I don't doubt what you are saying, but I still stand by my statement: "High RPM is TOUGH on an engine." It will most definitely decrease the life of ANY engine.

Yes, the small Fords have really slow piston speed, making them a more revable engine, but high RPM puts amazing stress on all rotating/reciprocating parts of an engine, most especially the connecting rods.

Spinning up a 289 occasionally is no problem, but constantly running it to 6,500 will most definitely effect engine life.

 

The flip-side of that is that high torque is hard on everything else. Those tuner cars that make bookoo power from little motors and turbos usually don't have enough torque to break the cheap drive train parts but they make lots of power at the top end. Drop the clutch on a 428 CJ at 5000 RPM with slicks and you'll stress a few parts (and maybe twist the car a little too). But I know this is somewhat off-topic!

 

It appears that we are beginning to compare oranges to apples. The little rice grinder engines have much less reciprocating mass thus lending themselves to high RPM operation. With those motors 6,500 might be routine, but there is some level of high RPM that will shorten their lives as well.

It wouldn't take a 428 to tear up drive train components by side stepping the clutch at five grand. A 289 could do the same thing albeit with a little less frequency. Side stepping the clutch is done either in a serious drag race car, or just simply equipment abuse. I started to say it was a driving style issue, but spinning a 289 to 6,500 often would also a driving style issue.

Spinning the 289 to 6,500 constantly, IMHO, borders on abuse, much the same as side stepping the clutch at 5,000.

I might be 60 years old, but I still drive my Mustang GT like a kid, but that stops short of constant over revving or side stepping the clutch at high RPM.

 

High rpm is WAAAAYYYY harder on an engine than torque is. WAY harder. Torque is just straight force, but rpm causes the mass of the moving parts to generate force on them that increases exponentially. Twice as much torque means twice as much force on parts(approx), twice as much rpm though means 4 times the force(just from velocity) for the same mass....only you need beefier parts to turn high rpm so they have more mass AND more velocity. And that's not even considering additional force generated from acceleration, every 1g off acceleration increased total force by the weight of the part being accelerated.

Turning high rpm regularly isn't abuse IF the engine is built to take it. Stock 289/302 rods, while they CAN turn high rpm, are unlikely to do so reliably when turning high rpm on a regular basis.

And creating torque is actually substantially less stressful on an engine than rpm is, and here is why. Torque isn't generated suddenly, combustion takes time so pressure builds in the chamber over time and slowly begins to push down on the piston and rod and crank(even though this can all occur in a small portion of a second). They can handle that, they're designed to. The piston will move around, the rod will flex, the crank will flex....but that's how they're designed. But the force is applied over time and then released, and it's not really a huge amount of force either.

At high rpm however the end of the exhaust stroke is where 90% or more of rod deaths occur. The reason is as the piston comes to the top of the exhaust stroke 1 or more valves are open and there is no resistance against the top of the piston. Velocity wants to make it keep going up through the head....but the crankshaft rotates around and the big end of the rod is then pulled in the opposite direction. The combination of the piston moving up the bore rapidly combined with the sudden change in direction at the big end of the rod creates forces pulling the rod in the opposite direction. The rod literally gets yanked, very suddenly, in opposite directions at both ends of the rod. This creates MASSIVE amounts of acceleration in g forces that multiplies the force created from the mass of the parts. Remember that lbs are created by 1g of acceleration(the Earth's gravity) times the mass. The sudden direction change at the top of the exhaust stroke in a performance engine often generates thousands of g's of acceleration, multiplying the force due to the weight of the parts thousands of times over...due in part to the exponentially generated force from the increased velocity(inertia) of turning higher rpm. That results in massive tensile loads on the rods, and if the material is not designed for it, or is continually subjected to those forces at it's limit of strength, the part will either fail outright due to exceeding the tensile limits, or it will fatigue until the tensile limit is lowered at which point it fails.

The extreme example of this is of course, a Formula 1 engine. They're now limited to 18,000rpm, and previously were turning over 20,000rpm. At 20,000+rpm they were generating 11,000g of acceleration at the top of the exhaust stroke, and even at the 18,000rpm mandated limit, still produce around 9,000g of acceleration. A cup engine on the other hand produces less than 6,000g of acceleration. The parts in the F1 engine must be much smaller to limit the total load....and the F1 engine is still subject to failure much more often than a Cup engine, but the Cup engine produces more than twice as much torque.

Forces generated by rpm increase MUCH more quickly than forces from simply making more power at the same rpm. That's why higher rpm performance engines are prone to failing much more frequently than larger lower rpm engines. Think about it, f1 engines(145cid V8) that produce 750hp and around 210lb-ft of torque fail FAR more frequently than an NHRA Pro Stock engine which is a 500cid V8 and produce around 1,350hp and a peak torque that's in excess of 650lb-ft.

RPM will kill an engine much faster than power will, that's why lower rpm blower engines don't spit rods out the side of the block like high rpm engines do.

Also, many of the newer import engines turn more reasonable rpm with blowers, less than 7,000rpm, and are much less prone to failure.

 

67mustang302,

Thanks for the detailed back up. Most people don't understand the EXTREMES that a connecting rod has to deal with. At high RPM when that rod goes from being compressed to being under sudden tension it is dealing with a tough proposition. I know that there is one whole course of a Mechanical Engineering curriculum that pays lots of attention to the stress problems of a reciprocating member. There's probably not a better example of this than a connecting rod in an engine.

 

I didin't intend to change the direction of the thread. My point was not to argue that high RPM is okay on the motor but torque isn't. I agree fully that high RPM is not good on a motor, especially run that way often. And I wouldn't recommend spinning a 289 to 6500 very often either (if at all). I was simply making an observation that while high RPM IS hard on engine parts, high torque (from big motors not wound so tightly) often breaks DRIVETRAIN parts (the 'everything else' I mentioned, NOT engine parts). So if you drive it like a drag car, you'll shorten you engine life by high-revving a smaller motor, but just as likely to be fixing transmissions and things like that if you abuse a car with a larger, lower-revving engine.

It's the nature of 'performance' cars, or performance driving, to be fixing things now and then (sometimes more now than then :-) ). If you are going to drive that way, you had better build your engine and drivetrain to handle it.

 

bottom line is you can fairly easily get 300hp out of a 289 if you are willing to take the time and spend some money. IF the OP wants more info on doing it, Mustang monthly had a great article a couple months ago where they got over 400hp out of a 289 without spending an arm and leg. That would be good reading.

 

Yeah, high torque output will break drivetrain parts, but what usually kills them really fast is shock loading, which comes from dumping the clutch. Even with low torque the shock load generated from a clutch side step can be multiplied many times over and break the trans all the same. And a higher rpm engine usually runs more gear and also ends up running the transmission at higher rpms, which means more heat in the gears.

The reality is one way or another performance engines can break lots of different things. But drivetrain parts are usually cheaper and easier to fix than engines(on street cars at least), and it's fairly easy these days to get very robust transmissions.

But building an engine to take high rpm regularly starts to rapidly consume money. I have $300+ into my rods alone and another $600 into my pistons. But, I'm not worried about blowing my bottom end up either when I run it out past 6,000rpm.