Rod Ratio

Long Rod, Short Piston

Rod Ratios is a topic that some of you know has me "pre-occupied". My Chevy experiences lead me to believe that the best combination for the olden days was the lowly 327. It had enough low-end grunt while giving me a very quick run to redline. The larger displacement small blocks yielded tons of torque but were just lazy reaching top end as a truck motor like the 454.

I was told that the performance difference is mostly based on the magical thing called rod ratio. Divide the rod length by the stroke to get this metric. Many agree that the 302 Z28 is a zinger with a ratio of 1.90:1. 1.70 is the ratio of Fordís famous 302 (5.09"/ 3"). My stock 327 pushed 5.7 / 3.25 numbers for 1.75. Ubiquitous SBC 350 and its 400ci successor are 1.64 and 1.48 respectively. Most endurance racers today run with a range of 1.9 to 2.2. Chevyís new NASCAR 358ci R07 engine has 3.25 stroke and varies the rod from 6 to 6.2" based on the track. It yields ratios of 1.85 to 1.91 to generate 9,300 to 9,600 rpm.

The theory goes something like this. Short piston rod length has a quicker turn around at both top and bottom dead center. Longer connecting rods stay a bit longer at the extremes and reduce the side loads thus less friction and wear because of a better angle to the crank also giving it more leverage. The delay allows slightly more time at the top leading to a more complete combustion. Better combustion translates to less detonation through less timing and permits a slightly higher compression ratio. Peak velocity is delayed somewhat for more filling on intake, grants longer overlap timing, but lowers scavenging of the exhaust gases at low rpm. Higher pin height improves the piston center making it more stable. You can also use shorter skirts therefore lighter pistons. Tensile loading is also lower as the speed is reduced at TDC.

Chevy started its 265 in 1955 with a ratio of 1.9 as a favorite of hot rodders. Drag strip "Screamers" were the 283. It reigned from 1957 through the mid 60s used the same 5.7 rod / 3 stroke as the early Z28 and the 265. Not all big blocks are dogs. 1958 348 had a 1.89 ratio. Mighty fine 409 had 1.71 but both the 454 and 502 slipped to a stump pulling 1.53.

Ford started out quick too. I remember Dadís 62 stick shift Fairlane with the 221ci small block, predecessor of the 260W, whose ratio was 1.80. The 260 and 289 had ratios of 1.79 (5.15/2.87) then they produced the 302 and 351W at 1.70. Alan Fosterís race winning Mustang uses a 351W stock rod ratio (5.95/3.5) but is highly modified for head flow and is internally balanced with a light rotating assembly. It screams down the straights at 7,200rpm and redlines well above 8,000. Our 351C (5.78/3.5) is fair at 1.65. Some of us have switched to a 6íí rod to get back to 1.71. Tom Todak runs his 380C with those Chevy 6" rods on 3.75 crank for 1.60 ratio. His objective is a reliable mid range torque machine able to take the taller gears in his ZF. Art Stephensí old 377C engine had no trouble getting the revs up quickly with its 6.125/3.7 for 1.66. But I eagerly await his new 408C engine with a 6/4 1.50 ratio. His right foot is about to test the heavily laden 4 bolt with all the right Australian BOSS goodies. The typical 427W uses 4.17 stroke and a 6.125" rod for 1.47 ratio: our Cleveland based 427 at 6.125/4.154 has a 1.485 ratio. Bill Wright lucks out by running one of these big cubes in a super light GT40. I wonder how fast it propels the car to 7500rpm? 911 Porsches are designed for 1.7 to 1.8 ratios. Remember that Viper does prove that even truck engines can deliver fast cars in the right combo.

The key seems to be the intake, exhaust, head, cam, valve train, and rotating assembly selected. The choices determine the flow possibilities so vital to extracting the desired response from the engine. Each designer chooses his own way to propel a car down the road. Horsepower is calculated by torque numbers, so, do you want big booms or lots of little booms? Unfortunately, I donít know what is best. I only hope that you will tell me of your experiences and preferences. I, with your help, can then plan the rebuild of my engine.

Rod Kunishige 2/5/2008