"Best" pre-cups for AAZ head?

[Jetmugg]

I have a spare AAZ cylinder head that I'm daydreaming about having ported, fitted with new valve guides, valves, springs, etc. to use on my race truck.  I'm planning to run this season with the engine set-up as it is now, but a worked-over AAZ may be in the future.

There are some cracks in the pre-cups as it sits right now.  If I go to the expense of having the head ported and machined, I'd like to replace the pre-cups also.  It seems that I've read about people using pre-cups from other engines (GM 6.2 maybe???) for better flow characteristics.

Any advice would be appreciated.

Also, for anyone interested, here are a couple of pics of my 1.5L - powered Rampage prowling the streets as a shakedown cruise for The Ohio Mile, and later, Bonneville Speed Week.





Steve.

[Gizmoman]

Sorry, can't advise you on pre-cups but that machine just looks fast!
I think you made that lady pee her pants.

[theman53]

http://shop.volkswagen-classic-parts.de/vwcp/Jetta-Vento/Vento/Antrieb/Motor/Brennkammereinsatz-VW-Dieselmotoren.html

If you want you could port them. I do not know about the GM ones working or not.

[vanbcguy]

Around here using GM precups would definitely be breaking new ground. Definitely a similar injector design so it passes the first test!

Sent from my HTC One XL using Tapatalk

[theman53]

Around here using GM precups would definitely be breaking new ground. Definitely a similar injector design so it passes the first test!

Sent from my HTC One XL using Tapatalk

They look similar, but if they aren't 100% fit in the head I would just rather port the precups like I did mine. That would be the only advantage is the higher flow. Open up the small end and ramp it a little more aggressive. Then you wouldn't have to worry about it falling in if you didn't get the outside machined correctly. The aaz has bigger openings and volume than the 1.6 did anyway.
Example :

[Jetmugg]

Thanks for the replies, guys.  Maybe it's my imagination, but it just sticks in a corner of my mind that there was a discussion about using the 6.2 cups.  Prehaps it was on another forum, or perhaps it came to me in a dream.

Anyway, not looking to reinvent the wheel, just thinking about how to make MOAR POWER.

When I get a little more time, I'll try to research the dimensions of the 6.2 units.

It may be a moot point, since the AAZ precups can apparently still be purchased in brand-new condition.

Steve.

[RabbitJockey]

andy2 experimented with gm precups on his failed compound aaz build.  i don't think he had any issues with them, but he also never really go far enough with that build to really say either way how they worked.  his thread is still on here somewhere.  he had to machine the hole larger to fit them.

also there are a few different sizes of gm precups,  if u search gm forums theres alot of info, but its not anything i ever took notes on, how ever i've definitely read about it, and higher powered gms had larger pre cups, even our vw's, the turbo diesels have a different swirl chamber, with a dimple on the bottom.

[Jetmugg]

Thanks, RJ.  At least I know that I didn't imagine the GM precups.

Like everything else, it's the details that make the difference, and it's not a good idea to overlook any of the details.

Steve.

[theman53]

after chopping up my old head that dropped a pre cup I would not ever machine the head out larger to accept a different one.  There really isn't much meat there. If I were going to do it I would go the other way and try to cut on the cup.

To me it doesn't matter because IT IS A DIESEL... But when you add a several CC to these precups as I have it will smoke white when the engine is cold. NO MATTER WHAT. I have had the timing as high as 1.05mm and now it is around .94mm, until the engine is up to temp it will have some white smoke and diesel smell. I did the math once and my compression is around 17, IIRC by the formula I used it was like 16.8 with the metal HG I used. I could find that formula and my numbers of the added volume again if you are really interested and figure it all up again. The moral is if you are picky about smoke don't drive a diesel.

BTW, it starts just fine down to 11f. I did it this winter, but I have a very fresh engine. It didn't like running until warm, but it started the first turn of the key.

I think it would be a great idea to copy the GM "performance" cup. VW designed these precups for somewhere between 48 and 80hp...I decided they needed opened up if you are going to build a bigger fire in them. Just like the intake design I think velocity and flow rule. The general coach's whistle design should be maintained, but I really don't think you can mess it up too badly, just make sure they are all the same volume when you are done.

[TylerDurden]

AIUI,

displacement / compression ratio = combustion chamber volume

400 / 23 = 17.391304347826086956521739130435

[Jetmugg]

AIUI,

displacement / compression ratio = combustion chamber volume

400 / 23 = 17.391304347826086956521739130435

Can you help me understand exactly what you are saying here?

[TylerDurden]

That's a rudimentary formula for calculating compression ratio. Example shown is the stock 1.6. Flip the formula for finding the desired info.

There are some spreadsheets available online for detailed calculation of bore, stroke, gasket thickness, etc., which can be used to project effects of changing the precup size and resulting combustion chamber volume & Cr changes.

[Jetmugg]

Gotcha.  I have made a few spreadsheets as well, taking into consideration gasket thickness, piston protrusion, cc's of the "Mickey Mouse" indention on the heads.  I've spent some time playing with various possibilities to calculate CR's and/or figure out if particular pistons will work in a non-standard application.

What would be really handy is knowing the cc's of the various pre-cups.   If anyone has that info, it could prove valuable to anyone putting together a non-standard combination of parts.

Right now, I'm inclined to rebuild my "extra" AAZ head with new AAZ pre-cups, but it's always nice to know all the options.

Steve.

[RabbitJockey]

Gotcha.  I have made a few spreadsheets as well, taking into consideration gasket thickness, piston protrusion, cc's of the "Mickey Mouse" indention on the heads.  I've spent some time playing with various possibilities to calculate CR's and/or figure out if particular pistons will work in a non-standard application.

What would be really handy is knowing the cc's of the various pre-cups.   If anyone has that info, it could prove valuable to anyone putting together a non-standard combination of parts.

Right now, I'm inclined to rebuild my "extra" AAZ head with new AAZ pre-cups, but it's always nice to know all the options.

Steve.

i don't have a link but do some searching, this info is on the forum some where.  if the forum search engine doesn't work go to google and type site:vwdiesel.net to search the site using google.

[carrizog60]

i was thinking at playing with the indents in the piston,by making a channel in the opposite direction of the one in the piston so the flame would travel also in that direction.
has anyone tryed the somender singh theory to see if any gains can be achieved?