How high do you intend to crank the Duke engine? The stock springs should be good for over 5000 RPM before the valves float. At that point the Duke is past redline. If you intend to rev higher don't expect the engine to last long.
Another thing to consider is the higher the valve pressures the lower the cam lobe life becomes. If you are dead set on "performance" valve springs try Summit Racing Equipment. They carry most everything.
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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, 3.4" Pulley, N* TB, LS1 MAF, Flotech Exhaust Autolite 104's Custom CAI 4T65eHD w. custom axles, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "
