[240Dave]

> Due to a major F$$k up by the machine shop
> that cleaned my intake manifold with bead
> shot and didn't wash it all out. (They
> actually only cleaned the as far into the
> runners as you can put a finger and left the
> rest of the manifold full of carbon, junk
> and hmmmmm... steel beads!)

> So the cylinders are being overbored, new
> pistons and rings, new gaskets. All paid for
> by them of course.

> This is a stock l28 and n42 head with stock
> cam and stock dished pistons. Can I put
> flattop pistons to increase compression and
> get a performance boost without doing any
> other major mods?

Wow, at least they are paying for it. I sure hope you get this **** engine running, I know it's frustrating to have to keep re-doing stuff, especially if you havent' even driven the car!! Anyways, regarding the pistons. You sure can put flat-top pistons in the block. They have the same pin height as dished, and the overall block height is the same for all the L series engines. So, no problem with the piston sticking out of the block at TDC.
As far as compression ratio goes, you would get a nice increase to about 10.0:1 using a stock 1.25mm head gasket. Using the 1mm HKS metal gasket would give you @ 10.2:1. With the increase in compression, and the overbore, you should notice a nice increase in power. This would be the limit for the compression on a street engine. Any further and you might need some better ignition, polishing, etc.... The compression also gives the exhaust a little more powerful bark also. You'll probably be sounding alot more like a Ferrari with the high compression, rather than a American V8.

 

[240Dave]

Sounds about right...

> Yeah, at least I do have something positive
> to get out of this. Like more horsepower.
> But not really the way I expected it to go
> what with scored cyl walls and shot pistons.
> Luckily the bottom end is still solid.

> What sort of horsepower increase can I
> expect out of the increase in compression
> and bore? 5 - 10 horsepower? I already have
> a header and 2.25 exhaust as well as a K&N
> ram air.

Usually, a 1.0 increase in c.r. is equivalent to a 3-5% increase in engine hp. Your engine is rated @180hp stock. With the additions you've added, I'd say you're @195hp. Add in the 1.5 c.r. increase, and you'd be @203-210hp. This is a hard thing to estimate, as engine hp is so dependent on so many other factors, such as volumetric efficiency, cam timing, etc....
Of course, with the added compression, you're engine will respond better to a performance cam or timing changes. So, when you increase the c.r. you've increased the potential for the engine to make more power, all you have to do is feed it.