Internals HP/torque vs. stress theory
 

*Please read the following throughly*

I have a question regarding the theory of stresses acting on the internals of an engine. Particularily the VQ35 in our 350zs.

I am going to make up a hypothetical situation in order to see if my thinking is correct about this. The situation involves two VQ35 engines. Both are turbocharged.

Constants of sitauation
1) Each engine is pushing a peak power of exactly 400whp and 400lb/fts
2) Same exact compression between each motor
3) Exact same displacement
4) No power adding differences between motors what so ever (i.e. turbo size, pullies, or plenums.)
5) Exact same boost pressure
6) Exact same A/F ratio

Variables of the Situation
1) One engine is making its peak power at 4300 rpms and one is making its peak power at 6100 rpms both dropping off after that point and losing power

In the VQ engine all power is derived from the combustion which takes place in each of the cylinders. This means that there is no supplementry form of power such as an electric motor (i.e. hybrid car) or other form of energy. Given this fact and the given situation I have come to this conclusion. If this situation were to happen hypothetically, according to my reasoning I would think that there is more stress on say the connecting rods in the engine producing its peak power at 4300 rpms.

My reasoning is that the hp and torque numbers are derived by the power of each explosion in the cyclinders. So there for there are more explosions per second in the 6100 rpm motor than the 4300 rpm motor. This would lead me to believe that the hp/torque value of each explosion is higher in the 4300rpm motor than the 6100 rpm motor

math:
400hp/6100 rpms= .0655 hp/rev
.0655 hp/rev / 6 cylinders/rev= .0109 hp/cylinder

400hp/4300 rpms= .0930 hp/rev
.0930 hp/rev / 6 cylinders/rev= .0155 hp/cylinder


This leads me to believe there is more stress on the internals in the enigne making powr at 4300 rpms.

this is just a hypothetical situation. Please give me your input.

Thanks, you just gave me a headache.

JK, but I get what you're saying. For a motor to make equal hp at a much lower rpm, there must be more pressure in the cylinder.

Im not sure what your getting at? You basically described the difference between a turbo setup and SC right?

Turbo makes full boost and Trq right away where a SC'er makes full boost and HP at the top of the RPM band.

Good thinking,but you are mistaken, for a number of reasons. The engine producing power at 6100rpm is ALL ELSE BEING EQUAL, more stressed than the engine making power at 4300rpm.
Some basics:
Engine rpm is a whole lot more destructive than boost. The inertial forces acting on the piston at 6100rpm are FAR greater than the boost pressure acting on the piston at 4300rpm. Add extra compressive force from the boost at 6100rpm, and the loads are increased in comparison to the motor at 4300rpm. As an example, in a high revving NA VQ the acceleration of the piston from TDC on the exhaust stroke at 8500rpm is 29,663m per second squared. The inertail loading from that puts like 6000 lbs of stress on the conrod. Boosting 400whp at 4300rpm would never even come close to that.
Second, boost will never put that load on because the "bang" isn't instantaneous. While fully compressed, the extra air pushed into the cylinder will not ignite all at once, in fact, the piston will be a third of the way down the cylinder by the time the greatest increase in combustion force occurs from boost, almost by the time the conrod is at 90 degrees to the crank throw, which is why boost makes so much torque. This is also why cramming so much extra air doesn't apply that much more loading onto the piston, because the compression force is offset by the expanding room for combustion as well as the application of inertial tension forces pulling the piston in the same direction the combustion process is pushing it. Convienent huh? of course, this is why igniting slightly before the piston is at tdc produces and extra "push" on it, and too early produces and extra hole in the block.
So, in your example, all else being the same, producing power higher in the rpm will be more damaging than the lower rpm. Now if you really want to compare SC vs turbo and which is more stressfull, then there are a lot more factors involved.
Will

Are you refering to trq peak or hp peak? No VQ35 should be making peak power at 4300rpm, unless there is something seriously wrong with the tuning, or the build-up and setup of the motor.

An engine produces trq, and we derive hp from the trq number. Trq should be the basis for your hypothetical scenario. :D

In any event, an engine produces peak cylinder pressure at the trq peak, rather than the whp peak. However, interial loads are greater at higher RPM. THere are several forces and stresses acting on the motor, and your hypothetic doesnt address them, so this question cant really be answered.

I am not a mechanical engineer, nor do I claim to be one. But cylinder pressure, and intertial loads are just two piece of this equation.

In the most basic of terms, and engine that makes peak trq at 4100 rpm is under more stress than a motor making peak trq at 5100rpm. That said, intertial loads are significantly greater at 5100rpm vs. 4100rpm. So again, this question cant be directly answered. :(

The president of Spoon talked about this in a JDM video I watched. Resolute's post #4 is right on. An increase in piston speed creates exponential lift in cylinder pressures. For example, if cylinder pressure at 1000 rpm is X, then cylinder pressure at 2000 rpm is more than 2*X.

You can see this theory in action in the S2000 motor. The amount of work done to get it to safely rev to 9K rpm is amazing.

This is not true at all, especially for an FI car, and even in the case of an NA car. For instance, if are 1/4 throttle at 5000rpm, cylinder pressures are far lower than 3000rpm and 5psi of boost.

Intertial loads increase as RPM's rise, but cylinder pressure may or may not increase as RPM's rise...in fact....cylinder pressure is highest at the trq peak, rather than at redline.

After reading it, that was my take too. Then I realized it was just a question based on accelerated stress, I think :)

Ok, after reading it along with some of the responses and changing my answer a few times (because of all the open variables) this is my take on it.

I concur with you (quamen), I believe there is more stress on the motors internals when going from standstill to whatever at an accelerated (pressure/boosted) rate. IMO, it’s quite simple, the physics that is… faster acceleration applied to whatever, will always require more energy, which can be broken down into many forms. The motor creating X amount of power at 4300 will create more accelerated stress than the motor creating X power at 6100. But in the end it’s the same, the same amount of wear and tear and energy consumed. One created more energy sooner, thus more stress early on.

After thinking about FI and racing over the past year, I have come to the conclusion that I can do both safely, ie with the Zs longevity in mind. IMO, our cars were not designed to handle all the power produced my FI, they’re just not engineered for this, ie up-grade extravaganza with anyone over 8psi. But I think you can have both. You just have to do more roll on racing, thus eliminating the accelerated stress on all the stock components!

Inertia is your friend against accelerated wear. Yes, if you do it often enough though, all things become equal… LOL!

Zquicksilver

Then why can't we rev our engines to 10K rpm on a stock block? :icon8:

Inertia is NOT your friend, it's the enemy for longevity. without turning this into a SC vs Turbo, boost vs NA, and without looking at loads on the drivetrain, susp, and other parts of the car... just focusing on the engine and what will cause more stress on the reciprocating assembly, it will be the the higher rpm. It is the inertia of the piston at 35 meters per second that makes it so stressful to stop and accelerate back the other direction over and over again. The stress placed on the conrod is caused from inertail loads, it switches to compression stress, to tension stress, and back again as the piston is pushed and pulled up and down the cylinder. The faster this happens, the greater the acceleration to change direction, and the loads induced from this rapid accel are far more than anything applied by the compression forces of the combustion process.
halitosis, I think you have the right idea about inertial forces increasing exponentially with rpm, but the wrong specifics, Sharif is right. Peak torque occurs when the Ev of the cylinder is highest, this means more "air" is crammed into the cc per swept volume of discplacement than at any other time in the engine's working rpm. this also means that the mean effective pressure , the all important MEP, acting on the piston will thus be highest at this rpm and is why the peak torque is at this rpm. The cylinder pressure is not increased or decreased with a linear relationship to rpm, it is affected by the Volumetric efficiancy, which builds up to and drops off from a particular speed based on a number of variables.
The MEP of the engine, or the amount of "air" sucked into the engine to be burned, is not a very large influence on the compressive stress of the connecting rod. In fact, as stated earlier, the connecting rod is actually having tension forces applied to it as it accelerates away from TDC on the powerstroke, and is so aided by the compressive forces of the combustion. This compressive force is not instantaneous, but applied over the length of the power stroke. Therefore, boost is not all that bad for an engine with the key being "ALL ELSE BEING EQUAL". Therefore power, not even torque, applied with a peak at 4300rpm, while low for hp (high for a diesel i guess if that's what your comparing) will cause less stress than the same motor revving and making peak power at 6100rpm.
Finally, some might wonder why the power peak is important over the torque peak for this example, and the reason is it doesn't really matter when it comes to this example. The question is about load, and the rpm is more detrimental than the torque created by the max MEP value when it comes to piston loads. So a VQ wich putters out at 4300rpm will still be making torque over an increasing number of rpms as the revs climb, the same as a VQ making torque over 6100rpm will continue to do so with less efficiency as the revs climb, and in either case, the diminishing MEP of the cylinders will not be the limiting factor of engine reliability.
Will

EDIT: consider this also, that the effect of MEP and boost are independent in a turbo, the cylinder pressure increase will peak at the combined peak of the engin'e Ev and compresser efficiency. And since the compressive loads from boost are spread over the entire power stroke, until the ex valve opens of course, this is why raising the CR of a motor, is NOT the same effect as boost. Compression loads are not a big deal for piston loads, it is the inertial loads that are. If I can find a link, I will post a great article for the SAE by Garret on race engine durability with low rev turbo motors and high revving NA motors with the same specific outputs. Great reading for those who want more info.

You most certainly could free rev the motor to 10,000rpm, as long as your valve springs provide enough seat pressure to close the valves fast enough. I would be willing to bet you could free rev a completely stock motor to 10,000rpm.

But apply some load to that (ie...drive the car), and it will grenade the motor.

Sharif and Resolute thanks for clarifying what I could not explain... good write up.

While we are on the topic, how are F1 engines fundamentally different from consumer engines to allow for redlines over 12K?

See this and related links
http://auto.howstuffworks.com/question381.htm
http://auto.howstuffworks.com/champ-car13.htm

The short answer is, in every way. They have spark plugs less than an inch and use the piston crown to conduct for a spark, they have rediculous rod lengths and super short strokes to keep the all-destructive piston acceleration low. They rely on high revs and as many pistons as rules allow to make hp from a set discplacement. They use barrel throttles and as many valves as rules allow controled by hydraulic actuation to vary timing and lift. And they don't last longer than two race weekends. If that:)
Will

Good discussion........

Resolute,

I agree inertia is bad (the way you’re viewing it), that's why I said "if you do it often enough though, all things become equal". Of course more repetition (tugging and pulling) is going to create more wear and tear. I agree with the points you made on rpm, friction, compression, tension, wear and tear on the components of a motor... But I was speaking of accelerated stress in the form of pressure, within the cylinder, created by boosting. And its ability to create pressure between parts ie bad friction of some sort in the end. Based on your response though, maybe it’s not as bad or harmful as a I thought. This was my answer to what I thought was the question originally posted. :cool: Too much pressure can destroy things or make diamonds... LOL!

Also while speaking of the top of my head, I was considering wear and tear on other components while making my point. Which I shouldn't have!

In the end, on an average Z, I would prefer a linear TQ curve slightly better than stock. It's just more predictable...

Zquicksilver

Great read. Got a link for that?

+1, that would make a good read...