2V vs. 4V???

[Chalky]

So here's a question thats always had me wondering. Back in the day SVO were developing a 4 valve, twin cam head. We've seen the pics, and I've actually seem one of these heads, so I'm wondering; what advantages does a 4V head have over a 2V head and what elements of the design create this effect???

[brsvo]

I'm no pro but the basics are better mixing in the chamber plus centeraly located spark for a better more efficient burn. Never mind the port flow and possibilty of individualy adjustable cam timing changes.

[TheSVOTrust]

http://www.a-car.com/valves/4valves.html

[MustangRacer18]

Most people don't know the answer to this....the age old question of why 2V produce more torque and 4v more power. Well for the same engine displacement...2V engines, due to packaging, can have a smaller bore and longer stroke. Longer stroke equaltes to a higher mean piston speed which is the driver for turbulance in the cylinder which drives up flame speed. You want as much turbulance in the cylinder (not in the port) as you can to drive up flame speed to keep combustion as close to a volume=constant process as possible. Furhtermore since it is of smaller bore the flame has less distance to travel again getting closer to V=ct combustion (higher temps, pres). Lastely longer stroke - smaller bore, has better heat losses properties...less heat is loss per engine cycle again good for torque. All these things are at a lower rpm where frictional losses havent gotton huge yet. But an engine is nothing but an air pump! Air is hard to get in fuel is easy. HP is an indicater of acceleration performance (not torque! I wont get into why on this thread). HP is essentially the rate at which work can be performed. HP is trq*enginespeed/5252 (5252 unit converstion for engine speed in rpm and trq in ft*lbs). HP is also directly related to the mass flow rate of air comming into the engine. Mass flow rate of air is more or less in proportional to valve area. At higher rpm you need all the valve area you can get to get the air in at higher rpm. This requires a larger bore and smaller stroke. This hurts on flame speed and heat losses but getting air into the engine is EVERYTHING! HP IS GOD and so the 4v engine will produce more HP but it will occur at a higher RPM which makes it harder to drive compared to the higher trq lower rpm 2V thats set up for grandma :D. There you go!

[oneowner88lx]

I had a '91 Escort GT which had a 4 valve head. However, there were "throttle valves" in the one of the intake ports on each cylinder. So at low RPM it only breathed through one intake port. Then at a certain RPM the "throttle valves" would open and the engine would breath on both intake ports. With this set up the engine had both low end torque and top end power. Not to mention the cam profiles of the intake valves were tuned; one for lower torque and the other for top end power. That Escort was a pretty strong runner.