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Old 01-20-2017, 08:27 PM   #1226
wv2.5i
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Quote:
Originally Posted by Celery GT-5 View Post
They don't seem to be blowing up as much anymore. It's kind of like how 02 wrxs were blowing tranny's left and right and now they don't seem to so much. Early adopter fukboiz suck at driving


Haha nice, maybe I'll reconsider.
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Old 01-20-2017, 08:55 PM   #1227
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Lmao the driver mod comments have always cracked me up especially in any sort of performance oriented car or when it's said on a forum, 90% of the ones on the forums are the ones that are pushing cars harder than average drivers.

Short of someone downshifting too far and grenading a tranny, doing 20 launches a day and driving WOT pretty much all the time I don't think the driver has anything to do with it on a stock car.

There are a lot of people that all drive differently out there, we are all snowflakes and we are are ignorant if we think in stock form that isn't accounted for.

Mod your motor and push it past stock levels its not the driver and it's not going WOT at lower revs than 3k that's the cause it's the fact you you've modded your motor outside predefined tolerances.

I guess the bottom line is read the data formulate an opinion but stop telling people "you're doing it wrong" lol.
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Old 01-21-2017, 01:15 AM   #1228
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I still think its the shoes you wear that determine if the car blows up or not.
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Old 01-21-2017, 01:31 AM   #1229
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Well, I think its the soles really. That combined with lace color.
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Old 01-21-2017, 09:11 AM   #1230
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Oh and that....lol
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Old 01-21-2017, 09:21 AM   #1231
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Originally Posted by wv2.5i View Post
Lmao the driver mod comments have always cracked me up especially in any sort of performance oriented car or when it's said on a forum, 90% of the ones on the forums are the ones that are pushing cars harder than average drivers.

Short of someone downshifting too far and grenading a tranny, doing 20 launches a day and driving WOT pretty much all the time I don't think the driver has anything to do with it on a stock car.

There are a lot of people that all drive differently out there, we are all snowflakes and we are are ignorant if we think in stock form that isn't accounted for.

Mod your motor and push it past stock levels its not the driver and it's not going WOT at lower revs than 3k that's the cause it's the fact you you've modded your motor outside predefined tolerances.

I guess the bottom line is read the data formulate an opinion but stop telling people "you're doing it wrong" lol.
IDK.. I have a few friends that have these cars. I've ridden in said cars... In 30 minutes time they've beaten on the car harder than I have since I brought mine home. Do not consider that as a factor.... Not on board with that thinking. When it comes to my car.

But I do agree the people here are pushing the cars pretty hard as a whole compared to the average person.
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Old 01-21-2017, 10:38 AM   #1232
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Originally Posted by ericdet View Post
IDK.. I have a few friends that have these cars. I've ridden in said cars... In 30 minutes time they've beaten on the car harder than I have since I brought mine home. Do not consider that as a factor.... Not on board with that thinking. When it comes to my car.



But I do agree the people here are pushing the cars pretty hard as a whole compared to the average person.


Yeah that's kind of why I put short of just really flogging it 24/7 I don't think the stock cars should fail with low miles....I would expect it to fail sooner, for curiosity sake what the heck were those few people doing?
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Old 01-21-2017, 10:56 AM   #1233
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Quote:
Originally Posted by wv2.5i View Post
Lmao the driver mod comments have always cracked me up especially in any sort of performance oriented car or when it's said on a forum, 90% of the ones on the forums are the ones that are pushing cars harder than average drivers.

Short of someone downshifting too far and grenading a tranny, doing 20 launches a day and driving WOT pretty much all the time I don't think the driver has anything to do with it on a stock car.

There are a lot of people that all drive differently out there, we are all snowflakes and we are are ignorant if we think in stock form that isn't accounted for.

Mod your motor and push it past stock levels its not the driver and it's not going WOT at lower revs than 3k that's the cause it's the fact you you've modded your motor outside predefined tolerances.

I guess the bottom line is read the data formulate an opinion but stop telling people "you're doing it wrong" lol.
lolwut?



pushing a motor past stock levels and going WOT (especially at lower revs) is definitely mutual.

if you beat the **** out of your mechanical machine, it's going to break sooner.
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Old 01-21-2017, 01:14 PM   #1234
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Lolwut is right...

My point is again that I've seen on multiple threads people being called idiots for doing things to stock cars that are within the parameters of what is expected for 90% of owners. (Using the power available In Lower rev ranges being one of them along with about 800 other things and of course wrong shoes)

All of the different styles of course will relate to longevity of your mechanical machine even at stock form, but my point which is being taken literal it seems is that the biggest factor with modded motors are the actual mods.

Anyways I'll go back to my reading and non posting lol.
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Old 01-21-2017, 09:01 PM   #1235
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When we're talking low rpm boost are we talking 5-10 PSI or WOT whatever it'll let you have boost?

I drive my '17 many highway miles on the cruise and it'll see 5-7 PSI on some big uphills when maintaining 70-75 MPH. I live in the mountains and generally take it fairly easy on it with the occasional blast for kicks. I try to keep it under 10 PSI if that.
In short where does the low speed deto occur?
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Old 01-21-2017, 10:01 PM   #1236
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Originally Posted by tramp View Post
When we're talking low rpm boost are we talking 5-10 PSI or WOT whatever it'll let you have boost?

I drive my '17 many highway miles on the cruise and it'll see 5-7 PSI on some big uphills when maintaining 70-75 MPH. I live in the mountains and generally take it fairly easy on it with the occasional blast for kicks. I try to keep it under 10 PSI if that.
In short where does the low speed deto occur?
The issue comes into play when you're cruising in 5th or 6th in the lower 2000's or below, and abruptly go to WOT without downshifting, spooling up the turbo to its boost limit before allowing the RPMs to catch up. That is the situation that creates huge loads on the internals. You can spool up your turbo pressure in a high gear provided you do it more gradually. If you want instant power, downshift...SC
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Old 01-21-2017, 10:40 PM   #1237
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Thanks. That's what I thought but I've been wrong before.

Honestly I'm so impressed with this little 122 cubic inch beast. It's taken some time to learn to drive it well but worth the investment.
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Old 01-21-2017, 11:55 PM   #1238
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by putting a high load on the engine at higher rpm, the inertia reduces the stress on the rods. by putting a high load (wot) @ low rpm you're making powerful combustion (due to compressed o2/boost and loads of fuel) shoving the rods down slowly but forcefully, which puts compression stress on them.

it's common sense if you think about it, your legs hurt more and you need to work harder if you ride your bike in an incorrect gear. your legs = rods, your feet = pistons.

in emergency situation it happens, everyone's goes WOT at low rpm at some points. i just wouldn't make it a habit. for people used to n/a cars it's easy to think "oh if i WOT at low rpm i get better mpg and engine doesn't work so hard" but nope, compress air to 20 psi, add fuel, make spark, very big boom.

worse yet, with DIT engines, we also have the possibility of LSPI

ALSO, oil pressure isn't adequate at low RPM & high boost condition so lubrication can be an issue. wouldn't be so concerned about n/a engines but something to think about on a boosted engine.

tldr don't WOT at low rpm unless you're in 1-3 gear.

Last edited by mishapopa; 01-22-2017 at 12:05 AM.
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Old 01-22-2017, 07:10 PM   #1239
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by putting a high load on the engine at higher rpm, the inertia reduces the stress on the rods. by putting a high load (wot) @ low rpm you're making powerful combustion (due to compressed o2/boost and loads of fuel) shoving the rods down slowly but forcefully, which puts compression stress on them.
I got to thinking about this and it doesn't make a lot of sense to me.

To my thinking, torque is a static measure of force applied to the engine. And that force more or less directly equates to compression (or bending, depending on the crank angle) of the rods. In other words, 100 ft/lb at 2000rpm applies exactly the same compression stress as 100 ft/lb at 6000rpm. Tensile stress (stretching of the rods due to inertia as the piston is pulled down from top dead center) increases dramatically as the RPM gets higher.

So I went and did a bunch of reading, including a paper discussing dynamic stress and connecting rod engineering, and also a bunch of discussions of the causes of connecting rod failures.

The paper - to my layman eyes - seemed to confirm that compression stress is independent of engine RPM, but tensile stress increases with right RPM. Also interestingly, in reading a bunch of discussions, it seems that most connecting rod failures are tensile failures that occur not under acceleration, but when the throttle is shut and tensile stress is at its highest due to inertia. Also, it was mentioned in both the paper and many discussions that maximum compression stress occurs at highest cylinder pressures.

So the question to me is why it's bad to go WOT in higher gears (and nothing above disputes the often repeated statement here that you should not do this - just the technical reason).

And here's where I go from summarizing what I read, to reaching a conclusion (and I would welcome any good arguments to dispute my conclusion; I am not an expert in this area).

The total stress on a connecting rod accounts for the total stress over a complete cycle, both compression and tensile. The maximum stress therefore occurs when peak torque is developed at the highest RPM. In that way, WOT throttle at low RPM is *not* more stressful on the rods than WOT at high RPM. In that way, I would think that your engine experiences its highest stress either during a dyno test or during a drag race.

But even if you are a total lunatic on the road, you can't possibly spend that much time producing maximum torque at high RPM.

But now consider the difference between higher and lower gears at normal driving speeds. In a high gear, the engine needs to produce far more torque to achieve the same vehicle acceleration as if it were in a lower gear.* By driving around in too high a gear, you are causing the engine to produce more torque than it would otherwise produce for the same vehicle speed in a lower gear. At low throttle settings, especially if you are not making boost, this is probably not a big deal at all. If you routinely floor it in a high gear, though, you force the engine to produce maximum torque for an extended period of time. Even this shouldn't be a big deal, though, since tensile stress is minimized at low RPM and tensile stress is more likely to cause failure.

So that leaves me to wonder about LSPI or detonation being a significant factor in failures. Have there been any instances of rod failure in an FA20DIT where the pistons showed no signs of detonation? Alternately, is it possible that there are simply some cars out there with weak rods that would have failed even unmodded?

And I can't stress this enough: I am not an expert in this area, and I did not sleep at any special hotel last night. I just put in about an hour of thought and reading into it because my curiosity was piqued. Do not change your driving habits or mod plans based on anything I've said. If you break something, it's not my fault.

-Wade

* At the risk of starting something, horsepower is a measure of work, not torque which is just a measure of instantaneous force. It is horsepower that moves a car. Horsepower is defined as force over time, or torque * RPM in the case of a car. Therefore an engine at 2000rpm has to produce twice as much torque to power the vehicle as the same engine at 4000rpm - not accounting for driveline efficiency.
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Old 01-22-2017, 08:44 PM   #1240
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Just using your words in my response. Maximum compression stress happens at peak cylinder pressures. Peak cylinder pressure will be greatest at low RPM's. In high gears you're putting the greatest load on the engine. Putting the car in sixth gear at 2500 and hammering the throttle and keeping it there will put the most load while simultaneously getting the highest cylinder pressures.

A Dyno run in 3rd gear will never see the load you will see in 4th and so on. Put a stock car and the Dyno in 4th and watch it struggle all the way through.

There are a few stock cars out there that have failed. But that number is much smaller after the ECU updates and better tuning has come along.
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Old 01-22-2017, 09:19 PM   #1241
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I think there's more to consider, you might start by checking out why lugging is bad for a motor, and why it can be really bad for turbo motors (and especially really bad for the EJ25 as it has a larger bore vs stroke).

This is a decent start to get the overall concept:

http://www.roadandtrack.com/car-cult...ad-for-engine/

Keep in mind the FA20 DIT has a very high static compression ratio and decisions on how the motor operates, and transmission if it's a CVT, favor emissions and fuel economy. IMO manufacturers are having to push motors harder to conform to stricter standards. Mazda is coming out with an ~18:1 sparkless gas motor for '18... It does seem like government policy is pushing a lot of these new technologies, it costs us all but it's probably a good thing.

The case may be that making these high efficiency motors more reliable might cost too much vs the cost of honoring warranty replacements.
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Old 01-22-2017, 09:42 PM   #1242
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Quote from the above referenced video---regarding LSPI.

"it means........you can't use ignition timing to get rid of it."

The biggest problem. The only cure is to avoid it-period.
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Old 01-23-2017, 04:46 PM   #1243
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Quote:
Originally Posted by ericdet View Post
Just using your words in my response. Maximum compression stress happens at peak cylinder pressures. Peak cylinder pressure will be greatest at low RPM's. In high gears you're putting the greatest load on the engine. Putting the car in sixth gear at 2500 and hammering the throttle and keeping it there will put the most load while simultaneously getting the highest cylinder pressures
So much going on here...Torque is thought of as twist, but really it's just a measure of energy, averaged over a cycle. That's misconception number one. Peak torque is actually not measured by a dyno, as it's too instantaneous to capture.

Compression isn't causing fatigue (it doesn't), but buckling is. Take a stick and push straight down on it. That's your rods at low RPM. Now imagine the thing the stick is on is moving down and exert your force at the same rate. The bottom of the stick moves away, reducing the amount the stick (rod) buckles.

The other half of that is that tension also fatigues the rods. If you go back and forth between buckled and stretched, its going to give out in a spectacular fashion eventually. The stuff you were reading was probably more to do with NA motors where revs will kill the rods before cylinder pressure in most cases.

Eric, you're right that low RPM presents more cylinder pressure, and therefore more compression in the rods. When you slow the engine down, you also increase the amount of time the rod spends loaded at a weird angle. More of the force goes into spinning the thing around at high RPM because the expansion happens at a more similar rate to the motion of the piston.
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Old 01-25-2017, 10:38 AM   #1244
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It's been a busy few days, so I've not had a chance to spend any time on this, except to watch the video. It says that lugging the engine is bad, and I agree. It spends a lot of time on LSPI and has some good information from a Toyota study on why it happens.

But regarding high throttle at lower RPM, it says not to do it, but doesn't offer anything about why specifically with regard to the rods. It does mention that the engine works harder and builds higher temperatures. It also talks about how it can contribute to LSPI.

But then it goes on to say that LSPI is somehow worse the pre-ignition at higher engine RPMS. Pre-ignition is bad, no doubt. But the reason LSPI seems worse has more to do with when it is likely to happen, than the damage it does.

Again, I am not arguing that it's not bad to go WOT at low RPM. I'm just trying to understand the technical reason. I hear that compression stress happens at maximum cylinder pressure, and that is surely true. But I cannot find any reason why peak cylinder pressure occurs at lower RPM.

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.

But I cannot think of any reason why 100 ft/lb at 2000rpm puts any more compressive stress on a connecting rod than 100 ft/lb of torque at 6000rpm. In both cases, sufficient cylinder pressure occurs to cause the engine's output shaft to produce 100 ft/lb of torque. No more, no less.

Oh, and I thought that I would include a link to the paper that I referenced above discussing stresses applyed to connecting rods:

It is here.

It specifically addresses all of the force applied during the combustion cycle, including the effect of sideways loads, etc. as the crank is at different angles. But if I read it correctly, it says that compression stress is independent of engine RPM while tensile stress is strongly affected by engine RPM.

Apologies that this seems to have gotten quite a bit away from the purpose of the thread. I'm just curious about the physics going on here and would be happy to take it to a more appropriate place if anyone has suggestions.

-Wade
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Old 01-25-2017, 10:53 AM   #1245
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^Piston velocity is slower at lower rpms.

There is an explosion happening in the combustion chamber that's intended to move the piston, what do you think the difference are with a slow vs a fast moving piston?
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Old 01-25-2017, 11:04 AM   #1246
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For some of you, think of max cylinder pressure at low revs......TIME.

Lower revs allow more TIME for pressure to develop and it lasts longer vs. high revs. So, the pistons and rods we more pressure for a longer time when you are running a high gear and go WOT.
In a lower gear, the car accelerates faster, the revs are higher, the pressures are a bit lower and last shorter.
Notice max torque is lower RPM's and max HP is higher RPM's?
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Old 01-25-2017, 11:29 AM   #1247
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Again, I am not arguing that it's not bad to go WOT at low RPM. I'm just trying to understand the technical reason. I hear that compression stress happens at maximum cylinder pressure, and that is surely true. But I cannot find any reason why peak cylinder pressure occurs at lower RPM.

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.

But I cannot think of any reason why 100 ft/lb at 2000rpm puts any more compressive stress on a connecting rod than 100 ft/lb of torque at 6000rpm. In both cases, sufficient cylinder pressure occurs to cause the engine's output shaft to produce 100 ft/lb of torque. No more, no less.
In your response again.... Engine torque is a measure of cylinder pressure.

Do you have an AP?

Put your car in 6th at 2500 and go full throttle. Then at the same speed go down to 3rd or 4th and go WOT. Check out Calculated load.

You'll quickly see why in a little turbo car why Peak cylinder pressure will occur at low RPM. A lot of the thought process in what you're saying can be applied to Naturally aspirated situations, but is not entirely relevant in this one.

Also it's true that force is force no matter the RPM. But I'd also be quick to point out that torque is diminishing quickly over the Rev range in small turbo cars. My car makes almost 100 ft/lbs less from 3800 to 6000 RPM. Imagine the difference from 2800-3000 to 6000 RPM with the turbo fully spooled and sitting there because the load is too great to overcome.

Last edited by ericdet; 01-25-2017 at 11:36 AM.
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Old 01-25-2017, 12:29 PM   #1248
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I think this thread has more theories than actual stories.
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Old 01-25-2017, 01:10 PM   #1249
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People say torque kills the rods because that's the only measuring stick they have. It is force caused by pressure that actually buckles them, but who has an engine with pressure transducers to actually measure that (no one)?

Pressure is the problem, so lets take a step back and think about what causes pressure.

#1 engine load: g/rev, how much reactant you have in your chamber (cylinder), O2, fuel, the ratio of the two

That loading, moles of reactant, will determine the max energy you can get out of the combustion cycle. ( in truth initial temperature and pressure when the reaction starts matters too, but lets not go into that becasuse it is an unnecessary complication to understanding what is going on here)

That energy come from heat, from ideal gas law Pressure times volume equals number of moles of substance times a constant times temperature:

PV=nRT

this is a closed system so n is constant, lets look at 2 moments in a combustion cycle lets say 1 is just before ignition and 2 is when combustion is "complete" by saying that:

P1V1/T1=P2V2/T2

Change in temperature, T1-T2, is the energy from combustion which determined by engine load.

Volume is controlled by crank position (also cylinder geometry, but that is constant)

Pressure will be whatever the other two variable force it to be.
______________________

Lets pretend combustion speed is constant (it isn't, but we can pretend and still understand what's going on)

Combustion speed and engine speed will determine what the change in volume is between V1 and V2,
combustion takes .01s, how far has the crank moved in that time, that is Delta V.

Therefor it is clear that if RPMs are low DV will be smaller than if RPMs are high

If DV is small then DP (change in pressure) will be large!

______________________
Let's go deeper and consider the effects of ignition timing

think about a circle on a wall, you trace the circle at a constant speed, the vertical position on the circle is what determines cylinder volume and any moment in time. When you are near the top of the circle, consider the top 20 degrees, change in height is very small.

Thus it is clear why if you fire at top dead center at low rpm your cylinder pressure will be immense.

If that happens, cylinder pressures and temperature are too high you will get spontaneous combustion at multiple places in the cylinder at once, no longer is there an organized pressure wave from top to bottom, it is chaotic.

That is detonation. Detonation is bad but it is related to ignition timing and you can vary that with parameters (rpm, load, ect) in your ECU to start combustion later and keep the pressure and temperature in check below the threshold for detonation. If you ECU senses detonation it "pulls timing" which means it starts ignition later.

What is pre ignition? That is that is spontaneous combustion before the ECU even planned for the spark plug to fire. That means that it is not controllable (directly) by altering ignition timing cutting load would work of course, but the ECU would have to know it was happening. Worse still the ECU is not listening to the knock sensor before ignition is planned (why would it, should just be noise)

Finally we make it to Low Speed Pre Ignition and if you've been following along at this point you understand why it is so damaging. A combustion event while the cylinder is not moving or maybe even moving up, making the volume smaller, is going to mean extremely high pressure
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Old 01-25-2017, 01:18 PM   #1250
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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!
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