GM continues to design and produce push rod (Over Head Valve or OHV) engines while most of their competitors have gone to over head cam (OHC) configurations for their performance engines. Is there a known reason for this? Their smaller engines utilize overhead cams, but the larger V8 engines continue to use pushrods (Gen III/VI [LSx] and Gen V [LT1] Small Block).
Car and Driver asked this very question to GM's Sam Winegarden, chief engineer for small-block V8's.
The advantages of a push-rod valvetrain mentioned in the article can be summarized as follows:
It allows for a more compact engine layout
The camshaft sits in the "valley" of the engine, making use of free space. Because the engine doesn't have to place the camshaft above the cylinder head, the overall dimensions of a push-rod engine are both shorter and narrower.
A more compact engine dimension frees up precious space in the engine bay and helps keep the vehicle's center of gravity low.
It results in a lighter engine
All else equal, one camshaft will have less rotating inertia than the two or four camshafts found in OHC designs.
Fewer parts in the valvetrain = less weight.
It costs less to build
Fewer parts in the valvetrain = less cost. The article mentions an estimated $400 saving per engine.
Put a DOHC head next to a pushrod head and the answer is immediately obvious. The heads from pushrod V8s seem impossibly tiny if you've only ever seen DOHC heads before. And since the block doesn't actually take up that much space in the overall size of the engine, shrinking the heads makes an enormous difference in the overall size and weight of the engine.
The size and weight of a 5-6L pushrod V8 is comparable to the size and weight of a 1.5-2.0L DOHC I4 motor. If you're trying to make a fast sports car and have a choice between two equally heavy engines, one of which makes 200whp (honda F20 for an extremely high power all-motor example) and the other makes 400whp (chevy LS6 for example), which would you choose? Unless gas is extremely expensive or you're being taxed on engine displacement, it's always going to be the more powerful motor.
Honda F20c/F22c1 Motor Weight (w/accessories): 326lbs.
GM LS1/2/3/6 Motor Weight (w/accessories): 385lbs
Fuel efficiency is similar as well.
The OHC engine has advantage over the OHV engine at higher revs .The same is true when comparing side valve to OHV .Smaller engines need the revs more and have more to gain by being OHC .The cost penalty of OHC is less on an inline 4 which is the most common small car engine.The OHC engine uses up more space in relation to its cubic capacity .Remember that the Gen 1 smallblock chev is the most transplanted engine .If you are concerned about volume you could argue that if you designed a OHC engine that occupied the same amount of space you would end up with less cubic inches .In fact it could be argued that on a normal heavy street car your overall performance could be worse .It is totally plausible that OHC gives you more power per cubic inch albeit at higher RPM where engine wear and internal friction are big factors But OHV gives more HP per pound .Remember also that many people have big cars for towing where torque is more important.The OHC engine has cambelt[s] and these belts did not last as long as expected so OHV has a definate maintenance advantage.
protected by Pᴀᴜʟsᴛᴇʀ2♦ Mar 14 at 10:11
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