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Discussion Starter · #1 ·
Hello,

I want to understand why larger valve openings on your intake or exhaust cam will help your car? I do understand that larger will make a car flow better but is that the only reason? Would making on set larger help more no a turbo or supercharger? would making both larger should the size be the same? would making them larger hurt your NA performance?

Please I am a sponge!
TragiC
 

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It would help getting replies if your question could be clarified a little: I assume you are talking about cams with longer duration or higher lift ("larger valve openings on...cam"). Even if you meant the actual size of the valve, and not modified intake/exhaust cams, the general answer I think is that the more your engine can flow, the more power will be produced. Think of your engine as a pump that sucks in air/fuel and expells exhaust. More air/fuel in (whether by radical cams, bigger valves, or FI) and then out, more power is produced. Either bigger valves or modified cams can accomplish this end. Increased flow needs to be matched by intake and exhaust modifications that will meter the proper amount of fuel and allow spent gases to efficiently exit. Making intake valves larger on a 2 valve head can increase flow up to the physical limits of available room in the head which is why multivalve cylinder heads are so popular-2 "medium" sized intake or exhaust valves per cylinder can flow more than the largest single valve that might be fitted in a particular cylinder head. Some engines use 5 valves-3 intake and 2 exhaust to flow per their particular design. If you look at the underside of a twin cam 4 valve head such as on the Zetec engine, you will see the four valves and then see why 2 intake and 2 exhaust valves can use more of the space available for induction/exhaust than 1 intake and one exhaust valve would. There isn't much room there for larger valves so flow improvements (meaning more power) can come from porting and polishing the head for better and smoother flow, installing high lift/high duration cams as appropriate for a particular engine (considering possible interference issues), and of course FI. Flow is where it's at-just about all high performance engines are of the crossflow multivalve type-16 valve 4cylinder engines, 8, 12 and 16 valve V-8 engines, the new GM Vortec L 5 and L 6 multivalve engines all use crossflow induction/exhaust. You don't see the old type of 4 or 6 cylinder engines that had exhaust and intake valves on the same side of the engine anymore, they just didn't flow nearly as well as a multivalve crossflow engine. Even a moderately powerful engine like the stock Zetec produces more power than old 4, 6, or 8 cylinder engines with double or triple the cubic inches. Of course, things like higher compression, better fuel, stronger components, fuel injection contribute to the higher power output of modern engines but it all comes down to flowing more air/fuel in and to produce more power.
Of course there are other things beside just "flow" to consider-you won't find 3" exhaust and a 70mm tb on a small 4 cylinder twin cam high performance engine like the Suzuki 1.3 unit it the Swift GT. That unit put out a lot of power for it's size, but had appropriately sized exhaust and intake. Also, look at the stock exhaust on the Civic SI-produces a lot of power but what a small exhaust pipe.
For NA applications, modified cams or bigger valves (where possible) will flow more and give you more power, provided of course that you can meter the proper amount of fuel and also have a low restriction exhaust.
I have heard of 12 cylinder engines (don't remeber which one) with 5 valves per cylinder. I don't even want to think of how much a valve job would cost on such an engine
 
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