Abbott Racing "Big Valve" Cylinder Heads
A common misconception is the exhaust needs to be ported as much or more than the intake. Research on our own IHRA Top Sportsman BBC engine plus other engine combinations has proven that a smaller exhaust port will make more power and will make the engine rev gain quicker than a large port, even on engines with superchargers and nitrous. A key to quick acceleration in the car, whether it is on the drag strip or the autocross track, is a quick accelerating engine. The smaller exhaust ports keep the velocity of the air high enough to let the motor accelerate as quickly as possible. An exhaust port too large leaves the engine sort of "out of breath" at the upper portion of the exhaust stroke in the engine. You may ask, why not keep a smaller intake port too? The reason the intake needs to be larger is because that is where the air and fuel enters the engine. It needs to flow as much as possible to get as much air and fuel into the combustion chamber to make a more powerful explosion. The difference between a set of heads working on naturally aspirated engine and engines with power adders is in the camshaft. Nitrous engines will run a little more exhaust duration. Engines equipped with turbochargers and superchargers will need the smaller exhaust port, less exhaust duration on the cam, and a wider lobe separation to keep the velocity up in the exhaust tract. The reason a wider lobe separation and less exhaust duration is used on high boost forced induction applications is because the exhaust is pushed out of the combustion chamber by the boost of the turbo or supercharger. If the lobe separation is too close, there is too much overlap of the intake and exhaust valves being open and some of the new and unused air/fuel charge is pushed out of the chamber into the exhaust.
UPDATE: R&D on the big valve cylinder heads is complete and we have come to the conclusion that the larger intake valve is not beneficial. We tested with valve sizes ranging from 1.86"-1.88" and the amount of air moved through the ports changed very minutely and lowered the efficiency of the heads. The smaller 1.84" valve should be the best candidate for the best power and best accelerating engines.
