What to do? 3.1 conversion on a 2.8 Fiero block...

[82-T/A]

Hey guys,

I rebuilt my 2.8 from my 87 Fiero into a 3.1 a couple of years back. The car has some excellent low-end power, but I really didn't do my homework when I put this motor together. I went with some crazy cam that produces tons of power up to 4,000 but basically dies after that. I also went with OEM replacement pistons for the 3.1 which dropped my compression to something like 8.4:1.

I'd really like to just re-do the motor and correct my mistakes.

First things first, I plan to go with an H272 with 1.52:1 roller rockers. (I already have the roller rockers).

However, as far as pistons go, what are my options? I'd really like to have a mid 9 compression ratio. What are my piston choices? I currently have a .040 overbore in there.


Thanks guys!

Todd,
2006 Pontiac Solstice
2002 Ford Crown Victoria LX
1987 Pontiac Fiero SE / V6
1984 Pontiac Fiero 2m4 SE WS6
1981 Pontiac TransAm WS6
1973 Volkswagen Type-2 Transporter

[IsaacHayes]

I imagine you have iron stock heads still then? Not sure what comppresion would be ok with those with normal gas. The aluminum head 3.1/etc I think had 8.9:1, and the 3100's had 9.6:1... Since you probably didn't swap 3.1 heads and just ran with fiero top end, your heads are iron right? Iron can't take as high comppresion due to not disapaiting heat as well, so that might be somethign to look into?

[82-T/A]

I imagine you have iron stock heads still then? Not sure what comppresion would be ok with those with normal gas. The aluminum head 3.1/etc I think had 8.9:1, and the 3100's had 9.6:1... Since you probably didn't swap 3.1 heads and just ran with fiero top end, your heads are iron right? Iron can't take as high comppresion due to not disapaiting heat as well, so that might be somethign to look into?

Thanks Issac, yes they are the stock heads. The stock Fieros compression was also 8.9:1. I actually lost compression when I went with the 3.1 OEM pistons.

I was thinking I'd like to see a compression ratio of 9.2-9.3 realistically.

I don't know that much about supercharging or turbocharging, but I know that quite a few people turbo their stock 2.8s. Typically they run 4-5 PSI of boost and don't modify their heads. That said, do you think I'm safe? I suppose my alternative is to deck the heads, but I'd really rather just do it the right way...

[IsaacHayes]

9.2-3 should be fine I would imagine. Im no expert, but you don't want ping that's for sure, and if the motor isn't making tons of power, I can't justify paying more for higher octane fuel to stop pinging you know?

Turboing/supercharging an iron head motor is like on the gm 3800 S/C. Iron head is more durable, but you'll ping quicker than an aluminum head. That is why the 3800 S/C is 8.5:1 and requires high octane fuel. (don't let anyone else tell you other wise, scan the car for knock retard, most have it even with high octane stock!).

In all reality you'd probably be fine with even 9.6:1 like the 3x00 motors, but you might have to run higher octane fuel to stop pinging... I have to run min 89 in my aluminum head 3100, but it does have 180k on it. If it pings, it does the "knock retard" and pulls timing, and the motor is a dog then at partial throttle. I'm guessing your 2.8 has a knock sensor, so you would loose timing too, plus you don't want to ping anyways.

Hopefully someone else can chime in with what they think you would be good to run at. But 9.2/9.3 isn't a whole lot over 8.9, so I think that's a happy medium.

Just something to think about, hopefully I've helped!

[sharkey]

the iron 2.8 does not have a knock sensor.

if you want to keep the extra displacment and have higher compression, about your only bet is custom pistons. realy the only options for the 3.1l ironhead is oem replacment or hyperutectics from trw (if they still make them), but even those are only 8.9:1 or so.

as for boosting it, id get a cam with a less overlap, and just run good fuel. around 8psi with that low a compression even on cast pistons should be just fine.

as for compression and fuel, my ironhead is 9.5:1, and i get no ping at all on 87, but i run 89 most of the time (what car guy builds a motor up and runs cheap fuel???). i plan to boost the thing around 10psi (i have forged slugs, molley rings and stainless valves, itll take it) and i realy dont anticipate having and detonation issues after tuning.

[BillS]

8.5 is great if you want to turbo.

9.5 is just fine on pump gas.

Don't boost compression if you ever plan on a turbo.

And you will not get as much benefit from the cam you are going to use (which is NOT that suitable for a turbo, BTW) unless you do a bunch of head and manifold work. Relieving the exhaust alone gains you ~8-9 BHP, and there are gains to be made in the intake and heads. Once you've set them up, the cam will make much more difference - it's no good setting up to breathe fire if you have to suck through a straw.....

[IsaacHayes]

Do you have upgraded springs? Just a thought since high rpm with a cam can cause valve float...