What pistons can I use?!

[82-T/A [Work]]

Hey guys....

I recently built up a motor for my 1987 Pontiac Fiero V6. Originally it was a 140hp 2.8 V6/6 with MPFI. I learned that I could drop the 3.1 crank and rods into it. So I did just that. I did a number of other things, higher output camshaft, etc. But I noticed a drop in compression. I went with a set of slightly dished pistons (as apposed to the highly dished pistons).

My compression went from 8.9:1 to 8.5:1. However, I installed .040 overbore pistons, and a few other things. I did notice a good deal more torque from the increased displacement, and it definitely pulls hard. However, I'd really like to get my compression back so I'm guessing I may have to buy a set of pistons and tear the motor apart again.


I have TWO options:

1) Look for a 3.4 block from a 93-95 Chevy Camaro / Firebird, drill the starter bolt holes, and rebuild the bottom end.

or

2) Rebuild my current block over again with better pistons.


Now, for #1, my question is. What is the stock compression of the 93-95 f-body 3.4 V6/60. I can apparently just drop that RIGHT into my Fiero, and everything bolts up (with the exception of the starter bolts). If the compression is high enough on the 3.4 (I'd like ~9:1, but anything up to 9.5:1 is a bonus) I may not bother looking for aftermarket pistons and just go with OEM replacement pistons, or stock compression pistons.
My other question for #1 is... if I was do go this route, where can I get a GOOD rotating assembly, or should I just reuse the one that I get with the block. I'm thinking the next time I do this, I REALLY want to do it right. I want something badass. Like what's in the SBC in my 81. I want Oliver Rods, completely balanced rotating assembly to within 1/2 a gram. Any of you guys know where I can get something like this?

For #2, if I was to rebuild my 3.1 (or 3.2 I guess), where can I get a set of pistons that will give me the equivelent compression that my motor had originally.. at the very least 8.9:1. Like I said though, I'd be happy happy all the way up to 9.5:1.


FYI, I do not plan to ever turbo this car, or supercharge it. My goal is for the engine to look COMPLETELY stock (which it does), but just have more power. If you want to see what I did to my motor, you can look in the pictures on the link in my sig. I rebuild the 2.8 into basically an HO 3.2. I wasn't THAT impressed. I think in the Fiero, with everything i've done. I'm guessing I'm probably putting out ~30 more horsepower. I guess I'm probably running low 15s. I could have done a much better job, but I just didn't know any better at the time. NOW, I want to re-do it, and do it right.

Thanks!

[RacerX11]

Todd,
If you stay with the 3.1 bottom end, you will need to get custom pistons made to raise the compression. If you are already running the 3.1 iron head small-dish pistons, this is the best you can get with off-the-shelf parts. The only other option is to mill the heads.

If you go with the 3.4 Camaro engine, you can use 3.4 DOHC pistons to raise the compression ratio up to around 9.6 or so. It is still streetable, and will run on pump gas. I am not sure what the stock CR is on those engines. I used these DOHC pistons in my 3.4 engine, and had good results.

The problem you will run into is the stock Fiero MPFI intake is very restrictive, especially when you start increasing displacement. You will pick up more low-end torque and mid-range with the 3.4, but your top-end power won?t be much better than the 2.8 or 3.1.

Marty

[82-T/A [Work]]

RacerX11, what you say REALLY REALLY does make a LOT of sense to me now. One of the things I DID notice about the Fiero after making all these modifications.. and comparing the OLD performance to the NEW performance was that, well.. the top end DOES feel flat, it really does only feel marginally better than what my 2.8 originally was.

I'm really glad you mentioned that becuase it adds a whole new variable into the puzzle.

Putting into consideration what you have said... if YOU personally were going to do this swap. 3.4 with 3.4 heads, Fiero intake (but 1.8" intake valves instead of 1.7".. I know a place that does that). Lets say I have my throttle body bored out to 57mm (up from 52mm) and that I have my entire intake pieces port-matched (which I do actually).

WHAT characteristics would you look for in a cam.

Would you go with one that improves LOW TO MID RPM power? or MID TO HIGH RPM power?


Thanks!!!

[RacerX11]

Are you dead-set on keeping the iron heads and Fiero MPFI setup? If so, you might want to look into having the intake extrude-honed. Putting a larger TB on won?t help much without opening up the intake.

If you aren?t too attached to the MPFI setup, here are my two favorite ideas:

1. If you are going to go through the trouble of swapping out pistons in a 3.4, you would be better off doing a swap to the gen3 aluminum heads and intake, and using the 3400 pistons. You will get a 9.6 CR, and much better top end power. The aluminum heads are lighter (by ~30 lbs for the pair), and can handle higher compression without detonation. This raises some other issues, such as computer control, but I won?t get into that here.

2. Another option I am considering is to use a modified 2-barrel carb manifold with a truck TBI unit. The carb manifold has shurter runners than either the Fiero MPFI manifold, or the standard 2.8 TBI manifold. It is really well suited for high-rpm power. The normal 2.8 TBI setup doesn?t make much power, because it is designed for torque. With an adapter plate, you can use a large 5.0/5.7 truck TBI setup that will flow enough air for 200+ HP. With a 3.4, the TBI setup, and a decent cam, you can make good torque and tons of power. You can use your stock Fiero ECM and distributor (it will need some chip tuning to run properly with the TBI). I thought of this after talking with several guys running the Holley 500 CFM 2-barrel carb on their 2.8 and 3.4 engines. They really praise the powerband of this carbed setup. I?m and EFI guy, so the larger truck TBI setup represents a nice replacement for the Holley carb.

For a cam, I would look into a Crane H260 or H272 (or equivalent cam from other manufacutrer). These are still very streetable, but will give decent power as well. You just don?t want to go too wild.

Marty

[82-T/A [Work]]

Thanks Marty,

I am kind of dead set on using the Fiero intake manifold. I mean, it's very much a major component of the Fiero (at least as far as aesthetics go). It says FIERO engraved in the intake plenum. I just want it to all look original.

More specifically about the cam, I wanted to know exactly what TYPE of cam you would be looking for if you were in my situation. Lets say you had the 3.4 with Iron heads and the Fiero intake.

Lets say that you were me, and you were going to go out and purchase a new camshaft to compliment this set-up.

Would you

A) Purchase a cam that would improve low to mid range power.
or
B) Purchase a cam that would improve mid to upper rpm power?
or
C) Purchase a cam that improves the overall performance of the motor in it's stock power-band location.

In particular, my throttle body was professionally bored out with it attached to the plenum, so the opening of the plenum was actually enlarged all the way into the open chamber of the plenum.

I guess then, my only need really, is to have the runners, and lower intake slightly enlarged, and then port match it with my plenum (easy) and have the cyl heads port-matched to the intake manifold.

If I purchase a cam that improves low to mid range power, will my high-end power completely die? Or will it STILL be better than the 2.8s was being that it's a 3.4 now?



Thanks... I appreciate your responses. Please just bear with me.. I'm still trying to figure out exactly what kind of characteristics I want to build / look for in my motor swap.

[RacerX11]

If it were me, and I wasn't planning to do anything real radical to open up the intake, I would go for plan C. Even though you had the TB bored all the way into the plenum, it will still be restrictive. The long, narrow runners are the real bottleneck on the Fiero intake, not the TB. Your TB will help, but not as much as a bigger intake, or extrude-honed intake. This would be the only real effective way to open them up without cutting them open. Everything I have read on the Fiero forums points to disatisfaction with the Fiero induction system once they start modding the engine. Most people elect to keep it, for the exact same reason you do. If you over-cam it with option B, you will be losing low-end torque, but you will never reap the benefits at the higher rpms.

If you wanted to put some work into it, it would be possible to cut open the Fiero upper plenum from the bottom side, and modify it so that the runners are shorter, and the plenum is larger. If done right, it wouldn't be visible once installed.

Either way you go, you will have more HP at the topend than the stock 2.8, but because the lowend will be so strong, it will feel like it runs out of breathe. There isn't much to do about that.

Marty

[Fierofiend]

Hey there...

Just wanted to chime in.

You could use the 3400 block and you won't need to drill starter holes. You would be going to a much stronger block this way, with the roller cam. You would still need to get the DOHC pistons to bring up the CR, and custom pushrods, (5.85x3/8th) but in my opinion, you are ending up with a better block.

You then bolt on the fiero heads, fiero plenums, and run the fiero ignition. You would still need to get a chip burned to maximize performance, just as you would with the camaro 3.4.