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01-25-2017, 12:55 PM | #1251 | |
Scooby Specialist
Member#: 395882
Join Date: Jul 2014
Chapter/Region:
MWSOC
Location: GR, Michigan
Vehicle:2016 WRX DGM |
Quote:
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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01-25-2017, 12:56 PM | #1252 |
Scooby Newbie
Member#: 158185
Join Date: Sep 2007
Chapter/Region:
NWIC
Location: Boise, ID
Vehicle:17 WRX CWP |
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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01-25-2017, 01:07 PM | #1253 |
Scooby Specialist
Member#: 393629
Join Date: Jun 2014
Chapter/Region:
W. Canada
Location: Calgary
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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. |
01-25-2017, 01:56 PM | #1254 |
Scooby Newbie
Member#: 415322
Join Date: Mar 2015
Location: Cleveland, OH
Vehicle:2015 WRX Galaxy Blue |
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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01-25-2017, 02:41 PM | #1255 | |
Scooby Specialist
Member#: 327563
Join Date: Jul 2012
Chapter/Region:
MWSOC
Location: Cincinnati
Vehicle:2018 Civic Type R 2008 N54 335i |
Quote:
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01-25-2017, 03:07 PM | #1256 | |
Scooby Specialist
Member#: 393629
Join Date: Jun 2014
Chapter/Region:
W. Canada
Location: Calgary
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Quote:
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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01-25-2017, 03:59 PM | #1257 | ||
Scooby Specialist
Member#: 395882
Join Date: Jul 2014
Chapter/Region:
MWSOC
Location: GR, Michigan
Vehicle:2016 WRX DGM |
Quote:
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:
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01-25-2017, 04:27 PM | #1258 |
Scooby Newbie
Member#: 144106
Join Date: Mar 2007
Chapter/Region:
MWSOC
Location: MN
Vehicle:2015 WRX Lightning Red |
Were there any signs that things weren't good w/the engine before you brought it in for the recall?
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01-25-2017, 04:29 PM | #1259 | |
Scooby Guru
Member#: 139693
Join Date: Feb 2007
Vehicle:Dura ngo 95 horrorshow |
Quote:
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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01-25-2017, 04:39 PM | #1260 |
Scooby Specialist
Member#: 393629
Join Date: Jun 2014
Chapter/Region:
W. Canada
Location: Calgary
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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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01-25-2017, 05:19 PM | #1261 |
Scooby Newbie
Member#: 408438
Join Date: Dec 2014
Location: OH, USA
Vehicle:2016 WRX STi CBS |
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01-25-2017, 05:38 PM | #1262 | |
Scooby Specialist
Member#: 393629
Join Date: Jun 2014
Chapter/Region:
W. Canada
Location: Calgary
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Quote:
give this a read if you're interested: http://www.ukintpress-conferences.co..._Alewjinse.pdf |
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01-25-2017, 06:31 PM | #1263 | |
Scooby Newbie
Member#: 408438
Join Date: Dec 2014
Location: OH, USA
Vehicle:2016 WRX STi CBS |
Quote:
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01-25-2017, 09:20 PM | #1264 | |
Scooby Newbie
Member#: 426075
Join Date: Jul 2015
Location: Snohomish, WA
Vehicle:2015 WRX Silver |
Quote:
- 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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01-25-2017, 09:35 PM | #1265 |
Scooby Specialist
Member#: 222893
Join Date: Sep 2009
Chapter/Region:
NESIC
Location: Revere, MA
Vehicle:2016 WRX DGM Limited 2002 WRB/2012 SWP Hatch |
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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01-25-2017, 10:05 PM | #1266 | |
Scooby Newbie
Member#: 126901
Join Date: Sep 2006
Location: Front Range CO
Vehicle:2016 STi ISM |
Quote:
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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01-25-2017, 10:08 PM | #1267 | |
Scooby Newbie
Member#: 126901
Join Date: Sep 2006
Location: Front Range CO
Vehicle:2016 STi ISM |
Quote:
I assumed the rod failures were all from fatigue, not sure if there was any plastic deformation. |
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01-25-2017, 11:08 PM | #1268 |
Scooby Specialist
Member#: 408776
Join Date: Dec 2014
Chapter/Region:
BAIC
Vehicle:15 WRX EFR 7064 |
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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01-25-2017, 11:13 PM | #1269 | |
Scooby Specialist
Member#: 393629
Join Date: Jun 2014
Chapter/Region:
W. Canada
Location: Calgary
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Quote:
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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01-26-2017, 01:54 AM | #1270 |
Scooby Specialist
Member#: 408776
Join Date: Dec 2014
Chapter/Region:
BAIC
Vehicle:15 WRX EFR 7064 |
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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01-26-2017, 07:40 AM | #1271 | ||
Scooby Specialist
Member#: 395882
Join Date: Jul 2014
Chapter/Region:
MWSOC
Location: GR, Michigan
Vehicle:2016 WRX DGM |
Quote:
Quote:
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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01-26-2017, 11:17 AM | #1272 | |
Scooby Newbie
Member#: 426075
Join Date: Jul 2015
Location: Snohomish, WA
Vehicle:2015 WRX Silver |
Quote:
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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01-26-2017, 11:26 AM | #1273 | |
Scooby Newbie
Member#: 426075
Join Date: Jul 2015
Location: Snohomish, WA
Vehicle:2015 WRX Silver |
Quote:
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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01-26-2017, 11:40 AM | #1274 | ||
Scooby Specialist
Member#: 393629
Join Date: Jun 2014
Chapter/Region:
W. Canada
Location: Calgary
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Quote:
Quote:
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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01-26-2017, 11:55 AM | #1275 | |
Scooby Specialist
Member#: 395882
Join Date: Jul 2014
Chapter/Region:
MWSOC
Location: GR, Michigan
Vehicle:2016 WRX DGM |
Quote:
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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