Forged vs Billet Aircooled Aluminum Cylinder

[LeroyAchoy]

Which is better?

I am looking at two sets of cylinders one set is a billet that is CNC machined.
The other set is Forged then machined.
Which is the better process.
These are for my 911 porsche engine
Thanks Leroy

 

[swall]

All things being equal, the forged part will be stronger. This assumes that we are talking the same alloy and heat treat for forged vs billet. But "better" is up to your pocketbook and your time frame for availability.

 

[hydrae]

The billet will have the consistant properties throughout the section e.g. at X distance from the edge of the billet the strength will be Y, and that value will be consistant with billets from the same lot and even lot to lot.

A forging will be stronger but will have less predictable properties such that random imperfections may creep into production without tight controls. Each unit will be different and analysis will need to be done on the method to verify there are not weak spots in critical areas created by the forging. It can be assumed that this analysis has already been done on the forged part you are looking at.

Hydrae

 

[EngJW]

Are the stresses in the engine so high that it will know the difference between a forging and billet?

The most important thing is how the design is executed: port shapes, heat transfer, quality of machining, etc.

Note that aluminum cylinder heads for aircraft engines are castings.

 

[GregLocock]

I agree with EngJW's implied cynicism.

Are billet parts made from the usual cold drawn bar? If so that is a fairly intense forging process in its own right, although I suppose the subsequent heat treat probably destroys any improved grain structure.

It would, of course, be quite easy to make a forging that is less strong than a billet part, - how will you ever know?



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[LeroyAchoy]

How about the abilty to shed heat? Provided the had they both had the same amount of cooling fin.s
Would having the cylinders Cryo treated, help with the cooling?
I plan to run a fairly high compression ratio.
Leroy

 

[GregLocock]

Well, here's a neat test.

The electrical conductivity of a metal is very closely correlated to its heat conductivity. So, if the cryo parts are more electrically conductive, they should be better for heat transfer.

So, get a bar of alumium before and after it is cryo tested and test whether the resistivity has changed.

Let us know the results of the test.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

Another test would be to get some long and identical bars.

Cryo treat half.

Put them all in a heated oil bath, taking care that the oil is stirred to get uniform temperature.

Place the bars in the oil, taking care to insert them all exactly the same distance.

Plot time temperature at each end of all the bars.

If the results are scattered, repeat the test, taking care to eliminate variables.

If the results show two distinctly different results in sets of treated vs untreated, publish your results here, and sell them to your cryo treater.

Cylinders need stiffness rather than ultimate strength. If they blow out and become barrel shaped, you cannot maintain ring seal. In a speedway midget with an air cooled VW based motor, we repeatedly blew centrifugal cast iron cylinders out of shape and lost ring seal. We ended up using a very heavy wall centrifugal cast iron cylinder. I don't think aluminium, whether cast, forged or billet would have survived more than a lap or two.


Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[allencr]

I'd be concerned with the type cylinder in it, - pressed in steel, cast iron or plating - chrome, nikoseal(sp?), many others, and its fins.
A lot of Porsche aftermarket stuff is more advertising then engineering..

 

[LeroyAchoy]

Both sets are plated.
What is the best Material to use?
I will ask the manafucter what type of Alluminun they are using.

Thanks Leroy

 

[swall]

A few comments--1) Yes, it would be helpful to know what alloys are involved. The choice may be best made by picking the appropriate alloy rather than billet vs forging.2) Aluminum billets are just big logs of extruded aluminum--little additional mechanical working, except for straightening.BTW, most aluminum bar and shapes are extrusions, rather than rolled.3)No known benefit to cryo treating of aluminum.

 

[magnograil]

The original Mahle cylinders are nikasil plated. Several places in the US now do it and also repair it. Check the advertisements in Cycle News.
Something I would check on the billet cylinders is whether the fillets are maintained at the base of the fins. On both types, what the wall thickness is compared to the Mahle.
If the billet manufacturer states the alloy is "aircraft aluminum" then it is probably something cheap like 6061.
The 60's/70's racers used cyrogen treated connecting rods claiming the molecules packed closer together preventing fatigue failure. No explainations on how they stayed packed after returning to ambient (or higher) temperatures.
Not just Porsche aftermarket parts are non-engineered hype.

 

[icrman]

I'm curious, are these cylinders sleeved?
If so then are you describing a heat muff??
If so then the main concern is just heat transfer and expansion issue not a strength issue. If there is a sleeve then what material is it made of?

 

[MikeHalloran]

Castings and extrusions can and do have internal flaws, that sometimes become apparent during machining, and sometimes only become apparent in service. Examples of flaws are closed seams or actual voids; both are a bad thing to find in a structural part in service. A less common example of a flaw would be a foreign object, e.g. a micrometer caliper in an aluminum extrusion (actually happened).

The single most important reason for specifying a forging is that if an internal flaw exists, the forging hammer will make it apparent, so the chance of an internal flaw appearing in service in a finished forged part is very small.

For suspension arms, I'd strongly prefer forged aluminum parts. For engine cylinders, so long as I got a warranty that covered collateral damage and other costs like replacement labor, I wouldn't have a strong preference.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA