Not having the flap valve will limit the effectiveness of manifold heat during warmup, but any heat is better than none. There shouldn't be a problem with overheating the intake because there will be a non-restricted flow down the rear pipe even when the engine is at normal running temp. A stock manifold with a second outlet added and the original heat riser flap is still the best choice, as long as a baffle is welded into the manifold between the 2nd. and 3rd. exhaust ports. The baffle doesn't even have to be a total seal- - - -it just needs to block most of the flow between front and rear. That's the way the 6 cylinder Corvettes did it, and those manifolds are scarce and expensive now. Far more of them were sold over the counter at Chevy parts departments than were ever installed on Corvettes.
Jerry
