Piston crowns 2

[willeng]

I need to test piston crown shapes for better efficiency is there any substance that will handle the combustion temperatures,pressures for short term experimentation purposes, EG a few quick dyno runs. I am after something that can be reshaped & renewed quickly.
Thanks.

 

[Metalguy]

Well, aluminum *is* pretty soft and easy to machine.

 

[willeng]

Metalguy:

Agreed aluminium is soft & easy to machine.

If you have any suggestions other than using numbers of pistons i would appreciate your thoughts.

I thought of welding them then machining, shaping etc but i thought they would distort to much, especially if it was done more than once.

They are only cast pistons unfortunately, if you could tell me a welding proceedure that would not distort them i'll give it a go.

I have Mig & Tig welders.

 

[Metalguy]

Most cast piston alloys are tough to weld. Can you use a forged piston instead? What is the bore dia.? Unless it's model-airplane size, you'll want to MIG weld it. Be sure to use some preheat and sock the amps. to it. Wire #4043 is a general-purpose filler metal for many alum. alloys.

 

[willeng]

metalguy:

Bore is 81mm, i don't think there is a forged piston available for this,it is an ugly dished cast piston that has to be used---rules.

What preheat temp 200-250deg C.

In my original post i should of wrote, piston bowl shapes.

Thanks

 

[patprimmer]

Can you machine the bowls a bit deeper and wider, being carefull to leave enough strength in the ring land area, machine aluminium inserts, then use a few small capscrews through the original crown from below, into the new crown inserts.

I would think you might need 6 or 8 small screws to do the job.

This would allow you to remove the inserts to modify them.

You would need to think about thread strength of the aluminium at high temps and the inertia over TDC on a trailing throttle during high RPM downshifts.

You might even consider countersunk head bolts from the top, but the heads of the steel bolts might form hotspots and cause detonation. If the bolts were from the top, and the original crowns had thread inserts, you could remove the inserts without removing the pistons.



eng-tips, by professional engineers foe professional engineers

Regards
pat

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

 

[willeng]

pat:

You have given me a million & one ideas with your post, theres plenty of material to play with. Well done once again.
Thanks

 

[SMOKEY44211]

Pat: You beat me to the punch. Great minds....... ---Phil

 

[Tmoose]

>snip< ....a welding proceedure that would not distort them ..............
molten metal solidifes, then shrinks some more as it cools off.
Welding always = thermal stresses
Welding usually = distortion
Many distortion reducing welding techniques &quot;balance&quot; the stresses against each other to partialy cancel out.
Distorted ring grooves compromise piston seal.
I think my dyno tests with pistons modified by welding might be tests of how distorted the ring grooves became, rather than directly from chamber shape effects.