stroke it. And again, rather than casting a special block to keep up with marketing every time someone wanted to add some inches, piston, rod and crank changes are the more economical solution (Chevy 427 to 454). Yes new blocks were cast to accommodate larger bores (i.e. 413 to 440 MoPar; 427 and 428 Ford) or very long strokes (i.e. 425 to 455 Olds) but many just went the crank rods and piston route (i.e. Ford 289 to 302; 352 to 390 - even the 428 has the same deck height as the 352).
This isn't 1950, and ford, gm, pont, buick haven't had a ratio that big since 1950, no bb gm has ever seen the high side of 1.6 EVER.
The nascar motors are 1.85 to 1.9, and again if YOU are building a motor to hang out at 8000 to 10,000 rpm then use the 440 rod, this isn't a super speedway motor he's building, he is building a motor to work within idle speed and up 7500 of which the lower RR is key to making better power.
Using the 400 rod makes the RR 1.7 which is better on the street or strip since the ability to move more air by piston speed is helpful here.
Using the 440 rod you are keeping EVERYTHING about the 440 in a block that is simply losing 30 lbs. So again no one is building the 451 correctly when they use the 440 rod..
It seems that when this 1st started back in the late 70's early 80's it somehow got
AFU when it became popular in the late 80's which everyone started doing it wrong and building a lighter 440, and all the gains were lost.
1 way of increasing power with a motor not able to see 10,000 rpms, you increase piston speed, how do you do that, stroke it and keep the same rod length or use a shorter rod and get into the 1.6 or 1.5 ratio, you increase piston speed and move more air and take advantage of better more aggressive cams on the street.
The piston speed in a F-1 motor is almost exactly the same as that of a nascar motor, a 355" nascar motor turns at 9000 rpm to make 850hp and a 140" F-1 motor turns at 19,000 rpm to make 800 hp.
Making the speed brings the power in, however when you make the RR lower you make it breath heavier, and you need a good intake and heads, and you need a high flowing dual plane if you're going to use that and you need to address the heads and use a good head or do some serious porting.
Most drag motors are in the 1.3 to 1.5 range now, that 1.8 and bigger RR is for nascar and F-1, F-1 is at 2.0 RR, if you are going to idle at 6000 and make power from 12 to 20k then use the large RR.
Nothing is harder or more severe use on a motor than that of the marine world and there is where endurance is measured, not in street cars being sold to public for 2000 rpm use, marine motors are also in the 1.4 and 1.5 RR.
You want performance you have to think performance not what ma mopar did in the factory to sell cars and they had 1.8 RR, even the fixed hemi doesn't have 1.83 RR anymore its at 1.74.
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