Making sense of Flow Numbers
Simply looking for the highest "peak" flow number is an all too common mistake which has lead to less than desirable performance. Heads with large runner volumes tend to flow well at high lifts. However if your cam has 0.500" peak lift, then it makes no sense to select a head which flows great from 0.600" to 0.700".

In general, the best overall performance is obtained from a cylinder head which has the most "average flow" within the lift range of your cam shaft. A valve spends more time at mid-lift (between 0.200 and 0.500 thousandths) and relatively little time at peak lift. A head which shows good flow in the mid-lift range will outperform, at the track and on the dyno, a head with simply a higher peak lift number. Brian Tooley, President of Total Engine Airflow, and one of the principle designers of the Holley Systemax head, states, "The objective is to get mid-lift airflow as big as possible. Peak flow numbers are not important."

It is wise to look for the best average mid-lift flow numbers (refer to Table 1 Intake Flow). Two heads clearly stand out. The TFS Twisted Wedge flows an astonishing 199cfm average flow between .200" and .500" lift. The AFR also flows an impressive 194cfm in the mid range average. Discounting the larger 2.02" TFS valves and revised valve angles, the AFR cleary is the top head in the Street class. Compared to the stock 5.0L engine which averaged under 130 cfm, the AFR delivered a 30% increase (about the same as the increase you can expect in horsepower). The Brodix 5.0 head came in second at 186 cfm.

Table 1: Intake Flow (CFM 28" H20)
lift (in.)	TFS*	AFR	Brodix	Holley	Edel	GT40x	World Jr.	Stock
.100	62	60	61	56	56	56	54	48
.200	131	125	125	119	114	105	105	92
.300	193	188	175	166	166	154	141	130
.400	230	226	215	200	200	192	170	141
.500	242	238	230	219	214	210	186	155
.600	242	248	233	228	215	224	195	158
Avg.	199	194.3	186.6	176	173.5	165.3	150.5	129.5
*TFS 2.02" intake valve, revised valve geometry.

Choosing a New Head
There is no question that any of these after market street heads will outperform the stock heads. The most important factor in choosing a new head is performance for your engine. Cost, reliability, and ease of installation are also issues but there is little difference between manufacturers in terms of reliability and installation issues, “bolt on” installation is the norm, and pricing is fairly even - in a range of $995 to $1,300. The TFS heads run at the low end of that range, while the AFR's run at the higher end due to the CNC port work from the factory.

Performance from either of the top heads is impressive. Our 1992 Mustang project "Green Machine" with a stock cam, mildly ported Cobra intake, and TFS heads ran 12.50's at nearly 110 mph in quarter mile testing.

However, since the TFS head uses a 2.02” intake valve and modified valve angles it sits on the edge of the “street” and “strip” categories for our comparison. The rotated valves in the TFS heads can cause piston to valve interference problems when using cams with increased lift and duration with stock or domed pistons. In the ‘92 LX Mustang a cam with 220 degrees duration and 0.520” intake lift fit without any clearance issues. Larger cams will require flycutting the pistons.

As for my two favorites, each achieves its excellent performance in a different way. The Trick Flow head uses a revised valve angle and centerline to minimize valve "shrouding", a problematic condition on small block Ford heads whereby the air flow around the valve is restricted due to the valve being too close to the edge of the combustion chamber and cylinder bore walls. The AFR heads use five-axis CNC porting after the casting process to deliver immaculately shaped ports and runners, while retaining the stock valve geometry. This is an advantage for those wanting to install large cams without dealing with special pistons. This is a great head as a “bolt on” to a stock 5.0L while having plenty of room to grow if your power demands increase.

For our most recent project, the decision came down to trying something new. We've had good experience with TFS heads in the past, but having never played with AFR heads seemed to be reason enough to give them a shot. In a future article we'll detail the installation of the AFR heads on a 5.0L Cobra engine, and report the track results. Stay tuned! F/M

Exhaust Flow (CFM 28" H20)
lift (in.)	AFR	TFS	GT40x	Edel.	Brodix	Holley	World. Jr.	Stock
.100	52	48	49	44	58	57	47	41
.200	100	97	105	91	93	95	101	78
.300	142	134	141	132	130	126	125	95
.400	168	161	158	160	155	152	140	102
.500	181	178	162	171	172	166	148	106
.600	189	189	167	176	183	176	154	105
Avg.	148.7	142.5	141.5	138.5	137.5	134.7	128.5	95.3