Over head cam 8

[enginesrus]

I am having difficulty seeing the logic in using that tech for average everyday cars, especially in the past before the advent of VVT tech. It is such an expensive way to accomplish the task. Expensive to manufacture and when needing repairs due to warped heads, worn cam drive systems and the like. But in this day of disposable engines it makes things a bit more logical. Problem is these all seem to be over priced inexpensively built engines, so that makes the disposability a bit hard to take in the pocket book. For racing and performance its a go, not for grocery store cars.

 

[RodRico]

enginesrus,

Personally, I don't feel overhead cams are a big cost adder, especially in the four cylinder engines that dominate the market these days. By eliminating pushrods and lifters, overhead cams keep everything tighter which helps high RPM performance valuable in short stroke engines (which are popular to keep vehicle weight down and frontal profile small). Fuel economy relates directly to both economy and emissions, so keeping frontal area (drag) low and fuel efficiency high has direct impact on the marketability of a car these days. Bring back $1/gal gas and eliminate emissions/economy requirements, and pushrods/lifters would likely still be with us.

Rod

 

[MikeHalloran]

Nobody wants a 2.0l four with less than ~200HP these days.
At that power level, the engine has to breathe really deep.
With big ports, there's no room for pushrods.


Mike Halloran
Pembroke Pines, FL, USA

 

[enginesrus]

Yeah and who cares about stuff like this.

https://www.youtube.com/watch?v=eOIvT_NsCbw

And one of many of different brands.

 

[GregLocock]

I must confess that it seems to me that toysrus seems to keep asking very opinionated questions and paying little attention to the answers.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[BrianPetersen]

What happened "in the past" is no longer relevant aside from many of today's engines being descendents of that past.

The engine in my car is a Fiat 1.4 MultiAir. This is 16-valve SOHC with Fiat's MultiAir variable valve timing and lift system. (MultiAir takes the place of the intake camshaft)

It's a descendent of the 1.4 DOHC 16-valve with a different cylinder head for the MultiAir components.

That's a descendent of the entire FIRE engine range that Fiat developed in the 1980s. FIRE = "Fully Integrated Robotised Engine" = designed for automated machining and assembly. Now, everyone does that, but it was revolutionary back then. The original engines in the FIRE range were 4 cylinder SOHC 8-valve, and some of those continued in production long after the 16-valve and MultiAir variations were developed.

The FIRE engine range replaced an assortment of older designs, some SOHC 8-valve, some pushrod. The original architecture for some of the engines it replaced dated back to the 1950s if not before.

Among other manufacturers, I don't think Honda has ever built a pushrod engine for automotive or motorcycle applications.

The only VW pushrod engine that I can think of is the old air-cooled flat-four. Pushrod made sense with that layout, but the inline engines (which started in the early 1970s with the VW Golf Mk1), OHC makes sense. It's an understatement to say that the VW air-cooled flat-four is obsolete in today's world.

 

[romke]

maybe enginesrus wants us to go back to simpler engines like the three cylinder two strokes that were built by Saab, DKW, Wartburg etc. Very simple, cheap, dirty and unreliable, but usable as a grocery car.

 

[dgallup]

Honda actually made a 4 valve push rod transverse V twin motorcycle back in the late 70's and early 80's. Just throwing that out as a historical anachronism. They even turbo'd it back when people thought that was a good idea.

I know Honda made a claim that they could build a 4 valve engine for the same price as a two valve because the smaller lighter valves were lower stressed and thus could be made from lower cost material.

I'm a big fan of DOHC 4 valve engines in my motorcycles but at the moment all my vehicles have push rod V8's. Chevy continues to develop the LS series and they are great in terms of packaging efficiency. You would never get a big V8 DOHC engine under the hood of my C6. I'm pretty pleased with the 28 mpg highway it gets too, the best of any car I've ever owned.

Going back to the Ford Modular V8 DOHC in the video, I'm not aware of any egregious problems with that engine. When they are made in the millions, every engine family has some that break. Every engine has to be produced to a cost that consumers can stand. Every car company has to sell products in a very competitive environment. If there are bad failure modes people stop buying the products. Fords F150 is the best selling vehicle in the country and has been for a long time. Statistically, vehicles are getting more reliable every year. I don't think toysrus has a leg to stand on.


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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[enginesrus]

Greg I see the answers. And something else I have seen in my time, is some engineers that have no time behind a wrench, or a machine tool. OHC especially the higher end DOHC's not the geo metro's is very labor intensive to work on as far as timing chain and head R n R, overly complex on some brands. I am not complaining about power potential of the design, its the lack of durability (material choice and design), and difficulty to repair. The LS and the Hemi are great examples of not choosing the OHC design.

This is quoted from above.
""" What happened "in the past" is no longer relevant aside from many of today's engines being descendents of that past.
"""
I think this is a problem in engineering these days, no one has learned from what the old guys toiled to improve back in the days of slide rules and rotary calculators. And is why there has been such a rash of problems that have cost some owners of the new age cars thousands. Things learned years ago seem to have been forgotten in some cases. And some things thought as new ideas are really old ideas.

 

[BrianPetersen]

Lack of durability (material choice and design) is not a pushrod-vs-OHC issue. Difficulty to repair is not a pushrod-vs-OHC issue, either.

Vehicles nowadays are designed to go down the assembly line easily, they are not designed to be serviced afterwards easily, and they are ALL like that. The engine is designed to go together in automated and semi-automated fixtures on its own without anything else around it, then it gets mounted on a subframe together with accessories and transmission and steering and suspension parts, then the whole thing gets plonked into the bodyshell as a single unit. The fact that you now can't reach parts of the engine for access without dropping the subframe ... is not their concern. And they are ALL like this and it doesn't matter if the engine is pushrod or OHC.

Fortunately, stuff generally needs less fiddling than it did in the old days. The days of needing periodic valve clearance adjustments are gone. The days of needing to fiddle with breaker points and condensers are gone. Spark plug replacement intervals are often 100,000 km or more. It's not common to need to remove a cylinder head any more. (I've never had to have the cylinder head removed on any car or truck that I've owned, and I've taken a couple of vehicles past 400,000 km.)

The Chrysler Hemi perhaps is a good design but so is the Pentastar V6, and those are DOHC 4-valve with VVT. Changing spark plugs on those is a bear ... part of the intake manifold has to come off. But that's not the DOHC's fault. Fortunately, it isn't specified until 160,000 km.

Would the Pentastar be a better engine if it were a pushrod design? Well, considering that the single Pentastar engine design replaced a raft of prior designs, some SOHC, some pushrod, and it makes more power and is lighter and is more efficient than any of them ... probably not.

Would the Hemi be a better engine if it were DOHC 4-valve with VVT? It wouldn't have the same marketing, that's for sure. One thing of note is that if it kept a 90-degree layout, it would be wider (the Pentastar is a 60-degree V6 and this is not an issue). Yes, pushrod cylinder heads are more compact. But this is mostly only a 90-degree V8 issue. For an in-line or a 60-degree V6 ... The DOHC heads fit in all the applications that matter.

I think Chrysler make the right choices for both of these.

 

[LionelHutz]

Do you have any proof of this widespread lack of durability in OHC engines?

 

[SwinnyGG]

The way these threads usually seem to go is that any request for data to back up the OP's wild assertions about engineering are met with bluster followed by a rapid and permanent exit from the thread.

Then two weeks later there's a new thread about some other way that every engine designer in the world is doing it wrong.

 

[SomptingGuy]

Ah yes, but it's sometimes good to question whether we are here (i.e. at today's implementations) by evolution or by design.

Steve

 

[SwinnyGG]

We're here by both- the evolution of design.

 

[RodRico]

I don't think people appreciate the extent to which evolution in simulation and manufacturing technology combined with the evolution of emissions standards has influenced engine design. Engines aren't the way they are because designers are stupid!

 

[SwinnyGG]

Engines aren't the way they are because designers are stupid!

Well put.

 

[LionelHutz]

RodRico - exactly, even though some people still insist the high point of engine design was the end of the '60's.

I'd also like to know what in the newer engines can't be fixed which makes them a throw away item.

At any rate, mistakes can still happen (design and build) and not every single engine built will run 500,000 miles trouble free. But, some of that can be attributed to usage and maintenance. The hard work of the design engineers have produced MUCH better products then we had even 15 or 20 years ago and engine reliability today is way superior then anything produced 20+ years ago. Engines today easily go 100,000 miles before requiring any maintenance beyond oil changes. That would be the point where the plugs had been replaced 10 times and the engine was needing a rebuild 30-40 years ago.

 

[SwinnyGG]

At any rate, mistakes can still happen (design and build) and not every single engine built will run 500,000 miles trouble free.

Right on.

The other thing that I think a lot of the worshippers of the past fail to understand (or just ignore altogether) is that failures today get public attention they way they couldn't 50, 30, or even 15 years ago.

In 1980, if Ford released a bad batch of Mustang engines out into the world and they all blew head gaskets, there was no Jalopnik trolling MustangForums.com and then writing articles about it.

The much-lauded Focus RS head gasket debacle got a ton of press, but it's a problem that cropped up on less than 150 cars. And Ford is replacing probably 15,000 head gaskets because of it. In 1980 (or 70 or 60) an issue that grenaded 100 engines out of 100,000 wasn't even a blip on the radar. Now it's cause for picketing in the streets.

 

[enginesrus]

The lack of durability comes from the need for the long snaked timing chain and the plastic guides it rides on to support and guide on its path up to the heads and back down to the crankshaft. There are plenty of stories about the various engines that need this timing chain drive system fixed at very low mileage figures. And in some cases there is enough destruction to require total engine replacement.
Pentastar ? Is the exhaust manifold still an integral part of the aluminum head on those engines? They have done so many redesigns on that engine and had to increase the warranty mileage to help sell them.
I guess not many here have turned a wrench on push rod vs DOHC engines since there seems to be a lack of knowledge of what is required to say remove a head from one or the other.

 

[GregLocock]

So where's your data?

Cheers

Greg Locock


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