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Old 01-25-2017, 12:55 PM   #1251
playslikepage71
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Originally Posted by wadeh View Post
Think of it this way. Engine torque is a measure of cylinder pressure. Torque is a measure of force, and it has nothing to do with being averaged over anything. Power is the effect of torque over time. Consider applying a wrench to a stuck bolt. Applying force to the wrench is torque. But no work is done until the bolt starts to move. I agree that a dyno cannot measure torque directly. It measures power by looking at the effect of the pull on the dyno. It then calculates torque.
YOUR ENGINE IS MOVING. If you take POWER and DIVIDE it by TIME (RPM) you get WORK not FREAKING TORQUE. Torque is the lazy way to describe what's happening. Each piston is doing work and that's being translated through the crank mechanism into torque, but if your engine stops moving, no torque for you.

The reason low RPMs are bad are not because of higher BMEP (that's cylinder pressure averaged over the stroke, and gives you this "torque" figure that has nothing to do with rods blowing up), but because of higher PEAK pressures. When the volume stays small for longer, as in the piston moving slower at low RPM, the pressure goes up enormously for a time, until the piston starts moving down to allow for expansion. At higher RPM, the piston moves much sooner, in terms of time, and peak pressures don't get as high. The ~same amount of energy is added to the system over the stroke, but the high compression force in the rod isn't there because the burn doesn't complete until the piston has retreated.

If you are having trouble understanding how torque and power are truly related, this is a the best video I've found to explain it:

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Old 01-25-2017, 12:56 PM   #1252
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Really informative post, uofime! I've always wondered why people said that low rpm torque is bad. Just seemed like something that was repeated without anybody actually understanding it.
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Old 01-25-2017, 01:07 PM   #1253
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the whole concept of a turbo charged engine is high oxygen due to highly compressed air + lots of fuel = big boom. the way a boosted engine shoves down the pistons is very different from the way an NA engine does it, waaaay more pressure/force. our tiny 2L motors making V8 power isn't magic, everything's got drawbacks. if air was composed of pure oxygen, everything would be on fire, o2 is a very powerful gas.

the stick analogy is good and someone mentioned the amount of time the rods spend at awkward positions had a good point too.

this isn't a simple topic, which is why analogies are good. this involves chemical, mechanical, and electrical engineering. i've done stress analysis and see hvac compressors failing due to high load and low (discharge) pressure all the time whether it's the connecting rod or an oil return issue.

point is... and let's dumb it down, don't WOT at 1500 RPM in 6th gear.
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Old 01-25-2017, 01:56 PM   #1254
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Holy crap, there are some smart mother effers driving WRXs apparently. Great information and write ups. If this info prevents one person from grenading their rex then it was well worth it. Thanks, you brilliant bastards!
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Old 01-25-2017, 02:41 PM   #1255
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Quote:
Originally Posted by playslikepage71 View Post

If you are having trouble understanding how torque and power are truly related, this is a the best video I've found to explain it:

https://www.youtube.com/watch?v=vX8wlkSDwSY
If you want to throw away hours of your life you will never get back... Then get this "game".
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Old 01-25-2017, 03:07 PM   #1256
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Holy crap, there are some smart mother effers driving WRXs apparently. Great information and write ups. If this info prevents one person from grenading their rex then it was well worth it. Thanks, you brilliant bastards!
a large demographic of wrx/sti owners work in the engineering and IT industry. not really the reason i personally bought this car but i find subaru gives you good value and good value usually GENERALLY (i gotta stress this because there are a lot of dumb subaru owners) attracts the more intelligent crowd. combine educated and car enthusiastic people and you get a pretty decent community.

hopefully this information prevents from people grenading their engines but i doubt it. we all do stupid things when we get behind the wheel. it's a machine... break, replace.
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Old 01-25-2017, 03:59 PM   #1257
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Originally Posted by GrownupGoKart View Post
Holy crap, there are some smart mother effers driving WRXs apparently. Great information and write ups. If this info prevents one person from grenading their rex then it was well worth it. Thanks, you brilliant bastards!
I'm not thousands in debt for nothing! I just hope there's not too much misinformation in between the smart mother effers' posts. There's a lot of it on the internet and I see a lot of the old "my uncle's cousin's bloodhound's nephew told me..." crop up here more than I would like. There's a lot of people mistaking generalizations for absolute laws. There's also a lot of high level math and physics that even I balk at when I try and read papers from SAE, etc.

That said, I think we've come to terms with a few simple rules:

1 - The best oil is one that is changed often
2 - Keep it stock or pay to play
3 - Downshift
4 - Ethanol is our friend, but just like people, these cars only like it in moderation
5 - It's just the throw-out bearing

Quote:
Originally Posted by mishapopa View Post
a large demographic of wrx/sti owners work in the engineering and IT industry. not really the reason i personally bought this car but i find subaru gives you good value and good value usually GENERALLY (i gotta stress this because there are a lot of dumb subaru owners) attracts the more intelligent crowd. combine educated and car enthusiastic people and you get a pretty decent community.
I'd say there's a huge dichotomy in the WRX/STi crowd. If it looks ricey and runs like ****, they probably aren't an engineer/IT professional. That isn't always true, but my engineer friends all have really tastefully modded Subarus. More often than not, they tend to be JDM replica crazies that spend all day looking for the right dome light lenses so they can almost have a real Ver 6 STi to beat off to.
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Old 01-25-2017, 04:27 PM   #1258
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A little late in posting but my engine failed the compression test back when the recall happened in March and had a new engine put in. Engine had about 28k miles on it at the time. It's been running great since then though!
Were there any signs that things weren't good w/the engine before you brought it in for the recall?
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Old 01-25-2017, 04:29 PM   #1259
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More often than not, they tend to be JDM replica crazies that spend all day looking for the right dome light lenses so they can almost have a real Ver 6 STi to beat off to.

LMAO. I never got into that. Wheel changes, LED's, yada, little interest. ICE, yes, power yes, suspension and brakes yes. Cosmetic crap, not unless it's for protection. I do like putting rubbers on my Subaru though (Clearbra) cuz the paint is thin and chinze.
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Old 01-25-2017, 04:39 PM   #1260
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yeah... functionality over looks any day, it's a car. and i'd be lying if i said it's the best looking car.
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Old 01-25-2017, 05:19 PM   #1261
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worse yet, with DIT engines, we also have the possibility of LSPI
Why is a DI engine more prone to LSPI than a port-injected motor?
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Old 01-25-2017, 05:38 PM   #1262
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Why is a DI engine more prone to LSPI than a port-injected motor?
DIT motors seem to be most prone to it because of the high compression ratios and running high boost. direct injection is great, you get more efficient combustion, thus more power, better mpg, better response, but in the world of science there's drawbacks to everything and anything.

give this a read if you're interested: http://www.ukintpress-conferences.co..._Alewjinse.pdf
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Old 01-25-2017, 06:31 PM   #1263
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Originally Posted by mishapopa View Post
DIT motors seem to be most prone to it because of the high compression ratios and running high boost. direct injection is great, you get more efficient combustion, thus more power, better mpg, better response, but in the world of science there's drawbacks to everything and anything.

give this a read if you're interested: http://www.ukintpress-conferences.co..._Alewjinse.pdf
Thanks for the info! That presentation has a ton of great details in it.
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Old 01-25-2017, 09:20 PM   #1264
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Originally Posted by playslikepage71 View Post
YOUR ENGINE IS MOVING. If you take POWER and DIVIDE it by TIME (RPM) you get WORK not FREAKING TORQUE. Torque is the lazy way to describe what's happening. Each piston is doing work and that's being translated through the crank mechanism into torque, but if your engine stops moving, no torque for you.

The reason low RPMs are bad are not because of higher BMEP (that's cylinder pressure averaged over the stroke, and gives you this "torque" figure that has nothing to do with rods blowing up), but because of higher PEAK pressures. When the volume stays small for longer, as in the piston moving slower at low RPM, the pressure goes up enormously for a time, until the piston starts moving down to allow for expansion. At higher RPM, the piston moves much sooner, in terms of time, and peak pressures don't get as high. The ~same amount of energy is added to the system over the stroke, but the high compression force in the rod isn't there because the burn doesn't complete until the piston has retreated.

If you are having trouble understanding how torque and power are truly related, this is a the best video I've found to explain it:

https://www.youtube.com/watch?v=vX8wlkSDwSY
Nothing in that video contradicts anything that I have said. In particular:

- Power is the relevant number regarding moving the car, not torque. It says that specifically.
- Power = Constant * Torque * RPM (which contradicts your second sentence above, btw...but that said, after some more reading, I agree with you that we are using the term torque, when we should be using work. You have convinced me on that.)

One thing that you said above is enlightening and I had not considered. Specifically, an engine at lower RPM experiences high cylinder pressures for a longer period of time than an engine at lower RPM. This undoubtedly changes the stress experienced by different parts of the engine and is detrimental to the life of the engine.

The one thing that I still cannot reconcile is how all of this relates to the rod failures that we've been hearing about. Pretty much everything I'm reading says that (assuming proper lubrication and no defects) rods specifically are damaged by tensile stress associated with high RPM, not high cylinder pressures. EvenWikipedia cites over-revving as a cause, but does not mention low RPM operation.

Some people have rightfully pointed out that small turbo engines experience higher cylinder pressures than naturally aspirated engines (and this likely applies to all forced induction engines). But diesel engines experience even higher cylinder pressures, and compression stress on the rods in those applications is not generally cited as a particular worry.

Ultimately, I am just trying to understand why we're seeing rod failures. Telling people not to use lots of throttle at low RPM is good advice and will likely prevent certain failures, but I don't see the connection specifically to throwing rods.
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Old 01-25-2017, 09:35 PM   #1265
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My guess would be the 5 or so seconds that the rods get compressed with each stroke they go plastic and then all it takes is one downstroke to separate it.
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Old 01-25-2017, 10:05 PM   #1266
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Ultimately, I am just trying to understand why we're seeing rod failures. Telling people not to use lots of throttle at low RPM is good advice and will likely prevent certain failures, but I don't see the connection specifically to throwing rods.
Add in the shock/pressure wave from knock / lspi on top of high cylinder pressures... I don't know a ton about diesels but afaik this isn't an issue with them while it is for a gas DIT motor.

Also, consider a highly stressed DIT whose ECU has adapted to it's environment, then put in a bad tank of gas... the ECU will compensate but not perfectly...
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Old 01-25-2017, 10:08 PM   #1267
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My guess would be the 5 or so seconds that the rods get compressed with each stroke they go plastic and then all it takes is one downstroke to separate it.
Maybe a fraction of a second...

I assumed the rod failures were all from fatigue, not sure if there was any plastic deformation.
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Old 01-25-2017, 11:08 PM   #1268
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The best analogy I like to give to people regarding torque at low rpm is riding a 18 speed bike. Use the hardest gear possible to start moving from a stop and let me know how that works out for your thigh muscles. Until you get moving, it's a struggle...and you aren't even moving. Once you get going, hey, not so bad. This should be common knowledge!
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Old 01-25-2017, 11:13 PM   #1269
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The best analogy I like to give to people regarding torque at low rpm is riding a 18 speed bike. Use the hardest gear possible to start moving from a stop and let me know how that works out for your thigh muscles. Until you get moving, it's a struggle...and you aren't even moving. Once you get going, hey, not so bad. This should be common knowledge!
yep... sounds like something i said but it either wasn't caught or people haven't rode a bicycle before. except with a turbo charged engine it's like being on steroids trying to ride the bike in the hardest gear up a 60 incline.

feet = pistons... legs = rods... go ride a bicycle and think about it apply thinking to engine. don't do steroids though.
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Old 01-26-2017, 01:54 AM   #1270
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steroids are not bad for you, much like boost. you have to know what you are doing or it will hurt you in the end.
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Old 01-26-2017, 07:40 AM   #1271
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Maybe a fraction of a second...

I assumed the rod failures were all from fatigue, not sure if there was any plastic deformation.
They are most likely fatigue failures, given that people aren't just turning up the boost and having it blow up instantly. People are getting 5-10k+ miles out of the motor before the rods pop.

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Originally Posted by wadeh View Post

One thing that you said above is enlightening and I had not considered. Specifically, an engine at lower RPM experiences high cylinder pressures for a longer period of time than an engine at lower RPM. This undoubtedly changes the stress experienced by different parts of the engine and is detrimental to the life of the engine.

The one thing that I still cannot reconcile is how all of this relates to the rod failures that we've been hearing about. Pretty much everything I'm reading says that (assuming proper lubrication and no defects) rods specifically are damaged by tensile stress associated with high RPM, not high cylinder pressures. EvenWikipedia cites over-revving as a cause, but does not mention low RPM operation.
I think you missed half of it. Not only does it see the high pressure for longer, but it also sees higher pressures. Combustion is not instantaneous. It happens during most of the power stroke. The burn gets further along before the cylinder really starts to move down. Here's a diagram showing the general shape of the curve. That peak is the deathknell for rods. Slow down the crank, and that shape shrinks along the X and grows along the Y, as the rate of combustion becomes more similar to the dwell time at TDC.



Compression isn't fatiguing the rods, but them doing this is:



Do that a bunch, then rev the thing out to redline a bunch (which will slowly fatigue the rods as well, albeit no more than the stock setup) and eventually the rod is going to go. That's why you will see some bent rods and some that look like they just flew apart. The failure can occur in either mode, but the buckling mode is a new one introduced by the higher than stock boost at low RPM possible with bolt-ons.
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Old 01-26-2017, 11:17 AM   #1272
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yep... sounds like something i said but it either wasn't caught or people haven't rode a bicycle before. except with a turbo charged engine it's like being on steroids trying to ride the bike in the hardest gear up a 60 incline.

feet = pistons... legs = rods... go ride a bicycle and think about it apply thinking to engine. don't do steroids though.
As it happens, I bicycle commute 30 miles per day. My route includes a couple of significant grades.

My personal dyno curve does not look like a car's dyno curve at all. I produce most of my torque at a low RPM, and it drops off steeply at RPMs rise. But because of the way that the math works, I probably produce my peak horsepower at 75rpm or so. The thing is, that I *can't* physically make as much torque as the RPMs climb. If I could, my legs would take much more abuse mashing the pedals at high RPM than they do at low RPM.

Riding in too high a gear on the bike, I have low RPMs and need to make more torque to get the equivalent horsepower that I would get if I gear down and make more RPMs. The good news is that if change gearing to require less torque, I can pretty easily raise the RPMs enough to get the horsepower I need.

As I said in my very first post on the subject, an engine does need to make more torque at a low RPM than it does to make the same power at a high RPM. There is no dispute there. The difference between an engine and the bike analogy is that an engine *can* make lots of torque at high RPM. There is no way to demonstrate that with your legs on a bicycle.
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Old 01-26-2017, 11:26 AM   #1273
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Not only does it see the high pressure for longer, but it also sees higher pressures.
Yes. I get this.

But would those higher cylinder pressures not result in an increase of the calculated torque (work) that is reported by the dyno?

In other words, isn't this effect accounted for in the dyno graph? When we say that an engine produces X amount of torque at 2000rpm and the same amount of torque at 5000rpm, that it's seeing the same cylinder pressures accounting for all of this?
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Old 01-26-2017, 11:40 AM   #1274
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I produce most of my torque at a low RPM, and it drops off steeply at RPMs rise.
exactly

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If I could, my legs would take much more abuse mashing the pedals at high RPM than they do at low RPM.
this is why red line exists

there is an optimal range to floor it just like there is an optimal range of rpm and torque made for your legs
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Old 01-26-2017, 11:55 AM   #1275
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Originally Posted by wadeh View Post
Yes. I get this.

But would those higher cylinder pressures not result in an increase of the calculated torque (work) that is reported by the dyno?

In other words, isn't this effect accounted for in the dyno graph? When we say that an engine produces X amount of torque at 2000rpm and the same amount of torque at 5000rpm, that it's seeing the same cylinder pressures accounting for all of this?
No, because that assumes peak pressure is directly related to torque, which is untrue. Think about a single cylinder engine. The shaft doesn't produce a constant torque because it only has firing pressure 1/4 of the time. On top of that, the peak torque might not occur at peak cylinder pressure because of the small ATDC crank angle. Anything near TDC will have a really small moment arm to translate the force into torque. Work is Force * Distance, sort of like how torque is Force X Distance (scalar vs vector multiplication). If you graph the force (pressure) vs the crank angle (distance), its the area under the curve that will tell you how much work (torque) the engine is producing at a given RPM. As the piston comes down, the pressure is reduced, but the length of the moment arm increases, like so:



More concisely, BMEP (brake mean effective pressure) is a calculated value based on torque output and engine geometry. Actual cylinder pressure is all over the place and some calculus turns it into constant power.
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