What SixShooter and miata2fast said.

There's an excellent section in Corky's book which discusses the effect of inertial load on the rod and piston. The short version is that the inertial loads to which the rod is subjected to at high RPM can actually be greater in both the tensile and compressive states than the compressive load which is imposed on it by the combustion process.


To more directly answer your question, think about the upper half of the exhaust cycle. At the point where it crosses through 90° BTDC, the piston begins to rapidly decelerate and then at TDC it reverses direction and begins accelerating towards BDC on the intake cycle. During this time, the rod is under tension.


Of course, sometimes the rod is subjected to both compressive and tensile stress at the same time. Physics is a bitter mistress...


I don't think it matters for the compressive test. From my perspective, it would seem that load is all borne upon the main portion of the journal. I'm not an ME though.

Did more digging today, it is going to take a bit of time to get these tests setup.

All the collaborators have been contacted and we are working together now to get things rolling. I'm no ME either Joe, not yet and not for a while, and I'll be acting as the student researcher. Today, I contacted a number of Different rod companies, and had great fun tracking down who makes what for who. Unfortunately, I did make some promises not to list the manufacturer of a number of rods we buy with names on them. I will leave that up to 'you' if you want to know.

As well, myself and Shannon, the career machinist at our school, got our last person in on this test, Andrew, a 25-year automotive engineer who formerly worked for Dodge's SRT division and alluded to his works on the Viper ACR and race teams... needless to say the introduction of about 2 hours today consisted of a lot of automotive theory from crank scrapers and dry-sumps to Mean effective pressures and combustion forces vs. Rpm, as joe stated.

We are working on formulating a series of tests, currently, with Andrew's expert input, we are considering the following:
-Initial magnafluxing for imperfections
-Brinell harness test
-Grain structure analysis
-Composition analysis
We will then, after this first battery of tests, well, actually, I will... create a to scale computer model of every rod design in order to determine the stress points under load.
We will then section off each rod into an undetermined number of sections and conduct tensile testing on each section, and compare results.

So far, I have Carrillo rods, M-Tuned Rods, and 949's rods they supply as well, on my list.
Thus far, I have determined a few key points, but I'm withholding these until I can compile everything in a proper report. I was able to speak with representatives from Carrillo and 949's supplier in-depth today, and I gotta say, the fella who supplies 949 seems to be a very stand-up fella, and was very forthright in any info I requested. I must say I have a better feeling about that product today than I did a week ago.

I'll be speaking with Carrillo's QC manager at a later time, as well as a few of the engineering staff at the other firm at a later time.

That is all I have for today, This appears as though it will take a good deal of time, but I'll try to keep my posts few with more technical data from here on out. Thanks for the interest all.

Any update at all ?
As someone mentioned: the strength of the bottum of the Mazda engine is strong enough to take most abuse.
Since my powergoal is quite moderate I am NOT interested in strenght (as long as it is good enough). I am interested in getting the weight down. I am also on a budget build so I would look for the Eagle rods. But are they much lighter then stock, is it worth the swap ? I took my engine appart to check it's condition (bought the car with a broken engine, now got another one from the junk and had to check it over), so if I wan't to do the swap, I can do it now. But would I bennifit other then get my wallet a bit lighter ?

See my answer to your other post. This testing they speak of will take some time and they have not yet started.

Ok, But I do like to know: do the rods save any weight ? I know they do on Volvo's.

This is an interesting thread. I would love to see the results. I won't be building anything extremely powerful, but it will be good info. I'm sure the data will help a forum member make a decision on what brand they buy, on of these days, if they actually do a forum search.

You can buy a kit for $1200. With all the goodies inside. Bearings, pistons, gasket, rods. I have a fucked engine and one that is opened up for inspection from the junkyard. IF i'd like to do to it, I have to do it now (not as in this minute but the comming months).
I do beleve they are stronger then stock, but will it get the weight down of the moving parts ? I have a piston and rod pulled out of the engine and they are not that heavy...

Any plan to do fatigue testing? Small-end or big-end?

It would only take 10-20 rods of each type and access to some super expensive equipment to a good S-N curve for each type of rod.

That would be some useful info...

Any updates or ETA for results?

Subscribed. I remember seeing something once (this was for a turbo kawasaki) that said the stress on the rods at full tilt was more than the that of running 18 psi at full power. And that's taking a motor designed for 74 hp and putting 250 through it.

As far as real advice, rods in a miata are big, pistons you can skip, I think. Once I went to good rods in my motor, I never really had issues. Perhaps the issues are different with the 1.6.

Found the section in Corky's book that I was looking for. Hopefully he won't mind my reproducing a small excerpt from it here.


Now, the relative magnitudes of the two curves will vary with engine design. A big diesel engine running high boost at low RPM might well produce a greater compressive power load than either inertial load. A gasoline engine running at high RPM, OTOH, will look much more like the chart.

The last two sentences are interesting, as Corky makes the argument that tensile stress is the real killer, and that compressive stress (especially that resulting from combustion pressure rather than inertial force) pales by comparison.

A big diesel has big lumpy pistons. So I actualy think the same rules apply.

But all the bad rods I have seen are bent. Some turbo engines are known to produce wobbly rods when too much torque is produced at low rpm..

The bent rods on MT had waterinjection. So they may be the result of an liquid-lock.. But all 4 doesn't make sense.

Posit: Tensile stress (a product of high RPM operation) causes fatigue. This reduces the strength of the rods, and thus their ability to resist compressive loading. In a naturally-aspirated engine, compressive power loading is not sufficient to lead to deformation. However, in a turbocharged environment where both BMEP and peak cylinder pressure are several times greater, the weakened rods eventually yield and undergo inelastic deformation.

Just a theory.

Doubtful. The quantity of water that we're shooting at the engine is pretty trivial, generally much less than the quantity of fuel. I can't think of any failure modes of the WI system (as typically employed) that would plausibly support a conclusion of liquid lock without leaving some very obvious evidence, such as part of the nozzle broken off and rattling around inside one of the chambers.

We're at a point in the lifecycle of the turbo Miata where there's a pretty solid body of evidence to support the conclusion that once a certain power threshold has been crossed, the probability of rod failure increases exponentially. What we don't have is a good explanation of why this occurs, or any empirical evidence to differentiate the relative performance of one aftermarket rod from another.

These are interesting times, indeed.


Nor I. Never took thermo, properties of materials, or dynamics. I just like learning about new things, particularly in an environment rife with speculation and uncertainty.

Likely knock/preignition - there's not enough leverage to move the piston/car out of the way, and plenty of peak pressure and bam, there goes a rod.

What's the dead volume in the engine? Pretty high. 2 liter / 4 = 500 cc @ 10 to 1 compression that's 20 CC's before you'd have an issue. Quite a bit of water, imagine how long your WI tank would last at 20 CC's per ignition event.

I've bent 2-3 rods at a go, and it was because I wasn't fueling and didn't pull timing.

Yessir. Forces are going at LEAST as the square of the RPM, so the jump from 6000 to 7000 is 36% more load. 8,000 rpm is 70% more load than 6,000. So, people making big power always go for "a few more revs", but they have no idea how much more stress that makes over just running more boost and better low end breathing.

I'm guilty of liking this approach myself. Sure, you get power linear with flat torque, but that's a hard thing to accomplish at some point.

Ah! Here is the answer that actually means something! It's that last bit that is important.

One thing I'd say we do know - pistons are an issue way before rods are. Not a lot of people with rods and no pistons have issues, so the smart money is rods then put more cash into known weak points.

Did you mean "Rods become an issue way before pistons are" ? Either way, That's what I have figured by reading what others have posted.

Sorry for all the curious minds, no deadline or ETA on the project yet. I have just basically gotten familiar with the equipment we need to use, and we did some hardness tests on the Carrillo's only. Still need to get some of the other aftermarket sets, and factory sets.

Read through all the comments, and I was going to mention I am more thoroughly convinced by Mr. B that tensile testing is the significant factor, despite boost pressure and the likes. He likes my theory on the exponential increase in gas expansion, basically, but still believes rpm is going to be the major stress on any motor exceeding 7200rpm, which a lot of people do with built motors and SEM, If I'm not mistaken.

I'm hoping to get this through before Christmas, but I honestly wouldn't hold my breathe.

In like Flynn.

Getting a very wide powerband is nice.. So reving it up is a good thing as long as you do not lose to much in the low revs.

So actualy there are more factors on engine failure (the rods):
1. Revs
2. Torque
3. Detonation or too much timing

I decided not to change my rods, but now I have to think it over as I like 2 of them and will dyno into the third..

Thanks about clearing the WI story.

500 grams for each rod was mentioned for the eagle rods, I still need to put my stock rod on a scale.

I agree...maybe even a FE3...

Subscribed.
I was talking to my local machinist about this today. He said that eagle rods and a few other manufactures forge the rods in China, and have them shipped here for final prep. He said Carrillo however does all of their work in the USofA. It would be interesting to see if Chairman Mao's steel has gotten better over the years.

They're steels seem to be okay, it's their machining that suck. My work gets tons of their forgings from China, that we machine here in the states. Any finished product (forged and machined) we buy from China turns out to be crap. They'll put their best machinist on the job for the prototype, we'll approve the part, and then they'll throw dozens of machinists getting paid $.1 an hour to mass produce, and they're crap. But forgings alone seem to be OKAY. Okay because USofA forgings seem to have slightly better strength, but the products my company makes doesn't need the little bit of extra strength, I'd like it for my rods though.