Ever wonder why the LT5 power curve is shaped like it is? The factory manifold length from plenum runner entry to valve seat is approximately 380mm (15") long. It's a rather tortuous path, with some awkward angles along the way to fit under the C4 hoodline. The intake length was designed to provide strong wave tuning at the factory torque peak (4800 rpm), and also the intended peak power speed (6000 rpm). There are actually several tuning peaks, which can be seen in this graph. The actual power and torque values shown are irrelevant as the simulation was done with 4" stub exhaust pipes to isolate the intake action only.
The relative strength of each peak is affected by the valve time-area--larger cams and valves optimize the later tuning peaks. This model matches results seen on an engine dyno with my initial 427 buildup. Due to the long duration camshafts, my engine actually had peak torque at around 6000 rpm. At that speed, the pressure in the intake port exhibited four distinct peaks. One of these four peaks arrived at the back of the intake valves just as they were opening, while another peak arrived just before the valves closed. The second peak provides good charge "ramming" to boost volumetric efficiency at 6000 rpm.
There is potentially another, higher, tuning speed around 7500 rpm, which would feature three pressure peaks during the cycle. In theory, that tuning speed should offer the strongest ramming effect. Most highly developed four-valve racing engines use the three-peak intake length. So, why didn't my 427 show a VE boost up there? The answer is that there was not enough intake time-area to take advantage of that higher speed. The engine was choked by valve area and lift before reaching the next tuning speed. That was with .500" lift / 255 degree intake cams!
So my conclusion is this... the factory LT5 intake length is really well optimized for most applications. It provides a nice torque boost at 4800 and 6000 rpm, and for all-out race builds with larger cams, should allow for a peak power speed of 7500 rpm. I believe the factory length will outperform a short-runner sheetmetal manifold at any engine speed under 8000 rpm. The only drawback is that the factory manifold is contorted to fit under the C4 hood, which does cause some pressure loss compared to a straight tract.
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