Modifying Stock AAN Pistons

[mushasho]

The idea is to "add accumulator grooves to the stock pistons to aid the prevention of blow by and aid piston stability under high boost figures."

How many folks here have experience with such mod and can comment on it's effectiveness...




pics taken from the s2forum...

[amd is the best]

Interesting. I know nothing about it... Is there any benefit to doing this over just going aftermarket?

[Hank]

That is kindof my take as well. Modification by a shop would be 200$ or better, plus you are going to want to put new rings in (100+$). That is 300$ or half of what nice lightweight forged units cost in an overbore piston that allows for fresh bores.

[Wheeljack]

Looks like some more dredging of JP's thread...some good stuff in there.

It is one of those things good for a DIY'er with the tools and know-how to mod OEM slugs on the cheap, but as Hank said, probably not too cost-effective to be outsourced.

But the function of the groove is proven and accepted in the industry; in fact, it is an option when ordering aftermarket pistons from JE or the like...

[quattro87]

I guess I could go read the S2 thread, but is another compression ring used there?

[Wheeljack]

I guess I could go read the S2 thread, but is another compression ring used there?

No, John. It is just there for the gas volume. It is intended to keep pressure spikes from happening in between ring 1 and 2 (from blow-by), by averaging out the pressure more from that additional volume, hence the 'accumulator' function. This helps keep the pressure differential more consistent across ring 1, which should keep it from 'fluttering' and in more consistent contact with the cylinder wall.

Some info from various piston manufacturers:

Accumulator Groove is the groove between the 1st and 2nd compression ring. It does make the piston lighter, but the real purpose is more abstract. Pressure spikes that get trapped between the 1st and 2nd compression rings tend to unseat the top ring. This action encourages ring flutter and loss of piston ring seal. Past efforts to reduce ring unseating pressure have included increasing the second ring end gap. Now, with the addition of the accumulator groove, ring flutter can be controlled in all engines. The void created by this groove between the rings tends to average the normal pressure present, keeping the pressure low enough to prevent lifting the top ring while maintaining some preload on the 2nd (oil scraping) ring.

Accumulator Groove is a V shaped groove machined in the 2nd ring land to collect excess blow-by between the top and second ring. This groove collects residual gasses during combustion and alleviates top ring flutter while increasing ring seal. - See more at: http://www.cp-carrillo.com/Tech/PistonT ... EOAnJ.dpuf

...but you get the idea...

[quattro87]

So no ring at all....just a groove to increase volume between the compression rings. How deep are they cutting them? I've always been under the impression that lifting the top ring was usually a function of piston speed and that when the piston reaches tdc and reverses direction downward in the cylinder that the drag on the ring becomes a problem and unseats the ring. It would make perfect sense that the problem would only be compounded if there was pressure spikes between the two rings helping to push the ring upwards. Thats an interesting concept on the added groove however.