Are they any different?

I'm asking because I can pickup 2.8 internals from cavi owners for free when I am rebuilding my 2.8 for my Camaro so if they work together it will save me a bunch of cash.

Does anyone know forsure?

 

Yes, they are different, that is if you are refering to a 1987 to 1989 2.8, which you most likly are, since there was only one year of Cav that had an iron head 2.8 (1986).

The stroke is the same and the crank will fit in any large journal RWD 2.8L block (mid 1985+).
The rods are the same.
The pistons are quite different. They use a rather large dish to compensate for the smaller combustion chamber in the aluminium head.
Cam is the same as the "H.O." cam in the early 2.8 and later iron head 2.8s.

If you were looking for a performance improvment building a hybrid with the FWD top end and internals, is a gret improvment in flow and power output, especially if you retain the iron head pistons (Approx 11.5:1 SCR).
But I would skip the genII top end and go for the genIII top end, which flows even better.

 

I remember calculating the CR of the RWD 2.8 pistons + FWD heads = 14.6:1 cr or so..

The RWD 3.1 pistons, with the dish, would have a lower CR, but I don't think it's THAT much lower.

Around here, you can find people GIVING the RWD 2.8s away. Nobody keeps them - just keep looking around

 

Havent been able to find any 2.8's around here running to tell you the truth.

I'm not sure exactly what parts I need if I rebuild it yet as I havent pulled it but it is bottom end somewhere. COmpression is still good so it isnt rods, not quite sure what it is to tell you the truth so I'lll take it apart and find out.

 

Doward, recalculate. it is less than 12:1 for the 2.8 stroke.

 

here is some calculations for everyone.

A stock 2.8 motor is 8.9cr

add aluminum 28cc heads and you get 13.5cr

then also add 3.1 (dished pistons) to the 2.8 bore and stroke block with aluminum heads and you have 11.7cr

--------------

A stock 3.1 motor is 8.9cr

Add alum 28cc heads and you get 12.9cr

Keep the iron 50cc heads and swap to flat 2.8 pistons ( this is physically impossible because the flat 2.8 piston would come into contact with the head, it is 4mm taller) Don't know if a stock 2.8 piston has enough meat on the crown to have it milled down. But if so you could set the CR whereever you want (maybe 9.5-9.8 range if strong enough) and reduce the piston mass for a lighter weight assembly----better to just have custom pistons made so you have adequate top ring thickness on the crown.

 

You need to recheck your info. The 2.8 piston, regardless of generation does NOT come proud of the block, so in no way, other than a rod breaking/rod bearing failure, would the piston contact the head.

If you are suggesting to use the genII 2.8 piston with the iron head, even using the iron head head gasket (.040" compressed thickness), the SCR would be around 7:1 SCR, WAY too low to have a decent running engine.

I'd like to see your calculations on how you got over 13:1 using genI 2.8 pistons with the aluminium heads.

 

The 3.1 is a longer stroke so the 3.1 piston has a deck height 4mm shorter than the 2.8 piston + it is domed to lower the compression from the longer stroke. put the 2.8 piston into the 3.1 motor and you will have a flat top piston into the stroked 3.1 that has a deck 4mm taller

And you were saying?
My figures are dead-on accurate, you prove me wrong cowboy.
You are going from a 50cc head to a 28cc head.

 

I'd like to see how you didn't. Since you are challanging me, I would expect the same reciprical curtiosy from you to display an answer!
Here's mine
Use this chart- http://www.csgnetwork.com/compcalc.html
Put in these figures-
staring with the top box
2 (change to mm)
89 (mm bore)
76 (mm stroke)
89 (mm head gasket)
1 (mm head gasket compressed= .040")
50cc /28cc (for the iron head) /(for the alum head)
0 (for the flat top 2.8RWD piston)
.575 (mm for the piston to deck clearance= .0226")

Note: The actual head gasket bore AND the deck height may slightly "vary" but these figures are close enough for government work and gets you right onto the 8.9:1 stock CR of the factory 2.8RWD. Once you switch the 50cc heads to 28 cc heads, the CR goes through the roof to 13.5CR