Pontiac Tiger Heads - Breathe Deep

Why is the short turn in the intake port so much longer in the new head as compared to factory?According to Don, the longer short turn reduces the sheering effect that occurs when the turn is too short, which causes the mixture to push across the valve into the opposite wall of the port thereby not using the whole valve area. The longer short turn provides better flame propagation by creating more of a cone or funnel effect around the valve.

What does raising the intake port entry do?It creates a straighter port to get a straighter shot to the valve thereby making more power and it keeps the fuel suspended longer. Fuel tends to drop out of suspension when making sharp turns Don related.

What does radiusing the port corners do?"It helps promote velocity," Don explained. He also related that the 310 cc port runner is conservative as many big-block heads for other brand engines have larger port volumes like 360 cc or more. But the port size here is smaller to maintain velocity and produce more midrange power. That's also why the ports appear tall and narrow.

What does raising the exhaust port do?In this case, it provides the 80 percent of intake to exhaust flow that many racers are taught is the optimal balance. Don believes however, that more power is made when that ratio is closer to 70 percent, which is just where the stock height exhaust ports generally flow.

On the heads with the stock height exhaust ports, why are round and D-ports offered?This is so racers stepping up from D-ports won't have to incur the cost of new headers as well.

Do the round exhaust ports flow significantly better than the D-configuration as is the case on stock heads?Don says that they probably do but he hasn't tested them yet. Since the same casting is used for both, if a racer buys the D-port Tigers and decides that he wants round ports, all he need do is gasket match the exhaust ports to the round port headers.

Don Johnston would like to thank is wife Cindy and the guys at the shop for all of their help in developing these cylinder heads.

Head Flow TestingThe head was tested on a SuperFlow 600 flow bench. All measurements are at 28 inches of water. Outside temperature was 70 degrees, the flow bench temperature 115 degrees and the relative humidity was 62 percent. The test fixture bore size was 4.25 inches.