GM Iron 290 Oval Head

This is a representative closed-chamber oval-port head. Even with its 50cc larger port volume, the small valves hurt airflow. This head continues to flow well beyond 0.700-inches of lift, which leads us to believe that larger 2.19/1.88-inch valves would help especially on the bottom end and across the rest of the board.

GM 049 Iron Oval Port

This Open chamber oval-port head saw use in a number of 402ci to 454ci big blocks from ’73-’84. Of the three stock iron oval-port heads we tested this one was the strongest. Again, like other production heads, if horsepower is your goal, larger valves are the way with these heads.

GM 702 Iron Oval Port

This is an example of an early 396ci and 427ci closed-chamber head used only in ’65 and ’66. This head virtually clones the GM 290’s flow numbers. While the 049 head flows much better it would drop compression two full points. The best set up here is to either give up some power and use the GM 702 head as a matching part number restoration piece or go with the better GM 049 head and Open chamber pistons to match.

Brodix OEFI Aluminum Oval Port

With a maximum flow of over 270cfm right out of the box and a "small" 255cc port volume, this Open chamber head is an excellent choice for making outstanding torque and reasonable horsepower. We’ve seen these heads ported and on a Lingenfelter dyno make as much as 650 hp on a 502ci big block. This head can be a serious street contender.

World Products Merlin Iron

This head is just what GM should have built for its Chevy enthusiast. Its design is very similar to the production GM heads but has been outfitted with larger valves. The small port volume makes for great streetable torque while larger valves accommodate all the extra flow these ports can handle. These heads do not come with any handwork to the bowls, which would help tremendously.

Dart Oval Aluminum

This head flows much like a larger rectangular port head up to the 0.500-inch lift mark. From there the rectangular port pulls away only because of its larger design. The E/I ratio of 75% was the best in this class, which tells us that this head would be ideal for an automatic transmission equipped street terror.

Edelbrock Performer RPM Aluminum

The RPM head features a large-volume oval port with a unique rectangular oval-port entry, or "roval" for short. This head sacrifices low lift efficiency just a little for its high lift flow numbers up around 0.800-inches of lift. This head would best work on pump gas 396ci to 502ci motors.

GM Performance Parts Aluminum

This head offers the highest average intake port flow and the highest peak flow numbers. The exhaust was very impressive creating an E/I ratio of 72%. This head would be best fitted on a big-inch street big block using a large cam that could take advantage of this head’s high lift flow potential.
 AFR 305 Heads



Canfield Aluminum

This is Canfield's first foray into the Rat motor market and the company has aimed this head directly at the street market with the smallest port volume of any rectangle port head. This head would be best applied on 454ci to 502ci street engines. Only one other rectangle port head outflows the Canfield at 0.300 mid-lift numbers, which makes this an excellent choice. The Canfield head comes with three chamber options (110cc, 113cc, and 119cc) and can be special-ordered with CNC porting.

World Products Merlin VR Iron

The Merlin 310VR uses a 0.130-inch narrower rectangular-port opening, and the smaller intake port volume contributes to greater mixture velocity compared to the larger 345cc intake ports. This head offers great flow potential below 0.600-inches of lift. Remember that none of the World Products iron heads come with any short-side radius work as is common with most aftermarket aluminum heads. This head is a great choice for those desiring more flow and greater torque. The Merlin and the Merlin VR heads are also available in aluminum as well.

World Products Jenkins Aluminum Merlin

This is a duplicate head to the original World Products Jenkins VR head except in aluminum featuring excellent low- and mid-lift numbers compared to some of the larger heads. These numbers indicate this head would work best on smaller displacement street Rats in the 427ci to 502ci that would add massive torque. The Jenkins head produces the best 0.300- and 0.400-inch lift flow numbers of any big-block head we've tested.

Holley Avenger Aluminum

This is the first of Holley's new Avenger series for the big-block Chevy and proved to flow very well. The relatively small 314cc intake port will add flow velocity to the inlet tract while the exhaust offered an impressive 82 percent intake to exhaust flow relationship at 0.400-inch valve lift. This is another head that offers excellent torque potential combined with a large intake port that will make some ponies.
AFR 315 Heads


GM 188 Iron

This is the current GM replacement rectangular-port iron head that was produced between '68 and '71. It offers good flow for its size, especially without hand blending under the valve seats. The exhaust port offers decent flow compared to the intake with a 66% E/I relationship at 0.400-inch lift.

GM 990 Iron

The 990 head is listed as Chevy's stock replacement iron rectangular-port head in the GM Performance Parts catalog but it is no longer available. There are still plenty of these heads out there in recycling yards and from after market manufactures.

GM Performance Parts 401

This is the smallest rectangular-port GM Performance head, yet it flows extremely well for its size. Having a very similar port volume to the 990 head the 401 head flows dramatically better, especially at higher lifts. With the combination of great flow and being lightweight, this head would make a great mid-priced aluminum head for a hot street rat motor.

Dart Pro 1 Aluminum

This head has the best E/I ratio of all the rectangular-port heads. Combine this head with a 500-plus cubic inch engine and a loose converter with some gear and you'll have a great combination. Dart also offers CNC porting options for this head as well if you're willing to shell out a few more bucks.


Edelbrock Performer RPM 454-R

The Edelbrock RPM head is based off of an iron rectangular port yet flows much better than a stock casting. With an exhaust port generating a 67% E/I ratio at 0.400-inch valve lift, this head would be a good choice for a mid-priced big block of 468ci or more. The PRM head has been designed to work on all three Mark IV, GEN V, and GEN VI blocks.

AFR 325 Heads


AFR 335 Heads


World Products Merlin Jenkins 345 Aluminum

This is the fourth 345cc rectangle port Rat head we've tested and three of the four heads are almost identical in flow. The Jenkins head offers excellent flow throughout the entire lift curve all the way from 0.200- to 0.600-inch lift. Plus, the World Jenkins head delivers the best 0.400-inch exhaust port flow of any of the other three heads. Plug in a single pattern cam with this head, and you'll make serious power.

Brodix -2 Plus Aluminum

This head tested very well in the mid-lift flow areas between 0.200 and 0.600-inches of valve lift. Brodix recommends using this head with more compression. It will accept all standard valve train parts but requires the use of longer head bolts. In addition, the exhaust ports have been raised 0.600-inch higher than stock, which perhaps will require custom headers or at least significant modifications to existing production based chassis headers.

Brodix -2X

Brodix recommends this head for lighter cars or large engines of 480ci to 572ci. A 0.250-inch longer intake valve is required, along with longer head bolts or studs. The exhaust port has been raised 0.600-inch which may require the use of custom headers. Brodix also offers different levels of porting for your choice of big block Brodix heads.

Brodix -4

The Brodix -4 is a competition-oriented cylinder head with raised intake and exhaust ports, requiring much longer valves and custom pistons machined to fit the 4-degree canted valves. Using a standard deck block, the heads require spacers or a tall-deck intake manifold. This head is a race-only piece recommended for 468ci to 572ci engines spinning 7,000 rpm or more.

Dart Pro 1 Aluminum

This head is very competitive with the larger 370cc port heads all the way up to the 0.600-inch lift and the E/I ratio is right in the ballpark at 68%. These heads may not appear as strong compared to numbers generated by the manufacturer's, but this is because of the smaller bore fixture we used for our testing.
AFR 345 Heads

AFR 357 Heads


Brodix -3

This head has the largest port volume of all the Brodix line and is recommended for huge big blocks in the 540ci to 605ci range. This is a race head that will accept standard intake manifold and headers, but does require a 0.250-inch longer intake valve and custom guideplates. The exhaust valve has also been moved 0.030-inch away from the intake, requiring different valve relief pockets be machined into the pistons. The flow numbers are great but this head suffers some on the exhaust side with a rather low 59% E/I ratio at 0.400-inch valve lift.

Edelbrock Victor 7760

This head is sold only as a bare casting with seats and guides and no machine work. The heads are competition-oriented with raised intake and exhaust ports and can accommodate intake valves as much as 0.400-inch longer than stock. We were only able to test up to 0.700-inch due to the short runners where the head was still well below its potential. The Victor is designed to work with all three versions of the big block.

Edelbrock CNC Victor

Edelbrock's CNC-ported Victor head offers the largest port volume of all the heads we've tested and also offers the highest flow numbers at 0.600-inch valve lift with great mid-lift flow numbers. This heads would be especially good on a larger displacement big-block in the 502ci and larger range with a big roller cam. The outstanding exhaust-to-intake relationship is also strong enough to warrant a single-pattern cam.