The Cal-look Lounge

I need to find 5cc.
50cc chambers
1mm deck height
AA slipper skirts with 6.5mm crown.
82 x 94
42 x 37
Web 86b
1 5/8"

Full street car.

Currently 11:1
5cc in the piston would be 10.2:1 heads have been worked and ported so would rather not butcher the CC out of them unless a piston dish is specifically a bad idea.

Theory suggests a dish can centre the burn and make it more efficient. Anyone have any experience of this?

Round dish? 75mm diameter 1.3mm deep would do it :)

Luke

Here's a pic of mine in the rough machining stage. they are cut in a bathtub shape to try to match the chamber shape. The idea is to keep the two squish bands intact and focus the burn in the center of the chamber, while creating turbulence at the same time.
Mine also incorporated a flame port in the head to further spin the mixture around which makes a more efficient burn.
I couldn't be happier with the result! The heads run cool and burn everything that goes in. Valve adjustments have been minimal and it has a huge cam in it.
Engine now has 29,500 km on it and still seems brand new  :D

Thanks Martin, that's exactly the kind of feedback I'm after. How many CCs did you get in that dish?
Do you have a picture handy of the finished piston? Also the chambers?
Looks exactly like what I am picturing :)

Look closely and you can see it written on the top left piston in red marker  ;)

Ah ha! derp... Thankyou...

I'll weigh things up, not sure I have the tooling to machine a slot like you have...

I think Steve made most of his own tools. Here's a pic of it being dished on the mill.

I do it all the time.  I try to keep it less then .070" deep, and I make sure the diameter of the circle does not over hang onto the squish flats.  I have done a couple oval shapes, but a regular circle in the lathe is far easier and way quicker.  Your 75mm x 1.3mm deep looks about right. 

Brian

Got any pics Brian?

These were the ovals I did.  I will be doing a set of circles next week.  I will get some pics of my fixture and the dish measurements while I am doing it. 
(https://www.thesamba.com/vw/gallery/pix/1202991.jpg)
Brian

Looks almost exactly like mine, nice!
Someone should CNC these and sell them already done.

I'm going to go for 63x1.55mm, basic round dish.
Should get me to 10.2:1

Why round, because it's easier?
Are you going to have the edges like what is shown (with a flat bottom), or more of a gradual parabolic shape (like what VW did with some factory pistons)?

I had to have some swimming pools machined in mine to get the compression down as well.  This is 17CCs

Same scenario as Taylor for me - made by JE

[attachment=1]

Taylor, spooling, thanks for sharing! they look awesome.

Martin, yeah flat bottomed. Purely because I have the means to do that myself.

Here are some quick pics of my easy piston dishing fixture.  It registers in the machined step in the bottom of an AA piston.  Slip fit.  The pin is scrap nylon tapped for whatever bolt you have around.  Tighten the bolt into the pin, and suck the piston against the fixture.  Chuck on the round aluminum fixture and check the run-out just above the top ring.  I made this one to work for 92 and 94mm AA's.  I have another for T4 stuff. 
(https://www.thesamba.com/vw/gallery/pix/1703570.jpg)
(https://www.thesamba.com/vw/gallery/pix/1703568.jpg)
(https://www.thesamba.com/vw/gallery/pix/1703569.jpg)

Brian

I wouldn't want to lose the squish bands like that.  :o

Martin are you referring top Spoolin' post?

(https://i.imgur.com/WyGX5QZ.jpg)
(https://i.imgur.com/CNsFLge.jpg)
(https://i.imgur.com/ncpWRxF.jpg)

This is exactly how I do them as well,  My thinking is that if you are going to do a full circular dish you have lost your squish band and tight deck... you may as well just shim the barrel out or use super thick copper shims, you will have the same result.

That's true, but then your pushrods are longer.  😛  made good power.   We'll see if it overheats.

Highlighted in red is 70mm diameter, my dishes are 65mm specifically to avoid losing the squish :)

(https://www.thesamba.com/vw/gallery/pix/1702313.jpg)

Is it really that much more work to do it properly?

Not if you have the time and money to scan and/or program the cnc, or do it manually in a Bridgeport. But the 65 mm dish as shown above works well. The tiny loss in squish on the non plug side is so little that it doesnt matter.
How do i know that works well, - because these engines do not need much advance to perform well, and the emissions are fine. Very little unburned gasses emit the pipe.

T

And you don't need the extra cc's.
These are not one-off customs are they?
You'd think it would be easy to set up for one size piston and crank them out with different depths.

Was that a question for me? If so then YES, it would be much more work seeing as I don't have a suitable radiused end mill to machine a bathtub... The round dish was the best with what I've got to work with.

Thanks for your Input as always T

Luke :)

If it still has the squish intact, it will be great!  :D

I've maybe lost 1 or 2 mm on the none-plug-side at most... >95% of the squish is still intact.

Thanks for your input :)

Do what properly?

JE made these for me

(https://www.thesamba.com/vw/gallery/pix/1704885.jpg)

Nobody dishing pistons manually - like head porting is traditionally made? If you only need a couple of cc's it should be quite an easy work, easy to measure and quite easy to do close-enough shapes in four pistons.

It should be also quite easy to mimic the combustion chamber shape. But is it necessary, as head chamber shape is formed for max flow from valves, good swirl etc. Piston shape only affects these when it's quite close to the head, at least if the piston crown shape isn't very radical.

If we don't consider piston shape's effect to generate more flow or swirl, shoult the piston bowl should be centered / radiused around spark plug but leaving the squish area intact? Then the flame front starting (...hopefully) from the spark plug, reaches piston more at the same time the flame front travels sideways. At least in my head this reduces knocking and allows for more ignition advance?

Is the squish band as important with 1750cc injectors and 40psi boost ? 

My dish is 37cc so to keep the band would mean deep deep pockets in the piston

Fiatdude has the right idea (see his JE pistons) and at least is keeping one side of the squish band intact. It's a balancing act keeping the CR down. But the advantage kicks in with the squish working to do the same thing as keeping the CR down, while at the same time making more power. The deck has to be tight, as tight as possible for the squish to work though.

As it happens, just a couple of weeks after asking anybody dishing pistons by hand, I just got to rebuild/finish an engine with 78/94, stock rods and quite nice small port 42/37,5 heads that have chamber volume of 47cc. With W120 cam and the engine going to split screen bus for some touring, nice deck height (~1,5mm) would get a bit too much static compression even for my taste (10,4:1).

About 6cc's should be easily grindable, just 1-2mm of depth on chamber area, but what about the shape. Should I mimic head chamber in even height (roughly as it is pictured here in 944's pistons but way more shallow) or should I shape it somehow more wicked? At least I think the squish area should be left intact - after all without some fancy shape I just could add a shim under the barrels of coppers to head to increase deck height. That's also a bit boring ;)

(https://rennlist.com/upload/porsche_944_turbo_pistons1.jpg)

They should all match so making a radical shape will make matching more difficult. 1.5mm ( .060" ) is too much deck height.

Why is that?

Tighter deck makes more power thru turbulence and also lowers pre-ignition. Try 35 to 40 thou (.1mm) instead.
The way it was explained to me: take a coke can and put gas in it and light it on fire, then set it down on your bench. Then do the same thing, but this time slam it down as hard as you can.  :o

In some engines and applications, yes of course. But I don't think all street driven VW engines apply here. I have run .060 deck for years. As far as evidence of pre-ignition, aside from the obvious audible clue, I've yet to see any, using this clearance. I've run street motors with 0.040, that were "ragged edge" and faced the music with some of the results. Again, these were put together and used, knowing they weren't going to set longevity records. My old "almost daily driver" 94 x 82 ran 53cc and 0.070 deck, 34 deg total and had no evidence of pre-ignition.

I have that tight deck in two current motors. One is my wife's 65 which was a high mileage low CR Berg 1776 with stock intake and exhaust (on the road since 1999). That engine had a top end rebuild about 4 years ago (was over 50,000 miles on it and bores were oval) with the high CR tight deck and it has given the car an new life with way more power than before and has an aggressive diesel sound to it now. The stock late type distributor was modded by lightening the springs to deliver quicker advance.
It has .030" deck and you can hear the pistons kiss the head when it's started dead cold (that's why I would recommend .035"). Go-karts have used this trick for more power for years.

The other engine is my turbo 2332 with around 8.5:1 which has about 20,000 miles on it now and has been the love of my life and so much fun to drive.
I wouldn't hesitate to do a tight deck on any engine again, as there doesn't seem to be any downside.

Are you saying deck tight enough to hear the piston hit the head is better because the motor will make more power with higher compression?

Partly. I haven’t tried high compression on a stocker running regular gas, but I’d guess you’d have knocking. With a tight deck you can run high compression and get power without having to advance the timing like with the low compression Berg setup.
Mock it up to target the deck and see what the CR is. Then, if you need more cc’s, dish the pistons if you’ve already unshrouded the valves.