High Engine RPM Weakest Link

[Born30YrsLate]

Got a question for you engine builders out there. What is the weakest link when building a high rpm big block...let's say 7000rpm range. My initial thought was the stroke, but I was told by another person that it was bore diameter because the larger the bore diameter the lower the natural frequency. I really don't want to go through all the calculations of shear stress and strain or for determining natural frequencies so I thought someone has to have lived it or have some working knowledge of this subject.

[Salvatore]

I would say the valve train may be the weakest part.

[larry_csa]

I would have to say rods. The bigger the bore the heaver the piston the more stress on the rods. 396s are turning 8,000 to 8,500 427 are in the high 7s. I have a L-88 coming off the Dyno this week or next and I'll let you know the HP range

[3macs1]

I'm with Sam. Any big block we destroyed racing usually valve train started the distruction. Years ago stock engines were known for dropping valves.

[larry_csa]

10 deg valve locks and good valve springs will stop that. Valve flutter is what drops valves

[3macs1]

Agree 100%. I was referring to stock BBC. They ate valves.It was rare to see a rod failure come back to the shop unless stock bolts were used in the rods. Oil pump pick ups cost a lot of high rpm engines as did the plastic coupler on the pump drive shaft.That was years ago however not up on it today.

[69z2x4]

I will have to agree with valve train and rod bolts. I took a 454 LS6 out of a 3 year old Chevelle and changed the pistons, cam, springs, rod bolts and had that motor in three different Camaros over the years with only one rebuild. Shifted it at 7000 rpm and it went through the traps at 7000. Sold it to a guy in IL. One in a long line of my many big mistakes.

[Keith Tedford]

In the L72 and L78 engines the valve springs couldn't keep the valve in contact with the valve train at much over 6K rpm. The valve would actually be free falling shut. Eventually the keepers would pound themselves through the retainer and you got a dropped valve and usually a broken block. That's how most L78s died. I must have had horse shoes as our L72 engine saw 6500 countless times. My racing buddy just shakes his head and cringes when I tell him the stories. http://www.yenko.net/ubbthreads/images/graemlins/blush.gif I never heard of any bottom ends letting go in street use.

[nuch_ss396]

Keith,

What spring retainer, keeper, and spring combo's would you
recommend for a stock L/78? I'd like specifics if possible.
With all the $$ I have wrapped up in my current L/78 project,
I'd hate to drop a valve and trash my engine.

Steve

[WILMASBOYL78]

Steve, sent you a PM regarding parts.

...if weren't for weak valve springs, there wouldn't be so many CE motors http://www.yenko.net/ubbthreads/images/graemlins/naughty.gif

wilma

[3macs1]

Share with us wilma. I would like to know what is being run today .I know what we ran years ago with the zl-1 cam in the l-72 and like Keith I never lost one. Only BBC I blew up was one of those black crate ls-6's and it was my fault for not talking it apart prior to racing it.

[Keith Tedford]

We used L88 springs although there may be better springs available. I doubt that an L72 or L78 were turning their best ets at rpms over 6K anyway. A friend's L78 with flow benched heads, and a better cam turns its best ets shifting at 6K. Over or under and his times suffer. Take it for what it's worth. Our L72 saw 7200 a couple of times, so these engines certainly are capable of the rpms. Time slips tell the real story. Most people might be very surprised at their ideal shift point.

[larry_csa]

Sorry: I misunderstood the question. I thought we were talking about a Hot rod motor.Valve spring pressure is a major problem with BBC. too much pressure and you roll a lobe too, little and they won't run.