by David Grasso

(Taken from  High-Performance GM LS-Series Cylinder Head Guide by David Grasso)

The intake manifold is the most important element in swapping from a cathedral to a rectangular port head. To match the runners, this factory LS3 (PN 12610434) or an L76 manifold are the most popular. It is important to note that there are some minor differences between the L76 and LS3 intakes, such as a screw-in MAP sensor instead of the Gen III–style clip-in; though none are of any real consequence to swapping over. These factory manifolds come as shown here with a 90-mm electronic throttle body, injectors, fuel rails, and gaskets. (Photo Courtesy General Motors)

Since the factory L92 cylinder heads were released, one of the hottest topics has been how to convert older (cathedral port) LS-series combinations. Thankfully it is a pretty simple head and intake swap, which lends itself well to fairly high horsepower combinations on a modest budget. And since all LS blocks have the identical (four-per-cylinder) head-bolt pattern, all LS or LSX heads are compatible. (Some aftermarket blocks have two additional head bolts on the outside and inside of the standard ones). The only exceptions are when certain heads have valves too large to fit in a particular bore. The L92/LS3 heads, for example, must be used on a 4.00-inch-or-larger bore, and the LS7 requires at least a 4.125-inch bore.

In order to utilize such mammoth intake valves in such scarce real estate, General Motors went with an offset intake rocker on the L92 head, which means converting it also requires sourcing at least a set of 8 intake rockers (if not a full set of 16). The LS7 uses a unique rocker system due to its offset intake valve and 12-degree valve angle, which is not compatible with either the LS1/LS2 or the L92/LS3. Beyond this point, the rest of the conversion largely depends on the previous chosen components and the application.

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All rectangular port heads use offset intake rockers to accommodate the massive valves. With factory rocker setups you can get away with just changing the intake rockers; however, changing the whole set or going with an aftermarket variation are also options. In either case, extra clearance may be needed on the valve covers.

With factory valve covers, some grinding may be required to clear the offset intake rocker. This is particularly true on older LS1 designs (on the passenger side mainly), though LS2 and LS6 or later-style valve covers should have no issues. Aftermarket valve covers are usually made to clear bulky aftermarket rockers; if they don’t clear, a simple set of spacers from UMI can be used to rectify this problem.

The much different intake runner shape also necessitates a different intake manifold. It is important to note that the factory intakes come complete with injectors, fuel rails, gaskets, and a 90-mm electronic throttle body. However, neither the injectors nor throttle body is compatible with a factory LS1 wiring harness and computer. This is most commonly rectified by purchasing injector plug adapters (such as those from FAST or Caspers Electronics), a 90-mm cable throttle body, and a cable bracket.

Because the LS3/L76 now places the MAP sensor at the front, just behind the throttle body, a MAP extension such as this one from Caspers is also needed. You may choose to extend the wires themselves, but for $34 you can save yourself the hassle.

Experienced builders, particularly those who are accustomed to building high-compression engines with larger camshafts using tight clearances, may be wondering one or both of two things: how are these heads going to match the valve reliefs on the pistons and how well will the old cam work with the new heads?

If high-compression pistons with LS1-style valve reliefs are in the engine, there is a slight misalignment due to the offset and much larger intake valves with the new heads. In a more street-friendly engine with plenty of piston-to-valve clearance this doesn’t factor in, but those running lots of duration and lift with a smaller chamber may need to swap out the pistons.

Because the valve sizes and combustion chambers are different, it may be necessary to check piston-to-valve clearance in applications with larger camshafts. Without changing pistons there is a slight mismatch in the pistons’ valve reliefs, which can affect clearance.

Generally speaking, cathedral port heads have much better exhaust flow than rectangular ports, which is why cam grinds tend to have more exhaust duration with a rectangular port. For optimum performance, changing cams is recommended, though not necessarily a must. (Photo Courtesy Comp Cams)