Deck height, compression ratio, and valve relief flat top pistons, In a 2.0 Options

[emerygt350]

Ok, my question of the day:
Put my new 96mm pistons and cylinders on and measured deck height, .033 all the way around. Let's say .032 to be conservative.

I will not be running head gaskets.
The calculator gives me
8.8 CR at .033
8.6 CR at .043
8.4 CR at .053


The pistons have a valve relief (pretty large) cut. I am shooting for 8.4 CR.

Heads are in the shop so I can't check the CC yet.

Everything I read says .04 - .06 for deck height. With the valve relief do I need to worry about going lower? If I had dishes would you measure from the edge of the piston or the dish?

[Montreal914]

Not sure I understand how you can establish the CR with the gaps you are showing if you don’t have the head volume (IMG:style_emoticons/default/confused24.gif) . Also make sure you consider the valve pocket in the pistons as volume.

The 0.040” gap is not necessarily for interference between piston top and valve but piston top and head face.

Hopefully my comments can help answer your question…

[brant]

You have to measure the head volume to get compression ratio accurately

[rfinegan]

O.040 deck height is considered the minimum deck height for t-4 engines. Sure you can go lower or shim the bottoms of the cylinders to increase or decrease the CR(compression ratio)

However the area between the bottom of the head and the top of the piston will determine the squish area. Too tight, you will restrict the flame-front travel and really hurt performance.

some reference on squish:
https://www.thesamba.com/vw/forum/viewtopic.php?p=8865130

[emerygt350]

Thanks Finegan, that's what I am looking for. I am just modeling several scenarios until the heads come back, you guys are really obsessed with CR. I know the head will be between 64 and 56, so for the moment I am using 60, as I get more information I will revisit that. My concern is slapping the piston against something. Now I understand the where and why of the .04-.06. It isn't the valves...

[emerygt350]

Very interesting read. Reminds me of why I rarely look at thesamba unless I have to. Firehose.

So, it sounds to me sitting somewhere around 1.5 to 2.5mm is good. It looks like if I put in a .02 shim that will keep me around 8.4 CR as long as my CC isn't too big. That would give me a 1.45 mm deck height. Really seems like a ton of factors go into the 'optimal' deck height. Has Jake Raby ever said anything about it? Since it is a stock 2.0 head/cam and a really common 2056 improvement. might there actually be a 'best practice' for that combination?

[Jack Standz]

For what it's worth, our 2056 had pistons with valve reliefs that cc'd at 3 CCs per the manufacturer's specifications and real world measurements.

Also, I'd still check the valve to piston clearance as you wouldn't want them to hit. There are ways to end up with inadequate clearance, like the pistons are upside down or what's the actual valve lift, etc. Mock them up and measure a few times, use some play dough. Then order the spacers and re-measure when you get them.

[emerygt350]

For what it's worth, our 2056 had pistons with valve reliefs that cc'd at 3 CCs per the manufacturer's specifications and real world measurements.

Also, I'd still check the valve to piston clearance as you wouldn't want them to hit. There are ways to end up with inadequate clearance, like the pistons are upside down or what's the actual valve lift, etc. Mock them up and measure a few times, use some play dough. Then order the spacers and re-measure when you get them.

That's what I was thinking. I want to check the valve geometry so I will be putting it together to test that with full intent to tear down again. That valve relief is what is going to make for issues I think.

[rfinegan]

A tip on piston to valve clearance.

The closest relationship is not TDC for the pistons to valve, but about 10 degrees before top center(piston chases the exhaust valve as it closes) And about 10 degrees after Top Center (valve chases the piston as it opens. I think I got that right...

EZ to test

Place play doe /clay in the piston relief and tighten head. Rotate the assembly minimum of 2 complete turns to have the valve compress the play doe.
Remove the heads and measure the thinnest location on the play doe and that will be your piston to valve clearance.

0.080 exhaust
0.120 intake (I will check this in my notes) Memory only for now

[technicalninja]

Normal engines require .080 on intakes and .100-.120 on exhaust.
VW air-cooled might be backwards...

Quench (piston to cylinder head dimension is the single most important measurment in a modern engine that uses pump gas.

I want .035 minimum for a cast piston and .045 for forged.

I would not change quench to solve a valve interference issue. I'd have the valve reliefs "fly cut" if required.

[emerygt350]

Heads are back. They measured 55 CC but I am going to do my own to double check. I also want to measure the valve relief in the piston.

[rfinegan]

With stock valves and what cam are you using? The piston to valve clearance is worthwhile to check but stock components have LOTS of clearance. Unless something was not assembled correctly like the timing of the cam. Plus you said the new pistons have a generous valve notch already. The largest valve notch I was able to put together in my pistons was 0.050 inch based on the crown of the pistons from KB. Yes i did measure and called Keith Black tech support

[emerygt350]

Yeah. All stock but the piston/sleave combo. I got 2ccs on the valve relief, and 60 on the head. Not super confident on the head, lots of side spill etc. I am think between 58 and 60.

[emerygt350]

Did a real thorough job on the deck height today. Measured all the sleeves and tried to do some matching. I don't think it mattered much, it doesn't look like my case has ever been cut.

Bolted down nice and snug I got
.028
.031
.023
.028

So I guess I am pretty much stuck with a .03 shim. When I get ready to put it back together I will see if swapping cylinder/piston combo on the .031 with the .023 makes any difference. Looks like around 8.3 for compression ratio, which is what AA said for the combo.

[emerygt350]

Couldn't stop thinking about it, so I swapped the cylinder/pistons and measured again... No difference on the .023. Argh. If they were all .023 I wouldn't care. Maybe it is just measuring error.

[rfinegan]

Yes moving the cylinders around may help. Those deck numbers have some variation 023 to 031= 0.008 inch
Remember to measure as close to the center of the piston to keep the rock as small as possible (flat top)top of stroke
or measure on the highest edge ( dish) top of stroke and rock them all the same way to get most consistent measurement.
or

Average rock= highest-lowest/2 and use that deck measurement

[emerygt350]

I bet that is contributing. I didn't really consider the rocking piston.

[rfinegan]

here is how I did it..
https://www.youtube.com/watch?v=Wj3R2Zh-HNo

[rfinegan]

As far as rock , I would think if measured across the piston pins (center) right to left would not rock

but top and bottom would rock?

Are you mocked up with out rings or assembled?

[rfinegan]

yes look like 8.0 to 8.2 will work nice on pump/premium 93