Lancia 16v compression height

[1NRO]

Hello Guy,

Am I right in thinking that the standard compession height is in the 38.8mm region? Would this be something that you would adjust at all? If you do can you shine some light on the direction you would follow and why please.

Thanks,

Nik

[Guy Croft]

Nik, hi

according to my data taken from an unmodified Evo 2 Integrale the block deck height is 39.5mm and the piston compression height 39mm. It can be a good thing to run the piston low on a dished piston low CR engine because it keeps the piston weight down. The deck height is crank centre to block face.

Not sure what you want to know exactly, please add a bit of detail, thanks.

GC

[1NRO]

Thanks Guy,

I'm just trying to get an idea of how tight it might be possible to make the ring pack/compression height without compromising the durability, as you suggest weight is the motive.

When you refer to "block deck height " as 39.5mm I'm confused as then you say "The deck height is crank centre to block face." Does the 39.5mm refer to the piston being .5mm below the deck?

Regards

Nik

[Guy Croft]

In purely general terms you can make the land thicknesses pretty thin, their form is based on the pressure and material strength.

The top land has to be the thickest because it sees the full firing pressure and has to cope with, er, the odd detonation event. The lower lands can be thinner because the pressure below the top ring is less. It's quite normal to see a three-ring pack squeezed into a compression height of 25mm, using thin race rings, whereas of course the TC units have a very tall comp height, 38mm plus.

Be aware that the higher the cylinder pressure (eg turbo, supercharged, nitrous etc) the thicker the top land has to be. Top fuel dragsters with 7000bhp or so might have top lands 12mm thick - or more - and second 6-8mm. If you compare OE Vx and Integrale pistons with normally aspirated models you will see they're thicker there too.

Yes, you've read the deck ht/comp ht thing correct; the pistons do run low in many units, check out the calculator I've uploaded - the deck height (crank centre to block face) is taller than the sum of rod centre-centre length plus 1/2 stroke plus piston comp ht.

GC

[1NRO]

Thanks Guy,

It always helps to have a clear explanation.

Do you stick with the standard comp ht with the pistons you design? Anything different means rods to suit which complicates matters further.

Does piston crown below the deck allow for decking the block to flush or would you calculate a margin to retain the crown below the deck? Basically I suppose I'm fishing for squish clearance, more gasket manipulated I'd imagine but maybe fine tuned with a skim?

Regards,

Nik

[Guy Croft]

I recommend most strongly that any engine where the owner has gone to cost/trouble of fitting forged pistons has the head and block machined lightly (usually 3-12 thou") to assure flatness.

This has little impact on the setup height of the piston. I don't recommend running very close to the block deck (face) on any engine, don't see the need to do that. You have to have 1mm piston to head and don't want to run the risk of the crown of the piston nudging the head gasket. Most TC gaskets come with nominally 85mm bore - but they do squash a bit in the compressed state to under 85mm. So if your bore is 85mm or close to and the piston is running up thru the the gasket it better have a generous radius around the edge of the crown or it's going to hit it.

When I design a piston, in this case I guess a dished one (ie: one with a bowl in the crown) I examine how big the dish needs to be to give the CR I need - if the piston is close-to-flush - and if that dish is going to be really big whether it might be a better option to run the piston lower and reduce the dish volume, because a big dish makes the piston heavier. And a piston running flush and aiming for, dunno, 8 or 7.5/1 or lower is going to need a massive dish in that case. I ask CP to model my dish in 3D (because I don't have 3D cad) and they can thus show me its dimensions. Then it's an easy matter to integrate the dish into my design and ensure adequate under-dish thickness.

I would never advocate increasing the amount of squish, you are as likely to lose power as gain reliability. If you want more squish to reduce risk of detonation, you're pretty well wasting your time. Detonation of engines comes from over-lean, overhot, over-advanced, low octane fuel, too-high static CR etc etc - and if it's running out of spec in that sense - more confined squish will no way prevent a major det event.

The compression height of the TC range is very tall and I've never had to resort to shorter rods to get it all to fit in.

GC

[1NRO]

Interesting Guy,

1mm clearance is my aim, thanks for clarification. Its not so much that I wondered about increasing squish but rather how tight I can run.

Regarding comp ht I was thinking more along the lines of a longer rod needed to accomodate a piston with a reduced comp ht. You mention a comp ht of 25mm being not uncommon.

You touch on radiusing the edge of the piston which prompts another question in my mind. Do you "soften" sharp edges yourself prior to fitment or just leave them as manufactured? The valve cuts always look fragile to my eye.

Thanks,

Nik