Specific Question - ARP Head Studs on 2.7 Magnesium case - Insane?

[chris_seven]

Andrew,

I agree that the head expansion will produce a small amount of expansion but this won't do a great deal of damage as the pull out force increase will be quite low.

I would never increase the preload on a stud to a higher level than that recommended by the manufacturer without significant measurement and testing.

The increased static preload can only cause probems and be more likely to cause both stress relaxation and an increased tendancy to pull the stud out of the case.

It would be excellent if Henry would share his design calculations and we could then all give up worrying and be totally confident and stop all the 'tossing off'.

I do think, however, that these studs must create their own special Elastic Singularity and the math is probably beyond my capability to understand .

To be serious I think that there is a difference between finding a solution and understanding the solution.

[Lapkritis]

Henry, with all due respect I prefer to do my own homework rather than taking an answer that someone else found that works. There is thinking to be done outside of the fastener.

[Henry Schmidt]

Originally Posted by chris_seven

Andrew,

I agree that the head expansion will produce a small amount of expansion but this won't do a great deal of damage as the pull out force increase will be quite low.

I would never increase the preload on a stud to a higher level than that recommended by the manufacturer without significant measurement and testing.

The increased static preload can only cause probems and be more likely to cause both stress relaxation and an increased tendancy to pull the stud out of the case.

It would be excellent if Henry would share his design calculations and we could then all give up worrying and be totally confident and stop all the 'tossing off'.

I do think, however, that these studs must create their own special Elastic Singularity and the math is probably beyond my capability to understand .

To be serious I think that there is a difference between finding a solution and understanding the solution.

Chris, you know I'm not an engineer nor have I ever made such a claim so that is why I hired a company that specializes in fastener design to pencil out the specifications of our stud. There's no magic, just a reasonable design.

Originally Posted by Lapkritis

Henry, with all due respect I prefer to do my own homework rather than taking an answer that someone else found that works. There is thinking to be done outside of the fastener.

I wonder if you truly respect real world experience?
Building a 911 engine has a plethora of challenges and investing time in or trying to reinvent the wheel seems counter-productive in the rebuild process.
If you find something that works better than the solutions that have proven results, please let me know. I'm always looking for a proven "better way".
Cheers

[Lapkritis]

Thanks Henry. Yes, I do respect experience. That said I'm not here for a bay of pigs. There's hard data to collect and analyze and I have some of the best engineers with years of experience at Honeywell, Pratt-Whitney, Lockheed Martin etc helping me along and guiding me. You should be able to respect that they will have the ability to arrive at a recommendation for nearly any solution a 60yr old design requires. The largest root into the case as possible, as you suggest, may be a viable long term solution... but only time will tell as these units cycle under extreme pressure of mismatched materials pounding away. In 30yrs will these savers still be holding? A long term solution would be to eliminate the forces pulling on that root. Stay tuned, you may learn something new like you said.

-Andrew

[Henry Schmidt]

Originally Posted by Lapkritis

Thanks Henry. Yes, I do respect experience. That said I'm not here for a bay of pigs. There's hard data to collect and analyze and I have some of the best engineers with years of experience at Honeywell, Pratt-Whitney, Lockheed Martin etc helping me along and guiding me. You should be able to respect that they will have the ability to arrive at a recommendation for nearly any solution a 60yr old design requires. The largest root into the case as possible, as you suggest, may be a viable long term solution... but only time will tell as these units cycle under extreme pressure of mismatched materials pounding away. In 30yrs will these savers still be holding? A long term solution would be to eliminate the forces pulling on that root. Stay tuned, you may learn something new like you said.

-Andrew

There are no perfect solutions but there are solutions with proven track records.
Time will tell......oh wait. It seems I have Porsche engines that have won championships and run for 150,000 miles and more. I can name one belonging to a JPL engineer the had 190K on one of our rebuilds. I will bet none of your "experts" will be able to claim that.

I always get a kick out of the new guy who thinks he has some insight beyond that of the professionals who have built these engines for decades.

I wish you all the luck in the world on your first build.

[Lapkritis]

Your input here is no longer welcome. Good day sir.

[Henry Schmidt]

Originally Posted by Lapkritis

Your input here is no longer welcome. Good day sir.

You are an interesting twist.....
This is an open forum and as long as proper decorum is exercised, we don't allow individuals to censor other contributors.

I seem to recall you specifically asking for my expert opinion and when I suggested you go a different direction than your "as cheap as possible" solution you no long desire my advise.
Interesting to say the least.

[Lapkritis]

Good day sir.

[HawgRyder]

Ok...I have to ask a question.
Harley Davidson...for decades...has bolted the base of the cylinders to the block...and then the heads to the top of the cylinders.
This now seems like a probable solution for Porsches.
I know it would require some thinking...principally in the oil return tube...and timing chain areas...but is it possible?
It would have a slight effect on compression when the cylinder expands (in length) as the engines gets hot...but that in itself may be a good thing.
Brain working overtime...sorry 'bout that.
Bob

[Lapkritis]

Interesting notion. I think the issue might be weight hanging that far out. Are the old BMW motorcycle engines the same with base cylinder mounting?

[Walt Fricke]

Bob - I get your drift: If everything expands uniformly, no extral thermal expansion stress added to the usual combustion forces.

Do HDs use aluminum cylinders? Somehow hawg riding/wrenching has passed me by.

I'm thinking that the 911 aluminum cylinder walls are quite thin in between the fins, which might make the cylinder marginal for holding the forces of combustion (assuming a design could be worked out so a flange could extend out from the tip of the spigot over where the head studs now emerge, and feed the forces equitably).

Then there is the question of cycle length. As I understand it (from Carroll Smith), if you keep the tensile forces on steel below its elastic limit, you can get almost unlimited cycles from that material (maybe it has to be some percentage lower - but somewhere). Whereas, with aluminum, each flex ticks off one of the allowable lifetime cycles. I think of this when I look out the window of an airplane and watch the wing flex up and down - comforting myself with knowing the engineers have all that figured out long ago.

VW hot rodders - drag racers, actually - add a 5th stud to help keep the heads on their super stressed motors with Nitrous and Superchargers and whatnot.

Ultimately, all these forces, no matter what, have to be held in by threads in the case. I've not heard anyone say that the makers of inserts are wrong in saying that the wider the threaded hole, the more stress it can handle. Maybe that's a simple function of more area with the same strength of the case material?

[HawgRyder]

Almost all old bikes had iron cylinders...some of the newer designs have aluminum with steel (or iron) liners in them.
I believe the newest design (by Porshe I think) has aluminum/iron barrels...and a very good HP/weight ratio .
Bob

[chris_seven]

This debate is a bit like Torque and Horsepower - it will continue to run and run.

I have tried a couple of times to summarise the basic issues and I will have onle last try:

There is no real solution to pulling studs out of aged and overheated mag cases.
they were borderline even when fresh.

The use of a Big Cert or case Saver is the only thing that can help if you use any Ferritic/Martensitic Stud material.

This is the area where I disagree with Henry and have done for some time.

The dominant property that affects studs pulling out will just be due to the initial preload + the forces caused by expansion.

The difference in the two coefficients of expansion is the only significant driver.

Having a couple of extra threads in the case may give a 5-10% improvement and this may help move the edge of the borderline.

Using Dilavar does reduce pull out forces significantly and I am interested to understand why it doesn't give enough preload?

I can understand that this may be an issue with High Pressure Turbo Motors but normally aspirated engines won't develop enough peak cylinder pressure to lift heads.

If the engine needed the extra clamping force generated by expansion they would all tend to give problems when cold.

I agree with Henry that practical solutions are good but again where our opinions diverge is about 'proof'.

Good engineering finds solutions and then explains how and why they work. This creates confidence and improves the 'State of the Art'

I would also suggest thet it is usually the 'practical engineer' that refutes science and asks that we all believe in the 'Magic Fix'

I would agree with Bob about motorcylce engine construction but the secret os the cast iron cylinder. As soon as you use an Aluminium Cylinder you have the same problem.

the old BSA Victor used a Chromal Cylinder to replace the cast iron and this shortened crankcase life.


Walt,

The Fatigue Endurance Limit' of most steels is around 50% of the materials yield strength but this would only be valid for smooth bar 'fully reversed' loading. Once you introduce notches and loading that has a tensile mean stress then you need to really look hard at Stress Intensity Factors'.

It is true that Aluminium had no theoretical fatigue endurance limit but you could estimate that you could use up to 35% of the elastic limit for around 10^8 cycles.

This isn't great in terms of steel but does work for Civil Airliner Airframes which are relatively unloaded and the loading profiles used to test these materials and structures are based on wind gust dominated forces.

The main fatigue event of an Airframe is the pressure cycle in the Hull and the 'life' used to be around 100 000 cycles.

Fighter Aircraft are altogether different ans are manouver dominated and use entirely different test configurations.

The first time fatigue was considered an issue with Airframes was the Comet, there were some issues with the DC10 so the Science is relatively new.

Fatigue is not the main issue with case failures.

The forces that really cause the damage are the expansions being superimposed on the static load and it is clearly the heating cycle that causes the load to increase.

The material may also be prone to stress relaxation if the temperature is high enough compared to the local stress at the root of the threads. The relativley high mean loads could cause some microstructural damage that would locally weaken the material.

I have been convinced for some time that the mag cases suffer from this problem and there is a simple force/time/temperature realtionship that governs this behaviour.


If we knew the alloy used it is likley that Magnesium Elektron would have good design data.

I was surprised to learn that ARP recommend 36lbsft ot torque for their 911 studs as this will tend to increase this problem.

My conclusion is that there is no real way to redesign the case in this area but if we understand the basic mechanisms that cause the problems we can chose an optimium solution.

At the moment if I used Nikasil Cylinders on an old Mag case I would use Big Serts and Dilavar.

If I used cast iron cylinders I would probably use Big Serts and Steel - the type of steel is relatively unimportant.

On Aluminium cases I would generally use Big Sert and Steel Studs.

I have based these views on trying to understand the engineering issues and the basic material properties.

The solution used on most modern motorcycle enigines seem to be based around different construction methods with wet liner blocks and lots of short head bolts of small diameter.

[Lapkritis]

My mickey mouse calculations agree...

Non dilavar fastener, steel cylinder.
Dilavar fastener, aluminum cylinder.

My confidence, or lack thereof, in the dilavar fastener has me looking at non-dilavar fastener and steel cylinder. Simple as that for the sake of the case.

I think the diameter percentage increase of the saver is a good portion the strength improvement. The pitch may be the same but more biting with larger diameter.

The fatigue concern I allude to is based upon the evidence of stock thread holding then failing, timecerts holding then failing...now we're en vogue with case savers and not looking at the bigger picture of what is behind all the previous failures (thread pulling only). Everything so far works for awhile until the cycles get to it. If you cycle the engine to operating temperature and cool down often then the miles don't matter nearly as much.

[porterdog]

Originally Posted by Lapkritis

That said I'm not here for a bay of pigs.

Foul.

Originally Posted by Lapkritis

Your input here is no longer welcome.

Foul.

[Lapkritis]

Please keep this thread on topic instead of promoting quarrels. Thank you.

[Henry Schmidt]

CaseSavers have been used "en vogue" for 30 year by those qualified professionals who build Porsche engines in mass.
It's not a fad but a proven solution.
CaseSavers in combination with top quality studs and temperature control are in the real world performing flawlessly.

For those reading this thread who don't want to reinvent the wheel and want your best chance at success, there are professionals building these engine who have offered suggestions that will give you the best chance.

For those scratching on note pads who wish to spend the next 30 years trying to catch up on your own, good luck.

[Lapkritis]

The negativity and discouragement to innovate here is notable and disappointing. I expected more from reputable members. Do we all grow so proud of our accomplishments that we truly believe we know it all and then demand progress by all others to stop while touting those accomplishments? The feedback you provide is not constructive and does absolutely nothing to encourage development. Imagine if you received the same reception when you were developing your super-duper da ta da fasteners.

[porterdog]

Originally Posted by Lapkritis

I expected more from reputable members.

Perhaps you should lead by example.

[Daniel930]

Having spent my entire life in the Aerospace field, I have to respect Henry’s work; Porsche had issues with these studs even with their R&D departments. If the parameters are to push this engine to Mach 3 and the edge of space, of course it can be built better, but for the guy driving around the street and track the problems have been solved.