A minor amount of work by your local machinist can be worth some serious airflow, which is
'Breathe in . . . breathe out. Simple respiration is what keeps us alive, and your engine is no different. On a normally aspirated engine, atmospheric pressure pushes air in through the carb, intake manifold, intake ports, and intake valve on its way into the cylinder. If all engines were built with a straight shot directly into each cylinder, engines would certainly make a bunch more horsepower and torque, and life would be grand. But the reality of low hood lines and other packaging compromises creates a much more tortuous path to the cylinders. Each change in direction, twist, or bump in the induction path creates a flow loss that results in less power and torque.
The search for horsepower is literally a quest to eliminate these flow losses. Big cylinder heads with properly sized ports and larger valves along with more efficient intake manifolds and headers are all parts intended to minimize these flow losses. The problem with big intake-port heads and giant intake manifolds is they are expensive and involve some compromise to streetable power.
So what if we showed you a simple little trick that will work on just about any engine, costs very little money, and is just short of guaranteed to increase both torque and horsepower? Would you be interested? We'll take a wild guess and assume you're on board. Here's the skinny.
This is a Manley stainless steel intake valve with a 30-degree back cut just to the inside
One of the most critical areas in a cylinder head is the transition from the port to the valve and into the cylinder. This is why multi-angle valve jobs have evolved-the search for improved flow. Our simple trick that anyone can do is to add a 30-degree back cut to the inside diameter of the 45-degree seat angle on an intake and/or exhaust valve as a way to improve flow.
We talked with many cylinder-head specialists, and they all made one common point: Adding a back cut to the valves almost always results in improved flow. We decided to test that theory on several different cylinder heads. Jim Grubbs Motorsports helped us by allowing us access to the company's SuperFlow 600 flow bench, which is equipped with both the SuperFlow FlowCom digital software and Performance Trends' Port Flow Analyzer v3.0 software. It's a very useful program that not only records the data very quickly, but also averages and plots the results, making our evaluation process much easier and more accurate.
We began our test using an Edelbrock Performer RPM Oldsmobile head with a 30-degree back cut on both the intake and exhaust valves. The Olds' exhaust port did not respond to this modification, so we concentrated our efforts on the intake side.
We started by flow-testing several different cylinder heads from three completely differen
We also experimented with 35- and 32-degree back cuts on our way to 30 degrees to see which one worked best. From the results, it appears you could make a case for either 30- or 32-degree back cuts on the Oldsmobile head. The 32-degree back cut offers a slight advantage at 0.200- and 0.300-inch valve lifts.
Our testing revealed that, true to our previous research, this trick doesn't work in all applications. But in certain cases, especially when using stock or production-style valves, this simple machining operation can be worth an amazing amount of increased flow.
|Olds Edelbrock aluminum |
|2.072-inch intake valve |
| Intake |
|Gain w/ |
|0.100 ||68 ||71 ||70 ||70 ||2 ||3% |
|0.200 ||136 ||146 ||146 ||144 ||8 ||6% |
|0.300 ||190 ||196 ||208 ||206 ||16 ||8% |
|0.400 ||236 ||242 ||243 ||246 ||10 ||4% |
|0.500 ||232 ||237 || 237 ||237 ||5 ||2% |
|0.600 ||234 ||240 ||239 ||239 ||5 ||2% |
|Avg. flow ||182.6 ||188.6 ||190.5 ||190.3 ||7.6 || 4% |