Using a GM forged 454 LS6 crank. Should I use H series or P series bearings. Probably only going to turn 5400 but with twin turbos making 650 hp

even though you are using a gm crank i still would use the H bearing at that hp level.

h if you can get them for the gm rod...polish the face with news paper..they are hard on stock forged cranks..

can you explain how the h bearing is hard on a stock forged crank?

I believe most GM forged cranks are 5140 or 1053 nitrided steel. They were a stout crank for their day. I've been running them for a long time with superchargers and H bearings. Currently at 800HP and 6000RPM for 3 seasons so far, before that, 650HP and 5500RPM for many years. Not as strong as a 4340, but still a stout crankshaft. Thats all they used in racing for many years, some still use them today with good results.

When I pulled the motor apart, It had Clevite P series bearings std size in it.

ok i should rephrase that comment..if the crank was machined under its standard machine specs the nitrate goes with it..nitrate is only a few thousands deep the riding surface of the crank..i was told by the guy that does my machining that the h bearing has a thinner Babbitt material compared to the p series bearing and will make groves in a forged crank...ted said they are great for stock forged cranks but not if they are machined..

According to Clevite ... slightly different eccentricity, too. H May help prevent from spinning bearings.

i have used the h series in a lot of engines with gm cranks [5140forged] with no issues but none had undersized journals.i have a pair of 1000 hp supercharged engines out their with lunati 4340 crank that are .010 under ,not what i like but no issues .

What's the difference between the two?