4 valves vs. 2 valves
Posted by Jordan Gartenhaus on November 05, 1999, gartenha_ad_u_dot_arizona_dot_edu
I had a question concerning motors that feature a 4 vavle per cylinder motor rather than a 2 valve. The 1999 Mustang Cobra features a 4 valve V-8, and it makes
60 more horsepower than the GT version, which features a 2 valve V-8. What makes a 4 vavle motor better? Also, why did Subaru have a 4 valve I-4, and change
to a 2 valve? I was under the impression DOHC motors were superior to SOHC. If anyone can explain the benefit of 2/4 valve motors, I'd greatly appreciate it...
Posted by dave klipfel, deklipfel_AT_aol_DoT_com, Fri Apr 27 , 2001
re: 4 valve vs 2 valve. The name of the game
is velocity and turbulence/mixing of the intake charge at differing engine speeds. At low engine speeds, one intake valve
gives increased velocity, hence better gas mixing and better cylinder filling. If you open a second intake valve at low engine speed, the velocity drops dramitically,
leaving poor intake filling and a lean intake charge. The result is engine knock and less torque. On the other hand, at high RPM, breathing is the name of the game. The valves are open such a short length of time you need the maximum available intake area.
Therefore 2 intake valves work better at high RPM. All of this is of course subject to the exhaust system design. A proper extractor exhaust can make a significant
difference on a two valve system and a restrictive exhaust can nullify all the gains of a 4 valve system. These are SYSTEM designs we are talking about, both intake and
exhaust are related. Change one parameter and you must change the rest of the system or you have wasted your time. To wit, most guys who add street superchargers don't see
much need to have 4 valve heads because the supercharger is packing the cylinders as full as they can get at streetable RPM. The real world test is to compare the RPM vs Horsepower
(or torque)charts that come from dyno testing. Look at similar engine displacements with 2 valve and 4 valve heads. Then look at the curves for two entirely different engines with
the same horsepower ratings. For example, compare a turbo, 4 valve, 2 liter racing engine (500 Hp @ 10,000 RPM) to a naturally aspirated 2 valve 427 cubic inch street engine (500 Hp @ 5000 RPM). Now you
see the complexity of comparisons! There are no universal answers, just better or worse applications for what you want to do with your engine.
Posted by jk on October 10, 2000,
when you have only 2 valves, the air/fuel mixture entering the cylinder can be tangential to the circle of the cylinder, giving a high degree of swirl, better air/fuel mixing
and hence better performance at lower revs in an SI engine. At higher revs, enough turbulence is available to create good mixing, and so 4 valves are better, as they
allow greater airflow. Somwe advanced 4-valve engines, such as the honda VTEC, close off one valve at low revs and power demands, to increase the swirl. Hope
this is useful, JK.
Posted by Nabeel on April 19, 2000, nabeel79_ad_hotmail_dot_com
n reply to ur comment...benefit of having 2/4 v/vs. Actually the power of engine is related to the mixture of fuel.(as u specify sabaru) it have a SI engine. so as the no of valves increases
the mixture of fuel greatly increases the power of engine, but the fuel consumption is greater in more valves or ports. the pressure on the piston
provides the torque,torque is relatively less when the engine is at its idle,but increases with the speed of engine,assisting large carbrators,large section manifolds, large
valves or the no. of valves. however as the engine speed decreases the fuel mixture have less time to fill the cylinder by increasing the no of valves the flow can be
Posted by Srimal on March 17, 2000, srimalj_ad_hotmail_dot_com
4 ports better than 2-- more airflow.Could'nt the same 2 valves be made larger instead of going for 4 valves?
Posted by Will Baden on January 06, 2000
About the 4 valve system.
When you take figures of a 2 valve system you come to the evail that the larger the port the more horses you get. Well taking this fact they just expanded the system
from two valves to four valves to create that much more volume of ports to create more air flow. But this only causes the horsepower to go up. I dont know the
figures on the Mustang Cobra but i am almost pretty sure that the torque has probably gone down or has stayed almost the same than to the two valve system. In
other words you get more horses with bigger ports and why you get more torque with smaller ports.
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