Dyno Torquing Hemi Intakes

We had our Hemi test engine firmly affixed to the Westech dyno, and we figured what better opportunity to wring out some of the various induction-system configurations available for the Hemi. We had on hand a couple of Chrysler's latest intakes, both modern plenum-ram manifolds with forward-mounted throttle bodies, not unlike the type used on General Motors' LS-series of engines. Specifically, we had a late '05 passenger car composite intake and the latest SRT-8 aluminum plenum ram designed for the hot 6.1 production engine.

Both of these intakes are designed to take a large 85mm single-butterfly throttle body. Complicating these induction packages is the throttle-by-wire electronic-throttle-control system, in which the throttle opening is determined by the logic of the PCM, rather than your right foot. Actuation is via an electric motor, geared to the throttle shaft. Undaunted, we popped the cover off the motion-control system, filleted the intermediate gear between the control motor and throttle shaft, and deftly MIG'd a conventional throttle lever to the mechanism. Barbaric? Yes. Crude? You've got it, but it worked. Neither the adapted aftermarket engine-management system we were using, nor the Mopar Performance ECU has the capabilities of operating the electronic throttle. While this was a more than adequate approach for our dyno flogging, a more civilized solution would be to have a simple adapter plate milled to mount an aftermarket throttle body. A F.A.S.T. 90mm unit would be right at home here.

Since it ferries no coolant and has no archaic protuberances, such as a distributor stabbed through it, the Hemi intake is very easy to swap. In our case, it required just unhooking the EFI wiring, disconnecting the fuel line to the rails, and backing out the ten attaching bolts. We were eager to try the sexy SRT-8 manifold, but a fitment issue fouled our plan. The front cover assembly of the crate engine is based on production-truck componentry, and the layout is wholly incompatible with the front-mounted throttle body of the plenum-ram manifolds. In succinct terms, they hit. We made a substitution to the front cover assembly from a doner passenger car engine, and, fortunately, it bolted on with no undue complications to our accessory-free test engine. Be advised, however, that the accessory mounting, water pump, and drive system are entirely different; the water pumps will not interchange, and the drive offset of the crank damper pulley is different. In other words, the whole front cover/drive/waterpump/accessory system can only be swapped as a complete assembly.

With that hurdle cleared, we bolted on the SRT-8 manifold. While this intake will bolt on to the 5.7, it is designed for the much larger ports of the 6.1 Hemi. We knew that the stepped mismatch in port size represents the worst form of flow-path disturbance, with the intake's larger ports butting against a bottleneck at the smaller 5.7's heads. However, it was easy to bolt-on, so we figured we'd try it anyway. Naturally, the change in induction meant an entirely different tuning map was required. Since our test engine was being run with a fully programmable and tunable F.A.S.T. ECU and components adapted from a Ford 4.6 for testing expedience, the tuning task was handled by Westech's ace injection man, Tom Harbrzyk, with ease. Look for a real F.A.S.T. programmable engine-management system for the Hemi, which should be available by the time this reaches print.

Despite the port mismatch, what we found was torque in abundance, with gains showing up at 3,000 rpm and hanging in to 6,000 rpm. The midrange bulged to the tune of about 30 lb-ft, with a peak-to-peak gain of 27 lb-ft. Harbrzyk acknowledged the change by knowingly uttering, "runner length." Peak power, however, fell short of the 481hp number turned in by a previously tested four-barrel intake, with peak output now posting 473 hp at 6,400 rpm. The tally could only improve with congruity of runner size, so we plan on revisiting this intake after some reworking with a few sticks of epoxy and a session with a grinder.

Last on deck was the composite late-production 5.7 piece. It's as lightweight as it is homely in its natural semi-flat black patina. Similar in architecture to the SRT-8 plenum ram, but molded with longer and smaller runners to mate with the 5.7 Hemi's port dimensions, we expected a weak show. Far from weak, this intake proved to be a monster, tapping a mother lode of fat torque that was stunning. Torque was up compared to the other manifolds tested from the bottom of our pull at 2,500, and an advantage was carried to 6,400 rpm. Through the middle, torque swelled, showing a peak of 456 lb-ft at 4,600 rpm. Peak power checked in at 476 hp at 6,300 rpm--nice work, Chrysler.

What conclusions can we draw from this little junket? It seems as though the plenum-ram manifolds, with their equal length and long runners, are nearly ideal for massive torque production. We can't say that we gave the SRT-8 intake a truly fair shake, due to the port mismatch, but the potential showed through. The production plastic ram manifold is clearly a highly developed piece, making tremendous midrange torque, but clearly beginning to fade towards the top. The short runners of the MP single-plane aluminum intake were no match for the plenum rams in torque, however, up top, this design still held on to the edge for peak-number bragging rights. What's clear is that these MPI engines are sensitive to induction layout, with dramatically different power curves developing with changes to induction-tuned lengths and design. We hear that MP is currently working on even more radical intake designs, made without compromise with an eye towards maximum performance. Bolting on an intake with the new Hemi is better than we ever remembered back in the good old days.