Quote:
Originally Posted by oldman
There are several reasons why a DOHC engine does not get good fuel economy.
1) the port size is too big for good velocity at low throttle settings. Ford designed a whole new intake manifold to up velocity.
2) The 4 valve design does not have swril. That is why every VTEC (able to change LIFT) technology that is built for economy lifts only ONE intake valve. Honda even has a three stage VTEC, one valve, two valves low lift, two valves at high lift and duration on the D15 engine.
3) Forget EPA, real world economy is a low rpm engine able to generate a reasonable amount of torque at cruse speed. In my past life I was a hyper-mile guy just for fun going all the way back to the Chrysler leanburn systems.
4) a quadcam engine has a whole bunch of moving parts aka friction. Due to the small internal size DOHC engines usually have long stokes to make usable torque, so that piston has to fly pretty far up and down the bore. GM would destroke the engine OHV engine as you can see in the LS base 4.8 engines with the 83mm stroke, leading to a LONG lived engine with good fuel economy.
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So if DOHC are not good for fuel economy why are they the engine of choice in almost all cars that are made for good fuel economy then? I don't think any manufacturer even offers a OHV V-6 anymore.
Or are you talking strictly in a V-8 application
Either way like I mentioned that initial post was meant to be taken more as a playful jab and sarcasm lol