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Category: Categories > Engines > Engine Building > Tech Tip
|The real importance of connecting rod length
Posted: Tue Mar 11, 10:48 PM EST
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This is a subject that has brought much controversy over the years. While I have always been of the opinion that its importance has been somewhat overrated; as long as the rod is not way too long, or way too short, there is not that big of an effect. However, it can be a very important tuning tool in one area: Dynamic Compression Ratio. This is not the compression ratio that is normally stated when someone says for example: Ive got 12.25:1 compression.That is the STATIC compression ratio.
The static compression ratio is a function of bore, stroke, combustion chamber size, head gasket thickness, piston to deck clearance, valve relief depth, and other factors. The dynamic compression ratio also takes these parameters into account, as well as the intake valve closing point, intake lobe centerline, lobe separation, and the CONNECTING ROD LENGTH. Basically, the dynamic compression ratio uses the piston position in the cylinder when the intake valve closes, rather than the actual crank stroke. This is called the dynamic stroke length. Obviously, this is affected by connecting rod length. There are several ways to calculate the dynamic stroke length, and ultimately the dynamic compression ratio. I have several engineering books that get into this subject deeper than most of us want to get into. However, the easiest way is to go to this site http://cochise.uia.net/pkelley2/DynamicCR.html. At the bottom of this page, there is a download for a dynamic compression ratio calculator that is very simple to use. It has saved me a lot of time, compared to doing calculations manually using trig.
Using my engine as an example, I will illustrate the effects of rod length on dynamic compression ratio. I have a 351 Cleveland Ford engine that has a 3.5 stroke, stock 5.78 connecting rods, and a camshaft with an intake valve closing angle of 76.5 degrees ABDC. By the way, this is when the intake valve actually closes, not a .050 lift figure. This results in a dynamic stroke length of 2.4146. This is the number you use in place of the actual engines crankshaft stroke in the standard compression ratio formula. This gives me a dynamic compression ratio of 8.77:1. The static compression ratio of this engine is approximately 12.27:1. Take this identical engine, and put a 6.200 rod in it, and the dynamic compression ratio falls to 8.71:1. Not a very significant change, but it can be a good tuning aid. You could increase the static compression ratio some, which, combined with the significantly longer dwell time of the piston at TDC, could increase combustion efficiency, resulting in a power increase throughout the rpm range. Or you could shorten up the cam timing slightly, resulting in a flatter power band.
Using an example on the other end of the scale, here are the specifications on a 347 stroker engine I built recently. It has a 3.4 stroke, 5.4 connecting rods, and a very mild camshaft with an intake valve closing angle of 68.5 degrees ABDC, again, this is the point that the intake valve is completely closed. The dynamic compression ratio of this engine is about 8.49:1. The static compression ratio is only about 10.89:1. The combination of a very short rod to stroke ratio, and early closing of the intake valve combined to produce a dynamic that was nearly as high as the other engine, despite the fact that the other engine had a static of about 1.5 points higher. It is very telling that this engine produces 60 foot times as quick as 1.52, while my Cleveland is only slightly quicker at about 1.46 in the 60 foot. However, the similarity ends very quickly right there. My Cleveland engine covers the 1/8 mile in the high 6.70s at nearly 100 mph. The 347 runs 7.30s at about 86 mph, obviously lacking in top end power.
In defense of the 347, however, it will sit there and idle at 850 rpms all day, and have a manifold vacuum of about 17in hg. I wouldnt be even slightly afraid to put this engine into a daily driven vehicle. Most likely it would even get decent gas mileage. The Cleveland ,on the other hand, idles no lower than 1500rpms, has a manifold vacuum of only 8.5in hg, is very finicky in cold weather, and would not be very streetable at all. The 347, incidentally, has a better round win percentage than the Cleveland does, a function of the fact that it is much more docile. It does not, however, do as good of a job of catching people on the top end, and is very vulnerable to being caught by faster cars, due to the fact that it is all out of steam at the finish.
Well, I hope I have opened some peoples eyes on a very important engine variable. I started out making this a rod length tech tip, but ended up spending more time discussing dynamic compression ratio. The two are obviously interrelated, and one cannot be discussed without the other. Keep this tech tip in mind the next time you go out and buy that camshaft that is just a bit bigger. You may find that you lose performance, unless you follow some of the topics discussed here.