IcemanG17

Okay
How much better does a 4 valve head breathe vs a 2 valve head? Most people seem to agree that a DOHC 4 valve head will breath much better than an OHV 2 valve... But with the high outputs of many 2V pushrod V8's lately, it appears the advantage of the 4V head is not as great? I know some flow analysis has been done, but what is the typical gain...10-20%? NASCAR 2 valve heads flow really well, but that is a full blown race engine! So an SOHC 2 valve would be slightly more efficent than an pushrod 2 valve, due to eliminating the rocker arms, pushrods etc? So what I'm looking for is a general idea that a DOHC 4 valve engine approximately XX% more efficent vs a 2V pushrod engine of the same displacement.
Thanks
Brian

Daniel Dudley

917 racers also had 2 valves. 2 big valves. 4 or 5 valves can give you more overall size in a given area, smaller, lighter valves for higher revving, smaller, shallower combustion chambers with no hot spots, and more intake turbulence in the cylinder for more complete combustion. So you aren't just getting flow, you are getting a more efficient total package that can theoretically run higher compression, rev higher and burn cleaner and more completely at optimum air/ fuel ratios. How much more efficient it is I don't know, but my 2,5 litre 944S with 4 valves is about 30 horses more than a standard 2 valve 2.5 944 and gets 30 + MPG at high highway speeds. That's at least 15% more efficient. On the other hand, it flows too much at low RPM and doesn't make good low end power because the speed of gas travel in the runners is too slow. Redline is 1000 RPM higher. All this stuff makes way more sense if you think motorcycles or formula one engines with small displacement and super high red lines of up to 18,000 RPM. All of a sudden this stuff stops being hypothetical and becomes critical to engine management. Also, realise that Nascar engines are relatively big and deliberately restricted for flow by a plate with a hole in it. They could make way more power, but the tracks can't handle the speed, and the restrictor plate makes different engine designs more or less equal and less high tech. Porsche 917s made a lot of power with a lot of displacement as well, and they didn't have to ration the fuel either, so efficiency of that nature was not critical. So for high reving , lean running engines, the 4 valve wins hands down, especially when you turbo or supercharge. For simple, inefficient power, big cylinders and big displacement rule.

Herr-Kuhn

4V is the way to go...though a 2V head can also breathe really well. The biggest advantage of the 4V is the design of the combustion chamber...much less likely to detonate under high pressures...gives a cleaner more controlled burn.

Rich9928p

You need to look at the engine as a system. There are many components to making HP.

A major part of making power is the fuel and spark control. The S4 and newer 928s had the most sophisticated control, and the HP output shows it. The combination of better breathing, higher compression, more precise spark control with anti-knock control, resonance flap, and LH fuel injection is a good as it got.

Today, add variable valve timing, variable valve lift, much improved sequential fuel injection, and coil on plug spark our 4.5 to 5.4 - liter engines could be in the 500 hp. range!

mark kibort

pretty darn close by any standards. ( 4.7 2 valve 310 hp , 5 liter 4 valve 315hp!!)

those euro heads breath pretty well. 293rwhp out of a 10:1 ish 5 liter 2 valve, vs about the same for most decent running 4 valve 5 liters with the MAF vs the AFM too!!!!!!

i like the simplicity of the 2 valve. i bet if you a lot of money making the ports perfect and other head tricks, the 2 valves would be better. less losses , less mass to move around, and less parts to worry about when things break. have you ever seen how easy it is to remove the 2 valve cams?? you just pull them out !!!!

my ideal 928 would be a 5.4 liter 2 valve euro with the 85 LHjetronic I bet with some work, it could put down close to 350hp at the wheels!

mk