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Rod ratio for 390 stroked to 449
with the new 4.25" stroke scat cranks available it is tempting to do a stroked 390. One could easily cross bolt a 390 block and tap the core holes for screw in plugs and create a replica 427 engine.
Anybody know if it is better to use a rod length of 6.7" or 6.8" ? I have heard people mention rod ratio but I dont really know what it means. The rod ratio for the 6.7" length is 1.576 and for the 6.8" its 1.6. What else is a good or bad idea for this 390 to 449 stroker? |
Those rod ratio's are basically equivalent.
I'd choose a rod length based on the piston ring locations. (throw+rod) (2.125+6.8)=8.925 FE deck = 10.15 (or so) 10.15-8.925=1.225 for your ring package, which, in my opinion, is pushing things. Based on this, I'd go with the 6.7" rod if using the 4.25" crank. |
I'm putting one of the new Scat 4.25" cranks in a '64 LR block now and I'm using the 6.700" rods just to keep the pin out of the oil ring groove. The rod ratio for the 6.700" rods is 1.58 versus 1.60 for the 6.800" rods. Not much difference at all. The 6.700 rod allows me to run a conservative 1.34" pin height.
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Thanks for the feedback...the 6.7 rods seem to be the way to go based on the piston pin height distance.
Ted...have you picked a cam for your engine build yet? |
Because this is a restoration project, I'm being consevative on the cam. I've already got the EE-360 Isky ready to install which has 240° @ 0.050" and .530" lift. I checked the original cam as it came out of the engine and it was 228° @ 0.050" and .525" lift with 427 inches. The major difference is the original cam was ~113° lobe centerline whereas this Isky is ground on 108° lobe centers. I counting on the cubic inch increase to 482 inches to have the same idling characteristics as the original.
I've got the EE-392 in a 390 and it sounds like a double A fueler idling. Way too much for this buildup. |
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