Small Heads Keep Power Where We Like It

PT.03-572 lb-ft torque and 515 hp raises the bar.

Mike Petralia Jun 7, 2005 0 Comment(s)

We're sticking to a set formula with Major Mouse. And that formula is: Test only one part at a time to see what kind of effect it has on power. This way, we can find out if there's a hidden power producer lying around out there, or if the claims of ultimate power made by some manufacturers are just bunk. And since they're such a hugely controversial product today, we've tested yet another set of aluminum cylinder heads for Major Mouse (MM) Part 03-Test 03.

MAJOR PARTS SWAP
As is our usual way we swapped heads and then bolted MM back onto Speed-O-Motive's DTS dyno to flog the you-know-what out of them. We once again broke into little bro Danger Mouse's treasure chest of booty and stole the old set of Dart Pro 180s we'd tested on DM before. We made no other changes at all and plugged it back into the dyno. After warming up the Major and making sure timing was set the same as last month--32 degrees--we started our first series of dyno pulls.

DYNO POWER
The Major didn't respond all that well to the relatively small Dart heads and we don't exactly know why. When we'd last tested this set of heads ("DM31," SC March '05), it was on a stand-in engine we called "the Imposter" because Danger Mouse was on the disabled list. Although, that engine was built almost identically to DM, so we felt it was a suitable test subject. The Dart Pro 180 heads on "the Imposter" produced some good numbers. Even higher peak and average HP figures than some of the past's best heads, so we thought their performance advantage would cross over to the larger Major Mouse--but, it didn't. When compared to the peak and average numbers of the last two month's tests, we see that the Dart Pro 180's might not be the best choice for this large small-block. That's not to say they're a bad head. To the contrary, we've proven that they're a formidable power producer when used on a smaller displacement engine. And if your mind is already made up that your next set of heads will be from Dart, we're sure that you would not be unsatisfied with the power they're able to produce on any engine. This just further proves our point that making power is all about properly matching components. And that's why we're taking the time and spending the money doing all this stuff. So you will be able to make the best decisions on your next engine.

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Dart's Pro 1 aluminum cylinder head for small-block Chevys was the only test product for this month. The heads feature an 180cc intake port, 74cc exhaust port, 2.02-inch intake, and 1.60-inch exhaust valves, and a good overall average intake to exhaust flow ratio of 72 percent with its highest flow ratio of 85 percent occurring at 0.200-inch valve lift.

The 64cc chambers kept compression equal to the previous month's at 11.0:1. The Dart chambers also feature a very unique shape to enhance overall breathing capabilities of the heads.

Dart's very efficient exhaust port is raised higher than stock, giving lots of extra spark plug clearance. It's flow figures make it perfect for all types of performance usage.

The 180cc intake ports in our test heads did not receive xany extra work and were run as they came out of the box.

The Pertronix Ignition "Second Strike" box has served us well these past months. What it does is refire the spark plug at a selectable point after the initial firing. For example, the first firing is at 32 degrees BTDC and the "second strike" is 2 degrees later at 30 BTDC. That 2 degrees is called the Crank Angle Offset, or C.A.O.

We tried some colder spark plugs this time to see if it might help, but there was no power gained over the Accel plugs (left) we'd run before.

Major Mouse is looking great three months into its testing regime. The pistons are still showing signs of clean burn and swirl.

We've kept rocker arm ratios and performance constant with the same set of Comp Cams 1.6:1 Magnum roller rockers for every test.

We also tried some carb spacers this month, with no positive gains in power.

Finally, to make sure we covered all the bases, we always check, and sometimes recalibrate, the fuel curve of our Demon carb to see if we'd missed something. But, power remained constant using the same jets as last time.

MAJOR MOUSE SPECS FOR PART 03 - TEST 03:
As in our usual format, we left everything else the same and only changed cylinder heads this month. All other items in this test--i.e. timing, jetting, cam installed position, etc.--were duplicated from last month.427 cid, 11.0:1 cr, 4.125-inch bore Mowtown iron four-bolt block, 4.00-inch stroke Lunati crank, 5.850-inch Lunati rods, Lunati pistons (14cc dish) installed @ "0" deck, Lunati Moly rings, Dart Pro 180 heads (64cc chambers, 180cc intake ports, 74cc exhaust ports, 2.02-inch intake valves, 1.60-inch exhaust valves, angle plugs), Edelbrock Performer RPM Air Gap intake manifold, Edelbrock Performer RPM hydraulic roller camshaft installed at 108-degree intake CL (234/238 @ .050, 296/300 adv, .574/.584 lift w/ COMP 1.6:1 rockers, 112 LS), Demon 750 carb, 32 degrees total advance, "0" degree Crank Angle Offset (C.A.O.)*, 91-octane Shell gasoline. Tested on Speed-O-Motive's DTS dyno.*C.A.O. refers to the setting on the Pertronix "Second Strike" ignition box. It re-fires the spark plug after its initial firing, such as 32 degrees BTDC on the first strike, and then maybe once again at 30 degrees BTDC. In this case, as was the same last month, Major Mouse responded best to a C.A.O. of "0" degrees, meaning there was actually no second strike of the spark plug. In the past on some other engines we've found extra power by re-firing the plug around 2-3 degrees later, but not this time.

DART PRO 1 (180cc) FLOW NUMBERS
When shopping for cylinder heads there's lots of choices to consider and not every one of them will work best in every situation. So it's important to collect as much data as possible and then make your decision on what you've learned. When comparing cylinder heads, it's important to look at other figures besides just airflow. Port volumes, chamber shape and size, valve sizes, angle and locations, how much spring/lift they can take, and will it match your camshaft? These are just a few of the items that you should base your decision on, not just the power they produced in some magazine story. I also personally feel that a good indicator of how well a cylinder head will work is it's ratio of intake to exhaust flow, listed as "I/E flow" in this chart. Although this is not the "Holy Grail" of horsepower, it does indicate a head's potential to move air into and out of the cylinders effectively. A head with a ratio of less than 60 percent is not a very effective air mover, particularly since it indicates that it can move air in, but not out of the cylinders. A head with a ratio of over 90 percent, although rare, is capable of extremely good power numbers so keep an eye out for it. Check out the very high 85-percent I/E flow at .200-lift of the Dart Pro 180. This is an area of critical performance for any head, but particularly a street head that won't be run with much lift. Since the valve will see this area twice during every cycle, once on the way up and then again on the way down--as opposed to seeing max lift only once in every cycle--low-lift flow figures are often overlooked, but very critical to making the best power.

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