Valve Angle - Make More Horsepower

As a valve opens, a flow area is created by the combination of valve diameter and valve lift. This is called the flow curtain area.

The Ford Formula
Production Ford heads are especially limited in terms of flow on both the intake and exhaust sides, which seemed to us to be a perfect candidate for a back-cut test. This was by far the most amazing test we performed. In the results, note how the intake improved by an astounding 25 cfm, which is also 25 percent at 0.200-inch valve lift from 102 to 127 cfm. We did lose a small amount of flow at 0.400 inch and above, but for a mild cam with under 0.500-inch valve lift, flow losses above max valve lift are irrelevant. We didn't test a 32-degree cut, but it's possible there may be some slight flow improvements possible at the higher valve lifts.

We gained almost as much airflow at 0.200-inch valve lift on the exhaust as we did on the intake side, with a total improvement of 14 cfm that was worth 17 percent. This is especially helpful on a stock head with a mild cam. Based on these results, we'll never build another mild small-block Ford without back-cutting the valves.

Stock valves often have a ditch or undercut just behind the 45-degree seat that can disrupt flow. The stock Ford 351W 1.840-inch intake valve that we tested suffered from this condition. By adding a 30-degree back cut to this valve, we generated a phenomenal 25 percent increase in flow at 0.200-inch valve lift.

Small-Block Chevy Cuts
Anyone who reads automotive performance magazines and is into small-block Chevys knows the production, iron small-block Vortec head is reasonably priced and flows well. One reason is GM figured out that back-cutting both the intake and exhaust valves was worth significant flow increases. This may not seem like a big deal, but consider that back-cutting even high-production-rate valves costs more money, and even an additional penny on a couple million valves a year is a big deal to the OEs. So it's another example of the success of this simple machining operation when GM back-cuts its valves.

Since we know this trick works on a Vortec and virtually any other small-block Chevy iron head, we chose to test an Edelbrock aluminum small-block Performer RPM head to see how this idea would perform on an aftermarket cylinder head. Here, the back-cut improvements were not as dramatic on the intake side, but they were still helpful. It's also possible that a 32- or 33-degree cut might be more efficient, but we ran out of time to fine-tune our test. On the exhaust side, we saw a tremendous improvement at 0.100-inch exhaust valve lift, which could be worth some power. The theory around that is discussed in the accompanying "Low-Lift Flow" sidebar.

While stock valves do show a dramatic improvement with a 30-degree back cut, the best move is to step up to performance stainless steel valves available from companies like Manley (shown), Ferrea, SI Valves, Milodon, REV, and others. Of course, you'll want to back-cut these valves, too.

While a 30-degree back cut does not work in all cases, it sure was worth a bunch of flow improvement at low valve lifts when we tested these stock iron Ford heads.

Machine costs for back-cutting a set of valves will vary across the country. JGM charges $4.00 each ($64.00 for all 16 valves) to perform a 30-degree back cut.