I was blowing down the road and I usually cruise at 3500 RPM, why, I don’t know. It is a happy medium and you can hear the radio. I was wondering at how the GM engineers arrived at the set redlines and the yellow caution areas on the tack. Obviously the engine is screaming and this cannot be good for the motor right! Well, there is not that much difference between 4500 RPM and 6500 RPM in terms of the motions of the rotables, (pistons, crank, valve train, etc). Is there a destructive resonance that occurs, or is valve float the concern? Or does some fudge factor for warranty come into play.

Mark

 

The simple reply is piston speed. With a stock cast rotating assembly, you can only put so much stress on the crank and rods before something goes. Upgrade to forged crank and rods, and you can put more stress on the parts before failure. I have read that for stock parts, a maximum of 3,500 feet per minute or less is about right. If you use a forged crank with heavy duty rods and bearing caps you can probably get away with 3,800-4,000 feet per minute.

Notice that a shorter stroke can safely rev much higher because of the lower piston speed at a given rpm. Thats why you can rev a 327 (3.25" stroke) to 7,000 rpm and maintain about the same piston speed as a 350 Chevy (3.48" stroke) at around 6,500 rpm, or 6000 rpm for a stroker (3.75" stroke).

Crank stroke x rpm divided by 6 = Piston speed in feet per minute.

In reality it is the acceleration when the piston has to change direction that causes the stress, but this is directly related to piston speed, and not as easy to calculate. I found a post on the Nova list where someone calculated forces at different RPM levels. The stock 3.48" stroke puts about 1600Gs at 5000 rpm, 2300Gs at 6000 rpm, and 3100Gs at 7000 rpm. You can see that the force is about twice as much at 7000 rpm compared to 5000 rpm. It is this massive force that breaks stuff at high rpm.

 

Nic pics.... certainly drives the point home.... :smash:

but don't forget that with hydrolic lifters high rpm's tend to make the "valves float" and do a dance on the tops of the pistons.... solid cams can handle higher rpms because they do not have this float problem.... :cheers:

 

The real "killer point" is TDC at the end of the exhaust stroke - when the piston changes direction at that point, it isn't compressing anything, and the tensile stress on the rod is much higher than at the TDC direction-change point on the compression stroke.

Valve float on our engines seldom results in valves kissing pistons, unless a valve spring breaks or the keepers come out, as they're designed with valve clearance at TDC; most import performance engines, however, are "interference engines", where there is NO clearance between valves at max lift and pistons at TDC, and a broken timing belt results in a LOT of engine damage. Imagine the valvetrain dynamics of a current ("interference") Formula 1 engine running at 17,500 rpm - that's why they use a computer-controlled closed-loop 3,000 psi compressed air system to close the valves instead of springs (!).

 

The OEM redlines are based on valve gear limiting speed - lifter pump up on hydraulics and valve toss on mechanical lifters.

Bottom end life is usually a fatigue issue. Loading on the bottom end increases with the square of engine speed, so the more hi rev time, the shorter an engine's life before a catastrophic failure can occur. At 7000 revs the bottom end stress is four times what it is at 3500. Also, oiling is critical, and catastrophic failures can be initiated by oil starvation at high revs. The rods on early SBs are the weak link. The small bearing rods received an improvement circa '67 with that little hump of material you can see next to the bolt seats in the picture of the broken rod that Brad posted. Earlier rods did not have this, but even with additional material at this critically stressed point, failures can occur. SHP SBs did not receive upgraded rods until the 350 era. All 327s used the same rod and the pre-'67 rods are decidedly weak.

The later large bearing rods are superior, especially the "pink" rods used on mechanical lifter 302s and 350s, but if I was building a SHP or equivalent engine today, I would buy a set of Crower Sportsman rods - small bearing or big bearing depending on your crankshaft. This will make the bottom end bulletproof as long as it gets oil.

SB rod failure usually occurs adjacent to the bolt seat, but in the picture it's a bit above and symmetrical. I don't think the pictured failure is a common fatigue failure. I think it might have been initiated by oil starvation or a significant overrev like a missed shift.

Duke

P.S. I believe GM's most severe engine durability test is 200 hours at full load cycling between the torque peak and redline. That's eight days and eight hours of continuous WOT operation at high revs. The ZR-1 got this test and my Cosworth Vega also received the same test, but it might have been only 100 hours. Rather than using an electric motor, Cal Wade, the late Chevrolet engineer who headed up the CV development project used the engine itself to conduct the 10,000 rev flywheel burst test.

I don't know what the durability test was back in the 60s, but John Z might know. The only trouble with this test is that it is done on a dynamometer, so there are no dynamic loads that tend to starve the oil as what you see on a track or very spirited street driving.

I just finished reading Dave McClellan's new book and he stated that the ZR-1 program was delayed about six month, because oil starvation at high revs due to insufficient oil return from the head and crankcase windage forced a complete redesign of the engine!!!





[Modified by SWCDuke, 9:25 AM 8/23/2002]

 

Interesting to read about automobile engine testing. The tractor company I worked for in my youth had developed a test for their HD engines that consisted of the following:

Set both full open govener speed and fuel delivery 10% higher than the planned production setting (this would result in 17-20% higher power). Then run at full load for 25 minutes and then idle for 5 minutes. Repeat for 500 hrs.

We needed some higher hp engines than were practical for us to design, test, tool, and build so we approached a major engine company (they are the ones that always painted their machinery yellow). They proposed their new (at the time) V-8 diesel but got real nervous when they heard of our qualification test. Their concern, as I recall, was mostly due to the heat cycling between full load time and idling time. Seemed liked the pistons and heads were "on the edge". They did agree to it, however, and it did pass.

 

Damn good forensics on the4 failure! Since it can be hard to tell from the aftereffects of a violent failure, I don't know for sure what the exact secquence of events was. I was on the track Buttonwillow), but not at 7000 RPM. Just as I got through the esses, I saw some steam billow out from the hood. The car was making a bit of a strange sound, and I called over some of the others to listent and nobody could identify it. I checked my oil level and it was OK. Once the car had cooled, I added water and started the car. It ran (still making the odd noise) and I figured the engine was terminal, the only question was how soon.

Someone volunteered to follow me home, so we left and I drove at about 60. All the gauges looked OK, but about three miles down the road, BANG! I decided that it was time to call the flatbed for the long trip home.

Pulling it apart, I found lots of parts! There was water in the crankcase - and since oil is lighter than water, I was likely pumping water through the engine for those few miles and this is what caused the rod failure. The rod failure is what made the noise (and the nice imprint of the top of the cylinder on the combustion chamber) and made lost of metal pieces. But, I also had a broken lifter for #3 (#5 rod failed), and #7 and #8 had lots of water in them. #5 cylinder had a nice sized crack in the wall as well.

So, what happened first? My guess is that the cylinder holed, causing water to get into the crankcase. The noise might have been the piston rubbing the cracked cylnder wall, combined with water lubrication noises. My best guess is that the lifter went when the rod broke and impacted the crankshaft, and the water got into the other cylinders as the car sat since there was no evidence of hydro-lock effects.

 

It can be tough to determine the cause of a catastrophic engine failure, but if you have some experience and know what to look for, a most likely cause can usually be established.

Just based on the picture it appeared to me that the broken rod was an "effect", not the primary cause. The top of the rod shows what might be a bending failure, and this can be caused by a seized bearing - evidence of which will be visible on the bearing shell and crankshaft journal.

If a bearing seizes due to lack of oil - and water instead of oil can seize a bearing - the rod will see a tremendous bending load that can cause a bending failure.

The only engine I ever blew was in my E-production TR3 back in '77 when it broke a rod and threw part of it through the block, My investigation of the failure concluded that the most likely cause that initiated the failure sequence was a broken rod bolt.

In my professional engineering career, I was involved in some failure analysis work, so I have some experience on how to work through the evidence and come up with a likely cause.

It's too bad you didn't call the flatbed rather than trying to drive home. The damage would have been a lot less and you would probably have been able to save more of the engine. It sounds like there wasn't too much to salvage. I guess lesson is that if the engine starts emitting steam and making strange noises, you better call the tow truck sooner rather than later.

Duke