Advantage and Safety of Dishing AA Pistons

[gearheadgreg]

This is really just bench building, as I don't have the parts yet - But - I want as much compression as I can safely get away with, with as tight a deck as is safe for an every day use.

Building a 2074 (just started another thread about valve springs, but here it is again)
Forged 78 crank
Forged 5.5" VW Journal I beam rods with APR bolts
40x35.5 heads
AA Thick Wall 92s (B's)
Norris cam
Dual 40-45 IDFs or DRLAs

I would like to get over 9:1 (1971 Bus, 091 gearbox) with a deck of somewhere between .035-.045". I know Squishies are the easy answer, but I'm trying to keep the overall cost of the built down. I don't need to spend another 90% to get the last 10% increase. What I would like to do, is keep the combustion chamber as small as possible, while keeping the quench/squish band around the outside - not sacrificing deck to lower compression.

I was wondering if there are any issues with dishing the AA 92mm pistons (strength, thickness, etc) or is it a proven thing? I've got a VW machinist that I trust to do it, but my next question - if it is safe, how much benefit to shaping the dish to match (or strategically mis-match) the head chamber or not much difference from a simple circular dish?

Thanks!

[Ol'fogasaurus]

I have been thinking about an engine build also and have been slightly interested in AA pistons since some STF’rs started talking about them so I finally looked them up:

http://aapistons.com/index.php?main_page=index&cPath=15

http://aapistons.com/87x69mm-1641cc.html

If you read the second URL they say that the pistons are cast and made from Hypereutectic aluminum alloy material (http://en.wikipedia.org/wiki/Hypereutectic_piston) and http://www.mmsonline.com/articles/new-h ... nder-bores. I got this from a Harley post: http://www.nrhsperformance.com/tech_hypers.shtml. From a Ford site: http://www.fordmuscleforums.com/all-for ... -good.html. And this: http://answers.yahoo.com/question/index ... 6020808561. I got all this with a single search.

I don’’t know a good answer for you but in my un educated opinion: if you do try to machine them I would sure measure the crown thickness and take that into consideration.

I always thought dishing pistons were for reducing compression but then I am from back-in-the-day old skool.

Just to get some conversation started.

Lee

Update: I did an oops and said cast iron vs. cast.

[Marc]

Theoretically you'll maximize the efficiency by making the piston dish outlines closely match those of the combustion chambers (that way you get the maximum unswept volume increase for the greatest reduction in C.R. with the least depth cut out of the piston and the most quench area remaining). Is it worth the increased complexity compared to a simple round dish that is small enough in diameter to not reduce the effectiveness of the quench pad of the head? Only if you can't get the volume you're after without going deeper than you're comfortable with.
As I said before, if you want to be reasonably sure that the pistons won't smack the heads on overrun someday with a bore this large, you'd better not go less than ~.050" piston deck. The 1600 singleports which I used to build for offroad competition, with no more than a 30PICT-1 carb, came dangerously close to contact at .040" deck. Yes, they had more piston-to-cylinder clearance than a fresh street motor, but no more than would be normal by 50K miles.
Your target deck of .035-.045" is quite simply scary to me.

It's Saturday evening and I've had a few adult beverages...hopefully the following math is correct.
92x78 swept volume: 518.51cc
Unswept volume needed for 8.5:1 => 69.13cc
Unswept volume needed for 9.0:1 => 64.81cc
Deck-height volume at .040" => 6.65cc
Deck-height volume at .050" => 8.31cc

For the sake of argument, let's suppose the chamber cc's are near-stock at 52, and the ballpark you need to be in for total unswept volume is 67cc....gotta have another 15cc in deck volume + piston dish volume.

At .050" deck you'd need ~6.5cc in the dish. This could be achieved with a round dish that was 2" in diameter and 1/8" deep. At those dimensions, the quench pad should be relatively undisturbed and the piston crown should still have ample integrity.

Obviously if the chamber volume is greater the dish can be smaller in diameter and/or shallower.

If we were to use more pessimistic figures (let's say 48cc chambers, and even though I'm against it .040" piston deck - and the max desired C.R. of 9:1) it'd go like this:
10cc needed in piston dish volume. A 2" diameter round dish would therefore need to be nearly .195" deep - at this point I'm getting concerned, because in my experience anything over ~1/8" merits caution. The good news is that all you'd need to do is enlarge the diameter to 2½" - or shape the dish more like the chamber in the head - and 1/8" deep would again be sufficient.


So, what're the chamber volumes suppposed to be in these heads?

[gearheadgreg]

Ok - I give in on the deck height. I am used to watercooled VWs with brick shithouse blocks and cranks that don't move around nearly as much. That's why I am posting on here. If .050" is the closest it can be safely, than that's what I'll shoot for. Any experience shot-peening cranks and rods for the extra strength to prevent a bit more flexing to run tighter?

I don't know the dimensions on the heads yet - I haven't ordered them, but am about to. I've really been holding back until I feel better about having all my ducks in a row, though I don't see the heads changing in spec, even if I change cranks and rods and other stuff. They will be opened to 94s for the thick 92s, so I will have room to lean the chamber back if need be, but I would rather keep it tight and use piston dish to acheive that unless there is a large flow advantage to changing the chamber. I've got an old flow bench I need to dust off and fire up for this project, I think.

They are from Kaddie Shack, and even though I can buy some at wholesale, Jeff has treated me right and his prices are good, and he has more info about his heads than some of the wholesale suppliers. Here's a pic:



Thanks as always guys.

[Marc]

On an ACVW the stiffest component is the crankshaft - everything else is basically wrapped around it, and therefore subject to whatever distortion-producing forces originate from it. Type IV and aluminum Type I blocks are stiffer than magnesium alloy so you can safely run tighter clearances with them, but with a mag case and 92mm bore I'll stick by my guns and say that less than .050" piston deck is asking for trouble (maybe not tomorrow, but certainly within a few years). The theoretical advantage attributed to "squish" motors is due to the increased turbulence caused by running the piston-to-head clearance as tight as possible, and if you have lazy, low-velocity intake ports it does help demonstrably - but IMO the effect is overrated, if you have intake ports which are correct for the application there'll be ample swirl and little to be gained by tightening the deck. I can recall running out the last two races of a season to lock in the championship with one head that was so badly torched it belonged in the scrap barrel (had to run over .250" deck to match the 11.25:1 C.R. of the other side, I can't even remember how many fins we had to cut off)...and the lap times were as good as they'd been all year.

Piston rock and crank/case deflection are the primary concerns, connecting rod treatment won't help (aluminum rods are another subject, but for all practical purposes all iron/steel rods are length-stable in our regime). Shot-peening is another separate discussion, but the short version is that it does no measurable good unless done "just right" (media/radius/velocity) and 99.5% of those offering the treatment fall short of the mark. Your proposed engine won't need any special treatment to the rods beyond basic polishing out of any stress-risers found.

I'm not denying that "quench" is a factor when designing an engine, I just want to present the point that its effects are largely theoretical and a pragmatist would be unwise to trade off the known liabilities of too-liitle-deck in search of nebulous gains.

It's your build, but please don't go under .040-.045"

P.S. impossible to judge accurately from the photos, but if I was to wager I'd say those chambers are around 56cc. Once you have them in-hand you'll be able to calculate the piston dish requirement, but my gut feeling is that it's not going to be a big problem for you.