Notices
C4 Tech/Performance L98 Corvette and LT1 Corvette Technical Info, Internal Engine, External Engine

Flat or dish

 
Old 07-11-2019, 03:07 PM
  #1  
mlm0
CF Senior Member
Thread Starter
 
Member Since: Dec 2012
Location: Georgetown TX
Posts: 1,307
Received 51 Likes on 45 Posts
Default 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
mlm0 is offline  
Old 07-11-2019, 11:51 PM
  #2  
mtwoolford
CF Senior Member
 
Member Since: Jan 2009
Location: folsom california
Posts: 3,284
Received 134 Likes on 128 Posts
Default

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?

Last edited by mtwoolford; 07-11-2019 at 11:57 PM.
mtwoolford is offline  
Old 07-12-2019, 12:05 AM
  #3  
mlm0
CF Senior Member
Thread Starter
 
Member Since: Dec 2012
Location: Georgetown TX
Posts: 1,307
Received 51 Likes on 45 Posts
Default

Iím 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 canít remember for sure what they were

Last edited by mlm0; 07-12-2019 at 12:53 AM.
mlm0 is offline  
Old 07-12-2019, 01:49 AM
  #4  
65Z01
CF Senior Member
 
65Z01's Avatar
 
Member Since: Dec 2000
Location: SE NY
Posts: 83,237
Likes: 0
Received 154 Likes on 146 Posts
Cruise-In II Veteran
Default

Web pics show flat top with 4 valve reliefs.
65Z01 is offline  
Old 07-12-2019, 03:14 AM
  #5  
cuisinartvette
CF Senior Member
 
Member Since: Oct 2004
Location: Valencia Ca.
Posts: 69,052
Received 1,275 Likes on 1,164 Posts
St. Jude Donor '05
Default

call a piston mfr. at0 deck falt top youd be lucky to get a real 10:1 which isnr much
Build plans?
cuisinartvette is offline  
Old 07-12-2019, 07:22 AM
  #6  
mlm0
CF Senior Member
Thread Starter
 
Member Since: Dec 2012
Location: Georgetown TX
Posts: 1,307
Received 51 Likes on 45 Posts
Default

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.

Last edited by mlm0; 07-12-2019 at 07:24 AM.
mlm0 is offline  
Old 07-12-2019, 07:24 AM
  #7  
Pwnage1337
Tech Contributor
 
Pwnage1337's Avatar
 
Member Since: Oct 2008
Location: Custer, WI
Posts: 3,396
Received 66 Likes on 59 Posts
Default

Stick a bore scope in a spark plug hole?
Pwnage1337 is online now  
Old 07-12-2019, 08:56 AM
  #8  
mlm0
CF Senior Member
Thread Starter
 
Member Since: Dec 2012
Location: Georgetown TX
Posts: 1,307
Received 51 Likes on 45 Posts
Default

Great idea. Never thought of that. Will pick one up today
mlm0 is offline  
Old 07-12-2019, 10:36 AM
  #9  
vader86
CF Senior Member
 
vader86's Avatar
 
Member Since: Sep 2001
Location: Athens AL
Posts: 51,142
Received 338 Likes on 283 Posts
Default

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.
vader86 is offline  
The following users liked this post:
mlm0 (07-12-2019)
Old 07-13-2019, 05:11 PM
  #10  
dizwiz24
CF Senior Member
 
Member Since: Sep 2001
Location: NEwhere Ohio
Posts: 10,216
Received 89 Likes on 85 Posts
Default

My 93 had flat tops with 4 half valve reliefs cut into them
dizwiz24 is offline  
Old 07-13-2019, 06:23 PM
  #11  
cardo0
CF Senior Member
 
cardo0's Avatar
 
Member Since: Oct 2002
Location: Las Vegas - Boycott Hypocrisy!
Posts: 6,309
Received 229 Likes on 219 Posts
Default

Originally Posted by mlm0 View Post
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.
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.
cardo0 is offline  
Old 07-13-2019, 08:25 PM
  #12  
89dd
CF Senior Member
Support Corvetteforum!
 
89dd's Avatar
 
Member Since: Jul 2012
Location: NYC
Posts: 402
Received 12 Likes on 4 Posts
Default

Flat with valve reliefs.
89dd is offline  
Old 07-18-2019, 12:05 PM
  #13  
corvette_bob
CF Senior Member
 
corvette_bob's Avatar
 
Member Since: Jan 2001
Location: Dayton Ohio
Posts: 1,290
Received 3 Likes on 3 Posts
Default

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.
corvette_bob is online now  
Old 07-18-2019, 12:23 PM
  #14  
mlm0
CF Senior Member
Thread Starter
 
Member Since: Dec 2012
Location: Georgetown TX
Posts: 1,307
Received 51 Likes on 45 Posts
Default

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


Last edited by mlm0; 07-18-2019 at 12:24 PM.
mlm0 is offline  
Old 07-18-2019, 01:11 PM
  #15  
28uniflite
Junior Member
 
Member Since: Jun 2018
Posts: 15
Received 0 Likes on 0 Posts
Default

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

28uniflite is offline  
Old 07-18-2019, 01:23 PM
  #16  
mlm0
CF Senior Member
Thread Starter
 
Member Since: Dec 2012
Location: Georgetown TX
Posts: 1,307
Received 51 Likes on 45 Posts
Default

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

Last edited by mlm0; 07-18-2019 at 01:53 PM.
mlm0 is offline  
Old 07-18-2019, 01:25 PM
  #17  
28uniflite
Junior Member
 
Member Since: Jun 2018
Posts: 15
Received 0 Likes on 0 Posts
Default

Originally Posted by mlm0 View Post
Same as mine. Do you happen to know the piston vacuum?
Sorry, I do not
28uniflite is offline  
Old 07-18-2019, 02:17 PM
  #18  
mtwoolford
CF Senior Member
 
Member Since: Jan 2009
Location: folsom california
Posts: 3,284
Received 134 Likes on 128 Posts
Default

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.
mtwoolford is offline  
Old 07-18-2019, 02:30 PM
  #19  
mlm0
CF Senior Member
Thread Starter
 
Member Since: Dec 2012
Location: Georgetown TX
Posts: 1,307
Received 51 Likes on 45 Posts
Default

Not sure what Iím 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

Last edited by mlm0; 07-18-2019 at 03:06 PM.
mlm0 is offline  
Old 07-18-2019, 10:51 PM
  #20  
mtwoolford
CF Senior Member
 
Member Since: Jan 2009
Location: folsom california
Posts: 3,284
Received 134 Likes on 128 Posts
Default

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.
mtwoolford is offline  
 


Thread Tools
Search this Thread
Quick Reply: Flat or dish


Contact Us - About Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

© 2019 MH Sub I, LLC dba Internet Brands

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.
 
  • Ask a Question
    Get answers from community experts
Question Title:
Description:
Your question will be posted in: