Classic L-series build

[KoHeartsGPA]

280z http://www.rockauto.com/catalog/x,carco ... ttype,5620

Not familiar with the dish off-hand, but 86.1mm.

Flat top, it would put me over 10:1 CR, but......, I'm wondering milling option.....say...a 4.32cc dish to bring it to L18 ish?, hmmmmm....

[okayfine]

Picture shows a dish, at least for the ITM piece.

[KoHeartsGPA]

You're correct, I missed that!, I'll investigate

[KoHeartsGPA]

3.10mm deep x 74.00mm dia, I wonder how many cc that translate to..., anyone know the formula?

[James]

Pi r squared x height.

3.14159 (3.7cm)squared x .31cm 13.332 (I hope that is correct)

[okayfine]

And if you minus out the radius, that's probably the L20B dish

[James]

Could you machine a small dish in the flat top piston (assuming piston top thickness could take it)?

[KoHeartsGPA]

And if you minus out the radius, that's probably the L20B dish

Yup

James, my dad used to build these type of motors back in the 70s in Argentina for racing, he told me today that they did all sorts of mods but never messed with the integrity of the pistons, it can be milled but it'll be limited depending on material and thickness of the top, he strongly argued against doing so, piston failure would bring catastrophic outcome, you will likely lose the whole engine.

[James]

It would be interesting to see a section of the dished piston compared with the flat top.

[James]

If you took 1 mm off the entire top it would be about 5.8cc.

[okayfine]

Closest I can come:

piston cutaways.jpg (128.71 KiB) Viewed 3387 times

.040" (~1mm) isn't a lot. If you could leave the piston ring area at full height and just make a dish, you're probably fine. It doesn't appear that the dished L16 piston and the flat-top Z piston have different crown thicknesses.

[tr6racer21]

Julian,

I would agree. Spoke with my machinist yesterday, and he said as long as the piston crown is thick enough, no worries about machining a dish. This is going to be the route I take with this build. The other option is to use forged pistons. Modern forged pistons can have higher silicon content and thus have expansion rates which require tighter tolerances. For now though I need to do some calculations based on the (2) L20b blocks that I have and several of the heads, to come up with a firm solution to the need to get to max compression for pump gas. I will be posting figures soon.

[KoHeartsGPA]

Julian,

I would agree. Spoke with my machinist yesterday, and he said as long as the piston crown is thick enough, no worries about machining a dish. This is going to be the route I take with this build. The other option is to use forged pistons. Modern forged pistons can have higher silicon content and thus have expansion rates which require tighter tolerances. For now though I need to do some calculations based on the (2) L20b blocks that I have and several of the heads, to come up with a firm solution to the need to get to max compression for pump gas. I will be posting figures soon.

Yeah, keep it under 1mm milling work and stay away from Chinese made pistons, I just don't trust their "quality" more like, lack thereof..., I be waiting on your figures, Chip

Julian, thank you for those diagrams, I will too bring it up to Fred and see what he thinks about milling, maybe the piston that has 2.8cc dish (KA24) is the best candidate?

[okayfine]

KA pistons are 89mm? Bigger bore adds to the compression ratio.

Diagram above comes from the How To Modify "bible," available in PDF at The 510 Archives if you can't get a paper copy. Covers a lot of what you're looking for, and the revised "How To Hotrod" will cover a bit of the L20B as well.

[KoHeartsGPA]

So now with all those ideas tossed, and with what I got on hand, I'm liking the L28 flat top...

86mm bore L20B with L28 flattops, U-67 head......................... 10.30:1 CR.

If milling .5 to 1mm it will reduce the CR, all needed is calculation of how much to mill....