OK, guys, here goes nuttin.'I've raced NHRA Stock Eliminator for the last 25 years.My car of choice was a pretty good combination at the time, it was a '68 Chevelle 2 dr HT with a 327, a Powerglide, a 12 bolt with good axles,a spool, 5.38 or 5.57 gears, nine inch by 29.5 radial slicks, and this combo has the small valve 3917290 low compression small valve heads. It was rated at 250 horsepower, NHRA refactored it to 245, it runs J,K or L /SA. Depends on weight. That was a very common combination in the early '70s, still a few of us racing it today. It started out going 12.60's, it has gone 12.20's at Winternationals a few years ago.
The 68 cars were the first year of the large journal 327, although the sanctioning body lets us use the small journal version as well. The early 250 horse 327,62-64 used either a 520 or an 896 head,same as the 63-67 283's, it makes over 10 to 1 CR on a 327.The 68 290 head is a one year only affair that turns out to be about 8.75:1 CR, and that's after the block is decked to specs and the heads are CC'd and cut. The 275 horse 327 came with a 461 casting, a 291 casting, maybe a few others.The'68 engine has a 462 or a 292 casting. I can't use the earlier heads on a 68 car, but earlier cars can use the later heads. I sold off all my 461 heads some years ago.The small journal 327 used a 2" rod bearing like a 283 did. That wasn't the problem. The early rods don't tend to be very strong at the big end where the beam flares out at the journal area.The large journal rod was the exact rod put in a 350. It has 3/8"rod bolts, as to 11/32" bolts in the smaller rod,another improvement. Length was 5.700"in both applications.Pistons were the exact same in both versions,the earlier crank and rods were lighter than the large journal, but the additional strength of the large journal parts more than offset the additional weight. We are allowed aftermarket rods from an approved aftermarket parts list that NHRA released, so I use a forged Manley rod. We bush the small end,the pin diameter remains the same at 9.270"diameter,they weigh around 555 grams,never a problem yet. The OEM steel crank isn't a problem,I've collected a few large journal steel ones, even have a couple cast ones, and a few small journal cranks as well.The 307 engine that came out in 68 until 73 used the exact same cast crank as the 68-69 327's, the same large journal rods, and the block was a large journal 3 7/8" bore block. A steel 302 crank and some good rods and pistons would make a pretty stout 283.If you want to build a restoration, stock parts are very adequate. Any time a small block has to turn past 6500 repeatedly, the life span of the rods shortens drastically.If you magna fluxed, resized, used SPS type bolts, shot peened and did everything to a stock rod, it would be OK to 7500, but you would still start with an OEM rod. The question is-how many hundred thousand miles did they accumulate in regular service?In the four cycle engine, one cycle operates under little stress. The second cycle confronts increasing stress on the compression cycle,then the power cycle pushes it even more.The exhaust cycle is the worst, as there is suddenly no stress on top of the piston with an open exhaust valve. The piston arrives at high speed at top dead center,and the only thing stopping the rod and piston is the two rod bolts. The big end of the rod flexes a little bit, as well. How much does the rod stretch?If I set up a 327 with .030" or less total deck with gasket, race the car, then pull a head, you can see an imprint of the outline of the combustion chamber on the piston.It's faint,but it's there. Now. An engine turning 2000 RPM's rotates the crank 33 1/3 times a second, 100 times in 3 seconds,2000 revolutions per minute. 500 minutes is 1,000,000 revolutions of the crank. At 50 miles per hour, it probably rotates about that speed, so in 8 hours and 20 minutes, the million turns point has been reached. If you drive faster, it occurs at the same point on the map, but happens sooner. If you leve San Diego and head north on the 5, you rotated the crank a million times when you go through Sacramento.Now, at that slow of an RPM, stretching and relaxing of the rod would be minimal,but that's still 500,000 duty cycles on that rod. How many times can you flex or stretch a steel part before it fatigues and breaks? Now, if the piston is way heavier, it happens sooner because the fatigue loads are worse.
If I build another 409, I would forget about a 6" OEM steel forged rod. I would get the strongest 6.135 big block Chevy rod, maybe even a 6.300" rod, use a shorter than OEM pin, and get the piston as light as I could. OEM big block rods usually needed quite a bit of work to get them crack and stress free, quite often took a lot more than 8 rods to get 8 good ones, not worth the effort, use the best aftermarket forging they make. That heavy piston at 7000 RPM broke a lot of rods and motors. It's amazing they work as well as they do.
If a car is used in long distance Interstate highway use, with an average RPM of 2500, how many revolutions did it turn in 100,000 miles?That's assuming normal use. Bet it would amaze you!
