3.1 pistons in 2.8

[Fog]

Hello.

Since 3.1 was stroked from 2.8 the bore is the same for them. I really need some information about stock piston height. I am planning to reduce compression to about 8.0 using those dished pistons on 3.1 or 2.8(Gen 2.) since building a turbo setup for my Fiero.
Could anyone point out a concrete measures and sizes of the "dish" of those pistons.

Thanks.

[v6h.o.]

IIRC the piston dish for the GEN II (aluminum heads) was 25.5 cc's.

i believe the piston height in the bore for these was 0.00725 in the block.

[Fog]

Heh. According to my info stock Fiero engine was 8.5 CR, ang useing that as a baseline, that engine with GenII pistons will have a CR of 6.34

[v6h.o.]

yeah but the chambers for the Iron heads are what? fifty something cc's??

the aluminum heads have a 28cc or so chamber..
the argument is always between 27-29cc's for the Gen 2 and gen 3 alum heads.

[v6h.o.]

are your fiero pistons the slightly dished style or are they the true flattops?

i know some of the Iron heads had a small dish like around 10cc's or something.

[robby1870]

Yeah, from what I have seen, most of the iron heads, had the 50 something cc chamber, and and the Gen II had like 26.38cc or something small. I think the iron head piston dish was 11.9cc. Im showing the Gen II 2.8 having a 21.2cc dish and the Gen II 3.1L having a 26.4cc dish

[Fog]

So, Fiero had 12cc dished pistons for CR 8.5, head volume is 51cc and if I would install Gen2 25.5cc dished pistons, I would have 7.2 static CR?

7.2 would be ideal for my application.

[RacerX11]

OK, here ya go. First off, you can't use 3.1 pistons of any vintage with a 2.8 crank, and vice versa, unless you use custom rods. The different compression heights of the pistons are used to compensate for the stroke differenece between the two cranks.

The 2.8 HO iron head in the Fiero has flattop pistons with a compression height of 1.599". This results in a 8.9 CR. The non-HO 2.8 iron head pistons are flattops with a compression height of 1.578. This results in a 8.5 CR. The Gen2 2.8 dished pistons with a ~21cc dish, and a 1.608" compression height would give you ~6.9 CR with the iron heads. This is WAY too low CR.

Why do you think that 7.2 CR would be ideal for your aplication? Are you planning on running 25+psi of boost? If so, the stock pistons won't hold up very well anyway. I think most people running serious boost stick with ~8.0CR on the iron heads, and ~9.0-9.6CR with the aluminum heads. If you go much lower than 8.0, you will really start to hurt your off-boost performance and response, as well as increase turbo lag.

If you don't want to spend the money on custom forged pistons, here is what I would do. Use the low output 2.8 pistons, and the thicker head gasket from an aluminum head 2.8 (0.060" vs. 0.040" for the iron head gasket). By my calculations, ths would get you down around 8.1CR.

Marty

[RacerX11]

Yeah, from what I have seen, most of the iron heads, had the 50 something cc chamber, and and the Gen II had like 26.38cc or something small. I think the iron head piston dish was 11.9cc. Im showing the Gen II 2.8 having a 21.2cc dish and the Gen II 3.1L having a 26.4cc dish

All of the 2.8 iron head engines had flattop pistons. The 3.1 iron head had dished pistons with a 12cc dish, and the 3.4 iron head had dished pistons with ~8cc dish.

Marty

[robby1870]

Thanks, Marty, I wasnt really sure about the 2.8 iron head pistons

[Fog]

...If you don't want to spend the money on custom forged pistons, here is what I would do. Use the low output 2.8 pistons, and the thicker head gasket from an aluminum head 2.8 (0.060" vs. 0.040" for the iron head gasket). By my calculations, ths would get you down around 8.1CR.

Marty

THANKS! this kind'a detailed info is like air to breathe for me right now

Anyway, I'm planning to go 8psi for street use and 10-12psi for strip.
Why I'm thinking about using non-aftermarket pistons, is that delivery costs to Estonia are WAY high. Trying to get most of the work done by myself. Total budget I can pay for the pieces is about 400$.

[Fog]

It is going a little OT, but a couple of q's more:
I know that under boost the ignition timeing must be changed, so is it absolutely nescesary to have MSD box, or can this be done elseways. Since the project is ultra-low cost, I tought to just install the air temp sensor after the turbo. Also, I'm planning to use stock ECU with modified tables, SBC injectors and 2bar map.
Is there anything else to take in consideration?
Also, Marty, can you point out safe boost levels for both, 8.9, 8.5 and 8.1 CR engine. I know this depends on many factors, but as a guideline.

Thanks. Fog.