Ok this will probably just make its way to the bottom real quick, but here's my situation. I bought 2 rebuilt 3.1 shortblocks. Both have the Alum head pistons(approx-25cc dish) this would lower the compression ratio down to about 7.5. I could buy iron head pistons and bump that up to about 8.8(approx-12 to 14cc dish) but I dont want to buy new pistons. I know running a turbo would increase the CR but is it worth it? Or should I just get new pistons? I really dont want to put anymore into these engines. How bad will a 7.5 CR hurt my performance on the street. I got the engines for too cheap to pass up even if they have the wrong pistons.
TIA

------------------
JOEJOE Aol:SSFiero
Two 88FORMULAS Red & Yellow
SSFiero@Aol.com

With 7.5 compression, you could run 20 psi or more on pump gas. That would be a wild ride. Be sure you put it together with copper head gaskets.
With compression that low your gas mileage would suffer, but I don't know by how much. The engine would be a bit harder to start on cold mornings (Fuel injection does a good job hiding that).

Keep in mind that if you go too low in compression your low end torque will suffer until the boost comes on. I think 7.5 might be getting low enough that you may not like the lost of torque down low.

Of course once the turbo spools up........

If it was me, I'd change the pistons. A set of 3.1L hypereutectic pistons for the iron head motor aren't that expensive.

However, if you like to experiment, a turbo running say 12-15 lbs of boost may be enough to produce lots of top end power....just don't expect much low end power until the turbo spools up.

I have a 3.1 turbo with 8:1 and run about 5 pnds boost. Id try it with the lower compression and crank up the boost to see how it is before Id do anything. Mine with boost up to 12 or so starts knocking, so Id bet you could get away with 20 pnds or so pretty ez.

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

Thanks for the quick response. I paid $125(bought quite a few things to get that low) for each engine so Getting a set of pistons really isnt a big deal. I just didnt want to get too into it. Knowing me, I will want to change the pistons, then cam, etc. I was hoping to keep the engine build under $500. I guess I could be cheap, but thats probably not a good idea. Oh well, for now Im going to stick a stock 2.8 in. I have an extra 88 cradle So I can start to build on that and take my time.

------------------
JOEJOE Aol:SSFiero
Two 88FORMULAS Red & Yellow
SSFiero@Aol.com

Books I've read don't reccomend going below
8:1. The problem being poor starting, poor response, and maybe even increased emissions. If ya want more boost do it right, more fuel and INTERCOOL properly

I've seen some intercoolers that are air/airconditioner for the syclone and typhoon. The owners say the intake manifold is cool to the touch even after multiple hard runs. I'll post the link when i find it again

------------------
84 Fiero Phantom GT
Vortec 4300 CPI 5 spd.
Best ET 14.4 @ 95 mph
Go'in turbo
[IMG]http://groups.msn.com/_MSNUserPicture.msnw?user=4263~3AAAAAANAC8f6DB1mJPXynGb2o!x!!qH*bfPKgd!Xna6ZtYoAOPn8dyA$$~3AAAAAAOAE24QzxH*RX3b8oIfVSdl4KmI9PkY6ChdVd1w$&pgmarket=en-us[/ IMG]

[This message has been edited by vortecfiero (edited 06-26-2002).]

I would agree with the above posts on reasons to use more than 7:1 compression ratio. Yes with that low of a C/R you theoretically could run more boost but the 60* V6 won't be able to hold it. At 7:1 C/R you will have very poor low end responce. At boost levels above 10 psi,the likelihood of throwing a rod or at the very least blowing a head gasket increases 10 fold.
IMO, 8.5:1 is the ideal C/R for all around street/strip driving. I run 8-9 psi boost w 8.5:1 pistons and get good results. The low end is strong and the power comes on quick.

------------------
87GT 3.4 Turbo Best 0-60 5.2 seconds
http://turbofiero.fierojoe.com/turbo.htm

quote Originally posted by vortecfiero: I've seen some intercoolers that are air/airconditioner for the syclone and typhoon. The owners say the intake manifold is cool to the touch even after multiple hard runs. I'll post the link when i find it again

Could you post a link with info on these systems, please? Thanks.

Whats stock compression on the 2.8, and how much boost can you safely run?

Stock is 8.9:1. Maybe 6PSI without something to cool the air charge like an intercooler or water/alky injection. Even then you need provision for a knock sensor.

You shouldn't have much of a problem with the dished pistons especially if you are going to run moderate boost to compensate. I used an 88 2.8L with the dished pistons and cast iron heads as a quick fix for a blown stock motor. With the reduced compression ratio the car still ran nice. I have just finished compression testing a 2000 3100 that I'm turbocharging and installing over the next two days. My curiousity is with the compression readings from the dished pistons coupled with the cast iron heads: 180 lbs/sq.in. I expected it to be lower than that. The heads are from an engine that would not work and do not appear to be O.E. from a fiero though they were removed from one. It's possible they were shaved a little. The valves still had paint on them from the machine shop. I suspect the engine was damaged beyond what a head job could correct and the owner let the car go. I have also read 8.5:1 compression to be optimum for turbocharging, I have also read articles on turbo cars with compression as low as 7.5.
For a stock piston and moderate boost that should keep you out of trouble and happy. You have the engines, it wouldn't hurt to go ahead and use them. You are replacing a smaller higher compression engine with a larger lower compression engine. It should be close to balancing out.

quote Originally posted by Sootah: Whats stock compression on the 2.8, and how much boost can you safely run?

Ed has dialed in 10psi on Predators car with no intercooling and 20thou shaved off his heads. making 200hp and 250lbft at the ground. the stock engine is fine, with proper timing and fuel management 15psi wouldn't even be a problem with a decent intercooler, with liquid/air 17psi would be ok.. but the paper seals in the middle of th eintake pelenum may pop... water injection or meth/water would work even better mixing it 1:1. an msd 6BTM, rising rate FPR, ad decent injectors... along with programmable management.. from dynotuner, apex-1, HKS (Camp) or TEC3.

[This message has been edited by 1FST2M6 (edited 06-27-2002).]