It is not a question of a big block being restricted to low RPM. To prove this just look at what is under the hood of many Pro Stock cars in the Chevy camp. They all run a 500 cube big block Chevy and all of them leave the line at 8,600 and shift above 10,000 RPM. So why can they do that and not you?

They go in a different direction than most engine builders. A Pro Stock engine has a bore of 4.765" inches and a stroke of only 3.500" inches. So it is in effect a slightly over-bored SBC 350 engine. This is why it can rev like a small block.

As the length of your stroke goes up so does your piston's speed in the engine. The piston still has to go from the top of the cylinder to the bottom of the cylinder twice to complete one cycle. The longer the stroke the faster the piston has to go to get there and back in time to start the next cycle.

This causes two issues when attempting to spin a long stroke motor to high RPMs. The first is one of kinematics (good old Newtonian physics). There is a tremendous tension in the rod, on the piston pin, on the rod bolts and on the oil film that keeps the bearings from touching the crank as a rod is stopped dead at the top of the bore. Similarly the same parts are in tremendous compression at bottom dead center when the crank tries to accelerate the mass of piston, rod, and rings from a dead stop to several thousand feet per minute (faster than most bullets). While the piston is whistling up and down the bore at mid stroke it is traveling at ridiculously fast speeds. Which brings us to the second problem with high RPM and a long stroke engine.

We expect the piston rings to seal the piston to the cylinder on the power stroke and for those same rings to reverse direction and flip over to scrape the walls of oil while still asking them to seal a hard vacuum to suck in a fresh charge. The faster the piston travels the less chance they have to seal (which is why my 582 with it's 4.385 inch stroke never made the one thousand normally aspirated horsepower I expected of it).

This doesn't mean I can not rev the snot out of it. It just means I am wasting time gas and money attempting to do so. The faster and higher you rev a motor the more strain it is placed under and the faster it will wear out. If you use super strong materials to build your motor it should physically stay together as you rev the snot out of it. It just won't be making as much power as you would like it to make because physics is working against you.

Big Dave

 

Thank you for the explanation

Thanks Big Dave, that's exactly what I was looking for. Great explanation! I guess my next question how to figure out what safe RPM range is for my motor? I have a cam card and a dyno sheet as well as a couple of hand written parts receipts (not very useful).

Should I follow the suggestions of 4500rpm limit? The dyno sheet shows them pulling all the way to 6200 rpm, but the torque starts to fall off around 4500.

The motor still has the stock cast iron heads, but I'm not sure if there is any port work is done, I can't find receipts showing any. I do know its got a forged bottom end.

Thanks again, I really appreciate the speedy response. On another note, I wonder if my father was a member on this forum. I know he sold a lot of parts to the Impala and Monte Carlo communities over the years.

 

A BBC has a pretty stout bottom end. The block is strong and even a two bolt can hold up to 600 HP. All big blocks use forged rods, but the PASS engines all had 3/8th inch rod bolts. The HiPerf motors got even bigger rods with 7/16th inch rod bolts.

A cast crank can hold as much power as a forged steel alloy crank can, the difference is in how they fail (which is inevitable, if you race it). A cast crank takes out the block when it lets go; a forged crank gets bent out of shape and begins to vibrate the car apart.

Every big block I have blown up except two were lost to valve train failures, not the bottom end failing (Keep in mind I raced competitively for almost half a century, so I tended to go through motors much more frequently than say your dad might have). I probably lost seven to sucking a valve due to a keeper coming off from harmonic vibrations shaking it apart, or from a valve spring breaking. With others I broke off the rocker arm mounting stud and the pieces fell into the engine. Others bent push rods and the push rods then wrapped around the cam breaking pistons that then broke other parts. All of this carnage was caused by high RPM abuse (aka racing).

I have learned that the secret to building a big powerful BBC is to throw money at it. Buy nothing but premium parts, and pay attention to cleanliness and details to be sure it fits together correctly (this is called blueprinting). That really pays off. I just tore down three engines (including my 582) that I have raced for over five years. Aside from a little piston wear the engines all still looked nearly new inside. My solid roller cam had some visible wear on the lobes, but I was running really stout PAC alloy springs, and modern engine oil is lacking in DZZP. I was actually surprised that the roller tappets lasted as long as they did without a failure.

Your 468 if it had been built by GMPP would have had a 5,800 Red Line, but you could have held it on the mat at 6,400 RPM (were your cam falls over and dies) all day without the engine grenading, because the same engine with four bolt mains and a solid cam has a 7,200 RPM red line. Like I say the bottom end is about bullet proof but the top end is suspect.

When your cam floats that is when push rods get bent, and springs break, or keepers come off. Valve float is a loss of control of your valve train they are floating around bumping into things that they shouldn't be touching like the other valve (they open towards each other) or a piston. I gave up on hydraulic cams when Isky invented his Red Zone lifters. I have run a solid on the street and the track ever since then. With a solid lifter and good springs valve float is just about eliminated.

Big Dave

 

Assuming any care was put into building the motor, pulling it past 4500 rpm should be no problem. If it was built correctly with matched parts, it should be able to rev safely to at least the HP peak on the dyno sheet. Driving around at 5000 rpm all day is a whole different story.

 

Assuming any care was put into building the motor, pulling it past 4500 rpm should be no problem. If it was built correctly with matched parts, it should be able to rev safely to at least the HP peak on the dyno sheet. Driving around at 5000 rpm all day is a whole different story.

That is why I have and recommend an OD transmission. My 4L80e is able to withstand up to 700 horsepower as is without having to add any aftermarket parts other than an increased RPM stall lock-up converter. A TKO 600 will meet your manual needs and it is even stronger. Transmissions are actually rated by torque not horsepower, but no one talks about torque as we live in a horsepower marketing filled world.

Big Dave

 

What idiot told you to keep it under 4500 RPM? Chevy's have always been good RPM engines. Even Pontiacs which aren't known for RPM go to 5500. Big block Chevy's can hit 6000 RPM all day long. Waaayyyy back when, I had a 70 LS5 with the 70 LS6 solid lifter cam and I often hit 7000.

 

Thanks

I know the engine was put together by a world class engine builder, I've meet him and he has over 100 vintage mustangs. He's built a couple of motors for my dad and I am pretty sure it was blueprinted.

I figured it was BS, just a few people in the same circle said that.. I have looked into the TKO 600 but it won't fit the stock console, so that leaves me with the TKO500 or a GV Over Drive which I plan to do sometime next year if money permits. I already have wheels/tires, brakes and a ridetech coilover suspension going on as soon as I have time to install it.

I don't drive the car much, maybe 500 miles a year, and its typically just cruising, no hot rodding. With a car like that, you don't need to drive fast.

Thanks for the feedback and support guys!

 

If you drive less than 500 miles a year it will be hard to justify the price of any OD tranny. You certainly are not going to wear the motor out any time soon (figuring most rebuilds will be trouble free for about 70,000 miles in normal use that puts your motor in need of a rebuild around the year 3413).

Big Dave

 

od tranny

700r4 cut my rpms in half at 50 mph.