Upper RPM Limit TAFC/TAD

What really defines the upper RPM limit of current blown alcohol TAFC and TAD engines these days, and roughly what are those limits at now?

Valvetrain? Reciprocating assembly? Increasing parasitic losses versus decreasing power gains? Part attrition (which ones)?

Something else?....

Just curious what people's various thoughts and experiences were.

Thanks.

 

The size of your wallet & how much qualified crew help you have.

Chris Saulnier - Team Tigges
Mechanic Falls, Maine

 

Will, " low compression", how low?

 

Will how high do the 3.750" stroke guys spin typically, and are they faring better in terms of rod life?

 

Thanks Will.

I'd like to throw out a question to the whole group here. Maybe it's a dumb question, but here goes........

In these instances where people are trying to run up to let's say 11,000 on a regular basis with blown meth, I am assuming they are replacing the rods out of an abundance of caution after 10-12 runs just to be on the safe side. I totally get that, because the alternative is expensive.

Assuming that everything is totally perfect inside the engine, and assuming that a rod failure were to occur due to these high RPM issues, then:

1) What clues can be looked at or measured on a rod just prior to failure, to predict a potential for failure? Is it a gradual onset of symptoms type deal, or is it just a sudden unexpected failure out of the blue avoided only by replacing rods at a fixed number of runs whether they show evidence or not type deal?

2) In the case of a rod failure scenario as described above, has anyone done a deep dive into what the failure mechanism actually was? What failed first? Why/how? Bolts? Main cap? Beam to big end? Again, assuming that the focus is purely on the rod itself, and that everything else was perfect and can be ignored.

Just sort of curious from a forensic engineering point of view about this. Any thoughts or insights would be much appreciated.

 

Thanks Will.

That is quite useful and interesting information. So if I understand correct, the hairline cracks on your 43 run rod were in/on/within the main cap structure, near the bolt head of the nearest main cap bolt? Any particular side, or both? I am assuming an aluminum cap like the rod material?

Anyone run tool steel rod main caps bolted to AL rod bodies? Kind of like the older Pauter rods?

Interesting stuff.......

 

Another thing we keep an eye on is the bearing pins on the cap side. They can start to get loose or get rolled edges.

Chris Saulnier - Team Tigges
Mechanic Falls, Maine

 

There is a guy in Virginia that makes aluminum rods with steel caps. I believe you know him as he has a couple, three too many camshafts in his top sportsman car. He told me that aluminum rods flex at the parting line and it stresses the bolt and bolt holes and eventually they will break. He also told me that he has a go no go gauge that he uses to check the rod bolt holes and without the steel cap at x amount of runs (I forget the amount) the go no go gauge will fall into the bolt hole if the rod had an aluminum cap on it. With the steel cap on it, the go no go gauge still fits into the hole snugly like it was new. Now he uses these on top sportsman stuff and isn’t seeing the abuse of a TAD/TAFC, but if I had a disposable budget like some of the Alcohol guys, I’d have a set in an engine to see if they are worth anything.

 

The RPMs of the alky cars do not like the added weight of a steel cap

 

Steel caps with aluminum rods (Bi-Metal rods as they are called.....) have been used for decades in many engines all over the world with great success, over many thousands of miles, and heat cycles, and high RPM excursions (motorcycle). The physical science is not new in this regard.

Pauter used to make them for big inch applications. Patrick currently makes them for same. I'd like to see an FEA analysis of one in the application we are talking about. On my to-do list.

I'm not saying that this the appropriate answer, just one answer out of many possibilities.

Why did they do Bi-Metal rods in the past? : 1) Space constraints at the time. 2) Failures of the big end AL cap at elevated RPM.

More food for thought.

 

Just some quick math, the differential between coefficients of expansion of aluminum & steel is .0000058in/in per degree F. If we assume 70°F ambient & 400°F big end rod temperature after a run with a 3" bolt-bolt spacing, that is an .00057" differential.

That seems like quite a bit.

Chris Saulnier - Team Tigges
Mechanic Falls, Maine

 

Who’s gonna be the first to try a set of these?