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Old 07-02-2010, 09:29 PM   #3 (permalink)
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Originally Posted by Arragonis View Post
Secondly are there things we can use to promote this - for example I've seen posts on people using different oils to reduce friction in engines. Does this make a big, measurable difference ?

And finally are we (outside the US perhaps) kind of wrong to giggle when people still make OHV engines in modern cars ? Are SOHC units better ?
WRT oil, yes, using an eco oil can reduce friction and improve economy. The 2010 Prius uses 0W-20 oil and supposedly loses 1-2 mpg if you switch to a heavier weight.

Millions (billions?) of OHV pushrod engines have been made. They are proven and reliable. The limitations show up when you try to get maximum power from them. They typically have 2 valves per cylinder, which limits the flow, and the mass of the valvetrain limits how much you can rev the engine before the valves float. The valve springs have to push all the mass of the valvetrain back into position and at a certain RPM they can't do it fast enough. Heavier valve springs help, but there's a limit to high you can go here. Lightweight pushrods and rocker arms have been used too. OHVs usually use hydraulic followers (lifters)and oil-bath timing chains so no maintenance is needed.

OHC engines remove the pushrods and rocker arms (in some cases), reducing the reciprocating mass and allowing higher maximum revs to be achieved. It's also easier to add more valves per cylinder, improving volumetric efficiency. But they are more expensive to make. They also increase the overall height of the engine, so if your engine is upright or nearly so, it might compromise your aero efforts. OHCs usually use a timing belt that has to be replace periodically and some use cam followers that have to be fitted by hand and/or adjusted peridically, increasing the maintenance cost.

There are roller rocker and roller cam follower kits available to reduce friction in some of the OHV engines. The roller followers require a special camshaft because the contact point with the camshaft is on the same spot instead of constantly changing as with the flat followers.

Last edited by Patrick; 07-02-2010 at 09:36 PM..
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