2.8 3.1 engine question

[steelbender]

ok just want to clarify something here. could I use the crank and rods from a
91 2.8 fwd to make a 3.1 out of an 86 2.8 rwd? And can I use the 2.8 pistons?
I have a fwd 3.1 mpfi engine I can yank parts off of to put in my 86 2.8 rwd tbi block if they will fit.



Steelbender

[Superdave]

the rods are all the same, it's the crank and pistons that are different between the 2.8 and 3.1

so yes, if you swapped in the 3.1 crank, pistons and rods into the 2.8 it would now be a 3.1L.. if the 3.1 is a FWD aluminium head engine you will lose quite a bit of compression using those pistons with your iron heads. The CR calculator says 7.18:1..

your best bet is to use RWD pistons...

[steelbender]

Thanks Dave. We shall see how this turns out.

Steelbender

[powerdoctor]

That small comp ratio sucks .... you will go no where.

I did that along time ago to hurry up and get a car running, and I would not recomend it... unless you got boost???

[steelbender]

After some research I think I will be better off sticking with my 2.8
the iron head 3.1 tbi rates at 120 hp the 2.8 is 125

Diffrence in comp ratio maybe???

Steelbender

[Superdave]

where did you read that?

[powerdoctor]

I'm confused as well......

[geoffinbc]

After some research I think I will be better off sticking with my 2.8
the iron head 3.1 tbi rates at 120 hp the 2.8 is 125

Diffrence in comp ratio maybe???

Steelbender

No there is no way that is true. If anything the 3.1L has a SLIGHTLY higher compression ratio. Maybe a tenth or two tenths higher. But even if they were equal the 3.1L would make more power any day of the week.

[powerdoctor]

Due to stroke alone......

I totaly agree.

[WrathOfSocrus]

Not all 3.1's are equal. If you look at this link here :

http://en.wikipedia.org/wiki/GM_60-Degree_V6_engine

It shows the iron head 3.1 used in the Lumina APV's and such the LG6 at only 120 HP

Now that is a 3.1 with iron heads, TBI, and the very mild stock cam. If you look at some of the other comparable 2.8's such as the high output LH7 or L44 used a bigger cam (and apparently valves and fuel injection in the case of the L44) to produce 15 and 20 HP MORE than a 3.1

Generally speaking a 2.8 with the same heads similar rotating assembly will put out less power than a comparable 3.1. Using different parts can make a 2.8 (slightly) more powerful than a 3.1.

I'm sure if you compare the fuel economy of 2 comparable engines such as the LE2 and LH7 that the high output version would be less fuel efficient. Thats where most of the extra power comes from. Stuffing a little more fuel and air in the same spot.

[powerdoctor]

Exaclty ....

[steelbender]

yep that was the engine I was referring to. iron head stock 3.1 rated at 120 hp
compared to the iron head stock 2.8 tbi in my truck rated at 125 hp.
not comparing to aluminum head engines I am making an educated guess that the 3.1 pistons will yield a 8.5 to 1 compression ratio when used with the 2.8
iron heads Correct? however if I have my facts straight the tbi 2.8 in my truck with stock pistons is 8.9 to 1 Comp ratio.

This was my train of thought. Correct me if I am wrong.

[geoffinbc]

Well the van may have had a shitty exhaust or something. They only other 3.1L TBI engine was in the Isuzu Rodeo and it was rated higher than the 2.8L.

Remember also the 3.1L TBI still used a 2.8L throttle body and injectors. So you can un cork it further with a 4.3L Throttle Body on a bored intake manifold. In the real world a 3.1L will make more power, maybe GM rated the van low or maybe it really was low on power who knows.