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brandyv73's 1968 Firebird

2019 June
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Why Are 4 Valves Better Than 2

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Always interesting:


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Hi, Jim,

     Nice educational video on 4 valves vs. 2. I didn't have the sound on (but the pictures were good) Maybe the sound pointed out one advantage of 4 little valves vs. 2 big ones, that I was told about, whether accurately I can't say - better for air pollution reduction. In defense of the 2 valve OHV setup older Pontiacs use, I say the extra cost and complications of the extra moving parts are fine if you need them, but the Chev small block V8s seem to meet pollution standards while producing abundant power and excellent fuel economy with only 2 valves per cylinder. I remember a magazine comparison test report of a Ford GT, Viper, and Corvette. The 'Vette compared very well with the others in performance, except as to fuel economy, where it very significantly outperformed both - something like 20 mpg vs. 11 or 12. So it makes me wonder if we get more complicated designs for no real world advantage. 


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good stuff.

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2 small intake valves bring in more air volume. With greater velocity. Rather than With one large intake valve that actually creates a restriction and turbulence that disrupts airflow into the cylinder making it less efficient. Same thing with exhaust flow. Exhaust valve sizes are always smaller than intakes to help create backpressure in the cylinder. ideally you want the exhaust flow to be right around 70 to 75% of the intake flow. That is also why header tube size is important larger the tubes the less backpressure that's why they hurt low end power. It is all going out the exhaust. Especially on small blocks. That's why multi valve (example) ford 281 Cid (4.6) Cobra engines make as much power with 70 CID less than LT1 chevy. It's all about airflow. More volume and velocity = efficiency and power...


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