The real reason people are breaking rods

[Lex]

I have looked a lot at these blown engine issues. In the end it was all under our noses. The rods are plenty strong for this application and I have compared them dimensionally to both EVO X and SRT4 rods. They are no lighter and although the bolts are a mm smaller in diameter, the DISI engine was not designed for high RPM and we are not breaking rod bolts. I have looked at binding issues, hydrolock etc ... nothing was concrete.

So then I looked at the rod construction and noticed the small end is tapered. Interesting. Diesel engines taper the small end so that the bearing surface is lower so that the rod takes a high load. The rod I have seems to have some good wear on the bottom of the bearing caused by cylinder pressure. Interesting. Perhaps poor oiling. Perhaps too much pressure.



I then looked at how similar cars produced power and as it turns out no EVO or SRT4 motor makes as much torque as we do at such low RPM. Period.

The way this Mazda is calibrated is more like a truck than a performance car. We don't need more than 300 ft lbs of torque to make well over 400whp but Mazda didn't want that for this motor.

Everyone has been looking at hp gains for this car and have completely ignored the torque and where it is at its maximum and that it doesn't carry into the RPM band.

Further torque is the one that is DIRECTLY proportional to cylinder pressure.

Case and point, look at the tq and hp on this 450whp evo


Now look at Darksun's dyno. Again look at the torque he made.


Take a look at this recent DISI dyno. 375 ft lbs at 3000 RPM.
http://www.mazdaspeedforum.org/forum/foru...ue-maf-cal.jpg

Cylinder pressure is what causes the "force" that is then back calculated as torque by a dyno. The gases that cause the cylinder pressure as a result of combustion expand at the same rate no matter how quickly you engine is spinning. This is the reason that you advance spark timing - so that the flame front occurs at the appropriate time to provide as much force to the downward moving piston as possible.

Now, as RPM rises, the piston moves much faster. So then amount of TIME it is subject to the pressure of the expanding gases is less per each power stroke. The reason cars make torque during a certain part of the RPM band is because there is a relationship between how you can control the burn rate (which depends on air/gas inflow, mixing, lighting mixture) with respect to piston speed

So when you are making 300ft lbs of torque at 2500 RPM, the motor sustains the force required to makes that torque for longer each power stroke than if you made the torque at 4500 RPM. Let's not even talk about the fact that a dyno takes an average force applied which favors high RPM to low RPM. It's pretty damn hard to make the torque we do at such a low RPM. The principle is similar to lugging.

So our engines making 300 ft lbs of torque at 2500RPM and only 280whp are under a lot more mechanical stress than an F/I honda motor making 300 ftlbs of torque at 5000RPM and 400whp at 7500RPM.

Once RPM increases too much, inertial loads of the rod and piston break rod bolts. The counterpart is extreme torque at low RPM. Oil pushed out of bearings and extreme connecting rod loads are the result. This load applied again and again eventually results in bent rods and holes through blocks.

Once you open up the flow path to this car it makes MUCH more torque at an even lower RPM instead of breathing better at higher RPMs.

Have you seen the rods on a diesel? Here is an example of a TDI (yes diesel) dyno. Look familiar?



Here is an example of a 1.9 TDI rod and piston that makes 170 ft lbs of torque and 100hp from the factory. The rods are bombproof compared to us because THEY WERE DESIGNED to take high torque at low RPM.




Nuff said. I don't think anyone should be wondering why we break rods anymore.

Solution? There are many factors that affect how an engine produces torque and power. In order for the stock bottom end to stay in one piece, change the stock tune so that you don't hit 21psi at 2800 RPM. Help the motor breathe and move the torque curve further in RPM. Not only will the engine survive longer, but it will also make more whp. And everyone will be a hero, not just whoosh, who certainly didn't make the power he did by chance.

EDIT: This is an addition further explaining the low load rod breaking phenomenon:

I know that people are concerned with the part throttle blow-ups. These are certainly explainable - and I think the short article below from Hot Rod magazine clearly explains how fatigue and material impurity can lead to brittle fractures when components are under high stress. Brittle fractures start at a point of imperfection or impurity in the metal and propagate through. This is why some guys last longer stock and some don't. When a brittle fracture starts to happen, you won't feel anything until that fracture has grown enough to cause the rod to snap. The fracture will most likely let go under vacuum or when engine RPM changes (gear changes) since the rod is being "pulled apart" at that point if you will.

Cylinder pressure, high RPM, etc all play a part in exploiting a material flaw if it exists in the first place and bring it out of the wood work faster.

If you look at MS3 rods, you clearly see that the surface finish is not smooth (machined) like it is in aftermarket forged rods.

Stock Forged Steel Original-equipment forged steel rods are the next step up the strength and reliability ladder. Detroit-sourced OE-forged rods begin life as bars of carbon steel that are passed through a rolling die. The rolling process compacts the molecular structure and establishes a uniform, longitudinal grain flow. The bars are then heated to a plasticized state, inserted into a female die, and pressed into the near-final shape while a punch locates and knocks out the big end bore. In doing this, the grain flow at the big end is redirected in a circular pattern, like wood fibers surrounding a knot, and excellent compressive/tensile strength results. Finally the rod is put through a trimmer (that leaves the characteristic thick parting line on the beam), the big end is severed and machined to create the cap, bolt surfaces are spot-faced, then final machining and sizing take place. But there are some drawbacks. When the forging hammer hits the hot bar, heat transfers from the bar to the hammer causing a phenomenon called de-carb (decarburization). Here, trace amounts of the carbon in the steel migrate to the surface resulting in a rough finish full of what metalurgists call “inclusions.” An inclusion is described as anything that interrupts the surface of the metal, or a lack of cleanliness (impurities) in the material. The effect of a surface inclusion can be likened to a nick in a coat hanger. Bend it enough times and the wire will fail, usually right at the nick. The rough surface caused by de-carb affects the surface to a depth of 0.005 to 0.030 inch and is packed with inclusions that are a breeding ground for cracks. The old hot rodder’s trick of grinding and polishing the beams is a valid solution to this problem, though far too labor-intensive to ever be considered by Detroit. When it comes to inclusions caused by impurities, Detroit’s need to control costs can result in purchases of bulk steel that may (or may not) contain contaminants such as silicon that are not detected during manufacture. Such impurities can interrupt the grain boundaries between the parent molecules and lead to a fracture minutes or years after the rod is first installed in an engine. It’s a matter of luck and what kind of abuse the flawed rod is subjected to. Stock forged steel rods are an economical choice that should be able to handle one horsepower per cubic inch with quality fasteners, and as much as twice the factory-rated output if the beams are polished.

Edit July 30 2009: I am adding this reply I made to a member to the first post so it doesn't get lost.

We have seen some bent rods and some blown engines at wot or on the way to WOT that were outright compressive failures. The rod likely had no damage prior and simply was overwhelmed by compression forces. These are the "my line popped off and I made 30psi failures"

A rod that lets go during accel/decel lets go at that time because they are points where the rotating assembly is accelerated. F = MA remember. However, the rotating assembly goes through these transition points all the time on a normal motor without failure. So what can you conclude?

1. The rotating assembly has locked at that point due to hydrolock or mechanical lock.

2. The rod developed a gradual crack. Due to imperfections in manufacturing and high torque output, a crack is formed in the rod grows eventually letting go during these transition points.

I find #1 less likely than #2, but when the last guy that blew his engine tells me the hot side of his intercooler was filled with oil I do wonder. Remember that people blow doing stupid shit they don't mention as well so take it with a grain of salt.

So what have we learned here?

The pure MECHANICAL design of the rods and motor is fine for the power levels we are making on these engines given the rods have no flaws and nothing strange locks up. This is what DCR has been saying as well. If the actual mechanical limit was exceeded we would not have the whoosh, laloosh, darksun, and many others running in the 300-350whp/350 ft lbs of torque range for a long time. Pretty much everyone would blow and they would blow at WOT.

HOWEVER, something locking the assembly OR the more likely manufacturing flaws of the rod COMBINED with high cylinder pressures makes some of the "weaker" motors let go. This is why the failures happen at random times. You may be lucky or you may not be. High torque, especially high torque at low RPM will push you over the edge if you have a weak motor. So take your chances - or better yet tune and drive sensibly.

The SRT4s, DSMs all had similar issues, but after the car being around for 5+ years the weak ones were weeded out and the people that lasted were the ones that treated the car with respect and had a proper calibration.

STIs blow ringlands at stock power yet some make 450whp safely.

Don't be surprised by this. Parts have a tolerance. Let's say that at stock power 1% of cars blow. At 50% over stock power 4% of cars blow. At 100% over stock power 40% of cars blow - etc.

[gsrtype1]

Interesting thread Lex! I was thinken the Stanback gives you variablr boost control might ajust...

[devils10]

thanks for this post .. this info is very iimportnat to all our feellow speed owners

[SgtP]

with the standback you can adjust boost by rpm. im gonna give it a shot. maybe keep it a little lower from 2500-3500 then taper it up.

[Haltech]

This does make a lot of sense actually. But. you arent going to be lowering much torque by tapering the boost, thats for damn sure.

You can cut the ignition timing down to accomplish lower torque below 3000, but the trade off is, the car is going to be slow as shit.. and then bam, a shitload of torque hits the cylinder when you meet back to the ignition advance.

[SpdFreak]

Just thinking out loud. IIRC aaron found a difference in the way the 07-08 load maps vs the 09 load maps. One requested more load in the lower rpms vs the other requesting more load in the upper rpms. Load in this car equals boost. This car is load targeting. How about changing the load tables?

Then again, this engine was designed to make the grunt power under the curve. Mechanically it wasn't designed to make power up top.

Just thinking...

[Haltech]

Well the proper way to do it is with valve train mods. You could port/polish the heads, enlarge the valves, then regrind the cams to move the power upwards in the rpm.

However, once you build the lower with forged, it doesn't even matter anymore.

[madvillain]

thread of the year.

[steven88]

just out of curiosity, has anybody blown motors with upgraded fuel pumps?

[glocK23]

Originally Posted by steven88 just out of curiosity, has anybody blown motors with upgraded fuel pumps?

Yes, and with meth.

[Laloosh]

Is this suppose to be news of some sort? Darksun dyno is a not a good comparrison, he spiked the shit out of his turbo.

Want another comparrison, the caliber is making 530 lbs of tq down low on a reworked turbo.....no boom. all my standback tunes ramped boost up from day 1. TQ down low is the most over rated, dangerous thing imo. However, this motor is still to weak.

[mituc]

Originally Posted by Lex Once RPM increases too much, inertial loads of the rod and piston break rod bolts. The counterpart is extreme torque at low RPM. Oil pushed out of bearings and extreme connecting rod loads are the result. This load applied again and again eventually results in bent rods and holes through blocks. .

Lex, thank you very much for this post, still, I have some questions just to figure out how should we drive this thing, what we should not do, and so on, with mods or just stock.

Some of the guys with blown engines said that the rods bended under low boost/light acceleration and around 3k rpm. 3krpm is not high at all. 6500 yes, let's just say it's a bit high even though the rev limiter is at 6700 or 7000, but 3000?

While driving around town I find myself cruising at quite low RPMs (like, going with constant speed in the 1st or 2nd gear around 1200-1800rpm, or in the 3rd gear at 1800-2200rpm), and it's quite dangerous to hit the gas hard at such low RPMs - dangerous at least for the turbo but seems like it's dangerous for the rods as well. But at 3k RPM all should be just fine, no matter the boost level, pedal position, load, and so on.

P.S.: did anyone blow his motor at over 6500rpm?

[johnnyspeed6]

Excellent write my man ... makes sense ( of course someone had to shit on it ) Sooooooooooooo .... what ever happened to all the csutom cam testing that was going on 6 months ago .. I never saw any results on here about them .. looks like one way to go would be a diff. cam setup for sure.

[Frequentflyer]

Originally Posted by mituc Lex, thank you very much for this post, still, I have some questions just to figure out how should we drive this thing, what we should not do, and so on, with mods or just stock. Some of the guys with blown engines said that the rods bended under low boost/light acceleration and around 3k rpm. 3krpm is not high at all. 6500 yes, let's just say it's a bit high even though the rev limiter is at 6700 or 7000, but 3000? While driving around town I find myself cruising at quite low RPMs (like, going with constant speed in the 1st or 2nd gear around 1200-1800rpm, or in the 3rd gear at 1800-2200rpm), and it's quite dangerous to hit the gas hard at such low RPMs - dangerous at least for the turbo but seems like it's dangerous for the rods as well. But at 3k RPM all should be just fine, no matter the boost level, pedal position, load, and so on. P.S.: did anyone blow his motor at over 6500rpm?

I try to make it a habit NOT to nail it at low RPM (anything below 3) and when I catch myself doing it, I usually let out a, "FUCK". I tend to ease into it a bit. Not really the ideal "fix", but hopefully my motor will last longer. Granted, I'm lightly bolted.

So, with this theory, it really doesn't matter how much octane or meth you're running, because this isn't being caused by knock, just extreme cylinder pressure.

[ToledoSpeed3]

Good writeup. I still think our compression ratio is a good part of our problem. 9.5:1 is too high for a high boost application, especially when throwing rods have proven to be a problem. If you compare our motor with the CSRT, their compression ratio is 8.6:1. They even have a longer stroke than our cars (3.82 vs 3.7) with approximately the same bore which theoretically should put more pressure on their rods. They make 265ft lb torque down low (~2000 RPM if I remember right) and they aren't having these problems from what we have heard.

I hope we can find an easier way out, but it sounds like lower compression, stronger rods, or both may be required.

[Jarods7920]

Originally Posted by ToledoSpeed3 Good writeup. I still think our compression ratio is a good part of our problem. 9.5:1 is too high for a high boost application, especially when throwing rods have proven to be a problem. If you compare our motor with the CSRT, their compression ratio is 8.6:1. They even have a longer stroke than our cars (3.82 vs 3.7) with approximately the same bore which theoretically should put more pressure on their rods. They make 265ft lb torque down low (~2000 RPM if I remember right) and they aren't having these problems from what we have heard. I hope we can find an easier way out, but it sounds like lower compression, stronger rods, or both may be required.

You think its too high? I think you are a little off your rocker. DI is designed to be able to withstand high compression because of the fuel being sent into the chamber directly as it adds a cooling effect.

Lower compression gives you a greater ceiling for tuning thats about it. My port injection motor runs 9.5:1 compression. I have assembled a few motors that use 11:1 even 12:1 compression for boost. Now they are running Q16. But the compression is not our problem here at all. I mean if compression is just the issue here, then get a thicker head gasket and cal it a day.

I would look more too a stronger rod and a larger turbo. This engine needs more room to breath than mazda gave it for it to perform the way a performance minded enthusiast would want.

[06Speed6]

Originally Posted by Jarods7920 You think its too high? I think you are a little off your rocker. DI is designed to be able to withstand high compression because of the fuel being sent into the chamber directly as it adds a cooling effect. Lower compression gives you a greater ceiling for tuning thats about it. My port injection motor runs 9.5:1 compression. I have assembled a few motors that use 11:1 even 12:1 compression for boost. Now they are running Q16. But the compression is not our problem here at all. I mean if compression is just the issue here, then get a thicker head gasket and cal it a day. I would look more too a stronger rod and a larger turbo. This engine needs more room to breath than mazda gave it for it to perform the way a performance minded enthusiast would want.

We arent talking about detonation, we are talking about torque vs compression ratio

[Haltech]

Originally Posted by ToledoSpeed3 Good writeup. I still think our compression ratio is a good part of our problem. 9.5:1 is too high for a high boost application, especially when throwing rods have proven to be a problem. If you compare our motor with the CSRT, their compression ratio is 8.6:1. They even have a longer stroke than our cars (3.82 vs 3.7) with approximately the same bore which theoretically should put more pressure on their rods. They make 265ft lb torque down low (~2000 RPM if I remember right) and they aren't having these problems from what we have heard. I hope we can find an easier way out, but it sounds like lower compression, stronger rods, or both may be required.

This has NOTHING to do with compression ratio.. Its about cylinder pressure.

Take a concrete sidewalk. Depending on the size of the rebar used, will determine the weight it can carry. 1/4 inch rebar may be able to support 1 cars weight in a 6 foot section, but the second you place a big rig truck on the same section, the concrete cracks and breaks. To support that truck, you needed 1/2 rebar and a little different mixture of crete that can withstand a larger psi.

Same thing with rods, rods are rated for a certain psi, even forged. Mazda did not design the car with much overhead in it. Meaning, they have 15% over their rated power and thats it.

[ToledoSpeed3]

Originally Posted by Jarods7920 You think its too high? I think you are a little off your rocker. DI is designed to be able to withstand high compression because of the fuel being sent into the chamber directly as it adds a cooling effect. Lower compression gives you a greater ceiling for tuning thats about it.

Originally Posted by Haltech This has NOTHING to do with compression ratio.. Its about cylinder pressure. Same thing with rods, rods are rated for a certain psi, even forged. Mazda did not design the car with much overhead in it. Meaning, they have 15% over their rated power and thats it.

First, let me say, I agree with you Haltech and many others...its pretty obvious that the rods are the weak point and precious little is going to protect us against this limiting factor when striving for more power. Like Lex said, they are what they are.

Here is why I brought up our static compression ratio in this discussion. 1 psi of boost doesn't equal 1psi of added static compression. When you increase the static compression ratio of a motor by 1 psi, the cylinder pressure (i.e. load) goes up exponentially during the compression stroke. It's much more efficient to add the linear pre-cooled boost pressure as you can't remove the heat created by the higher compression ratio in the cylinder, its too late. I can't recite the equations off the top of my head, but I remember that if you compare 1psi of added static compression vs. 1 psi of added intercooled boost pressure, the static pressure will generate a shite more cylinder pressure (like 5 times more for that one psi), not to mention some serious temperature increases during the compression stroke. Yes, DI will help lower the temps, and so will meth, but it probably won't overcome that 1 psi of added static compression.

Who can say this isn't a participating factor in the differences between, say the CSRT motor and our motor and what their limitations are? In my mind (please prove me wrong if you think I am off my rocker), this is the difference between reliably running 15psi vs. 24psi during spurts on these internals...provided we have enough fuel and the tuning to make it work.

On a positive note, at least we have that 9.5:1 compression ratio to carry us through when we lower our boost at lower RPMs to protect our motors.

[Jarods7920]

Originally Posted by ToledoSpeed3 Who can say this isn't a participating factor in the differences between, say the CSRT motor and our motor and what their limitations are? In my mind (please prove me wrong if you think I am off my rocker), this is the difference between reliably running 15psi vs. 24psi during spurts on these internals...provided we have enough fuel and the tuning to make it work. :

Well with all this talk of caliber srt4s and evos and the like; we are comparting apples to oranges. Lets start comparing a DI motor to a DI motor instead of a port iinjection motor to a DI motor. I mean lets compare static compression ratios.

CSRT4 is 8.6:1
EVO 8 is 8.8:1

Cobalt SS (TURBO) 9.2:1 and the achiece a max pressure of about 17-19 psi stock
Fiat 1.8L DI turbo motor 9.5:1
Ford 3.5L ecoboost 10:1

I mean they didnt just throw high compression pistons in there because they thought it was cool one day. These engines are engineered for higher compression for a reason. Its not the compression. It weak rods and very little headroom in their design. Until we replace the item that fails time and time again...these will remain nothing more than theories. When someone has the gaul to replace the rods with a better set we will know the next limiting factor in this engine. For now thats the only concrete thing we have are broken/bent rods over and over again.

[superskaterxes]

I think I'm making the latest peak torque of any stock turbo car

[ToledoSpeed3]

Originally Posted by Jarods7920 Well with all this talk of caliber srt4s and evos and the like; we are comparting apples to oranges. Lets start comparing a DI motor to a DI motor instead of a port iinjection motor to a DI motor. I mean lets compare static compression ratios. CSRT4 is 8.6:1 EVO 8 is 8.8:1 Cobalt SS (TURBO) 9.2:1 and the achiece a max pressure of about 17-19 psi stock Fiat 1.8L DI turbo motor 9.5:1 Ford 3.5L ecoboost 10:1 I mean they didnt just throw high compression pistons in there because they thought it was cool one day. These engines are engineered for higher compression for a reason. Its not the compression. It weak rods and very little headroom in their design. Until we replace the item that fails time and time again...these will remain nothing more than theories. When someone has the gaul to replace the rods with a better set we will know the next limiting factor in this engine. For now thats the only concrete thing we have are broken/bent rods over and over again.

No, they didn't just throw in higher compression pistons because they thought it was cool. They threw them in because they wanted to have their cake and eat it too. The problem with these new generation higher compression DI engines is that they are built for efficiency while still claiming to get the power of a larger engine. This is the DI marketing hype. It's a fact, DI or not, higher static compression exponentially raises cylinder pressure and heat. My point is that we would have a bit more legroom in our factory block if it wasn't for the greedy design of this engine. This type of design may work fine for well planned stock configurations, but when power modding begins, there is less room for error and growth than in a classic lower compression turbo engine. Some engines are built stronger, but ours doesn't appear to be strong enough and that is why we are here discussing it. If we understand the weakness of our design, only then can we even try to prevent against it.

The 3.5 Ecoboost only runs up to 12psi stock for a reason. I couldn't find any boost info on the new Fiat 1.8L. The Cobalt is obviously built pretty well, but even they were smart enough to scale back static compression a tad and seemingly threw in better internals. Only time will tell.

[DaleNixon]

I'm hoping aaronc7's throttle smoothing fix will give me a little more control over this high boost in low rpm's business.

[H3llfir3]

I absolutely refuse to lay into it on my car until 4K RPM.

[Lex]

Just to clarify a few things here.

The strength of the internals is what it is. I am not claiming they are EXACTLY as strong as an EVO or SRT4. No one knows that without doing some extensive and expensive testing and the end result won't help anyone anyways. So what if those cars can stand 30 more ft lbs at the wheels? Our internals and their internals are comparable in the long run.

Further, add to that the fact that our fueling system is more failure prone than a regular tested and true port injection system. On top of this there are always the failures that are due to manufacturing flaws, mis shifts, water hydrolock through the CAI etc and you have the whole picture.

The point here is that the bigger horsepower cars that last on stock internals are making that torque much further up in the RPM band. This is the whole point of why they are able to make the higher hp numbers.

We make similar torque stock to a CSRT4 and just 1 or 2 breather mods push the MS3 to the 300 ft lbs at the wheels mark.

Here's driver311s old dyno with just a test pipe. There are many more similar to this.

This also goes hand in hand with why people that race and beat the crap out of their cars don't have a problem. No one starts street races at 2500RPM.

[SneakyP86]

I have been thinking on the fueling issue for awhile now and would like to see what you think LEX. I have the SB with the pro-fuel controller which is able to control meth or extra fueling. I was thinking of adding one large injector in the intake manifold freeze plug location. The way I was thinking of using it was since with the Sb you cantrol it by RPM, I was thinking of having it come on around 900 rpm (which is above idle) ramp it to full (or where it needs to be found by trial and error) by say 1500 than when the regular fueling starts to catch up start ramping it back down starting around 3000 rpm. I would love to hear some thoughts concerning my idea.

[myspeedy07]

i like it. I used this same concept to add extra fuel to my turbo 240sx running 6-50lb injectors. I used a SDS additional injector controller at the time.

[GQ_WhiteMS3]

So ... what can we do "tuning" wise to control this ... the throttle maps are obviously a start to limit that in the lower end, I guess lowering the WGDC in the lower rpms would help too.

Anything else to reduce torque / slow the spool on this small turbo.

[94jedi]

interesting take lex. but this doesn't really explain why people blow at part throttle in LOW RPMs', unless I'm missing something.

If people were nailing it, going WOT in 3rd gear at 2k rpm, then ok. but what about the guys that blow at part throt?

[GQ_WhiteMS3]

Originally Posted by 94jedi interesting take lex. but this doesn't really explain why people blow at part throttle in LOW RPMs', unless I'm missing something. If people were nailing it, going WOT in 3rd gear at 2k rpm, then ok. but what about the guys that blow at part throt?

The damage done to a rod may be done PRIOR to exiting the block. Perhaps on a previous WOT pull from low RPM doing damage to the rod ... then the added stress of a bent rod eventually sends it out the block.

[lidokrantz]

Nice work Lex !!!.

If you have any more words of wisdom to keep this motor out of trouble....please post up...

[Scatt Nasty]

so too much torque at too low of an RPM ?

So.... If I strap a retarded huge turbo on my car that doesn't even spool until 4.5k I'm golden ? *rhetorical*

[pidass]

but don't most with 3076 on stock bottom end still end up throwing a rod even though they have a turbo that spools much later?

[bf360]

Originally Posted by pidass but don't most with 3076 on stock bottom end still end up throwing a rod even though they have a turbo that spools much later?

there not big enough, idk anyone that blew with a 3076 maybe a 3071

[94jedi]

Originally Posted by GQ_WhiteMS3 The damage done to a rod may be done PRIOR to exiting the block. Perhaps on a previous WOT pull from low RPM doing damage to the rod ... then the added stress of a bent rod eventually sends it out the block.

I see. that makes sense. So, it's from previous metal fatigue.

[DaleNixon]

Yeah none of this really explains motors taking a shit crusing at -5 vacuum.

[bast525]

This thread is exactly why I just did this:

http://www.mazdaspeedforum.org/forum/foru...uys-think.html

I'm hoping this gives me a big extra safety margin... the car definitely doesn't pull hard until right around 3500 rpm now, no matter how low in the RPM I get on it.

[phillyb]

holy shit, great fucking thread lex. i have some pics to post up about fueling "issues" as well. but i'll save that for another thread.

[Lex]

Essentially the metal fatigues. The rods don't have a completely even structure - there may be inclusions, the metal lattice might have interruptions or distortions and we know the surface is not polished. These all lower the tensile strength and yield strength of the metal. So after the rod is "impacted" enough times close to the yield limit it starts to weaken or cracks develop. At that point it will end up breaking or bending.

If you look at streeunitmark's blow-up and jimmymacs you see the exact phenomenon. Mark had an uncontrollable spike of cylinder pressure and so did jimmy before a rod finally yielded.

This simply tells us that the rod is operating close to its yield strength and after a certain number of cycles it fatigues and gives in. Material science is very well detailed in describing this phenomenon and the strength of a part can go right down to the cooling/heating rate of the part during manufacture. Seeing as these are mass produced there will be some discrepancy.

Of course the stock turbo is too small on the car to really take advantage of a late spool. However having it spool at will as calibrated stock with full bolt ons and beating on the car (always working the rod close to the limit) will eventually result in loads which fatigue the rotating assembly.

My point is that if you're going to make 300-350+ ft lbs of torque, it's best to make it at a higher RPM where it will also give you a nice WHP result. On the stock turbo, I would eliminate the boost spike in lower RPMs and ramp load instead of having it hammer like a freight train. Your butt dyno won't be as happy but your internals will last longer.

Throw in the fact that high load/low rpm is also likely to detonate and you have a mixed bag of fail waiting to happen.

Bigger turbo guys have it easier because the slower spool time actually helps and you get better top end.

Remember the tuners from Puerto Rico and how they end up with really high hp cars on the stock bottom end? You know their secret? They bolt big ass turbos on their cars, make torque late, and make tons of horsepower. Traction is better and they are less likely to blow than when making all that torque down low and ALL of us with stock turbos make a ton of torque down low at 25% throttle.

[06Speed6]

At low rpm and low load the engine also runs as high as 60 degrees of timing, thats alot of stress right there.