Dishing pistons

[DWL_Puavo]

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?

[spoolin70]

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

[Martin S.]

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.

[DWL_Puavo]

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

[Martin S.]

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

[Jim Ratto]

1.5mm ( .060" ) is too much deck height.

Why is that?

[Martin S.]

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. 

[Jim Ratto]

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. 

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.

[Martin S.]

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.

[Taylor]

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?

[Martin S.]

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.