Modified Diesel pistons and rods in a turbo petrol engine...?

We all know that Diesel engines are stronger out of the box, due to the much higher cylinder pressures they have to endure.

I'm currently looking for some cheap pistons and rods for a turbo petrol engine build.. the two problems I'm having are finding some of suitable compression height and strength.

My reasons for looking at Diesel pistons is their strength and usually generous top ring land depth, which both gives me meat to play with to get the desired compression height, as well as also allowing more mass to absorb and transmit heat away from the piston crown.

I'm looking for suggestions as to why Diesel pistons in a petrol engine would be a bad idea; the largest down side I can think of so far is increased reciprocating mass and hence greater inertial loads on the crank.. also their gudgeon pins are significantly larger (for example, KR / ABF pins are 20mm dia while 1.6 TD pins are 24mm).

What are the disadvantages of larger OD pins? I'm thinking that with pin wall thickness adjusted accordingly a larger pin should actually be stronger than a smaller pin of the same weight..

Any thoughts / links to piston and rod weights would be much appreciated

 

Height above the top ring, or the lack of may be an issue once you have got the desired cr figured with oem parts in hand and machining taken into account.
Cant see any disadvantage with large pins.
May need checking for true due to the rejig of internal stresses after you hog out the centres more. (pins)
From memory, diesel pistons are pigs, skirts too.
And, the pistons with integral swirl bowl things are useless to you, so that rules out quiet a few.
Are s2s not an option also?

 

Thanks for your thoughts

The older 1.6 TD pistons would be the ones to go for I think; since they only have shallow features in the piston crown (1.6mm deep, according to Mahle):




Based on some half-a*sed photoshop measuring I reckon the top ring lands on these pistons is about 12mm deep, and would need around 7mm removing; leaving 5mm. Here's a quick chop:




And some VW 1.4 and 1.6 16v pistons for comparison:



I'm trying to find some pictures of each piston from the their undersides, to get a feel for how thick the crowns are above the pin.

Didn't think about pins distorting if machined; are they not pretty tough anyway? Might perhaps be better to try and source some diffierent, thinner walled pins if mass is an issue.

Thanks for the S2 piston suggestion; this is the route I'd take if building a KR turbo, but I'm not

Thinking the 76.5mm diesel pistons and 136mm rods could go into an AEE block (1.6 8v small yet tall block) with an ADX 70.5mm crank and a 1.4 16v head; hopefully giving a slightly oversquare 1.3 with a good rod ratio and enough strength to take a lot of boost

Other than sourcing the parts and making the head fit, my major concern is that while the head / rod ratio / bore & stroke will encourage the engine to rev, the extra mass of the diesel bits will do the opposite..

 

Is that a cast in, cast iron anti-fret/gall ring that ring 1 is located in? If so, you may have the crown open to this concentrically after machining, something to think about..

Edit, possibly maybe by the time you have it all done, a petrol piston may be as strong. Dont forget, better casting was carried out as time went on, so the alloys/manuf in new petrol pistons say, 95-00 may be considerably better than a 90s diesel piston.

Also dont forget, that hotspots are what you dont want. A crown should be as thin as is just needed. Just because its thicker, will mean its stronger, but in a petrol, you may have a big/many hotspot(s).

 

I believe it is.. not sure it's present on all pistons though.

Not quite sure I understand having "the crown open to this (the insert) concentrically after machining" - are you referring to machining that much of the crown away that I break into the insert at the edges of the piston?

The photoshopped image in my previous post is with 7mm removed from the crown.. leaving maybe 2.5 - 3mm of alloy crown above the insert, so assuming that the visible top edge of the insert represent it's upper-most point there shouldn't be any problems.

Would be good to get hold of a knackered example and section it..

 

Yes, thats what I meant

Your good a photoshop! I thought that was it as is! You should have loads there so,

 

Cheers. Unfortunately as far as petrol piston choice goes I'm fairly limited - it appears that not a lot of people make suitable items in a 75 or 76.5mm bore. 8v pistons have too much bowl volume so the CR is too low with the 16v head, and the 16v items are known for being a bit crap. Added to this, the rods to suit the 16v pistons are known to be especially crap

Regarding crown thickness, I suppose you want the centre of the piston to retain as little heat as possible by transferring it to the peripheries of the piston so it can disspate to the cylinder walls.. would be good to get hold of some pistons and get measuring

This leads thoughts to the profile of the crown; so far we're presumably considering it being machined flat, however looking at more modern piston designs it appears that raised portions near the edges of the piston is included for "squish" and "quench"..

Here's a custom Mahle piston for a similar application (1.3 16v G40) here (can't link to pics).

Cool, cheers

 

Just saw your other thread there so its making way more sense now

 

Cool

Subject to the completion of a somewhat convoluted payment process I scored these on German ebay earlier:





Did some calculations.. compared to the diesel engine with a stroke of 86.4mm and a max engine speed of 4500 rev/min, the diesel pistons with a 70.5mm (ADX) crank should be subject to about 37% higher peak acceleration at 6000 rev/min and 63% at 6500 rev/min.

I'll need to remove around 7mm from the piston crown which should lose me around 90g. Assuming an original reciprocating mass of 0.9kg per cylinder (still need to learn how to deal with the mass distribution in the conrod) that's a 10% reduction in the force generated by piston mass. This would indicate peak inertial loading would be around 25% higher than the diesel engine at 6krpm and 46% higher at 6.5krpm.

Will be interested to see how much they actually weigh as they're pretty fooking chunky.. I think if it ever gets built this engine will have to be a fairly low revving boost monster

 

Surprisingly these turned up earlier; not quite in the condition I'd have liked but I think they'll be useable.



Top ring land is 12mm deep; min crown thickness appears to be 18mm. I'll need to remove around 7mm from the crown, leaving a top ring land of 5mm and crown thickness of 11mm.




Piston crowns all have a few "custom features" not mentioned in the ad, but I'm not a*sed as they'll be machined away..






Pins are somewhat worn; not measured the journals in the pistons yet.






Mahle part numbers




Fat b*stard rods..


The pistons (minus rings) weigh in at 460g ea, the pins 180g and the rods are a portly 720g without big end shells

The total assy is 1360g compared to around 1000g for a Mk4 2.0 8v assy, which looks decidely anorexic by comparison Could do with weighing some ADX (donor crank) pistons and rods so I can get an idea of how much extra loading it'd have to take with these hanging off it..

 

Got to love vw's over engineering......I'm watching this project of yours with great interest

 

Cheers

Don't expect it to move very quickly though; a total lack of funds assures that. Still not sure about the diesel pistons and rods on account of the rods' weight. Can't see as a crank designed for rods that are perhaps 2/3rds of the mass of the diesel items balancing too well with their mass hung off it

Ideally I want to setup a spreadsheet that calculates the effective reciprocating and rotating masses within the engine.. unfortunely maths never was my strong point, and I've not had to use my brain in anger for years..

 

its good to see the calculations, your biggest problem is a heavy reciprocating mass due to the heavy components, which essentially means a lower red line similar to a diese so 6000rpm doesnt soound to badl. pistons are always the weak point in a NA gone FI engine, the conrods will be ok to about 250hp ,
best bet is to save the pennies for some forged gear with a piston bowl to lower CR a little if you want a drive that isnt simular to a TDI car...

 

Weight and thickness of the rings would be my reason for not going down this route
Jon