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Breathe
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Wed, 11 Sep 2002 09:32:09 Share Post by Email
In Reply to: why 16 valves?, nt moore, Wed, 11 Sep 2002 09:15:12
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Better breathing, really.
The idea of valves is to get air and fuel in and exhaust out. The bigger the valve area, the more air/gas/exhaust you can get in and out of the cylinder with less effort. Effort? Yes - it takes energy to pull air into and push exhaust out of a cylinder, and the smaller the hole, the harder the engine has to work. It may not seem like much, but it adds up.

You can make two valves big, but to do that you take up a large part of the cylinder head. Draw a circle, and try and fit as many circles into it without overlapping. If you can only draw two big circles, there is a lot of unused area. If you make the circles smaller and use more, you can cover more area.
With two valves, the spark plug is pushed off to the side, a non-optimal position. With a four-valve head, you not only can make more surface area available, you can put the spark plug right in the middle, which is a much better location for initiating combustion.

Some folks have even gone as far as making valves oval shaped to cover more area. LOTs of problems with that.

Why 5 valves instead of 4? Again, more surface area. You can also play games with valve timing, opening different valves at different speeds/engine loads.

OK, so why not make all engines 4, 5 or 6 valves/cylinder? Cost is one. That's more bits. And turning cams and opening valves takes energy, adds weight, adds noise. Valves extract heat from the head. You can't make too many holes in the head, because of structural reasons and heat removal. Cams affect the height of the engine, which determines how it will fit into a car, which is a function of styling, among other things.

There are many, many, tradeoffs with # and placement of valves, enough to fill a few books, more than just a mere posting. You can compensate for more/less valves depending on the intake/exhaust plumbing, which depends on if you've got a turbo, how the engine is placed in the car, where the suspension is (can't run an exhaust pipe through an A-arm).

My guess on the base VW going back to 8 valves is cost. It's a cheaper engine for a base car. The marketing guys figure that to capture the bottom end of the market, you've got to save money where you can. You want more power, get the 1.8T, which is a 20 valve (5/cylinder) engine. Or go for the VR6 (24 valves, 4/cylinder).





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