Overhead Cam 101: 4 VALVES vs. 2 VALVES

1. 4 valve head have tumble flow but no swirl. Effectively making the heads less responsive at low to mid rpm. But has Excellent top-end potential.
2. 2 valve has swirl & tumble flow combined and has the capability of combustion induced swirl! “Very important” creates better combustion efficiency vs. the 4 valve. This is why 2 valves out torque 4 valves @ lower rpm’s.
3. For the most part, 4 valves will out speed/horse-power the 2 valve. but the 2 valve will out accelerate in the mid range the 4 valve and will maintain better fuel efficiency.
4. The overwhelming BREATHING ADVANTAGE of the 4 valves even with the less than ideal combustion condition is still an enormous advantage over the breathing restrictions of the 2 valves.
5. The RPM potential of the 4 valves is due to the LIGHT VALVE WEIGHT! compare a 2 valve intake to a 4 valve intake and the difference is obvious, and combined with the Breathing advantages on the higher RPM ranges, it becomes obvious why serious racing engines utilize this configutation…
6. Serious 4 valves are very compression ratio sensitive! in a competition environment they need a high compression ratio to create a much improve turbulent chamber induced by this increase.
7. Supercharging or Turbos will induced this turbulent chamber condition that has an effect of creating an ideal combustible environment and contributes greatly to combustion efficiency…with its superior breathing potential over the 2 valve it is the runaway victor in a heads up situation.
8. The 2 valve combustion chamber does not improve that much more when pressurize by boost. it is at the point of diminishing returns…The intake restrictions is now on thge intake valve size, shrouding situation to the bore, and the obvious CURTAIN AREA of both configuration at a given lift! (CURTAIN AREA = The amount of window area that is opened to flow. 2 intake valves has more exposed window opening for flow, and not unlike the 2 valves 1 intake valve which has a shrouded situation that blocks additional flow activity adjacent to the cylinder bore that blocks the flow potential! FULL RACE 2 VALVE HEADS ROTATED THEIR INTAKE VALVES TOWARD THE CENTER OF THE BORE! THIS IS THE TREND IN PRO-STOCK AND NASCAR.

Case & Point: This is why your 1600 2 valve Toyota gets better mileage than your 4 valve.
I have corrected part of this swirl problem within the combustion chamber.

I built this 2TG for Rey Gonzales of W. Covina, CA. in 09/1988

These 2TG & similar engines when stroked tend to grenade at high rpm which is correctable. When stroking an engine, UTILIZE THE LONGEST ROD!!! The side loading on the cylinder wall creates internal friction and limits top-end. Cylinder wall stresses are exaggerated and the demands on the shorter rod will lead it to catastrophic failure to the bottom end. When Rod to Stroke ratio goes down, internal reciprocating drag goes up.

QUESTIONS? Ask away on the comments section below.