6bangerbill
Well-known member
You dont see dicussions of two plane manifolds on six cylinder engines.But in my mind they might be a good way to go. My first thought when fabricating a manifold for my ls1 hibred head for street, strip use was a four barrel set up. But after seeing pictures of clifford manifolds it seemed that the tuning dynamics were almost non-existant. The plenum is long, narrow, and the runners are various lengths.Its hard to find where the port stops and the plenum begines. One of the few advantages of a six is that the runner lengths are unlimited relative to the V8( with short ports facing each other).The tuning rpm for all v8's is well above practical street use.Looking at the new generation of v8's we see the ls1 with 17" long runners all scrolled into a large plenum.The late 4.9 six is designed the same way, but with about 24" runners(too long for performance).However, the ls1 and 4.9 are port injected,not carburated. Port injection solves all the fuel distribution problems us carburator guys have.A good idea, but how to get six 17" runners into a plenum that can distribute raw fuel at low rpm.Six banger racers of the 60,s and early 70,s had the right idea with a common plenum and 3 twos or two four barrel carbs. Mine had three twos with 15" runners.After I got an egine analyser, I played arround with my 60's combination substituting two four barrels and hit the two plane manifold key; horspower jumped about 10 hp and climbed even more after increasing the carburator size.Separating the front and rear plenums seemed to increase velocity in the venturies. How about 2 small plenums,about 100 cubic inches each not joined, with two 750 cfm carburators. Each plenum is now handling three evenly timed intake pulses for excellent fuel distribution for street use and the runners are more equal in length for excelent sonic tuning. I think maybe 750 cfm quadrajets with small primaries and stiff air door springs on the secondaries. Why are you laughing? Oh, 1500 cfm why not? A big hole in the hood maybe? This same manifold modeled with 13" runners made big hp at 6500 rpm. The 17" runners tuned at about 4000 rpm.
