How can I tell if I have a 3.1L swap? (Page 1/2)
xzeion MAR 18, 12:40 PM
I have had my Fiero for nearly a year now and in that time I have noticed a number of strange things and modifications that have been made by previous owners. I have the lower geared Grand Prix getrag 5 speed for example. I am 99% sure my car was converted from an automatic as I have a third "shift cable" going toward the front of the car among other little hints like no rev limiter.

My question: Is there an easy way to tell the difference between the 3.1L and 2.8L bottom end? Anything I can look at in the bay to tell? Only thing I have come to so far is to look and see if the heads are aluminum.

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87 GT in Blue
~ Drive what you love and love what you drive ~

Patrick MAR 18, 02:26 PM

quote
Originally posted by xzeion:

Only thing I have come to so far is to look and see if the heads are aluminum.



What does a magnet tell you?
pmbrunelle MAR 18, 05:33 PM
If you still have the RPO code sticker on the front left wheel well, you can confirm what transmission your car was equipped with from the factory.

If you have a stock-looking engine, you most likely have the iron heads.
GM gave the iron-head 2.8 engines flat-top pistons from the factory.
3.1s got dished pistons.
So, you could poke a borescope through one of the spark plug holes to see what you have.

The method is fallible, as if someone's already messed with the engine, all bets are off.

In general, the only way to reliably know the displacement of an engine is to strip off the heads, and measure its bore and stroke.

I don't know why you would need to know the displacement of your Fiero's engine.
fierofool MAR 18, 07:30 PM
Didn't the 3.1's have a crankshaft sensor like the 3.4? If so there will be a sensor or at least a place for one on the trunk side of the block near the middle freeze plug.
xzeion MAR 18, 07:34 PM
I checked the heads and they gave the same surface rust as my logs so they are cast for sure. So it's the 2.8

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87 GT in Blue
~ Drive what you love and love what you drive ~

Notorio MAR 18, 10:40 PM

quote
Originally posted by xzeion:

I checked the heads and they gave the same surface rust as my logs so they are cast for sure. So it's the 2.8




That just tells you the heads are cast iron. They fit on the 2.8, 3.1, and the 3.4. There are posts in the forum that tell you how to ID a 3.4 block from the outside but I don't know about the 3.1 on the outside.
fierosound MAR 19, 07:43 AM
The 3.1 was the 2.8 block (bore is the same) with a longer stroke crank and matching (shorter) pistons.
No way to tell from the outside.

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lou_dias MAR 19, 09:09 AM
dyno it, a 2.8 will only make about 165-170 ft*lbs of torque, a 3.1 about 185 and a 3.4 about 200.
xzeion MAR 19, 09:13 AM

quote
Originally posted by lou_dias:

dyno it, a 2.8 will only make about 165-170 ft*lbs of torque, a 3.1 about 185 and a 3.4 about 200.



Haha I was hoping there would be a stamp on the engine somewhere telling me. I guess I'll have to. My butt Dyno says 2.8 and the 0-60 is not very good. On the other hand it runs side by side with my buddies BMW 328i so it's difficult to say.

I guess I will just have to Dyno it

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87 GT in Blue
~ Drive what you love and love what you drive ~

lou_dias MAR 19, 10:17 AM

quote
Originally posted by xzeion:
Haha I was hoping there would be a stamp on the engine somewhere telling me. I guess I'll have to. My butt Dyno says 2.8 and the 0-60 is not very good. On the other hand it runs side by side with my buddies BMW 328i so it's difficult to say.

I guess I will just have to Dyno it


The 2.8 block could be rebuilt and stroked to 3.1 and then bored to 3.4L (after being stroked) so a dyno really is the only way to know what you are actually running...
Usually when going to 3.4L I rebuild the 3400 block with 3.4 DOHC pistons from something like a 94 Grand Prix with the 24v 3.4L DOHC motor. The higher compression using those pistons should get out over 200 ft*flbs. Some of us have even made as much as 250 ft*lbs but that would also require headers and a free-flowing exhaust.