Intake Manifolds - My Big Fat Obnoxious Torque Curve

Picking a g-Machine intake manifold for a Mopar big-block

By Andy Finkbeiner

The quest for a big street-friendly torque curve continues. In the March 2004 edition of PHR we were on the chassis dyno tuning the new Edelbrock AVS carb. The results from that test were satisfying with a peak of 452hp and 492 ft.-lbs. of torque, but we really wanted to try out some new parts to see if we could beef up the torque curve even more. Since our Dodge Coronet weighs in at over 3,600 lbs., we want plenty of torque on hand to haul us through the corners and over the hills.

Author David Vizard showed us last month how well the Edelbrock Air Gap Performer RPM worked on a 383 Chevy motor so we decided to try it out on a Mopar big-block. While the standard advice for years has been to use a dual-plane intake on a street motor and a single-plane unit on a race motor, this advice has certainly become more blurred over time. Several of the modern single-plane manifolds make pretty good torque on the bottom end, while the dual planes have continued to improve their top-end power capability. Add in to this mix the availability of good aluminum heads and big stroker cranks and it makes for some interesting bench races! But bench racing doesn't answer very many questions so we gathered up our parts and headed over to the neighborhood chassis dyno at McGee's Auto Service in Lake Oswego, Oregon.

The Manifold Lineup
Based on Vizard's article last month, we knew that the Performer RPM was probably the best pick for our combination since we wanted plenty of torque from 2,000 to 5,000 rpm. But since our 400 engine has been beefed up with a 3.875-inch stroke crankshaft, we thought that possibly a big-single plane would also work well. Lucky for us, Edelbrock recently introduced the Victor 383 manifold for low-deck Mopar big-blocks like ours so we ordered one of them and took it with us to the dyno. We worked with Bob Mazzolini Racing in Riverside, CA to get these manifolds as well as the other necessary items for testing our big-block Mopar. The guys at Mazzolini are Mopar experts and they carry most everything you would need to build up a killer big-block.

Manifold Spacers
The subject of manifold spacers is a tad confusing; at least, it is difficult to find clear-cut answers on the subject. Everyone seems to have a different opinion on how high a spacer to use or whether it should be the open or the four-hole configuration. While the theory may be confusing, the testing is fairly easy to do so we grabbed a selection of different shapes and sizes and threw them into the mix. One spacer that we wanted to test but couldn't find in time was a 2-inch tall tapered design. Fortunately, a buddy who has a CNC milling machine in his garage was willing to whittle one out of a chunk of aluminum for us on short notice.

Carburetors, Air Cleaner and the Stub Stack
We definitely wanted to continue to use the new 800-cfm AVS carb that we tuned up in the last dyno test session, but we also wanted to see how the older 800-cfm AFB Edelbrock carb worked on our motor. In theory, these two carbs should produce almost identical results since they share many internal parts, but sometimes it pays to test your assumptions. While the AFB and AVS carbs are similar, the AVS has the added feature of an adjustable air valve for the secondaries.

The air cleaner set-up on this Mopar big-block is a homemade ram air unit. By taking a large-diameter air cleaner from an Imperial and cutting down the lid, we were able to fashion a unit that draws cold air in from the hood while still looking somewhat stock. We improved the airflow capability a lot by using a 4-inch tall K&N filter element and then we dressed it up a bit with a reproduction pie tin from Mopar Performance. We bought the pie tin that said "383 Super Commando" since we liked the sound of that. No need to tell anyone that while our motor looks like a 383, it is actually a 466!

We recently stumbled across the fact that K&N is making a Stub Stack that fits the Edelbrock carburetors. They have probably been making this product for a while, but it was unknown to us. We had some success running the Stub Stack on a Holley carb years ago so we thought it would be a good addition to our testing. As it turns out, on our particular combination the Stub Stack cost us a significant amount of power. We tried to resolve the issue with jetting changes but couldn't sort it out before the dyno session was over. We called the K&N tech line to see if they knew what might be the cause of this power loss but in the short amount of time we had available, we weren't able to find a good explanation for our situation.

Cylinder Heads, Valve Springs, Rocker Arms and Pushrods
The cylinder heads that we are using are the fantastic aluminum units from Edelbrock with 84cc chambers. These heads work really well right out of the box, making enough power to put a drag race car into the nines if the rest of the combo is up to par. Our heads had suffered a few hours of abuse in previous dyno sessions so we pulled them off and sent them to Porter Racing Heads, a shop well known for their Mopar expertise, for some massaging. PRH did a beautiful job of touching up the valve job, polishing the exhaust ports and smoothing the bowls on the intake side. The flow numbers were increased by about 4 percent over the out-of-box numbers with final numbers being 292/219 cfm at 0.600 lift. These aren't huge numbers for a big-block head, but they'll get the job done in this kind of motor. It is easy to fall into the trap of "bigger is better" but bigger isn't always better when you're building a street motor.

While the heads were at PRH, we had them install a set of the new Comp Cams beehive springs. These beehive spring assemblies are 24 percent lighter than the Edelbrock springs which they replaced. Not only are the beehives lighter, but the conical shape can be very beneficial in reducing valvetrain harmonics. In the last dyno session we had noticed some wiggles in the power curve after 6,000 rpm that could have been due to valvetrain surging. So while we didn't know for sure if we had a harmonic issue, it seemed prudent to swap the springs while the heads were apart.

While the cam we are using is an old school Mopar Performance grind, we stepped up to the plate and bolted on a set of the super nice 1.60 ratio stainless steel rocker arms from Rocker Arm Specialist. While assembling the motor we discovered that our existing pushrods were a little bit bent so we ordered up a new set from the guys at Smith Bros. In keeping with the lightweight valvetrain theme we went with 5/16-inch diameter pushrods with 0.049 walls. In light of the extra stress placed on the valvetrain by multiple dyno pulls we had the pushrods made from heat-treated material.

The Test Mule
Since this is a chassis dyno test rather than an engine dyno, it is relevant that we share a few details on the rest of the drive train. The vehicle in question is our well worn 1965 Dodge Coronet test mule. For this test we filled the crankcase with Mobil 1 5w-30 motor oil and then used Redline MTL in the Doug Nash five-speed transmission. We conducted all tests in fifth gear, which with the Doug Nash box is a direct 1:1 ratio, not an overdrive. We also filled the Dana 60 rear-end with synthetic fluid in a bid to cut down the friction loss. Even after switching to synthetic fluids we noticed that the rear end requires about 25 ft.-lbs. to rotate. We would love to get a chance to tear into this assembly and see if we can figure out how to eliminate some of the power-robbing friction in this assembly.

The Results
The Performer RPM really outshined the Victor on our motor to the tune of 24 ft.-lbs. of torque. The Performer RPM made a best of 505 ft.-lbs. while the Victor could only muster 481 ft.-lbs. The Victor gave up torque and horsepower throughout the entire range and was only starting to catch up to the Performer RPM at 6,000 rpm. Evidently our motor just wasn't flowing enough air to keep the Victor happy. This means that the Victor manifold is aimed for really serious motors since our setup isn't a slouch. If we assume 20 percent for drive train losses, then the 464hp we saw at the rear tire translates into 580hp at the flywheel. So it is going to take a fairly serious motor to use all of the Victor's potential.

Our custom-built 2-inch tall spacer helped both the Victor and the Performer RPM manifold reach their best numbers, but it also was tall enough to cause a bit of a stumble during acceleration. Overall, we think the Performer RPM will work best for us if we use a 1-inch tall four-hole spacer. Having this spacer made from a non-metallic material will also lower the amount of heat transferred to the carburetor, which should help to make even more power.