It seems I see a lot of discussion on the number of valves an engine has. More isn't always better, it has to fit the way an engine is designed to run.

1.0 Valves have to be "weighted" to survive at the engines intended operating range. Just because an engine can "rev" out, doesn't mean it will necessarily produce the type of power you want or need. Many engines have seen performance increases with two, or three valve heads, especially under 10,000RPM where, we almost always see a road cars engine running.

2.0 An exhaust valve is typically set up to flow about 2/3 the cold volume of air compared to the intake valve. Hot exhaust gasses are less dense than cold (you hope) incoming air, and the exhaust vacuum combined with the incoming airs supercharger effect also help "push" the exhaust gas out. Dense incoming air needs a little more space to equalize the flow. An engine is really just an air pump. No sense in wasting flow where its not needed. Honda has use 3 valve heads to great effect on a variety of engines. Car and motorcycle.

3.0 Why four valves? Simple, the work best for high RPM engines because of reduced valve train weight. Yes, two intake valves breath better at low lift and RPM than one intake valve because of the increased circumference, but this is already accomplished with a three valve set-up! The exhaust valve(s)has/have the supercharger effect and exhaust system scavenging to help it out. The extra exhaust valve in a four valve engine is really there to help increase RPMs more than improve exhaust breathing. In fact, the engine could produce MORE power if it could somehow lighten a single exhaust valve enough to get rid of all the extra valve train friction. Once RPM's get so high, an engines internal friction eats up over 90% of the added power just to spin a little harder. Most four valve engines spin out a little faster to get to where the meat is since they do poorly at low RPM. This seems logical since the lighter weight of the valve train enables the engine to spin up that much faster. All in all though, unless you have serious RPM, three valves work fine.

4.0 OK, let do five valves. Yamaha did, Audi did, they all did at some time. But they never really got it working well for years. Notice the principal is the same, more valves for the dense incoming air (3),less for the hot thin exhaust gasses (2). The problem stemed (pun intended)from valve train friction. They never got enough friction out of the valve train to theoretically exceed a four valve set-up until recently. The friction just ate up the extra power instead of the rear wheels!

5.0 Lets go F1 racing! OK, here we see an advantage to a REALLY oversquare bore/stroke. HP really is torque x RPM, so we spin the dickens out of the engine to make power (pump more air per unit time). We don't really need a "powerband" since we're professional racers who never let off the gas (do we?)so high rpm is the way to go. Shift and floor! And, you need light valves to do it. The top of the piston is HUGE! How do we distribute a charge at 17,000RPM with two valves, let alone lose the additional breathing we gain with two intake valves at low RPM and lift? Use more valves, and distribute them around the combustion chamber. Oh, and remember, we need to GET to 17,000RPM soon enough to beat the guy next to you! But, light weight valve trains don't always equal low friction. Low friction comes from good design

Really, four valve heads used under 10,000 RPM aren't nesessary. The idea sounds great, but data doesn't back it up. It won't really hurt to have four valves, it's just that they are costing you money, mileage, and a tinney winney bit of power at the RPM's any road car sees.

The New V6 and V8 are touted to be a little less powerful than the old straight six and four valve eights. Well, once you rev them out they are! But this is more cam timing than the number of valves. With what MB knows about three valve heads, they could have outfitted the older engines with them and they would be jjust as fast, if not faster than the original four vave heads on top end power. Instead, MB went for lighter weight, more efficient fuel mileage (less friction). And biased the cams for much improved torque in a relatively big heavy car.

from reading your interesting article, I understand that an engine with 4 valves that is running under 10k RPM isn't really necessary due to the friction, but it's great if you're above 10k RPM, right? So does that mean that the more RPM's you have, the less friction?

Does a three valve engine rev faster then a 4 valve system? If that's the case, then can you kindly explain how come Hondas rev so high and so fast?

Also, new MB cars all come with 3 valves, and the rest of the carmakers are either running 4 valves or either 5, like Audi. Why the difference? Any reply would be helpful. Thanks.

Building a 3 valve engine that several different variations are possible cuts COST by $$$$$ & that folks is the only reason!! Certainly not performance or HP.

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MERCEDES BENZ MASTER GUILD TECHNICIAN
ASE MASTER TECHNICIAN
27 YEARS DEALER M.B. Shopforeman
190E 2.3 ITS RACECAR
1986 190E 16V