OHV systems have more moving parts for the valvetrain, which does not allow high rpm easily, so pushrods are more often found in low rpm, high torque engines.
OHC and DOHC have less moving valvetrain parts, which allows for higher rpm, so over head cams are often found in high rpm, high hp engines.
A pushrod engine can be made to rev high, but it takes more work because there are more valvetrain parts to lighten and strengthen. A overhead cam engine can be made for all low rpm torque, but then it might as well have pushrods because it is not putting its lighter valvetrain to good use.
Overhead cams also allow better airflow into the head. I know there are some current japanese motorcycles that have heads that flow so good, they do not need any valve overlap at all. This is why they can redline at 15,000rpm and still have a smooth idle. An extreme pushrod engine designed for high rpm hp will have so much valve overlap it will barely idle.
If OHC is so great then why.....
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quote:Good grief. Based on that criteria, the LS1 is the same basic architecture as the 440 big block that was in my dad's 66 New Yorker--or any pushrod V8 engine, for that matter.
Originally posted by Last_Z: Yes, the LS1 is "different" from the traditional small block Chevy, but really how much? Except for a few dimensions here and there, the basic architecture is the same.
quote:That's somewhat true. The LSX's are alot more advanced than most give credit. 15 and now 11 degree heads allow for awesome airflow that in some cases is more than comparable DOHC head flow numbers. Light valvetrains allow 6500rpms all day. Good head flow means power and that is what these engines deliver.
Good grief. Based on that criteria, the LS1 is the same basic architecture as the 440 big block that was in my dad's 66 New Yorker--or any pushrod V8 engine, for that matter.
Apparently some still believe that the technology behind an engine in only found in the hard parts that make the engine. Nevermind that the LSx engines were built using the latest in computational fluid dynamics. The pushrod engines may have more parts but the sum of the masses of those parts is usually the same or less than those found in OHC designs. Actuating 16 pushrods or spinning 4 cams, there is not much difference. For all intents and purposes there are no more advantages to using a pushrod engine than an OHC engine in terms of power/torque developement, it comes down to packaging, costs, and application.
quote:Well, we ARE talking pushrod vs OHC, aren't we?
Originally posted by G-Man II:quote:Good grief. Based on that criteria, the LS1 is the same basic architecture as the 440 big block that was in my dad's 66 New Yorker--or any pushrod V8 engine, for that matter.
Originally posted by Last_Z: Yes, the LS1 is "different" from the traditional small block Chevy, but really how much? Except for a few dimensions here and there, the basic architecture is the same.
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BlownF150 - Pushrods,lifters, and rocker arms, are reciprocating, not rotating, and are therefore much more inefficient. Those parts have mass and need to change directions frequently.
Yes we are. And no, the basic architecture is not the same. The Gen III small block was a clean slate design. Only thing it retains with previous generations is the bore spacing and roughly the same displacement. What makes these engines "old technology"? OHC engines came first.
CIB
Smaller package design
Cheaper to build
Better timing accuracy due to shorter cam drive
Less demand on oiling system
OHC
Control of valve float at higher rpms (which is important in smaller engines as they need the higher rpms)
Better NVH charateristics (at least when timing belts are used)
Production line flow (probably one of the biggest reason OHC designs are so popular)
Since it's obvious that you can make a pushrod motor rev and a OHC build plenty of low-end torque so in the final analysis it all comes down to cost.
quote:The lifters only rob engine power when opening the valves. The springs close the valves and return the lifters to the base of the cam circle, thus there is no drag on the engine.
Originally posted by mechtech: BlownF150 - Pushrods,lifters, and rocker arms, are reciprocating, not rotating, and are therefore much more inefficient. Those parts have mass and need to change directions frequently.
The springs are still placing a frictional load on the camshaft whether opening or closing though much less during the closing interval. OHC have lifters and rocker arms (for some types) as well or an equivalent system. Overall the parasitic losses are practically the same as with an OHC design as there are often more valves, more bearing surfaces, and more drive mass involved over a CIB design. Crank over a LS1 engine by hand and then do the same with a 4.6L DOHC...not much difference at all.
All that really matters for street engines are these items;
Is the manufacturing cost of the engine reasonable, for its application?
Is the engine weight and size reasonable for its application?
Is the engine reliable?
Does the engine meet the emissions requirements for the year?
Does it put out a reasonable amount of power for it's application?
Is the power reasonable tractable, for its application?
Does it provide a reasonable fuel economy, for it's application?
Whether it's got 2 pushrod actuated valves or 4 cam operated valves is meaningless.
quote:
Originally posted by VaderSS: All that really matters for street engines are these items; Is the manufacturing cost of the engine reasonable, for its application? Is the engine weight and size reasonable for its application? Is the engine reliable? Does the engine meet the emissions requirements for the year? Does it put out a reasonable amount of power for it's application? Is the power reasonable tractable, for its application? Does it provide a reasonable fuel economy, for it's application? Whether it's got 2 pushrod actuated valves or 4 cam operated valves is meaningless.
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BMW M-Series V10:
5 liters
500 hp
383 lb-ft torque
Chevy LS7:
7 liters
505 hp
470 lb-ft torque
Each type of engine has its advantages and disadvantages.
quote:I think it also depends on where that R&D money goes as far as fields of research. GM is a leader in nanotech materials and applications of those materials to production vehicles. I think China will start recieving more and more in terms of production based R&D while the core advanced technology R&D will remain in the US as that is where many private-sector businesses are.
Originally posted by JHZR2: I wonder what the domestic automakers' R&D budgets are... as a scientist/engineer myself, Id support domestic automakers interested in doing large scale R&D, for the purpose of maintaining a US R&D base... Everyone should be well aware that many US companies are offshoring R&D to china... but IIRC, automakers still do their R&D here at home. Boost those R&D numbers and theyll not only be helping themselves, but also helping the nation as a whole. JMH
JHZR2
Staff member
Excellentpoints... but one issue.
Most of the engineering type PhD students are foreign. Most are Chinese or Indian... China now subsidizes their students to go to school, and to come back to China to innovate. US companies at the same time are getting 'bang for the buck', and funding Chinese PHD students in china at the tune of ten pr one that they could fund here in the USA.
At some point, USA R&D manpower starts dropping bigtime, unless we 'import' some of these folks here. Sure a lot stay as of now, but there are becoming larger and more incentives for them to go back...
Wow, BlownF150, you only have 25 posts, but Ive seen a LOT of them, and theyre very smart and informative to say the least! Welcome to the board!
JMH
JHZR2
Staff member
quote:Two interesting comments you added there a while back. I wonder what the domestic automakers' R&D budgets are... as a scientist/engineer myself, Id support domestic automakers interested in doing large scale R&D, for the purpose of maintaining a US R&D base... Everyone should be well aware that many US companies are offshoring R&D to china... but IIRC, automakers still do their R&D here at home. Boost those R&D numbers and theyll not only be helping themselves, but also helping the nation as a whole. The other thing about the GC engines is interesting, except for one thing... the GC engine line is the consumer grade, non-pro engine offerings from Honda. We want to talk about low tech and all... lets consider... Honda's top of the line professional small engines, the GX line, are OHV type engines... ditto for the best from Robin Subaru, which is a superior small enginemaker as compared to Honda. JMH
Originally posted by Quest: 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. 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)
Thank you sir! Whole point of these forums is to learn and pass on information so I'd like to think I'm doing my small part.
We have a lot of foreign PhD students on campus but from the ones I talk to they are seeking jobs here. Dead right about trouble for the American scientist/engineer though. I think there was a faculty posistion here and out of 250 applicants maybe 5 were American from what I was told. It's a grim outlook for an engineering major. The competition is far more motivated than we are. I've worked in class groups with foreign students and it always amazes how much stronger their basic educational background is compared to ours. All this coddling of kids in grade school and letting them pass when 15 years ago they would have had no choice but to repeat the grade is killing our R&D talent.
quote:You may be very suprised if you saw them side by side.
Originally posted by CBDFrontier06: Your LS1 is certainly not smaller or lighter than my 4.0 all aluminum V6.
quote:Volumetric efficiency of the engine.
Originally posted by greenjp:quote:I really need some help here. What efficiency, other than fuel efficiency, is relevant? jeff
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'm with the guy that said all that matters is performance.
You can have all the hp/liter you want but if it's not faster, it's not faster. Besides HP is a B.S. number anyway as a function of torque and rpm.
100hp/liter 1.6L Honda 160hp - who cares, still slow.
How about $/hp or how about perf/$. Those are ratios that matter to me.
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