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My buddy has a 1998 Chevy Silverado 1500 with the 5.7 L Vortec motor. Cylinder 8 is getting weak and has low compression so I'm wondering if I can put a 1994 Chevy Silverado 1500 5.7 L TBI motor into the 98 frame? To put the 94 5.7 L motor in 98 what all do I need to change. And are the Transmissions different from 94 - 98
 

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This is across the great divide of OBD-I to OBD-II which is newer and a lot more complex.

The 94 is an LO5 that uses simple Throttle Body Injection and Swirl Port heads, the 98 is an L31 that uses complicated Central Port Injection and Vortec heads. The intake does not interchange between these head types.

The 98 uses high pressure fuel injection the 94 very low pressure, the pumps, plumbing, and pressure control systems are way different and not interchangeable in function.

The transmissions will bolt across those years the bell housing size and pattern at the block and the crankshaft flange is the same.

You can make this transfer but it is sticky and involved, basically the only thing that comes across is the short block but even here the cam’s are a lot different the 94, LO5 is very mild flat tapped, the mildest cam Chevy ever put in an SBC, it is used from 87 through 95, the 98 uses what essentially times as a roller version of the old 300 horse 327/350 cam. The computer might puke at this but having swapped the roller into TBI engines is successful so this may not be a big deal the other way.

The heads are different and the 98’s will have to bolt on the 94 block to accept the 98’s way different fuel injection, which is part of a highly integrated fuel and electrical system in the 98 so it is impractical to impossible to consider not changing the top end.

The distributors are different, the 98’s will have to be used and will fit to the 94 block.

Inside the head’s the rocker system is the same self guiding and the valve covers are the same. The 94 with the flat tapped cam uses the old long 7.8 inch pushrod, the 98 uses the shorter 7.2 inch roller lifter push rod. So with the 98 heads on the 94 block the longer flat tapped push rods must be used.

Up front the plastic timing cover will need to be used because it houses the crankshaft position sensor that the computer must have. This might be a fitment problem to clear the cam drive, it usually is worked with added timing case gaskets. You will find that the 98 has a little different fastening method to the block than the 94, you can substitute the 94’s bolts for the 98’s pins.

The crank position device and the damper from the 98 will have to come to the 94’s crankshaft. This should just trade.

Cooling is a bit different, the 98 pump will need to come to the 94 block. This as far as fasteners will interchange. What you’ll have to check for is the small bypass hole on the right side (as seen left to right from sitting behind the steering wheel). The 98 does not use this where the 94 probably does. If the passage is drilled on the 94 block it will need to be plugged to insure no leakage, this should be a tapped thread screw plug that must be at least flush with the machined surface or a bit deeper so that it doesn’t upset the pump mounting.

The 98’s exhaust manifolds must be used to get the EGR system functional. The 94 feeds off the right head into the EGR valve, the 98 feeds from a tube that connects the left side exhaust manifold to its EGR valve.

So yes the basic short block can be used but it must be completed to a long block in the L31 configuration and dressed with injection and ignition plus all sensors as the 98 is done.

Certainly if the loss of compression on the 98 is a valve this isn’t a practical solution but if it’s rings or piston it could be. The first thing is pull the head’s off the 98 to see what the problem is.

Bogie
 

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You can do it. It won't be smog legal (if that's an issue) and you'll be doing a buttload of work for a massive step backwards... literally about half the power, less torque, fewer MPGs, flat tappet cam. Not sure why you would do that when running Vortec longblocks are $250-500 in any junkyard.
 
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