Calculating conrod max rpm?

[Spetz]

Hi everyone,

I've been contemplating doing a DIY engine build as I've always wanted to build an engine myself.

I am wondering whether there is way to calculate what the max rpm a conrod can handle?

Currently the valvetrain and pistons are OK but the conrods are questionable.

The conrods are from a Mitsubishi 6A13TT.
It's a twin turbo V6 that revs to 7,000 or 7,5000rpm (not sure exactly).

I would like to use those conrods (same block/crank) without the turbos however with the possibility to rev to about 8,200rpm.

I am not sure if stress on conrods is rpm dependent, or boost dependent etc?

And what can be done to strengthen them?
Rods bolts, shot peening, cryo treating etc?

 

[Colt45ws]

Usually RPM kills rods, though weak rods can fail from boost. (looking at you ford with your powdered metal rods)
The weak point is either the rod bolts breaking, or if they are half decent, then its the small end, where the connecting rod is the smallest right under the piston pin. The rod will just pull apart from the forces.

 

[Kestas]

I've never heard of max rpm being calculable.

I've seen conn rod assemblies break in the middle, at the shoulder, in the shoulder of the cap, the cap bolt, and at the wrist pin bore.

 

[Trav]

May I suggest this book before getting into the weeds. Its cheap and an Ebook

https://play.google.com/store/books/deta...CFVELDAodYQYEjA

 

[Trav]

Originally Posted By: Kestas
I've never heard of max rpm being calculable.

I've seen conn rod assemblies break in the middle, at the shoulder, in the shoulder of the cap, the cap bolt, and at the wrist pin bore.


But piston speed is and that's where he needs to go first, lots of variables.

 

[spackard]

My dad would be able to do this, before he retired.
Finite element analysis. ANSYS was the program he preferred.
It would take a long time to model.
He'd be able to see the weaknesses though, and suggest design changes.

Problem is - you probably don't have access to the materials that went into building the conrod, so, couldn't do a FEA.

 

[Kestas]

I'm not intimately familiar with FEA, but I believe they use tensile strength and maybe yield strength for analysis. The data that is not broadly available is fatigue crack sensitivity for materials.

The popular conn rod materials I've seen are 11XX-series forged steel, ductile iron, and powdered metal. Some exotics use titanium alloy.

The parts are often shot peened, which markedly affects conn rod fatigue resistance.

FEA is a good start, but should always be followed up with testing, which uncovers unexpected weaknesses.

 

[ARCOgraphite]

You're not a pro builder? No?

FEA is not realistic for you.

Consult with those who build such engine and find what they've seen for failures.

You should make sure rods are straight, grind off parting flash, magnaflux for microcracks and shotpeen.

Rod bolt quality is VERY important and upgradable, Bearings and bearing oiling is big part too.

Have fun. But I don't know why you want to spin a TT 6 to over 7500 or even 7000.

Make big torque 4000 - 7000

youll be good to go - - - - F A S T

 

[4wd]

Originally Posted By: ARCOgraphite
You're not a pro builder? No?

FEA is not realistic for you.

Consult with those who build such engine and find what they've seen for failures.

You should make sure rods are straight, grind off parting flash, magnaflux for microcracks and shotpeen.

Rod bolt quality is VERY important and upgradable, Bearings and bearing oiling is big part too.

Have fun. But I don't know why you want to spin a TT 6 to over 7500 or even 7000.

Make big torque 4000 - 7000

youll be good to go - - - - F A S T


^^^^ Something like this … the designer uses FEA to find stress riser issues … Von what’s his name stuff … but guys who race learn to trust a brand or two for HP parts

https://www.ebay.com/itm/Manley-156mm-Long-Connecting-Rods-for-Mitsubishi-Evo-8-9-Eclipse-GST-GSX-DSM-2G/262001525149?fits=Make%3AMitsubishi&epid=1011857515&hash=item3d0082019d%3Ag%3APSUAAOSwU4FaPCJM&_nkw=mitsubishi+connecting+rods&rt=nc

 

[Spetz]

Thanks guys,

I meant more in the vicinity of "8,200 rpm is X percent more stressful on conrods than 7,000rpm" calculations?

Also would 7,500rpm + boost be more strenuous on conrods than 8,200rpm without boost?

And what (rough) percentage can rods be strengthened with different types of treatment?
Would balancing make life easier on rods?

I know of people who do run these rods to 8,000rpm but in street driven cars.
Mine is track only so 8,000rpm might be a common occurrence (once cams etc are ground to be able to get power in that rev range)

 

[Shannow]

Originally Posted By: Kestas
I'm not intimately familiar with FEA, but I believe they use tensile strength and maybe yield strength for analysis. The data that is not broadly available is fatigue crack sensitivity for materials.

The popular conn rod materials I've seen are 11XX-series forged steel, ductile iron, and powdered metal. Some exotics use titanium alloy.

The parts are often shot peened, which markedly affects conn rod fatigue resistance.

FEA is a good start, but should always be followed up with testing, which uncovers unexpected weaknesses.


Can do fatigue in FEA...here's an interesting paper
http://www.steamforum.com/pictures/t31pg105.pdf

Go to Case Study number 1, the L-2 blade failures. These were Parsons (GEC Erith) 500MW machines, that had some blade failures...the machines ended up being fitted with blade strain gauges, and induced current radio telemetry back in the 80s to measure actual blade loads, then that being fitted to a Campbell diagramme, and modifications of brazed and pinned detuning weights....worked well, and in the mid '90s the machines were fitted with new blade forgings with the detuning weights included.

I took over those machines in 2000, and 13 years later (after blade change in the '90s), had my first blade failure.

Got them re-modelled in modern FEA software, and the model took into account the material (a bit like 410 stainless)...the cyclic load due to gravity, and the blade passing frequency...calculated that the issue was a 20 node wave washer shape (discounted as an issue in the 80s), that would survive a minimum of 13 years until crack initiation in an environment with some chlorides...then should a crack initiate, failure in less than 40 days.

Was amazing...the entire study cost about $60k...

BTW, you like metal, I like engineering...Ducked outside to take a snap of some things I kept...can see the forged in detuning weight, and I hope, can see the "novel" Inconell weld that I came up with to join sections of buttstrap so that I could replace blades without taking rotors out, and having to heat treat the entirety of the 14 blade segment (and the titanium fish plates connecting blade packets)

 

[Shannow]

Originally Posted By: Spetz
Thanks guys,

I meant more in the vicinity of "8,200 rpm is X percent more stressful on conrods than 7,000rpm" calculations?

Also would 7,500rpm + boost be more strenuous on conrods than 8,200rpm without boost?

And what (rough) percentage can rods be strengthened with different types of treatment?
Would balancing make life easier on rods?

I know of people who do run these rods to 8,000rpm but in street driven cars.
Mine is track only so 8,000rpm might be a common occurrence (once cams etc are ground to be able to get power in that rev range)


Rev it 'till it blows and back off 500RPM...just jokes.

Rod and Rod bolt loads are about cubic, so 7,500RPM to 8,200 RPM is about 30% more load on the rods. It's likely with in the design safety factor, but the design is also normal driving RPM for 200,000 miles, not 7,500RPM every gear change for 200,000 miles...the latter operation is in the safety margin, and why a thrashed engine breaks unexpectedly.

Revs pull the rod parting line in, and can cause the oil film to be cut off...doesn't last too long there either.

Deburring and shotpeening help massively with fatigue life...but you don't know your start point with already used stock rods.

A mate used to run Yamaha 100cc go-karts. They had a normal rod, and a high rev rod...the latter was a LOT bigger and wasted a lot of energy starting and stopping it...the guys in the know used the stock rod, deburred and shot peened...and threw it out twice a season.

Boost...with a turbo should be a bit easier on the rods...I personally can't comment, no history.

 

[ka9mnx]

Call the manufacturer of the Rod. I'm sure they could guide you...