Having tested endless aftermarket and ported stock LS cylinder heads for our sister title Hot Rod, it seemed odd that the stock stuff was never included. A test of the popular factory cylinder heads has been a long time coming, and honestly, we are not sure why it hasn’t been done before. In the past, others have played with flow numbers and even measured port volumes, but we don’t race flow numbers or volumes, now do we? In fact, this test will illustrate exactly why you can’t translate flow figures into power production. At best, flow numbers indicate potential power, but we are getting ahead of ourselves.
The first order of business was to identify and round up a selection of popular LS factory heads. Second was to procure a test mule. The test engine was an easy choice, as the 5.3L was the go-to engine, especially for the power-adder crowd. In terms of cylinder heads, we included the 706 small-valve 5.3L heads, the 241 (LS1), 317 (6.0L truck), and 799 (as a stand-in for the hallowed 243) heads. The idea was to flow and measure each head, then follow up with the all-important dyno testing. We also took the liberty of measuring the combustion chamber diameter (distance across) to illustrate how the factory sized them for their particular target bore size. This might be another good indicator on how well they work on the small-bore 5.3L.
|706 5.3L Truck Castings|
|Valve Size: Intake - 1.89, Exhaust - 1.55|
|Port Volume: Intake - 197 cc, Exhaust - 69 cc|
|Combustion Chamber: 61.5 cc|
|Target Bore Size: 3.780|
|Chamber Diameter (distance across): 3.756|
Not surprisingly, the 706 truck heads offered both the smallest port and chamber volumes. The combustion chambers on the 706 heads measured 61.5 cc, while the intake and exhaust port volumes checked in at 197 cc and 69 cc, respectively. The port volumes were within a couple of cc’s of the 241 LS1 heads, and the peak flow differed by around 5 cfm despite the 706 heads having a smaller intake valve size. The smaller intake valve is the reason many LS owners dismiss the 706 heads, but enthusiasts should really look at something called the Coefficient of Discharge. The Coefficient of Discharge is a calculation of the airflow versus valve sizing, and gives a much more accurate picture of the power potential of these heads. Maybe at elevated power outputs and/or boost levels where maximum flow is needed the other heads might begin to shine, but just look at the dyno results to see how well GM designed the 706 heads to work on a 5.3L (even a modified one).
|241 5.7L LS1 Castings|
|Valve Size: Intake - 2.00, Exhaust - 1.55|
|Port Volume: Intake - 199 cc, Exhaust - 71 cc|
|Combustion Chamber: 68.0 cc|
|Target Bore Size: 3.898|
|Chamber Diameter (distance across): 3.821|
The 241 heads were used on 5.7L LS1 applications and are considered by many to be just a minor step up in performance from the 706. In terms of peak flow, the 241 heads offered 242 cfm at 0.700-inch lift from the 199cc intake ports, and 204 cfm from the 71cc exhaust ports. The exhaust flow offered by the 241 heads topped the other three heads tested here, but we suspect there would be variations among the different castings with different valve jobs and mileage. The 241 heads, compared to the 706 heads, offered larger 68cc combustion chambers, which meant a drop in compression of roughly 0.70 points.
|317 6.0L Truck Castings|
|Valve Size: Intake - 2.00, Exhaust - 1.55|
|Port Volume: Intake - 212 cc, Exhaust - 76 cc|
|Combustion Chamber: 71.5 cc|
|Target Bore Size: 4.000|
|Chamber Diameter (distance across): 3.923|
According to Internet folklore, the 317 heads used on the 6.0L truck engines share the intake port design with the 799 (243) heads. Our testing showed this to be at least partially plausible as the flow numbers were similar, though not identical. The 317 heads flowed more than the 706 heads by as much as 20 cfm at some lift points. The peak flow numbers differed by just 7 cfm at 0.700 lift. What made the 317 heads both kissed and cursed was the combustion chamber size. The massive chamber volume of 71.5 cc’s put them a full 10 cc’s over the 706 heads. This translated into a drop in static compression of 1.2 points (from 9.5:1 to 8.3:1). The 317 heads are often used on turbo small-block 5.3L builds, but the lack of compression really takes its toll on power. Check out the dyno graphs to see how much the 317s lost on this 5.3L test and you might think twice about what you choose for your next turbo LS. Designed specifically for a 6.0L application, the 317 heads also had the largest chamber diameter of 3.923 inches.
|799 6.0L (and 5.3L) Castings|
|Valve Size: Intake - 2.00, Exhaust - 1.55|
|Port Volume: Intake - 209 cc, Exhaust - 74 cc|
|Combustion Chamber: 64.0 cc|
|Target Bore Size: 4.000 (and 3.780)|
|Chamber Diameter (distance across): 3.876|
As expected, the 799 heads offered the highest peak flow, with 250 cfm coming at 0.700-inch lift—though a value not generally relevant to most LS enthusiasts. The 0.600-inch lift, or possibly the 0.650, numbers were more relevant, but the 799 heads still shined here with flow readings of 248 cfm and 249 cfm, respectively. Despite larger exhaust port volumes than either the 706 or 241, the 799 heads offered the least exhaust flow of the bunch, with a peak of just 191 cfm. The 209cc intake ports flowed as expected, but the 74cc exhaust ports were not quite up to snuff. Next to the 706 heads, the 799s featured the smallest combustion chamber at 64 cc. This put it within 2.5 cc’s of the 706, meaning there was only a minor change in static compression between the two. This was coupled with a 3.876-inch chamber diameter.
Check out the graphs for a full rundown, but the big surprise was that the amazing power of the legendary 799 (243) heads never materialized. Now, combine that with the fact that the 706 heads—the same set everyone throws away—made the most power, and we might have just turned the LS world upside down. It is no surprise that the 317 heads made the least power. Though they offer plenty of flow, they also started with a huge drop in compression (from 9.5:1 to 8.3:1), and therefore power. Though the lower compression might make them attractive for the turbo (or blower) crowd, there is a big gap in power that must be overcome before the 317s can start making more power than the standard 706s. At the very least, the 317s would need more boost to reach any given power level than the 706 heads, and besides, a compression ratio of 9.5:1 works well on a turbo application. The 241 heads performed slightly better than the 317s, and the 799s were better still. Of course, the question now is how would the four heads act under boost where the additional flow required might tax the maximum flow potential of the small-valve 5.3L heads? That, my friends, is another test for another day! CHP
Modified 5.3L Head Test: 706 vs. 317
We had high hopes for the 317 heads, but they were definitely the low man on the performance totem pole. The large combustion chambers were likely to blame, as the 10cc increase in chamber volume resulted in a drop of the static compression ratio by 1.2 points (from 9.5:1 to 8.3:1). The larger chambers make the 317s popular for boosted applications, but know you are starting with a significant deficit by replacing the 706 heads with the 317s. We have been told by a reputable source that the chamber design of the 317 heads is less than optimal as well, irrespective of the size, meaning if you milled it, it still would not produce the power you could get from the 799s, let alone the 706s.
Modified 5.3L Head Test: 706 vs. 241
Like the 317s, the 241 heads out-flowed the 706 heads, but once again, that airflow did not translate into power production. The increased chamber volume of 6.5 cc’s dropped the compression ratio by nearly 0.80 points. The 706 heads bested the 241 heads by roughly 10 hp, but down low the 706 heads improved torque production by as much as 20 lb-ft. Given the minimal difference in flow and lack of power production, the 241s don’t seem like much of an upgrade over the 706 heads. But on the upside, both respond equally well to porting.
Modified 5.3L Head Test: 706 vs. 799
This is the test everyone wanted to see, and the one every LS owner will be talking about. The 799 heads offered the most flow, bettering the 706 heads by as much as 20 cfm at some lift points. The chamber volume on the 799 heads was within 2.5 cc’s of the 706 heads, but the dyno indicated this did not translate into power on our test engine. The peak numbers were certainly close (473 hp vs. 471 hp), but the graph tells the story best. The 706 heads were simply better than the 799 heads through most of the curve. Would this change with the presence of boost? We think it might, but only at extreme levels that might tax the flow limit of the 706 heads. The question now is where that limit occurs.
Until we find that out, all hail the mighty 706 heads.