If OHC is so great then why.....

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Physically a pushrod engine is smaller, shorter and lighter so if they want a huge engine in a smaller engine bay, it makes sense. A OHC engine, specifically DOHC is still a more efficent piece of machinery. Someone who knows the technical info can chime in. Just saying, "My LS2 6-spd gets 28mpg on the freeway" doesn't mean it is a more efficent motor in making power which I think a lot of people tend to confuse. Note, I don't own a LS2.
 
Actually, some of the Honda "Vtec" designs composed of a partially modified hemi combustion chamber design ("hemi" stands for hemispherical BTW, just a gimmicky to describe a combustion chamber design). Also note that Sentras in the 80s and early 90s have hemispherical combustion chamber designs in all their 1.6L engines also. Ultimately, it's effort and committment that a company is willing to put into their engine R&D design. Honda typically puts 18~22% of it's company earnings back into R&D, and ditto with Toyota. One of the interesting side facts about the differences between some "yesteryears" design vs lastest offerings from Hondas are as follows: Honda GC series of lawnmower engine of the same hp output, when compared to a B&S Quantum engine, actually consumes considerably less gas (did the test over the course of 2 summers already, Honda hands down) Another good example would be Honda power generators(I shared a Honda EU1000 with a friend): this baby with roughly 2 liter (0.6gal to be exact) fuel tank, runs cleaner, quieter and lasts much, much longer (at one point we got close to 10hrs of runtime with roughly 200Watts load) than that of one that runs with Techumseh (Coleman generator)...
 
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Originally posted by Quest: Actually, some of the Honda "Vtec" designs composed of a partially modified hemi combustion chamber design ("hemi" stands for hemispherical BTW, just a gimmicky to describe a combustion chamber design). Also note that Sentras in the 80s and early 90s have hemispherical combustion chamber designs in all their 1.6L engines also.
I know BMW engines are Hemi's also but they don't go out and advertise it lol. I'm not sure about the latest engines with Valvetronic and what not though.
 
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Originally posted by windnsea00: ... Just saying, "My LS2 6-spd gets 28mpg on the freeway" doesn't mean it is a more efficent motor in making power which I think a lot of people tend to confuse...
I really need some help here. What efficiency, other than fuel efficiency, is relevant? jeff
 
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Originally posted by greenjp:
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Originally posted by windnsea00: ... Just saying, "My LS2 6-spd gets 28mpg on the freeway" doesn't mean it is a more efficent motor in making power which I think a lot of people tend to confuse...
I really need some help here. What efficiency, other than fuel efficiency, is relevant? jeff
Well for example an LS2 getting 28mpg is capable of doing so because of its low torque curve and high gear ratios. It's chugging along at 2k or less rpms. Wind that up to 3k-4k rpms and it will in no way be able to keep up in fuel mileage to a DOHC engine. Also 6.0 liters for 400hp while commonly a European brand can do that with a 4 to 5 liter engine that is DOHC. I know there is more technical info that I'm missing that hopefully someone can chime in. Generally speaking, a DOHC engine is more efficent at making energy per gallon of fuel to a pushrod design. That's why most of the world uses that setup.
 
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Originally posted by windnsea00: Generally speaking, a DOHC engine is more efficent at making energy per gallon of fuel to a pushrod design. That's why most of the world uses that setup. [/QB]
And because a pushrod design has more torque at low rpm, it follows that in the typical automotive application it will do well as far as highway (high gear, low rpm) mileage goes. In other words, for regular driving there's no need to wind up a pushrod engine to 3-4K rpm, so the mileage stays good. It does have an interesting impact on how you use your 'gas foot' though. I had a chevy with 3.4L V6...moving forward from a stop I would 'crack' the throttle and gradually spool up the rpms to get going smoothly. With my new Civic I just push my foot down aways and the engine spins up smoothly to about 3000 rpm before anything really starts going. Of course this has a lot to do with the 'drive by wire' throttle and gearing, but the fact remains that if I use the gas pedal on the Honda the way I did on the Chevy it almost feels like lugging it. Edit: I would like to add that I think it's just silly how people 'put down' pushrod engines. I for one have not seen enough to tell me DOHC is superior to a well-designed pushrod engine for your everyday driving situation. To me it's really just a choice of: More low-end torque / more compact = pushrod More overall hp per liter = DOHC The pushrod engines might not be as ricer/sexy as DOHC, but for regular people on the street I think they skin the same cat. This coming from a guy who's had a few of each. [ May 04, 2006, 07:59 PM: Message edited by: Matt89 ]
 
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Originally posted by windnsea00: I know BMW engines are Hemi's also but they don't go out and advertise it lol. I'm not sure about the latest engines with Valvetronic and what not though.
Anyone remember the K-car with the Hemi? Yeah, seriously. The optional Mitsubishi 2.6 had a hemispherical combustion chamber. The old Ford Escort 1.6 and 1.9 had a hemi head too.
 
I posted this in the bash GM thread, but I think it's more applicable here: OHC is best in the application where you have a tiny engine winding at crazy rpm to produce the horsepower needed to keep a vehicle moving at highway speeds. That is how you take advantage of OHC's efficiency advantage over pushrods. It also sacrifices low-end torque. Here in USA we want the low end too...so previously superefficient DOHC engines have been reworked with VTEC, increased displacement etc to provide low end power at the expense of fuel consumption. If we didn't want the low end, yes, tiny DOHC engines would be the clear way to go. But on the American landscape the pushrod engine still survives because the OHC engines TUNED TO MIMIC its performance consume the same amount of gas or more.
 
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Originally posted by Matt89: OHC is best in the application where you have a tiny engine winding at crazy rpm to produce the horsepower needed to keep a vehicle moving at highway speeds. That is how you take advantage of OHC's efficiency advantage over pushrods. It also sacrifices low-end torque.
But just because your engine is OHC doesn't mean your high winding, ex. Ford's modular engines. Even a good deal of BMW 6 cylinder and 8 cylinders aren't "screaming" down the freeway. That is more common with japanese/european 4 cylinders.
 
Could somebody please explain why a Pushrod engine would produce more torque than an OHC??? Engine output is dependent on Displacement, RPM and efficiency. Efficiency is dependent (among other things) on combustion chamber shape, Valve size and placement. The location of the Cam or how it acts on the valves does not enter into it. OHC designs tend to suit high rpm engines due to the fact that the simpler/lighter valve train can better follow a cam's profile at higher RPM's There is nothing inherent in the design that prevents an OHC engine from making torque! V type engines tend to favour Cam in block/Pushrod configurations, due to the fact that one cam can service both banks of cyls. V engines tend to be of larger displacement, and therefore Torquey, Not BECAUSE the have Pushrods!
 
Spitty. I will explain. It is not because of the pushrods or any such thing that OHV pushrod engines make more torque, it's simple flow dynamics. On non VVT engines, meaning conventional static valve timing only, pushrod motors are predominantly 2-valve per cyl. Having 2 valves instead of 4 means less air flow when the engine is revving high for a given engine size, but it also means higher air velocity when the engine is at a lower RPM. The flow dynamics of a 2-valve head lead to it having better low-end than high on conventional cam designs from factory (the ones the idle smoothly). Now, that in mind, the same thing can be achieved with a SOHC design using 2 valves, but eliminating the pushrods. Furthermore, DOHC can benefit from using variable valve timing and lift to give it better low-end flow without killing the high-end flow capabilities. In the end, a properly built DOHC with variable valve timing and lift, or **** variable cam timing, is going to be the BEST possible outcome. However, for most non-performance say truck engines, a nice mechanical no brainer cheap to make pushrod engine works just dandy. [Smile] Does that help? I have links if it doesn't.
 
I think that re-states what I meant. Pushrod engines tend to be used where torque is required. They do not produce torque due to their use of Pushrods. OHC engines are more suitable for High RPM applications they do not fail to make torque due to their OHC. Sure, there are examples of exceptions. I think it was Peugeot that produced the first DOHC 'Hemi' in 1912 But just TRY and tell Mopar guys that!
 
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Originally posted by 55: Anyone remember the K-car with the Hemi? Yeah, seriously. The optional Mitsubishi 2.6 had a hemispherical combustion chamber.
I had one of them. 1981 Plymouth Reliant K. %$#!@ pile of junk.
 
Can, is??????????? In general, pushrods tend to have the more reliable. longer lasting timing chains. No expensive belts to change. Would belt life be further diminished if they had to drive the weight of pushrods? Parts count favors OHC unless you count each link segment in a slightly longer timing chain. I think the Isuzu engine in my 77 Chevy Luv is a good compromise for a simple, reliable, cheap engine. It has a chain driven SOHC with not rocker arms riding on the cam and actuating the 2 valves per cylinder. Very little to go wrong. Still running strong after 130K of abusive, stop and go, short trips. The only real engine work was a head gasket. I did have the valves ground and valve guides replaced at the time.
 
If properly maintained with HDEO oils,the timing chains on Mercedes OM616/617 engine lasts over 200,000 miles with little or no stretch,of course,pushrods can also go bad if the valve clearance are not properly set but over all pushrod is a lifelong solution albeit little noisy.
 
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Originally posted by windnsea00: Also 6.0 liters for 400hp while commonly a European brand can do that with a 4 to 5 liter engine that is DOHC. ..... That's why most of the world uses that setup.
Our tax happy European friends tax engines based on their displacement. That and high taxes on fuel are why European automobiles tend to have smaller displacement engines. They're forced into making more horsepower from less cubic inches. BTW, the European, Jaguar, normally aspirated 4.2 litre DOHC 4 valve/cylinder aluminum V8 with variable cam phasing that I bought for my wife is only 300 hp and maybe 325 foot-pounds. Another $15K would have bought me forced induction and upped the ante to around 400 hp and 400 foot pounds. The types of European cars you are talking about start around $85K and go from there to whatever your bank account will stand, before taxes. It's sophomoric to try to compare those types of cars to a $20K Chevrolet with an OHV V6. If you want 400 hp and 400+ foot pounds for a price the average joe can put in his garage, a GM OHV V8 is hands down the best game in town.
 
I have a 5.4L SOHC Triton in my F-150 and I've never felt it lacked for low-end grunt compared to friend's trucks with 5.3L Vortecs and 5.9L Magnums. Keep in mind the 5.4L has a 4.165" stroke...whole lot of throw for an engine with its displacement. As such they aren't know for being rev happy screamers despite being OHC but they are smooth running engines. I don't see anything in that article that points to the Hurricane as being a pushrod mill either. Everything I've been able to dig up points to it being a large bore modular, which would fix one of the main weaknesses of the modular mills in terms of producing high hp figures without forced induction.
 
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