I am pulling the rod knocking 2.8 out of ANOTHER GMC Jimmy 4x4. This one is a 1987. In its place is a 3.1 from a wrecked 1991 Pontiac Firebird. I am staying with the 1987 GMC TBI heads, already had the machine work done on them. Turns out the 1991 Firebird 3.1 had been bored .040, with new flattop pistons and the crank turned 10/10 before I got it. The cylinder walls and crankshaft were spotless. I did however go with a good set of new bearings and a standard set of new cast iron rings. New gasket set and am bolting it back togather. On top the original 2.8 heads have bigger valves and ports that appear to flow more, plus have the studs in the right spots. I did a little bowl work in the garage and they back cut the valves and did a good 3 angle valve job. The stock GM TBI intake looks more like a tunnel ram than anything else. Each TBI bore feeds the cylinders on its side of the engine, no 180* crossover for this engine. The original EGR passage was so choked with carbon that the ECM gave a constant code 32. I had the intake bored to the same size as a small block TBI and adapted a 4.3 Astro front pull TBI unit with the 2.8 linkages. I am going to run a 14" x 3" open element. I could not find an aftermarket camshaft that I liked for this application and sent the 3.1 cam out for a regrind/rehardening. Looks like a new cam, but has the same specs as the Ford Racing E-303.

Duration@ .006-----282/282
Duration @.050-----220/220
Lift(w/1.5:1 rocker)-.467/.467
LSA-----------------114
ICL -----------------108
Powerband-----------1,800-5,800
Peak TQ---2,500
Peak HP---5,500
Shift @ 6,000

May seem like alot of cam, but these trucks have 3.73 gears, P225/70/R15 tires, and a torque converter that stalls around 2,800-3,200 rpm. It cruises at about 2,400 RPM doing 70 mph. It is destined for alot of travel in west Texas (80 mph speed limits)

This is going to be a fairly wicked 3.1 TBI for what it is.

I am eliminating the Emissions air pump by removing it, its bracketry, and using a shorter belt. The mechanical clutch fan is leaving as well. In its place I am using a Ford Taurus fan wired to run like an OEM fan. That is with a coolant temperature switch in the head and an A/C head pressure switch in the back of the R4 compressor.

The old 2.8 with the inop EGR got 24 MPG highwy and averaged about 19 mpg, I think the moded 3.1 can do better. Keep in mind that the old 2.8 didn't really have enough cam to pull 70-80 mph easily either.

 

nice!!

 

Right on brother.