Flat or dish
 

After a ton or research, I still do not have a positive answer. Does my 1989 corvette in stock condition have flat or dish pistons and If you know for certainty, do you also know the piston dome cc?

i am trying to do some calculations for a possible build and I need this information

thanks in advance to whom ever can provide it

Mike

you're research may turn up something different, but my research, along with actually looking at Chevy pistons of that era, suggest a (inverted dome aka dish) of minus 5 c.c.s, mostly from the four "eyebrow" cuts for valve reliefs. Aftermarket piston are available that are true "flat tops" with only two valve reliefs.

what are your goals (presumably) compression wise?

I’m shooting for 10 to 1.

my research told me everything from dish to flat and 6 cc to 18 cc. I was hoping someone on the forum actually had an 89 and saw what kind of pistons they had so I would know for sure. From what I can find out different years had different pistons. I had my heads off a couple of years ago and I took pictures of everything, but now I cannot find them and I can’t remember for sure what they were

Web pics show flat top with 4 valve reliefs.

call a piston mfr. at0 deck falt top youd be lucky to get a real 10:1 which isnr much
Build plans?

I just need to know exactly what I currently have without putting a head to check. I need to know so I can determine what I am going to buy for improvement in performance.

Stick a bore scope in a spark plug hole?

Great idea. Never thought of that. Will pick one up today

Should be flat with valve reliefs cut in. I used to know the size of the relief but have forgotten, but it should be available in a search on the compression or pistons of 1989. There were minor tweaks in later L98s.

My 93 had flat tops with 4 half valve reliefs cut into them

This a good experience for anyone doing a performance build/rebuild. You shouldn't be ordering parts until you have the block machine work done. Don't order pistons until the block is bored, decked and the head chambers are measured with liquid. And the last item to select/purchase is the cam. I wouldn't even buy rocker arms until everything was measured. To be precise you would need pistons to get the block decked accurately but you can calculate for a final deck height to select pistons. Maybe gaskets and seals but no reason to buy them early as plenty of sbc gasket kits on the part stores shelf.

Granted you most likely want a target cam or target cam duration to work with the drive line you have. But this target duration cam would no way be final. You will want to match the compression ratio for that cam so this is where the measurements and piston selection is required.

So what I'm saying is while parts shopping is part of the fun it's easy to get ahead of yourself. Myself I try to avoid a large dish piston but the cylinder head chambers are mostly what determine what you can use to make your target c.r. for your target cam duration. You may be surprised as I was to see how much the block deck height can make a difference in the final c.r.. Head gasket compressed height with different cylinder diameters are significant too.

FWIW I like to use 2 or more c.r./d.c.r. calculators to check my work and there are plenty of them on-line so it goes faster the more you crunch the numbers.

That's enough for me and hope it was for you to. :)

Flat with valve reliefs.

FWIW, most guys I know don’t use dished pistons unless there are specific reasons for them. The reason being is for achieving a proper quench for the exhaust. Try to get a good zero deck with your pistons at TDC, and, if you can, use a piston that will
get the compression ratio in the range you want. If flat tops make too much compression, there are pistons that are dished only on the combustion chamber side to provide quench as needed.
Best of luck on the build. I hope I’ve helped.

I just used bore scope not more then 15 mins ago and found I have dish pistons I have included a photo. I now need to find out what cc they are
https://cimg5.ibsrv.net/gimg/www.cor...fa0076f62.jpeg
https://cimg6.ibsrv.net/gimg/www.cor...c1ed3334f.jpeg

Here's a picture of my stock '89 from when I did a head gasket replacement this past winter.

https://cimg6.ibsrv.net/gimg/www.cor...30796e6e70.jpg

Same as mine. Do you happen to know the piston volume?

Sorry, I do not

In your contemplated build are you planning on reusing the stock pistons? If your possible build includes new pistons, most aftermarket pistons have known / published volumes. Head gasket thickness, and most important, combustion chamber volume are also variables to be considered.

Not sure what I’m going to do. Engine only had 50,000 on it. I just want to add some performance. I have all measurements exact stock piston cc volume

Does your engine burn oil? Absent something unexpected, at 50,000 miles the engine should be in pretty good shape and, money wise rebuilding the lower end might not yield the most bang for the buck. There's a ton of members here that can offer great advice concerning heads, cam, exhaust, etc for your L98.