Swapping a 255 HP ( net ) 5.7 L Vortec for a low compression motor that began life with ( net ) 150-180 hp. Sound great?
Even in top shape with great ring seal, the 350 heads will make about 50 hp more? That's IF you don't have issues with the compression increase, and detonation.
That's equivalent to running a 305 instead of a 350.
Shorter 5.565 connecting rods ( 400 ) 350 Vortec rods are 5.7 inches
Stock 400 - lame hydraulic flat tappet, slow opening / closing ramps. Stock L31 350, lame hydraulic roller, much quicker opening ramps. Regarding area under the curve, the roller cam reaches full lift earlier, holds the valve fully open longer, then snaps the valve shut faster.
64 cc heads on 400 = higher compression ratio, about 1.72 points more if you have the stock bore size. More if the bore is increased. More if thinner head gaskets are used than OE equivalent. More compression yet if the pistons are aftermarket and have less valve reliefs and no chamfer.
1970 400 had a stock CR of 9-1 from this source:
http://www.nastyz28.com/sbchevy/sbsum.php
Other years had 8.5-1, so dropping chamber size from 76 cc gets you to either 10.22 or 10.72 with the OEM compressed volume gaskets and the newer 64 cc heads, keeping the bore size 4.125"
If the pistons are OEM factory pistons in +0.030", advertised 8.5 to 1, you can take that 10.22 and raise it to 10.36 to 1 CR. If you had an original 400 with 9-1, and is + 0.030", you will have 10.86-1. Like what would have been GM service replacements from the stealer. Race fuel anyone?
If the engine sat for any length of time with old oil in it, the rings will be stuck, and the bearings will be pitted from the acids in the crankcase. You aren't going to take a stock engine that has sat for many years and expect to put it back to work without doing some work. It will need to come apart, unless you want to play hot potato with a grenade.
KB9907HC piston ( stock bore size, no oversize ), flat top with 2V relief , Fel-Pro 1045 gaskets, stock stroke, 0.010" machined off the block ( deck height ) and 64 cc chambers, I get 12.74 to 1 compression. Anyone else care to check the numbers?
With large swept volume, and small-ish combustion chambers, iron heads, you will need deep
dished pistons, likely custom to get the CR down to a streetable level... like 9.5 to 1.