Peak Torque High vs Low RPM

[TARM]

As we recently had a great example of the kind RPM levels we can reach peak TQ and HP at by Pontiacross engine setup. By the use of the guys at Swamps' engine dyno and CP testing equipment and excellent injectors we saw some impressive peaks on a stock bottom end. Also not a case of just a few runs that it held together but instead 63 WOT runs in one day.

It has long been held that by raising where your peak TQ hits it can greatly effect whether a engine lives or ends up in pieces. Long have we heard that if you bring peak TQ in down low all you do is break engines. The idea is that tq at low rpm puts more stress on the engine. While I think most all on this forum subscribe to this idea I wonder how many actually understand the real reasons of why?

Before we go any further there is a very good chance that this discussion could fall into yet another TYPICAL thread on what is HP and what is TQ. If you do not really understand the difference or prescribe to some of the old cliche created by those that do not understand it such as "HP makes you go fast TQ is what you feel" or "HP sells cars TQ is what wins races" Then do some searches on PSN or PSA and google. There are a number of sites that explain it in a number of different ways to make it easier to understand. But lets not get the subject at hand diluted and lost in hashing out the HP TQ quagmire yet again.



With that said, lets move on and take a look at some things to possibly consider when thinking about stresses of LOW vs High RPM peak TQ:

Here are some things to consider:

Would you say, forgetting about TQ or even Power for a moment, are High rpms or low rpms more stressful on engine parts? Why?

Is there any difference is the amount of energy it takes to produce the same amount of TQ @ high RPMS versus Low RPMS? Why?

What about producing the same amount of force(tq) @ lower RPMS causes more stress than doing the same thing @ a higher RPM? Why?

Lets see what various peoples opinions and thoughts are on what about producing a TQ peak @ a lower RPM puts more stress than the exact same TQ peak but @ a higher RPM level.

 

[JAP]

This is just my shot at this, because it has been SO long since I applied any knowledge into anything technical...

Would you say, forgetting about TQ or even Power for a moment, are High rpms or low rpms more stressful on engine parts? Why?

I think there is obviously a middle ground here... Too many RPM's on a large rotating mass could easily be detrimental if the balance isn't perfect, but in a perfect world, and in a diesel application, high RPM horsepower and thus torque = less stress due to lower cylinder pressures. Correct?

Is there any difference is the amount of energy it takes to produce the same amount of TQ @ high RPMS versus Low RPMS? Why?

Yes? My thinking here is Newton's First Law. It's easier to produce torque at higher rpm's if the injection system is capable, because it (in my mind) should require less fuel because of the already increased velocity of the rotating mass.

What about producing the same amount of force(tq) @ lower RPMS causes more stress than doing the same thing @ a higher RPM? Why?

This seems somewhat redundant? If not, I would like to know your thoughts?Lets see what various peoples opinions and thoughts are on what about producing a TQ peak @ a lower RPM puts more stress than the exact same TQ peak but @ a higher RPM level.

 

[golfer]

regardless of what anyone thinks... 6000rpm is hardly "high rpm"...

go find a big block that DOESN'T spin 6000rpm off the showroom floor...

so while our engines have heavier than gasser reciprocating components...we're NOT spinning these things to high rpm by gasser standards...

if high rpm were an (reliability) issue with the 7.3L...I'm prettttty sure we'd (all) have seen all the carnage on the interweb...piston pins pulling out of pistons...stretched connecting rods (LOL)...

the problem with the 7.3L is weak rods..weak in compressive strength....combined with a slow injection system...has forced tuners to try and make POWER at the available/conventional "diesel" rpm range...which makes more tq than the rods can handle...

make that same power at higher rpm...by DEFINITION the tq comes way down...more wheel speed (for the pullers), higher shift points (for the racers)...and the ability to spool large(r) turbo's and stay on top of them.

also...since hp is defined as WORK PER TIME (not per rpm)...but hit your stopwatch...TIME...

example:
cut the injection volume in half...and do twice as many injections...you just made the same (probably significatnly more) power at 2x the rpm as the 'other' guy...with less stress on the (this) engine...since this engines Achilles' heel is (too much) torque.

one of the questions was x amount of torque at low rpm vs the same tq high(er) rpm...and why one is harder on the engine than the other...

eh...aside from the fact that's it's unconventional to make peak tq at high rpm... while the physical stresses would be identical on the rod (etc)...the TIME that those stresses are on the rod can/will allow the weaknesses in materials manifest & show a failure.

a good portion of the time spent tuning is limiting our low end torque..again..we're not looking to neuter a truck...but on trucks that can make (do/have made) >2000lbft...big whoop...pull fuel in the lower rpm such that tq is at a reliable level (1000-1100lbft for forged rods is gravy)..and instead of making 1700lb ft, and stressing the engine...just hold that torque as loooong as possible...

and what happens when you make decent torque at higher and higher rpm? your hp just jumped up a ton.

1000lbft at 1500rpm= 285hp
1000lbft at 2000rpm = 380hp
1000lbft at 4000rpm= 761hp
1000lbft at 6000rpm= 1142hp

and to hopefully quell any torque versus hp arguments...

chose what fits your application...but I'm pretty sure this discussion is focused on 7.3L competition vehicles (this IS in the 7.3L Upgrades & Aftermarket section)...not tow vehicles...and not 16L OTR tractors..
with rods the size of my leg...that don't spin more than 1500rpm, and need 12 gears to make use of that. so shut it

 

[TARM]

I just had company arrive at the house so I can not get away for long but I want to point out a few things.


I am not talking about power but actual TQ. I fully agree with the issues with the 7.3.

One thing though HP is work done over time. You say that RPM is not a gauge of work over time but that is exactly what it is as far as I can tell. It is a revolution (work) per minute (Time) That is measurement of work over time.

Other than that I agree with what you have stated Dave.


The question though is one that transcends our motors but is one that has been placed over all engines. When every you get a bunch of gear heads together and ask what level or TQ breaks an engine time and time agains you hear its at low rpms. Now in our situation as Dave has pointed out we tend to try and make much more TQ down low to make up for injection speed. We then have the issue of weak rods.

Let me pose another question then: Is there any difference is the stress placed on the connecting rods for a given TQ amount at 2000 rpms versus say 4000 rpms or do they create the same stress on the rods? We can narrow this down even farther and say compressive stress. I think the answer is yes. I will get more detailed in my reasons and support for this position later. but I will say for obvious reasons Same TQ at a higher RPM creates higher wear rates but at lower rpms creates more stress.


Sorry guys but I have to go spend time with my guests. My better half would have my @ss if she knew I was in the office on the PC posting and spending time with PSA forum members instead of the people in my home. LOL

Be back in a few hours.

 

[golfer]

One thing though HP is work done over time. You say that RPM is not a gauge of work over time but that is exactly what it is as far as I can tell. It is a revolution (work) per minute (Time) That is measurement of work over time.

I wish this paragraph made more sense to me.

if "a" revolution was a metric of work...then we should be making the same ___(power/torque whatever) at 3000rpm revving in park...as we are at 3000rpm at full load/boost...

I wish you had phrased that differently.

 

[Hotrodtractor]

One thing though HP is work done over time. You say that RPM is not a gauge of work over time but that is exactly what it is as far as I can tell. It is a revolution (work) per minute (Time) That is measurement of work over time.

A revolution is not work. A revolution is a distance. Work is a force over a distance. Torque is a force. Power is a force over a distance in a given amount of time.

 

[TARM]

Sorry that is what I meant. I was in a rush and was going to quickly. I meant distance not work.


Since this has not really gone the direction I thought it would I will try to post my thoughts on it


At first I was looking at it that you have same compression pressure curve to produce the same amount of force ( tq) say 1000ftlb whether it be at a low rpm, say 2000, as you do at a faster RPM of 4000. These are only representational rpm figures. So it took 3500psi of CP at each cylinder to create the 1000ftlb TQ Nothing there is going to change from a low rpm to high. Then if you add in all the strain of higher rpms it looked to me that actually the same tq @ higher rpms created more stress.

As I started kicking it around a bit more I began to consider the CP and the actual crank revolutions and angles along with considering the actual instantaneous tq effects. Then I started to look at the actual combustion curve and time and how its spread out over the crank degrees of revolution, angles etc.. That is when things started to come together better or get more confusing.

Considering that no matter 2000rpm or 4000 rpm the speed of the actual rate of combustion is relatively happening at the same speed. Yet at the same time the crank and rods are moving in this example either twice as fast (4000rpm)or half as fast(2000rpm). Consider what that means for the force applied to that piston, rod and crank at the two speeds. The slower the rotating assembly is moving (low RPMs)the greater the TQ spike each part in the assembly is having to deal with as its all happening in fewer crank degrees of movement. When its moving faster (higher rpms)that pressure curve while still taking close to the same amount of time is now being spread out over more crank degrees as the assembly is moving that much faster. Just throwing some numbers out as examples only. Let say the pressure curve @ 2K is barely taking say 20 degrees ATDC to complete but then at 4-5K that same pressure curve is now spread out over say 40 degrees of rotation. Those are just examples to help illustrate the explanation and are not meant to be correct in ratio of degrees to rpms.


This lead me then to start thinking about how the 4 stroke cycles played into all of this as you have one power stroke per 720 degrees at each cylinder. So, even though TQ is not suppose to be a factor of time a 8 cyl 4 stroke engine has to go thru 2 rev or 720 degrees to complete a full power stroke cycle. So TQ as we measure it for engine performance is really an average created over that cycle. With CP @ crank angle I would think instant tq could be calculated. I would have to research to figure out the formula but figure it is doable and likely not that complex. Regardless the conclusion I came to is the lower the rpms the less combustion cycles you have for a given period of time. You also have that much more time between each combustion event and cycle. Yet you still are creating the same average torque @ that rpm. That would mean that the TQ spikes absorbed by the rotating assembly would be that much greater as they have to create the same average tq. As rpms increase the number of cycles increases and the time between them decreases there for it would create lower tq psikes to create the same average tq.

I guess the best way to frame this is that the stresses per cycle of revolution is greater at lower RPMS than at higher for the same mean tq level.


Now I have gone back and forth on this as I look at it from various points. The focus for me is from a stand point of stress on the individual parts of the rotating assemble per combustion cycle. For our engines rods and their fasteners seem to be the weak link so I focus on that and try to visualize the effects there. I am not sure if I am looking at this correctly or if I have over thought it too much to where I have got it all twisted. I may have not explained in the correct manner but I am hoping that those with more expertise in these areas can understand what I am trying to convey. Dave, Jason, Charles and many others of you that I am sure have far more knowledge about this kind of stuff than I.

 

[Gearhead]

Torque=cylinder pressure=compressive stress on rods. Horsepower has no bearing on this and that's why I have several pmr trucks out there above 475 horsepower while some people manage to kick out FORGED rods at 425 horsepower.

 

[TARM]

right I full get that. The question is does TQ at different RPMs cuase different levels of stress. The slower the rpms given that the combustion happens at basically the same speed the less degrees of crank is there to absorb that tq spike. The faster the RPMS the move degrees of crank movement to absorb the same about of tq spike. Or the initial force or instant tq from combustion. If I am thinking about this the wrong way please explain where I have misstepped

 

[CurtisF]

right I full get that. The question is does TQ at different RPMs cuase different levels of stress. The slower the rpms given that the combustion happens at basically the same speed the less degrees of crank is there to absorb that tq spike. The faster the RPMS the move degrees of crank movement to absorb the same about of tq spike. Or the initial force or instant tq from combustion. If I am thinking about this the wrong way please explain where I have misstepped

Torque is torque.

It doesn't matter if it's 2000 lbs/ft at 1000 RPM's or 2000 lbs/ft at 4000 RPM's. It's all the same.

Why? Torque has nothing to do with RPM's. So what you need to understand is that when you are referencing torque alone, you need to completely disregard RPM's. Remember that it's "power" which is a function of torque and RPM's.

In other words, torque is measured at a "moment" in time, not over a period of time, and not over a period of revolutions. As soon as you start measuring RPM's, you're now defining power, not torque.

If your rods break at 2000 lbs/ft of torque, then they will break at that moment, regardless of the RPM's.

Does that help?

 

[golfer]

To make the same torque at higher rpm requires a higher cylinder pressure.

 

[Gearhead]

To make the same torque at higher rpm requires a higher cylinder pressure.

Peak or Average? Comparsion of compressive rod loading? Linear relationship?

 

[TARM]

Thank you now this is what I am speaking of. I figured Dave as you have the ability to read cyl pressures and I am guessing seeing the actual pressure curve of the combustion event and you can also see where the crank angle is at during the various points in that cycle you would have good insight in to all of this. That is IF I can word what I am wanting to get across correctly so that you can actually answer them LOL

Matt, That is exactly what I am getting at and what kind of got me thinking down this path. There is a difference between what I have seen called instant TQ versus average or mean torque. then looking at that along with where the piston is and how fast it moving would effect how concentrated the tq peak is focused on the piston and thus the rod.


Pocket,

I understand where you are coming from. That is where I started. Tq is tq is tq. But the more I read and thought about it the more it becomes apparent this is not the case. We measure TQ at a crank of an engine. But now consider what is going on at the individual cylinder,piston and rod. Lets assume tq may be consistant at the flywheel do you think that pressure against the individual piston is consistant as its being pushed downward as that fuel is combusting? Or do you think there is a point of peak force from the combustion pressure that is then ever decreasing as the piston is moving downward? Now consider that the time it takes for combustion to happen is relatively the same. But now change the speed the pistons are moving (higher or lower) RPM. Now think how that may change how long or much of that peak force is pushing on that piston. Also there only 1 powerstroke as RPMS go up or down the down time between that one power stroke changes. SO if you are going to have the same mean or average tq @ the flywheel at slower rpms that peak force is going to have to be greater to get that same mean tq. This is at least how I am seeing it based on the data I can find and engine diagrams and animation.

 

[golfer]

Peak or Average? Comparsion of compressive rod loading? Linear relationship?

peak, since at high(er) rpm, the CP drops off very quickly with the speed of the piston.

 

[Gearhead]

I'd be curious to see an SAE paper concerning force vs time loading on connecting rods. I am led to believe that a higher load can be had for a short period vs a lesser load over a long period.

 

[389sixpack]

Plastic deformation in 1ms or 10ms is still deformation. The longer the applied load, the more deformation which will be cumulative until failure... I can probably find a paper on steel projo bodies, but not connecting rods - still the same effect.

 

[389sixpack]

Matt, Unless you're talking about elastic deformation, and the initiation of the cyclic stress and propagation of a stress crack??

 

[Hotrodtractor]

This is why fuel burn rate and combustion chamber efficiency is so critical. Anything that can be done to increase the burn rate, improve combustion efficiency and improve turbulent mixing of the cylinders can allow these motors to be turned harder and turned faster. The more efficent the burn (well atomized, highly mixed, fine droplets) the faster the burn, the faster the burn the more easier it is to create and maintain cylinder pressure at higher RPMs thus giving you more torque in the higher RPMs and higher power as a result.

This is also why at lower RPMs (say cruise and mid throttle positions) it is best to keep the ICP in the mid range and lengthen the PW out a bit - the decrease in combustion efficiency actually improves performance by lowering the peak cylinder pressures and prolonging the burn so the cylinder pressures are acting on the pistons for a longer period of time. Its all about the burn rate and how to control it.

 

[Gearhead]

Plastic deformation in 1ms or 10ms is still deformation. The longer the applied load, the more deformation which will be cumulative until failure... I can probably find a paper on steel projo bodies, but not connecting rods - still the same effect.

I'd have to go back and look at my text from way back, but I remember seeing something about sharp forces (time wise) vs lesser forces being applied over a longer period with the hardened steels and the latter able to bring about more deformation.

This is also why at lower RPMs (say cruise and mid throttle positions) it is best to keep the ICP in the mid range and lengthen the PW out a bit - the decrease in combustion efficiency actually improves performance by lowering the peak cylinder pressures and prolonging the burn so the cylinder pressures are acting on the pistons for a longer period of time. Its all about the burn rate and how to control it.

This concept is still lost to some. Also the parasitic losses from the increased pressure from the pump come into play.

 

[Gearhead]

Ever tried to hammer a nail in with a hammer vs a hydraulic press with the same amount of force? Especially when the angle isn't perfect, the same force applied over a longer period has a better chance of bending the nail vs just hammering it in. If you shock steel fast enough, it doesn't have time to react like it does when you apply that same force over a longer period of time.