I am building a 383 SB and was looking for some information on the pros and cons of using 6.0 inch rods as opposed to 5.7 inch rods. The car will be 95% - 99% street driven, and the rest of the engine will be built with this in mind (276 hydrolic roller cam, alluminum heads, dual plane intake, 650 cfm carb, 9.5 to 10.5 compression). Thanks in advance for any info.

Most recent Hot Rod magazine articles say go with the longer rods. However, this will increase the cost of building your engine since the block may need to be ground for clearance and you will need special pistons. Several companies like Lunati make engine kits with 6 inch rods.

Going with the 6" rods with a 3.75" stroke crank will give a 1.6 rod ratio. This is a good streetable rod ratio. A ratio as low as 1.5 has been used on some strong, well running motors. The theory is to have as long a rod as possible because it lowers piston speed, increases dwell time of the piston at TDC and decreases cylinder side wall loading. That's enough reasons for me. As far as availability of parts that's not a problem. All the major piston manufacturers have pistons for your setup. That's the good thing about going with proven, common engine designs. Go for the 6" rods. The only thing I would question when buying the pistons is how well the oil ring supports hold up. These are needed because the pin boss goes through the oil ring groove.

Joseph

I can't stand it, how does a longer rod lower piston speed in the bore? I agree with your other statements, improved rod angle, less loading on the thrust and longer dwell time at tdc, but that piston speed reduction has me scratching? http://www.camaros.net/forum/confused.gif

I thought piston speed was a function of rpm?

That one was a hard one for me to understand as well. It has to do with the rod angularity in the bore as the crank shaft turns. Not only is the piston speed reduced but also the acceleration of the piston from TDC to midstroke of the crank shaft where the highest velocity of the piston is achieved.

Joseph

If the piston speed is reduced with the longer rod at the same engine RPM as a smaller rod, would this increase the RPM range of a head with say a 190 cfm intake flow to that of maybe a 195-200 cfm head while still maintaining the better low end characteristics of the smaller intake head or is the increase not that significant?

I'm not so sure that piston speed, with respect to this discussion, will relate well to a head's working rpm range. When I think in terms of rpm ranges for heads I deal with port volume. Larger ports feed the engine at higher rpm when smaller ports can not flow enough intake charge. I'll need a little more convincing. Any takers.

Joseph

Check out the motor that Hot Rod built in their June 1997 issue and called "The 350 Engine Chevrolet Should Have Built". The crux of the project was the use of very long Ford 6.25" (I think) 6 cylinder rods. They have a pretty good explanation of the benefits of longer rods and better rod ratios and answer a lot of the questions raised here. In addition, they used the AFR 305 heads with 190 cc intake runners and 1.990 intake valves so they also address the question on intake volume.





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Scott
'69 400SB, Richmond 5-speed
'99 HD Road King Classic