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).
asked Mar 27, 2016 at 20:47
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1Is it a part of the culture? Like HD V-Twins and the BMW boxer perhaps?– DucatiKillerCommented Mar 27, 2016 at 21:41
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1They're dirt-simple, fit in almost anything (short heads) and make tons of power.– 3DaveCommented Mar 31, 2016 at 19:27
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damned if I know just stubborn I guess.....– Old_FossilCommented May 19, 2016 at 6:09
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3 Answers
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.
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While these things are true and is basically a good answer, I'm looking for the reason why GM has stayed with it. Is there anything GM has stated specifically which tells us exactly the reason they've stuck with them?– Pᴀᴜʟsᴛᴇʀ2 ♦Commented Apr 1, 2016 at 19:07
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2Zaid's answer seems to address the question from the standpoint of "why did GM people internally decide to make the decision" but the answer is completely non-surprising- less weight, less complexity, less volume, less cost for almost no lost power. As the guy from GM said, you're giving up a tiny amount of head flow in exchange for a huge weight/packaging gain.– Jim WCommented Apr 1, 2016 at 19:49
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This is very true. Basically what you and Autistic were saying as well.– Pᴀᴜʟsᴛᴇʀ2 ♦Commented Apr 1, 2016 at 20:03
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2Honestly I think we only ended up with all the small displacement motors because of the gas crisis in the 70s and then got DOHC, turbos, injections, etc because people wanted more performance from those motors. Before then, the only countries which invested heavily in such technologies were the ones with preexisting taxes on displacement like japan and italy. Similar tax shenanigans explain the historic popularity of big stroke engines in the UK (taxed bore) and diesel in countries like France (diesel got a lower displacement tax).– Jim WCommented Apr 1, 2016 at 20:11
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Beyond a certain point, efficiently using displacement is actually a pretty inefficient use of your weight/packaging/materials/assembly budget because it will require more deck height, more head (for cams, bigger ports, more valves, etc), more exotic materials, better balancing, more complexity (variable valve timing, variable intake resonance tuning, electronic controls, etc). The only reason you'd want to do something like this is if there is a massive tax on increasing displacement (either directly or through fuel cost) or you're racing and the rules limit displacement.– Jim WCommented Apr 4, 2016 at 14:43
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.
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Great answer, but I'm looking for something more authoritative. Has GM stated any reason specifically why they have stayed with the OHV configuration?– Pᴀᴜʟsᴛᴇʀ2 ♦Commented Mar 28, 2016 at 0:52
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Rumour has it that if/when the oil gets expensive GM will further refine the smaller OHC northstar V8 to replace the giant pushrod engines.– AutisticCommented Mar 28, 2016 at 1:54
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My 1.6L four cylinder Nissan engine is OHC and has a timing chain, not a belt. Commented Apr 26, 2016 at 13:37
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@Autistic the Northstar series of engines was discontinued in 2010. Any future OHC V8 from GM will likely be either a refinement of the current OHV "LS" V8 (used in trucks, Corvette, Camaro, etc), or a new design inspired by the "High Feature V6" (usually 3.6L) used in so many of their other vehicles.– JakeRobbCommented Jan 13, 2017 at 20:27