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bearwin
Samba Member
Joined: December 16, 2004
Posts: 278
Location: san diego,california
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 Posted: Mon Mar 20, 2023 9:23 pm Post subject: Dishing AA piston |
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hello, i just want to know what is the limit of depth is allowable to dish a 92 AA cast piston?
i was planning to dish mine, diameter of 70mm and depth of 2mm to get additional 8cc..
i got 043 heads (40x35) with 50cc chambers, i dont want to ruin a good set of heads, thats why im dishing the pistons..
engine 2180cc with engle w120 cam
thank you.. |
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VWporscheGT3
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Joined: September 01, 2006
Posts: 1985
Location: Gardnerville, NV
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| Posted: Tue Mar 21, 2023 7:49 am Post subject: Re: Dishing AA piston |
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I think the AA's have been measuring around .200 to .220" , someone will correct me if im wrong. that being said, you do not want to go any thinner than about .180" in the crown, any thinner than that and you run the risk of blowing a hole through it.
2mm equals roughly .080" so that just isnt in the cards , sorry man
Edit: folks have pointed out more than .250 in the crown so it is possible at that thickness. in the industry we always recommend to not going less than .180". doesn't mean that folks have tried and not been successful 
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Last edited by VWporscheGT3 on Tue Mar 21, 2023 1:42 pm; edited 2 times in total |
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Brian_e
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Joined: July 28, 2009
Posts: 2760
Location: Rapid City, SD
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| Posted: Tue Mar 21, 2023 8:50 am Post subject: Re: Dishing AA piston |
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i have done .080" deep in a circle on a smaller engine with 8.8:1. Not sure I would do it with more compression than that.
I have also made oval shapes to avoid the dish getting out into the flats, but adding more volume.
Have the chambers been worked on at all? Un-shrouding will usually get you around 2-3cc if you don't get carried away. Sinking the valves a bit will usually help some also.
Brian
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modok
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Joined: October 30, 2009
Posts: 25962
Location: Colorado Springs
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| Posted: Tue Mar 21, 2023 12:32 pm Post subject: Re: Dishing AA piston |
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| 2mm is risky with the cast pistons, imo |
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UK Luke 72
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Joined: September 07, 2011
Posts: 2796
Location: Little Britain
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bearwin
Samba Member
Joined: December 16, 2004
Posts: 278
Location: san diego,california
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| Posted: Tue Mar 21, 2023 5:56 pm Post subject: Re: Dishing AA piston |
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thank u guys..
modok, i think ill settle on 1.7mm deep.
ill get some 2cc on the heads |
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Glenn 
Mr. 010
Joined: December 25, 2001
Posts: 75124
Location: Sneaking up behind you
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| Posted: Tue Mar 21, 2023 6:20 pm Post subject: Re: Dishing AA piston |
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Why not just shim the cylinders to lower compression?
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Glenn
74 Beetle Specs | 74 Beetle Restoration | 2180cc Engine
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Vanapplebomb
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Joined: November 03, 2010
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Location: Holland, MI
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| Posted: Tue Mar 21, 2023 6:29 pm Post subject: Re: Dishing AA piston |
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| Glenn wrote: |
| Why not just shim the cylinders to lower compression? |
Because that increases the deck height/reduces squish. I have found tight decks usually run cooler and don’t need as much advance. I shoot for 0.030 - 0.040 measured at the top/bottom of the piston with it rocked out in the bore by pushing from behind.
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Glenn
Mr. 010
Joined: December 25, 2001
Posts: 75124
Location: Sneaking up behind you
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| Posted: Tue Mar 21, 2023 6:34 pm Post subject: Re: Dishing AA piston |
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| Vanapplebomb wrote: |
| Glenn wrote: |
| Why not just shim the cylinders to lower compression? |
Because that increases the deck height/reduces squish. I have found tight decks usually run cooler and don’t need as much advance. I shoot for 0.030 - 0.040 measured at the top/bottom of the piston with it rocked out in the bore by pushing from behind. |
And dishing the piston doesn't reduce squish?
I'm familiar with the .030-.040" range.
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Glenn
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esde
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Joined: October 20, 2007
Posts: 5469
Location: central rust belt
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| Posted: Tue Mar 21, 2023 7:33 pm Post subject: Re: Dishing AA piston |
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| Glenn wrote: |
| Vanapplebomb wrote: |
| Glenn wrote: |
| Why not just shim the cylinders to lower compression? |
Because that increases the deck height/reduces squish. I have found tight decks usually run cooler and don’t need as much advance. I shoot for 0.030 - 0.040 measured at the top/bottom of the piston with it rocked out in the bore by pushing from behind. |
And dishing the piston doesn't reduce squish?
I'm familiar with the .030-.040" range. |
you try to keep the piston dish positioned so that it's under the chamber. Keeping the tight deck around the edges squeezes everything towards the plug.
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RandyV
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Joined: November 23, 2019
Posts: 325
Location: So Cal
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| Posted: Tue Mar 21, 2023 7:35 pm Post subject: Re: Dishing AA piston |
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Just another thought and the reason I cut my pistons vs shims....
You get that beautiful fat sealing surface on the case side of the cylinders...(92mm thick walls as an example)....and then you throw a shim...(nobody makes a cylinder shim worth a turd anymore and certainly not fat enough to embrace the bottom of the AA T/W versions)....and now you gave up all that beautiful sealing area.
I probably wouldn't go past 1mm cut myself but would not hesitate to cut anything less in a street motor. |
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UK Luke 72
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Vanapplebomb
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Location: Holland, MI
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| Posted: Wed Mar 22, 2023 4:20 am Post subject: Re: Dishing AA piston |
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| Glenn wrote: |
And dishing the piston doesn't reduce squish? |
Correct, as long as the portion of the piston crown adjacent to the quench area of the head is not cut.
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http://www.thesamba.com/vw/forum/viewtopic.php?t=487021 |
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Vanapplebomb
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Joined: November 03, 2010
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Location: Holland, MI
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| Posted: Wed Mar 22, 2023 4:25 am Post subject: Re: Dishing AA piston |
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| UK Luke 72 wrote: |
| Glenn wrote: |
And dishing the piston doesn't reduce squish?
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Here’s the damage my dish did to squish… noteworthy?
Brian’s bathtubs are better, but so are his lathe skills.
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This is a great picture that perfectly shows how squish can be lost by dishing if the dish footprint overlaps the quench area of the head.
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vwracerdave
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Joined: November 11, 2004
Posts: 14934
Location: Deep in the 405
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| Posted: Wed Mar 22, 2023 8:06 am Post subject: Re: Dishing AA piston |
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It's amazing how many engine builders in the V-8 world are starting to use semi-hemi chambers, but the VW world considers it black magic taboo.
Way too much rocket science technology from a world class race car that doesn't make a damn bit of difference on a street engine.
You can't read a Chevy book and build a VW engine.
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Glenn
Mr. 010
Joined: December 25, 2001
Posts: 75124
Location: Sneaking up behind you
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| Posted: Wed Mar 22, 2023 8:31 am Post subject: Re: Dishing AA piston |
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| vwracerdave wrote: |
| You can't read a Chevy book and build a VW engine. |
Aren't VW engines just glorified lawn mower engine with 4 cylinders?
I hear that a lot.
I average about 40 hours to build a VW engine. usually 2-3 mock ups before the final assembly.
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Glenn
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Brian_e
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Joined: July 28, 2009
Posts: 2760
Location: Rapid City, SD
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| Posted: Wed Mar 22, 2023 8:46 am Post subject: Re: Dishing AA piston |
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| vwracerdave wrote: |
It's amazing how many engine builders in the V-8 world are starting to use semi-hemi chambers, but the VW world considers it black magic taboo.
Way too much rocket science technology from a world class race car that doesn't make a damn bit of difference on a street engine.
You can't read a Chevy book and build a VW engine. |
Here we go again... 
The new engine world also has canted valves, 4 valves per cylinder, very little valve angle, domed pistons, knock sensors, and EFI. Its hard to fit 4 valves in a chamber WITHOUT some form of hemi shape.
A modern engine design and a VW engine are two WAY different animals, with WAY different needs. Apples and oranges. No black magic or taboo.
It would be much more correct to said "you can't try to apply modern chamber tech to a 60 year old cylinder head layout."
It has been known and tested since the 1950's that a wedge chamber head with 2 valves works far more efficiently with .030-.050" squish. That will always be the same, no matter what the new cars do. I have yet to see any real V8 2 valve heads with a hemi chamber. Quite the opposite. All the real power maker heads run a super tight butterfly or heart chamber. With 2 valves, and a hemi chamber, the pressure recovery into the chamber is non-existent.
Many VW people, including myself have tested it on real running engines, and a tighter deck works better every time. Way better throttle response, easier to tune, less jet needed for the same power, MUCH more detonation resistance, better efficiency, and cooler running. Every one of those improvements is welcome on a street engine.
Its not difficult to do with new heads that come with larger chambers. Might as well make it correct the first time, rather than half ass throw it together, and call it good enough.
Brian
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Last edited by Brian_e on Wed Mar 22, 2023 9:07 am; edited 1 time in total |
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Brian_e
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Joined: July 28, 2009
Posts: 2760
Location: Rapid City, SD
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| Posted: Wed Mar 22, 2023 8:51 am Post subject: Re: Dishing AA piston |
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| Glenn wrote: |
Aren't VW engines just glorified lawn mower engine with 4 cylinders?
I average about 40 hours to build a VW engine. usually 2-3 mock ups before the final assembly. |
ALL 4 stroke engines function under the exact same principles, some just have more "accessories", and are more efficient then others.
You are quick. I have 40hrs. into most long blocks, and no less then about 6 mock-ups, and I thought I was getting pretty fast after so many.
Brian
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Alstrup
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| Posted: Wed Mar 22, 2023 9:00 am Post subject: Re: Dishing AA piston |
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| Brian_e wrote: |
The new engine world also has canted valves, 4 valves per cylinder, very little valve angle, domed pistons, knock sensors, and EFI. Its hard to fit 4 valves in a chamber WITHOUT some form of hemi shape.
A modern engine design and a VW engine are two WAY different animals, with WAY different needs. Apples and oranges.
It would be much more correct to said "you can't try to apply modern chamber tech to a 60 year old cylinder head layout."
It has been known and tested since the 1950's that a wedge chamber head with 2 valves works far more efficiently with .030-.050" squish. That will always be the same, no matter what the new cars do. I have yet to see any real V8 2 valve heads with a hemi chamber. Quite the opposite. All the real power maker heads run a super tight butterfly or heart chamber. With 2 valves, and a hemi chamber, the pressure recovery into the chamber is non-existent.
Many VW people, including myself have tested it on real running engines, and a tighter deck works better every time. Way better throttle response, easier to tune, less jet needed for the same power, MUCH more detonation resistance, better efficiency, and cooler running. Every one of those improvements is welcome on a street engine.
Its not difficult to do with new heads that come with larger chambers. Might as well make it correct the first time, rather than half ass throw it together, and call it good enough.
Brian |
AAMEN!
Also, bear in mind that just about ALL newer hemi heads are running twin spark plugs, - AND they have a slight squish shoulder on each side of the chamber. I wonder why(?)
I have built a couple of Twin spark type 1´s and am soon to start another 2275. When you learn how to design the enginbe it is amazing how much overall torque you can get, - along with some 7-15 hp more dependant on set up.
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Brian_e
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| Posted: Wed Mar 22, 2023 9:15 am Post subject: Re: Dishing AA piston |
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| Alstrup wrote: |
| When you learn how to design the engine, it is amazing how much overall torque you can get, - along with some 7-15 hp more dependent on set up. |
Or you just stay stuck in 2001, whine about other people trying different things, and eventually get left behind. 
Brian
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