Fbody fan 35yrs

I just removed the heads from a fresh, used L98 motor from GM. I was looking for flat top pistons and found dish pistons. I checked SDPC and the specs for L98 said dish.............. so all TPI L98 350s that came in Thirdgens have dish pistons?

xpndbl3

iTrader: (5)

dished pistons or do you mean, valve reliefs have been cut into the pistons?

Nixon1

That's awfully weird... This site's tech specs says the L98's have 9.3:1....which isn't exactly low compression. So I imagine (assuming the site is right) that either stock back then they didnt come with dishes, or the combustion chambers were super-small or something and provided a very high compression number, so they used dished pistons to lower it down.

TA-Wizard

ALL F-Body L98's have dish pistons with 4 valve reliefs. They measure out at roughly -12cc.

The only L98's that get true flat top pistons is the Corvette version with aluminum heads. They also have 4 valve reliefs.

RB83L69

9.3 is mighty low compression. It would be up around 10-10.2 with flat-tops.

They're defintely supposed to be dished, to lower the compression to such a cheap-pump-gas level.

Nixon1

Depends on your definition of mighty low. My 302 has smaller than stock combustion chambers and is around maybe 9.5:1. My old 3.1 Camaro only had 8.5:1.

RB83L69

9.3 fits my personal defintion of "mighty low". But you're right, grocery-cart motors have even less than that.

Nixon1

Lemme guess......you've owned some mighty fast N/A motors before huh?

Fbody fan 35yrs

Well, I thought that TPI motors being "hight output" would have flat top pistons to get to the 9.3 to 1. I checked a ZZ4 short block with flat top pistons at SDPC, and it raises the ratio .5 over the L98 with the dish. They show 9.1 with 64cc heads on the L98, and 9.6 on the ZZ4 with the flats and 64cc heads. This worries me as I am changing to vortec heads with 68cc chambers. I thought vortec heads had smaller or at least the same size chambers as stock heads?? Thoughts???

Nixon1

Lol....larger cc's equals better performance but lower compression. You'll definitely have some pretty mild compression with dished pistons and 68cc chambers... You could put some different pistons in....or consider that as preparation for forced induction...

Fbody fan 35yrs

I just ran the #s on my L98 heads, and they do have 64cc chambers. I hate it when you think you know how something is, and it .............. isn't. I'm sure there is someone out there that knows what you come up with when you use vortec heads on a stock L98 block?

smithtc

iTrader: (5)

My L98 pistons are -7cc 4 valve relief

poncho@home

actually the Vortec heads are rated at 64cc as are the stock L98 heads. The aluminum vette heads are rated at 58cc.

I have heard that the vortec heads actually cc a little less than 64, the set I cc'd came out to the advertised 64.

I used a .015" thick head gasket to give me a couple of points more compression. I should be at the 9.5 - 9.7:1

Fbody fan 35yrs

I am getting my #s from this web site: http://www.mortec.com/castnum.htm Vortec casting # 12558062 L98 casting # 14101083

poncho@home

both those castings show up as 64cc

Fbody fan 35yrs

I'm having a bad night................. you are right, they are 64cc. I looked down a line by mistake and saw 68 on another head.

RB83L69

I don't know about all that, like what you'd consider fast or whatever. Let's just say I've been building motors for a very very long time, and for a while I used to do it semi-professionally for semi-professional race teams. So I've seen one with a little higher CR than 9.3 from time to time. I try not to let my own ever get that low if I can help it.

Nixon1

I consider N/A 'mighty fast' to be 10's or quicker pretty much. Quicker than most street machines will ever see.

TA-Wizard

I have heard of this claim before....if true it would mean that the factory shipped this motor out with flat top pistons with -7cc valve reliefs, the compression ratio would be over 9.9:1.

I doubt this, but, with GM anything is possible.....LOL

Black 91 Z28

Stock L98 Pistions

TA-Wizard

Black,

I am curious.....did your motor have a steel shim or composition head gasket when you first tore it down??

OMINOUS_87

Steel shim on iron head L98.

Fbody fan 35yrs

The block in the photo has flat top pistons with valve reliefs. If what has been discussed earlier is true that isn't a stock L98 block. ???

smithtc

iTrader: (5)

http://f2.pg.photos.yahoo.com/ph/gta...cture_0330.jpg

Stock 89 L98

They have a "dish" and 4 valve reliefs that measure out to about -7cc.
Compression comes out to 9.47:1 using a 0.039 compressed gasket thickness. Believe the GM gaskets on my motor were a little thicker.

Russ-So Cal

With G.M. you might get anything. He said he found a steel shim with the head gasket. If the engine plant needed to build f-body l98 and didn't have the right pistons in stock to complete the order, they would do something like that on as many engines as necessary to complete the order. Holding up production on even a couple of engines could stop the production line at the final assembly pant. Stopping the line for anyreason gives a factory full of workers an extended paid coffee break!

Black 91 Z28

Just whatever came from the factory. I don't remember now cause that was last year.

Fbody fan 35yrs

I have heard that about production. Automatic cars got one cam, and manual cars got another for the most part, but some times you can find either cam in either car. The same is probably true of pistons. It helps explain why some cars just run better than others of the same year with the same equipment. The new L98 I have looks just like the one in the link in smithtc's post. I hate to give up that compression, but I got a good deal on the motor and it, as always, comes down to "how fast do you want to spend"

Beefy89

The pistons in the picture are FLAT TOP 4 VALVE RELIEF pistons. If you replace them with TWO VALVE RELIEF FLAT TOP PISTONS you will raise your compression ratio to roughly 9.75:1.

MikeDwhoROCZImports

iTrader: (1)

Seems to have been a lot of debate back in '04 over whether the pistons were -7cc or -12cc. Anyone confirm anything yet? I'm in the same boat...building an engine with a stock L98 bottom end, and some hard info on the piston CC could really help me out.

Fbody fan 35yrs

I can't tell you exact CC,s but I do know that at least some L98's especially in Corvette's had flat top pistons, and get you close to 10 to 1. Many if not most of the Fbody L98's had a dish in the top of the piston, and that puts you somewhere in the 9's to 1. Both pistons have valve relief's but the ones in the flat top pistons are a little deeper since they are physically closer to the valves. I sold L98 block I had and went with a Vette motor. It came from the factory with flat tops.

camarodarrell

I've heard the 87-89 L98's had 9.3:1 compression and the 90-92's had 9.7:1 because the 90-92's switched to speed density computers that gm felt would run more precise for more compression. That could explain why some L98's have dished pistons and some have flat tops.

MikeDwhoROCZImports

iTrader: (1)

Awesome, thanks guys! The engine I have has the dished pistons. I did some calculations this afternoon, and it seems it would make sense that the dished pistons are -12cc and the flat-tops are -7.

Damon

This topic has been gone thrrough several times on this board before. Around 1990 (some say as soon as 1989 in some motors) the factory switched from -12cc dished pistons to -6cc flattops. GM never changed the "official" compression ratio of the motor in the published specs but it was indeed higher (~9.8:1) on the later speed density motors.

I didn't want to believe it at first, but the vast direct and personal experience of memebers on this board has convinced me it's true. My experience is mostly with the 87-88 motors and they were definitely dished pistons every time I tore one open.

Orr89RocZ

iTrader: (20)

back from the dead....

check my motor pic.... does this look like -12cc dish? i guess it does block was cast in late 88 i think for the 89 model year. car is a 89 irocz L98

TraviZ

iTrader: (2)

I would think so. I also had an l98 in my 89 iroc-z, look to the right on the photo, you can see the same dish.

Orr89RocZ

iTrader: (20)

sucks cuz i was really hoping for flat tops

1989GTATransAm

iTrader: (3)

My 1989 had the same pistons from the factory as posted in the above picture. However as was posted above starting maybe in 1990 GM in fact did put in pistons with less of a dish. So the compression ratio was larger but not advertised.

Orr89RocZ

iTrader: (20)

that may be why SD cars ran faster than previous L98s. But i know they got a slightly weaker camshaft as well tho.

Damon

The pistons in both the pictures, above are the -12cc dish variety.

Dyno Don

iTrader: (7)

So far all of the '91-92 350's I have taken apart have had flatops w/4 reliefs.

Jproz1167

iTrader: (3)

I have an 89 L-98 with flat tops and my 90 vette L-98 had flat tops

five7kid

iTrader: (14)

'87-'89 L98 had 9.3:1 CR. '90-'92 had 9.7:1 CR. 12cc dish+reliefs vs. 5cc reliefs account for that.

Having reliefs is not proof of flat-tops. You can see both the dish and reliefs in the photos above. Look just like my Speedpro hypers, in fact.