What do you think is the same RPM limit for my stock 400 short block?

I am using the original untouched 1973 shortblock that has about 60K miles on it. With the current mods, it generates approximately 350 gross HP. Right now it runs out of breath around 4900 RPM, using some stock/rebuilt 1971 #96 heads and a custom ground cam (202/208 114 sep). I may upgrade to some milled & ported 6x heads from a 350 motor. My performance Engine Analyzer progams projects that the motor will then be making 396 HP at 5400 RPM, with 387 HP at 5700 RPM and 346 HP at 6000 RPM.

What is a safe RPM limit for my stock short block?

What do you think is the same RPM limit for my stock 400 short block?

I am using the original untouched 1973 shortblock that has about 60K miles on it. With the current mods, it generates approximately 350 gross HP. Right now it runs out of breath around 4900 RPM, using some stock/rebuilt 1971 #96 heads and a custom ground cam (202/208 114 sep). I may upgrade to some milled & ported 6x heads from a 350 motor. My performance Engine Analyzer progams projects that the motor will then be making 396 HP at 5400 RPM, with 387 HP at 5700 RPM and 346 HP at 6000 RPM.

What is a safe RPM limit for my stock short block?

I wouldn't run it past 5700.

Shift it at 5500.

Iv`e been runnin my 455 stock rod to 5600 for about 15 yrs. Missed a few gears before the MSD6AL also. Bearings look fine when i check em. Don`t see why a 400 can`t go to 6000. http://forums.performanceyears.com/g...s/icon_eek.gif

I agree with 5.5-5.6k and KEEP THE OIL CHANGED AND DON'T DIVE IT HARD IF IT'S GETTING ON THE HOT SIDE! http://forums.performanceyears.com/g...on_biggrin.gif

High RPM dont mean squat if you dont have the right springs.... Just my .02

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Ryche:
High RPM dont mean squat if you dont have the right springs.... Just my .02 </div></BLOCKQUOTE>

Well yes. If it all doesn't make it to 5,500rpm (valves, springs, cam, etc., etc.) then of course one of the other weak links will break. Poncho's tend to have bottom end bearing oiling problems.

Thanks for the input, right now my heads have new H.O. Racing VS-11 springs. If I put on ported heads, I am sure I will use adaquate valvetrain parts. I am more worried how the bottom end will hold up.

ive spun a rod bearing hi revving for too long...

I'd have to agree with the above answers. If you do go with better flowing heads, I would personally do a couple minor things to the bottom end- not serious modifications, for oiling of rod bearings. I'd check oil pump condition, bump the pressure relief to about 65-70 PSI, chamfer the rod journal oil holes a little, and use grooved-top-half main bearings (see prior posts about grooving the ends of the bottom halves). And of course pay attention to all the usual bottom end measurements- bearing clearances, roundness of journals, trueness of rods, rod side clearances, etc., and you should be good to about 6200-6300 RPM.