Why Chromoly vs Heavy Duty aluminum pushrods, for close to stock engines Options

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I'm just wondering why most people are running the chromolys instead of heavy duty aluminum (see links below) for close to stock applications that are not high revving or using stronger valve springs?

I couldn't find discussions about using HD aluminum (only stocks: http://www.914world.com/bbs2/index.php?sho...p;#entry1422350 )

In my recent 2056 build I used chromoly pushrods but didn't change to 911 swivel feet with 1.7 arms which I realize now was a mistake (still have 2.0 arms and new 2.0 adjusters). I'm getting ready to change over to the 1.7L arms and swivel feet.

I've been unhappy with the valvetrain noise I'm having, but I think most of that has to do with NOT changing to 911 swivel feet adjusters and 1.7 L arms. I can really hear this chatter with the 2.0L adjusters and I'm hoping that switching to 911 swivel feet will remove this because of the larger contact area.

As a result of changing the arms I get to re do valve train geometry and order up some new pushrods. One thing that has bugged me about the chromoly pushrods is they start at zero lash cold and end up at some amount of lash when hot.

After the switch I'm going to install my existing cut to length chromolys (which will be the wrong length) for a test run to see the sound difference just from the arm change and from there (with the help of this discussion) decide which route to go for pushrods. This conversation may be a moot point but I wanted to see what the reasoning is behind the use of the chromolys, the whole increasing valve lash thing has been bugging me.

I would expect that increased lash at operating temperature would reduce valve lift and duration slightly which would reduce performance. Since most people are trying to increase performance I wouldn't expect the chromolys to be advised. With the amount of care and thought that is given to every aspect of these engines by builders like Jake and all the others on this forum that recommend the chromoly pushrods I figure I must be missing something.

For close to stock engines it would seem the strength of the chromoly's is not the advantage but rather being able to set the lash to zero.

The .095 wall 2024 T3 Aluminum HD pushrods (58-62Ksi tensile) will be (theoretically) almost half the weight of the .058 wall chromolys (4130/4135 - 140Ksi tensile). Approx 3.8 grams/inch length for Aluminum vs approx 7.4 grams/inch lenght for Chromoly. Area difference is .08356 in^2 for .095 wall 2024 T3 Al HD vs .05776 in^2 for .058 wall chromoly HDs (assuming 3/8" OD)

I see there is a clear strength difference but again, for a slightly more than stock engine why would I want to use the chromolys when I'm essentially losing some duration and lift by having added lash at temperature in addition to the added noise I'm seeing (which again may go away entirely from the 1.7L arms with swivel feet).


There is also a price difference (aluminum HDs almost 2x as much as chromolys) but an extra $80 is not that much in an engine build.


As always, thanks for your insight and advice,

Andy


references:
HD aluminum: http://vwparts.aircooled.net/Heavy-Duty-Al...cnhdalprcut.htm

HD double taper aluminum: http://vwparts.aircooled.net/ACN-Dual-Tape...thdalprcut.htm)

Manton PDF: http://www.mantonpushrods.com/Images/Manton_Catalog.pdf