The OEM rod bearings debate
 

Hey guys! I am about to start my "adventure" of building a motor for my car. However after lurking the forums over the past year I have seen a few people comment on issues with OEM bearings. And like every other "problem" you see in a community of this size sometimes things get skewed out of propotion. Sometimes a small problem wasn't a problem and it was just simply a different issue altogether that looked like a problem.

So I thought it would be a good time to openly debate this topic as a number of us are starting to get ready and / or are putting a fair amount of money into a forged bottom end for higher horsepower applications. There are a number of manufactures such as Dynasty, Full Blown, and others who have "shortblocks" prepped and ready to go. All using OEM bearings. A number of companies are saying the OEM are just fine. But I have also spoked to a number of individuals who swear by the fact that the OEM bearings are a severe problem at 300+ WHP.

This topic can get a tad confusing because you see a number of people running 300+ 400+ 500+ on their cars without any issue from the factory. The reality is that we don't know how much these individuals actually drive at these horsepower levels. So how much of a problem is there with this?

I had started researching bearings and it is not an easy solution as of yet. I know @Don@Accelerated is currently producint modified rods that fit a custom set of ACL bearings he has worked on which is one option. I also know a local engine builder that has a coating technique to strengthen the rods (Although it is still in testing from what I know). I personally have contacted a number of manufactures.

Kings engine bearings has told me that they wont even start looking into bearings for roughly six months for this car. As they have a large list of other cars that are currently more in need of bearings. Lets face it not many people are really doing builds from the eyes of bearing companies. 50 builds is a very small number for a company to mass produce for.

ACL is closing their factory down in june 2014. This was announced a while back and at this time I don't think they have any desire to produce bearings for this car as there is little reason for them to "innovate" new products when they are shutting down their factory.

Mahle Is currently not producing any bearings. I will call them again today to ask if they have plans to produce for this vehicle.



Accelerated performance has told me they had a number of builds with bearing issues. I cannot find any reference on this forum to these individuals. However a lot of the car community simply is not on this forum.

I know @cf6mech has had trouble with his build that was built by AWDtuning that was pointing towards rod bearing failure. He has since rebuilt his engine using Don@Acceerlated's modified pauter rods with ACL bearings and as far as I know has had success.

You can follow cf6mech's posts on the topic here as when he was breaking ~400WHP he was starting to have issues.
http://www.ft86club.com/forums/showt...t=25673&page=2
http://www.ft86club.com/forums/showthread.php?t=44765

Are there any shops and / or users that have modified engines that would like to contribute info into this debate? I am looking to re-do my entire setup and looking to invest a "hefty" amount of money. I would hate something as simple as a bearing issue destroy my investment and ruin my day. :)

Surely there is somebody out there that has non-factory internals? I would love to see some input on anyone that has modified internals with factory bearings.

What you also do not know and cannot factor is:

improperly set clearances
bad line bore
installation error
driving habits
preventative maint

I could probably list another half dozen

I work on industrial machinery which utilize different types of lead,bronze babbit etc. If the lubrication is adequate, tolerances are maintained
and temperature is within range,a factory bearing will hold up no problem.

I have seen instances of bearings destroying crankshafts in industrial applications because the material was too hard.i would rather replace bearings than crankshafts

This is the exact reason I made this thread. I am hoping it is an install issue or another issue that "looks" like a bearing issue. I am literally about to pull the trigger on an expensive bottom end build. I stopped after reading about "possible" issues. I only have 1 confirmed motor with issues. And a shop that said that they had "other" issues but no on else has actually confirmed.

I would love to see the failed bearings. A number of vendors have told me that they are 100% confident in the oem bearings. I talked to Crawford earlier this morning and their staff confirmed that their "test" car is still running OEM subaru bearings and that the tach was reporting over 15k since that engine was assembled.

I also called ACL in Australia to confirm that they will in fact be closing down, they will in fact not be making bearings for this platform. They also confirmed that if they were it would be 2-3 years due to current desire for bearings for this platform. I asked them if I could purchase a one off set, or one off production for a high dollar value and was told it is unlikely that they would be able to do that with the factory shutting down.

It is starting to look like modified rods or oem bearings are the only two options for the time being, and likely the next 6-18 months. I am going to PM cf6mech and see if he has any photos of his failed bearings.

You might get better info if you post this in the forced induction forum. That said if you are doing a strong engine build wouldn't you look at upgrading these just for peace of mind?

If you read my post these do not exist, nor will they for some time. I literally offered ACL thousands of dollars to make a limited run set.

Power Enterprises is about the only bearing maker I know of that makes stuff that's "better" than stock (it's a different design and I'm not sold on it myself, but I've seen 1000+HP 2JZ's run them fine).

King is known to be, well, crap. At least in the other car circles I've run in.

Factory is usually more fine tuned than aftermarket (having more size variations to get the clearances spot on). On many other cars they're the go-to for the best available, and usually only not used because of cost or availability.

Keep in mind we're talking about hydrostatic bearings here, there's no real "wear" or contact taking place here. Generally if they're a crap bearing, they're not consistent in size and construction, as they're really only shims in such a system. Therefore, any failure is more likely due to other issues (detonation hammering them, oil getting too hot/thin, etc)

This is exactly what I was going to say. I'd avoid aftermarket bearing unless I 100% knew that the OEM ones wouldn't stand up to it and even then I'd decide if I'd want to risk spending all that money on a re-build that might need to be re-done.

There's so much that goes into re-building an engine, especially these newer ones with very specific tolerances, that I'm never sure I'd trust someone to do it perfectly, myself included. If it's not perfect it will need re-doing in the not too distant future and big cost again.

We have been building race motors for several years and have found today's factor bearings can take a beating. That being said your bearing are only as good as that time you take to set them up. There are a few coatings you can us to help all of your internal parts. We have had our motor open just to spec everything. We are building a motor for the 2014 season and we will be running stock bearings.

I will chime in on this post as I am one of the few that has had bearing failure that caused a short block replacement. Overall bearings are relatively strong even factory bearings. Based off of inspection, what I have found is an oil starvation issue and not really a bearing problem. I think people are over thinking this bearing issue and need to look more at the oil pump and oil combination. Also bearings shouldn't be that difficult to find as all you have to do is give most bearing companies the specs and they should have them made already (our rods aren't special). I can only speak for non-FI but it should still be relevant.

Just my two cent

I do think there is an issue with the OEM bearings at high hp FI applications,...I feel others aren't putting ACL or aftermarket bearing in because they are not available so they roll the dice. I do believe its an oiling issue, which an aftermarket performance bearing addresses with more oiling clearance. The stock bearing is hard, extremely hard, to hard for a performance application in my opinion. I had bearing failure on E85 with no detonation with a very sound tune on a built motor running close to 400whp Using Amsoil 10W 40. I'm now equipped with Accelerated Performance Pauter rods with ACL bearings and at 300whp @ 10 psi with no issues.

Thanks for chiming in! Do you have any photos of your failed bearings by chance? I have also been wondering about an oil issue. I had planned to with a friends shop and see if we can make modifications to the OEM oil pump to increase the output if possible. Do you by chance have any photos of your failed bearings?

Thanks for chiming in. I appreciate you getting back to me in my PM as well. Its good to see some people who have had failure come in here. I also know the company that built your motor builds a lot of specifically subie motors... so I also take into strong consideration if they had an issue as this is their "realm". Given this is a new motor, new clearances and overall different... but the overall manufacture is still the same. So many processes are still similar.

It is interesting to hear that they are just too hard, that I could understand. Just to keep as much info in the debate c6mech also mentioned the coating was coming off of the bearings. Was it flaking? Or was it coming off in smooth line similar to contaminants in the clearance zone?

While a little off topic, given the audience in here, this might be the best place to ask. Has anyone compared the FA20 oil pump to that of the FA20 Turbo? And does anyone know if the OEM bearings are 2 or 3 layer or possibly a newer tech such as sputter bearings?

When the WRX hits the ground we'll find out, we haven't had anyone dissect the JDM engine for us at this point so the WRX is probably when American tooners will try.

:lol:

Good point, thanks.

This whole scenario just makes me feel like its the worst time to build a motor haha. Oh the joys of being an early adopter!

Having opened a FA20 i would not fear it. i would take your time and go over everything or use a motor builder. setting the clearances not matter which bearing takes time. and for the oiling they are a few thing you can due to improve the psi. knowing the clearances of the stock motor i would not use anything thinker then 0w-20w

Appreciate your posts on this forum as you are really pushing the limits and helping everybody with your info. So don't take this post wrong please...

In my experience (I've had motors go on other cars with rod bearing problems) when a rod bearing "fails" it is typically due to detonation or oil problems (heat or starvation or both) and not solely the bearing. You presume oil issue due to not having detonation, how do you know you did not have detonation? Are you running an engine management system that literally logs constantly? Do you have knock sensors on the car? Sometimes detonation is very hard to hear with human ear when you are running hard due to other noises happening especially on modified cars. Not saying your issue wasn't oil caused, but wondering how you know it wasn't detonation.

Please bear in mind it is way, way easier for a tuner/vendor to presume a problem was caused due to an "oil issue" than due to a "tuning issue". One places blame on them, the other does not.

Not saying this is the case again - but I've had vendors pull the "oil issue" card with me, but ended up seeing detonation evidence on tear down. Fortunately I had an independent motor builder do tear down and I inspected the parts and at that point I could cut my losses and move on. Unfortunately, if the same vendor that tunes your motor tears your motor down after destruction and you don't inspect the tear down they can tell you whatever they want.

Just my wonderful experience with aftermarket tuners. I would say even in the very best situation if you or an independent motor builder tears down your destroyed engine one cannot always tell EXACTLY what the culprit was between all the different things that can be working in conjunction with each other to cause an engine to ultimately fail.

One thing people don't realize is that when you run E85 or race fuel cylinder pressures can go way up with enough spark advance. I know that sounds strange--say you have 300whp (or 300wtq) on E85, versus similar output on pump. The E85 is much less likely to knock, so it should be better for the bearings, right? Well, sort of.

Peak cylinder pressures is highly sensitive to spark advance, or rather the phasing of peak combustion pressure. Allow me to illustrate.

http://www.ft86club.com/forums/attac...1&d=1385214755

Here you can see a 300 engine cycle average pressure trace on this particular direct injected engine at an idle condition where knock is not really going to happen. This is all at constant torque, so other things were adjusted to make up for the effect of changing spark advance.

The top pressure trace has peak pressure occurring around 40 degrees ATDC firing. Spark is very late, after TDC. Now look at the Y axis. Peak pressure is about 9 bar.

Now look at the middle trace. Peak pressure is around 15-20 degrees ATDC, which at this speed and load is roughly MBT (minimum spark advance for best torque). Peak pressure is at 12.5 bar. Do the math--that's about 50% more cylinder pressure from retarded spark vs MBT, but engine output in the case is the same.

Finally, check out the bottom trace. This spark is way past MBT, something like 30 degrees BTDC. Peak pressure is about 15 degrees BTDC, way too early. Peak pressure is now about 19 bar. From the two extremes of unnecessarily retarded combustion to overly advanced combustion we double the cylinder pressure. Let's look at it with a pressure-volume diagram, (x axis volume, y axis pressure)

http://www.ft86club.com/forums/attac...1&d=1385214755

This relationship between non-knocking spark advance and peak cylinder pressure also exists at high loads. So what does that tell us? When we make our engine output relying on using knock resistant fuel and spark advance, it greatly increases cylinder pressure. What do you think that does to bearings? I'll say that again: Fully taking advantage of knock-resistant fueling with spark advance can be very stressful on the engine due to high cylinder pressures, and it's not knock causing the stress.

This engine was never designed for high loads (lots of airflow crammed in there at a given rpm) and high cylinder pressures. Think about it when you put E85 in there, boost it, and have a low knocking tendency fueling (race fuel, E85, maybe water/meth).

Thanks for this post. Sorta reinforces the last paragraph in my post before. Typically detonation or oil for bearings, but at the end of the day there are so many things it could be especially when considering combinations of things and all those possibilities. In this case you are saying possibly just the nature of much higher cylinder pressure, and I agree, that would stress the bearings more.

@arghx7 Thanks for the detailed post. I have always wondered how much of a pressure difference there was on spark timing. Do you have a reference for those photos? I would love to read more. :)

So when tuning, do tuners tune for power by adding timing and simply listening for knock?

I'm wondering if or how they know to tune for the best, safest power.

Here's something to check out. It's from a Ford study of using dual fuel pump gas + E85 capability in an 3.5L Ecoboost engine with PFI and DI. Ford Ecoboost E85 Study

Here are some more relevant charts.

http://www.ft86club.com/forums/attac...1&d=1385234537

For purposes of this discussion, BMEP can be converted to torque. 10 bar BMEP is 260 lb ft and 20 bar is 520 lb ft. CA50 is location of 50% burn. MBT is typically 6-10 degrees after top dead center. Add roughly 5 degrees for location of peak pressure (CA50 of 10 = peak pressure at roughly 15 degrees ATDC firing).

http://www.ft86club.com/forums/attac...1&d=1385234537

Example: near 20 bar BMEP on this engine, so 520 lb/ft of torque at a fixed 2500rpm. With pump gas only, sprayed through port injectors, we run -10 BTDC degrees spark (10 degrees ATDC). With E85 direct injection we can run 30 degrees spark.

The second chart is kind of complicated and hard to explain without reading the attached paper. Basically, they were dialing in spark, AFR, and E85 blend % to #1 not overheat the turbo and #2 not exceed 100bar peak pressure in order to protect parts in the engine. But you can see that they limited peak pressure to 100bar even when the engine wasn't knocking in order to prevent damage. They pulled timing even when they technically didn't have to, which then fed into the E85 blend ratio and Lambda so that the turbo didn't overheat. It's also a little complicated because you can go by average peak pressure across cylinders, average peak pressure over some number of cycles on a single cylinder, or peak pressure on a single cycle for a single cylinder.

Sub'd

Very interesting. Where can I find the first article? I am quite interested in the way they handled the torque compensation. I also would love to see the comparison of ignition vs AFR to pressure.

If I'm understanding correctly the reason the 500 torque is still driven is due to the "usable" cylinder pressure. As if you were to create massive pressure prior to TDC the duration of the high pressure is wasted during compression, and only partially usable post top dead center. This would visually show the same power output due to massive cylinder pressure and "shock" due to the "spike" of which the cylinder pressure increases. So you may be putting literally 2x the stress & at 2x the shock speed that would translate to possible bearing shift, or other issues due to the requirement for the components to respond to the pressure.

We constantly just think about power output and the spark as being "instantaneous" but the pressure has a duration. You could make 20bar pressure peak at BTDC, but by the time the stroke hits TDC you are only seeing 10Bar, both of these instances would see the same output. But the prior scenario would be showing 2x the stress internally.

I would be really curious to see what kind of ignition advance @cf6mech was having when his forged engine was struggling. If the engine was tuned past MBT the cylinder pressure could be significantly higher... However I don't want this to turn into a debate about who is a good tuner, and why. But he is one of the only to speak up and share his experience at this point. I don't think his crank was ever inspected he just replaced it but i cant verify that.

Would having a softer bearing be more resistant to damage via aggressive cylinder pressures by having more "flex" ability in them? If so that could explain the issues being seen. I have thrown a rod on E85, but have never taken the time to inspect it. I am curious to see what my bearings & crank look like. In the next few weeks ill inspect them. I also have logs of 'similar' scenarios showing my ignition timing and other variables that may be of use.

I really wish I had a setup locally that I could test on a dyno to see exactly where MBT was compared to my vehicles tune. I really want to find out what the limits on the OEM bearings really are as Don @ accelerated's solution seems to be the only available after market option at this time. And we have a number of people in the 400-600HP club on OEM blocks running just fine at this time. Although I am sure it is on borrowed time and for all we know it could be due to production inconsistencies. Does anyone know anyone that had an engine fail running on pump gas where you would actually see detonation? Everyone that I personally know that has thrown a rod or had a drastic failure was on E85. In which you may not see knock... you may just see a bent rod due to cylinder pressure.

Again the reason for this debate is a number of shops are saying the OEM bearings are just fine (such as crawford with over 15k miles on OEM bearings with forged internals). As well as a number of other builders who say they prefer OEM bearings when possible. Just so few people with built engines have popped their head up and shared their experiences.

Personally i'm starting to lean towards doing my build with OEM bearings at this point and tuning extremely conservative on ignition timing during any load beyond highway engine loads.

Adding timing, or adding boost in the case of forced induction. You can listen for it but most of the time with a stock ECU it's just a matter at looking at what the knock control system is doing

So there are two things we're talking about here. One is knock, which will cause pressure spikes, increased temperatures, etc. That is definitely going to cause problems for core engine components like pistons and maybe bearings in this case.

The other thing is essentially pushing the engine too hard without knock. At the risk of oversimplifying things, you can make more output by throwing more air at it and by adding spark if you aren't knock limited. Both have advantages and disadvantages but they interact.

Judging whether the tune is safe even in the case of no knock is not easy. No matter what there is trial and error involved. It's easier to understand if you have some specification for the parts. We can certainly guess that the engine can handle a max speed higher than the factory rev limiter, and there haven't been widespread concerns about tunes that have revved higher.

Load on other core components from pressure and temperature are difficult to judge though. You need some specification on what they can handle, which you'll never get for the OEM parts but you could get for the aftermarket maybe. You also need very expensive instrumentation (parts and labor involved) to read temperatures and pressures in the combustion chamber, or in the oil galleries.

So really all you can do is trial and error for this stuff. All the older platforms have gone through it already. Look at some forum posts for DSM's or EG/EK Civics from 10 years ago. People were working through the same type of issues.

Well lets face it too, most engine builders dont really share out or volunteer information. Its bad enough trying to get vendors to say anything anymore about testing. But one thing is for damn sure those claiming 400ish HP on here you wont see any logs or much proof of any sustained abuse like track time.

I mean you can throw boost and crazy timing at any motor for short periods but sooner or later something is going to give. I dont take any of these shops claiming high HP seriously. Namely if its just dyno speak. Nelsmar keep doing research and post back.

I was there for tear down,... I saw the suspect rod and crank, no detonation,....I have, and have had from the beginning a Zeitronix wideband for fail safe /data log/ and general monitoring of systems....... My engine wasn't destroyed,...it was one bearing in one rod.

That's great that you looked and also were running a wideband sensor AFRs/knock. I was just asking the question so we are sure the engine wasn't seeing det. Possibly it was an oil issue then.

Thanks for not taking my post the wrong way and as always for your information!

Also remember E85 is more stable... you dont see det as often. That doesn't mean the car wasn't tuned past MBT. And @cf6mech I am not in way shape or form saying that your tuner tuned it past MBT. But as far as I know it still possible to bend a rod via firing before MBT during compression stroke and have no detonation signs. At least thats what I have been told in the past... Someone correct me if I am wrong. I was just under the assumption that you could slowly bend that rod (or quickly) form being just past MBT and causing too much stress.

You see other users on E85 pushing 400+WHP on their car and they are still running. I would love to see a comparison of some cars that are and aren't running to see the ignition vs fuel at certain loads.

From my point of view for all i know cf6mech's new motor may just be stronger (or more flexible depending on how you look at it) than the previous one due to the bearing setup and more resistant to the shock of being close to MBT on E85.

Will do. I may just "gamble" on a set of bearings and just tune the car my self at this point so I can actually say what I did or did not do and show my actual tune in public unlike what other vendors would do. I am not the best at tuning in the world but then at least there would be "open" information. I am just trying to decide if I really want to jump in and throw money into a burning barrel at this point. So far I'm leaning towards yes... I'm having lunch with a friend who builds motors for a living to ask his opinion on the build and see if he wants to toss me a hand with my teardown. I plan on spending a lot of time on the dyno and actually checking my tune against MBT as I don't think my last tune was actually tested for MBT... (but that is a whole different discussion).

Hell if anyone wants to contribute to me doing a mid-build with OEM bearings I would gladly do lengthy tests on the dyno and post results of where i find MBT and report back testing. I put 25k miles on my car in 6-7 months. If anyone is going to find out how long term OEM bearings will last under conditions it would likely be me, especially considering my climate in my region. I'll obviously be trying to do this anyway but parts are pricey since the car is so new. So it will take me a while to piece everything together. ;) At this point I am looking towards buying an OEM long block. I am half tempted to test an OEM long block on the dyno for a bit and then tear it down and swap the internals. I wish more shops would post information on this. But then again this is how they make money, by having their reputable builds that they can sell by doing their own testing.

I also fully agree about this not being anything new. I am not new to the car scene. I remember when a number of motors were being seen as "weak" and little to no hope. Hell I remember the K series... that one was great. Everyone was complaining that they didn't accept any after market parts and were just horrible. And now look where the "threshold" is. I have a friend with a K20 that is practically stock that was putting close to the power of my first vortech setup.

Yeah, very possibly could have been what you state or oil like cf6mech thinks. I'm glad we can rule out detonation since nobody really learns anything there except for bad tune and everybody already knows that can destroy any engine no matter how robust. The whole cylinder pressure without det is very interesting topic though (especially with how E85 allows the tune to be so much more aggressive either NA or FI) as I had not thought of that until it was posted up, I just assumed it was det or oil.

If you want to tune it yourself, get help from the community, and also a little help from reputable tuners a solution like this probably would be the way to go:
http://www.ft86club.com/forums/showthread.php?t=46468

That is worthless to me personally. Remember OFT is just a "cable". Not a tuning platform or software suite. OFT is actually locked down extensively. I may however start contributing more to the open source guys and start working on a flex fuel setup for that. If you look in the ecuflash thread I have posted in there with trying to contribute with data logging ability. (i have built my own hardware data logger for this car that accepts ECUTek extended parameters). If you are not fully familiar with OFT please go find the OFT thread that was bashing on ecutek (whom im not a fan of either by any means, so dont think im on anyones side). Find my posts in there talking about what OFT really is. At least I think it was in that thread... As there is a number of people confused thinking OFT is a full blown suite for tuning. The developers of OFT use another companies software and literally just make hardware for reading / writing to the ecu. The owners were just kind enough to make some "optimized" base maps as well as an incentive to purchase their hardware. So i can't really "Contribute" to OFT as it is their project and their hardware and it is closed source. OFT is a device similar to the tactrix cable but slightly more advanced, and afaik doesn't follow the J2534 protocol which makes it a "closed" and propriety cable. Although this is of little use to know the difference to most end users since they are just using the OTS tunes provided anyway and aren't actually going to be tuning their cars them selves. However if I do testing on MBT thresholds on different setups that could be used on any platform may it be OFT + TunerPRO, the opensource guys, ecutek, whatever.

Sorry, didn't realize you want to actually change your tuning software via a community driven project. Thought you were just interested in tuning your car's tune/parameters which OFT does allow you to do I think (or whatever software it runs that is) versus something like Ecutek in which you cannot even change your car's parameters.

I didn't say what i wanted to do... I do however want control to edit my own tune in which ECUTek currently does not provide. OFT does not "allow" but if you use OFT to pull a ROM file you can use a number of software programs to edit the ROM just like a tactrix cable + ecuflash. (which is actually significantly cheaper but does not have ability to store roms or have a graphical display, and requires a laptop for flashing). OFT is not a platform, it is only a cable for datalogging & flashing an ecu.

But lets keep this on topic if you have questions regarding any of this please feel free to PM me and we can talk via there, or phone and ill gladly make sure everything is clear. :)

It doesn't necessarily have to be tuned past MBT to raise the cylinder pressure too high once you throw boost into the equation. Without cylinder pressure indication you can't be sure. There are limited options for that, but TFX engine technology does offer a system for this kind of performance use. It's going to cost a lot still. R&D is expensive no matter how you do it.

so @arghx7 do you have any charts showing the effect of increased mass vs the point of MBT? As in increasing air in the chamber which would result in increased fuel changing the density or mass of the atmosphere int he chamber which would change the burn rate and time it takes until the pressure reaches the piston? As well as different compression or deck height showing how it alters where MBT is obtained? Ive always wondered this.

You're talking about three different things: ignition delay (0-10% burn time), combustion speed (10-90% burn time), and combustion phasing for MBT.

In the ECU you set the spark timing. That's only when the secondary coil in the ignition system activates. There's a delay until the mixture really starts to burn--the most common way to represent this is the burn delay/ignition delay. That's the number of crank angle degrees from 0-10% burn. That's dependent on all sorts of things, but one of the big things is the amount of residual gas in the combustion chamber as a result of valve timing. Speed and engine load affect it, and once you talk about lean burn engines you have that playing into it.

The second thing is the combustion speed, which is often represented as the "bulk burn." It's the number of crank angle degrees from 10-90% burn. It's greatly affected by the design of the intake port (tumble and swirl flow) and also the geometric compression ratio. There's tons of other factors related to the fuel for example.

The last thing is combustion phasing. On a spark ignited, homogenous charge engine it's generally accepted that MBT is achieved when 50% burn occurs between 6-10 degrees ATDC firing. Usually the rule of thumb is 8. That doesn't really depend on a lot of factors: it's not that sensitive to engine load, engine speed, number of cylinders. I think a lot of it just the basic physics of reciprocating piston engines. The combustion phasing is not the spark timing. The spark timing required to get a 50% burn at 8 degrees ATDC varies according to changes in burn delay (0-10%) and combustion speed (10-90%).

Here are some charts from an experimental GM LNF engine (found in Pontiac Solstice GXP) showing peak cylinder pressure on E85 vs E0, running at MBT (50% burn between 6 and 9 degrees ATDC) on E85 and whatever combustion phasing was needed at borderline knock condition on E0.

http://www.ft86club.com/forums/attac...1&d=1385329362

http://www.ft86club.com/forums/attac...1&d=1385329362


But you'll never know what your combustion phasing, burn delay, combustion speed, or peak pressure is without a combustion analysis system. And without that info, it will be difficult to make a link between spark timing, peak cylinder pressure, and the bearing durability question. You need a cylinder pressure sensor installed, and then you need to run different spark timing under WOT pulls and see how that affects peak pressure with E85 or race fuel. That's going to cost money--a lot more than a set of aftermarket bearings.

Unfortunately it still comes back to trial and error... run a certain setup, see if it breaks, and speculate as to why it broke and what needs to be changed to prevent it from breaking again.

I'm attaching two papers. One has some discussion about combustion phasing/speed and engine efficiency. It's at part load condition though. The other is the study on an E85 version of the GM LNF engine that was in the Pontiac Solstice GXP.

You are an incredible resource, thank you for sharing with the forum.

I remember reading, it think it was in Crorky Bell's Maximum Boost but I could be wrong, that FI creates lower peak cylinder pressures then NA for the same power. As I recall it, the reason was that with FI more of the ignition stroke is used to provide power or something to that effect. Is there any truth to this, if so, would you mind expanding on the subject?