DOHC, SOHC, NOHC Performance benefits, or hype?

One reason I've heard is that the mass of the valve train is a lot less. The cam pushes more or less directly on the valve. With a conventional OHV engine you have lifters, pushrods, rocker arms, etc...a lot of mass that must be moved quickly. And it'll move only so fast. So, the OHC allows higher rpms, for one thing. I'd imagine there are lots of other good reasons for it not the least of which probably has to do with it being cheaper.

You're partially correct - OHC engines do allow higher RPM ranges, and also allow for multivalve setups more easily. However, OHV engines are _a lot_ cheaper to produce, as they only have one cam instead of 2 or 4 (as in DOHC V6, V8 setups) - I can't confirm this number but I think the difference between GM's OHV 3.5 liter and DOHC 3.6 liter is about $800. Whatever the number is, it's significant.

OHV engines are more compact than OHC engines because they have a less bulky engine heads, allowing larger engines to fit in smaller cars. One reason why GM's 6.0 liter OHV Corvette motor fits in the same space that their 3.5 Liter DOHC engine does.

OHV also accommodates cylinder shutoff technologies more easily than OHV setups.

So why are people rushing to buy DOHC rigs? For one, the hype - it sounds good and high-tech. But more importantly they allow for 4 valve heads more easily than overhead valves do. They do allow for higher RPM operation although this isn't really important for most overhead cam applications (most overhead valve engines can easily rev up to 6500 RPM

- The Corvette Z06 motor redlines at 7000).

I think OHV engines are really great. However, they aren't _quite_ as smooth as the best DOHC setups. But before knocking on pushrods and overhead valves, go drive a Corvette and then anything with GM's OHV

3500 with variable valve timing. Excellent engines, both of them.

Masospaghetti wrote in news:efsgqe$aeu$ snipped-for-privacy@news-int.gatech.edu:

Just like rear disc brakes on a road-going car, which are useless outside of the sales brochure.

As I understand it, DOHC allows more precise valve placement. Precision over valve placement is a bonus for emissions, mileage and power, since you have better control over intake fluid flow.

Didn't GM stick with OHV mostly because of their substantial investment in that technology?

One good explanation at

WÇY

Since you don't know something as basic as this, why are you insulting other people ? You seem quite ignorant "(no offense to you, though)".

Go read a book. Something written after 1920 should do. For instance, the Offenhauser engine that dominated Indy racing was copied from/inspired by the 1913 Peugeot engine which already had double overhead camshafts and four valves per cylinder.

Which allows higher RPM (= more power) and better cam profiles (more torque or more power or more efficiency, or some combination of those 3, depending on what the designer wants).

That too. Try to design a VTEC system with the cam shaft in the block. Or a desmodromic system.

simply a good geometry for the intake and exhaust (cross-flow)and the combustion chamber (hemispheric) when there is the restrictionof having the (only 2) valves parallel to each other.

To summarize:

- 1 overhead camshaft is better than a non-overhead camshaft, due to higher RPM and better profiles;

- 2 overhead camshafts allow better angles for the valves, better geometry for the combustion chamber and 4 valves;

There is an intermediate technology, using 1 overhead camshaft and small rocker arms to actuate 2 non-parallel valves.

About the only performance advantage of non-overhead camshafts is that the center of mass of the engine is lower.

There are several very good reasons for moving the valves and camshafts above the head. The biggest advantage is that it gives the automotive engineer a lot more flexibility in configuring the combustion chamber, aligning and designing valves, etc. It also reduces the mass of stuff that is rapidly changing direction several thousand times a minute.

That said, the marketing gurus took OHV, SOHC, DOHC, valves-per-cylinder, etc., and plastered the back end of cars with labels proclaiming those breakthroughs. Those were indeed significant improvements, but I would venture to say that no more than 1 in 1,000 people could really provide two sentences describing why they were a good thing.

Yes, there is a lot of misunderstanding about octane. And don't get me started about miracles-in-a-bottle.

Don't know who Stuart is...

You are kidding right....gawd.....

I would suggest that you take a few courses in automotive repair at the local night school and do a lot of reading.

You are contradicting yourself. In one paragraph you are saying that placing the camshaft up top has no benefit and in the next you are saying there is a benefit. As before you really need to read what you have written before pressing send. And placing the valves in the head or block and increasing their number does nothing at all to make it a stronger motor. It does allow for improved flow of air and fuel, better combustion and more efficient expelling of burnt gasses if the engineers gets the design right.

Why don't you do some homework and come back to discuss. Try reading up on basic engine technonolgy on places like Wikipedia and How Stuff Works as a starter. Then just starts reading lots of books on cars, automotive history, design, etc.

Or like the case of my diesel '76 Mercedes 300D, where the timing chain left town under heavy "get off the shoulder of the wrong side of the road (where I was due to needing to stuff a newspaper into a poorly placed box) and back across the centerline before something comes around that blind corner just ahead and creams me" acceleration. The camshaft landed with the #2 intake valve wide open, immediately followed by the #2 piston smacking into the valve and punching it straight up (directly into the cam follower/lifter tab and then the camshaft itself) breaking off the #1 and #2 cam-bearing towers, cracking the #3 tower, and snapping the camshaft into three separate pieces - one break between the #2 and #3 cylinders, just behind the #2 bearing tower, another between the exhaust and intake lobes of the #4 cylinder, just ahead of the #3 tower.

You wanna talk about "sudden loss of power" immediately followed by a severe "oh shit" feeling? I'm thinking that incident pretty much defines the concept! :-P

Timing chain and tensioner maintenance were a bit overdue?

probably. But check out the massive head size on a Ford Modular V8 DOHC compared to the GM Gen III (LSx) series. Sometimes, size matters. That's a lot of mass up high.

That said, the Gen III should have been DOHC, size be damned. And I own one in my Trans Am and love it, but in a high performance application pushrods make as much sense as drum brakes.

Ray

Likely - I'd only had the beast for a relatively short time - A couple of months, give or take. Whatever the underlying cause, it turned the car into a hulk - no possible way I could afford to put it back into running shape at the time.

Yes, those MB motors are startlingly expensive to do serious work on. Actually it seems like most MB components are pricey. A family member had to get an AC compressor on the 420 replaced - almost $5k!!!

That is true, but it only becomes an issue when engine RPM gets pretty far above 6500 or so. The Honda VTEC has good engineering reason to be OHC. The v8 in the Toyota Tundra has no *engineering* reason at all to be OHC.

OHC can have a lower parts count (although it often doesn't because manufacturers throw in things like roller cam followers, hydraulic lash adjusters, and more than 2 valves per cylinder which drives the parts count *UP*.) OHC engines have a bigger physical footprint because of the added machinery on the top of the head(s), but that can be mitigated by other clever design features (the Cadillac Northstar is a work of art when it comes to compact packaging). Many OHC designs suffer from less precise valve timing stability because of the extremely long chains or belts required to drive the cams compared to the short chain or pair of gears that can be used for cam-in-block pushrod engines.

Like I said- it all comes down to the intended RPM. If the engine needs to exceed 7000 rpm routinely., then OHC becomes a better choice. Below

6k, its all packaging and cost because either one can do the job perfectly well.

I think the evidence is against you there. I can't think of a single mass-marketed engine in the US with cylinder shut-off that is NOT a cam-in-block design. The Chrysler Hemi with MDS is a pushrod engine. The GM Gen-III v8s with cylinder deactivation are pushrod engines.

In the modern designs for cylinder deactivation, the deactivation is done by redirecting oil pressure at specially designed lifters, and that is very easily done with a cam-in-block engine because the lifters are fed oil directly from passages in the block. With an OHC, the deactivation mechanism has to be out in the cam follower- a much less direct path for oil pressure.

Hype and horseshit. Name me an OHC car that can keep up with a Gen-III powered C6 Corvette, at the same price point. And the Charger SRT-8 with its pushrod Hemi handed a whole bunch of cars costing a least $40k MORE a big dose of humble pie in a recent magazine test. And lets not forget the Viper, which eats OHC V12 exotics for lunch with its pushrod V10. "Performance" does not equal "High RPM." Torque matters too, and if you got it, you don't need to wind out like a mad mosquito to perform.

Sorry. I'm an asshat.

Sincerely though, thank you for the links. I'll definately check them out.

-phaeton

Perhaps you misread or misinterpreted what I wrote, but don't worry about it. Thanks anyway for the answers and discussion. You've been helpful.

-phaeton

Not hype and horseshit. They wanted a low hood on the C5 which ruled out the extra size of an OHC design.

The Gen III kicks maximum ass - I have an 01 TA that does the quarter in

12.18 with nothing more than a 100 shot and sticky tires. (13.11 on the motor.) And it gets 34 mpg on the highway.

A redline of 6200 isn't bad either, but torque and horsepower has ZERO to do with where the cam is located. That's cam profile and bore/stroke. I'll repeat that - an engine fitted with a cam of X lift and Y duration will make Z horsepower at Q rpm no matter WHERE the camshaft(s) is located. PERIOD.

But... Locating the cam right over the valves allows for a lighter valvetrain and more rpm, which is why you can get redlines of 8-9000 rpm. Make same torque at higher rpm=more HP. (you know, how motorcycles make 150 hp from 600 cc....)

I agree that an 8000 rpm redline in a pickup is silly, but in a Vette that's not so silly.

BTW... it's not a GenIII in a C6. It's a GenIV. (LS2/LS7 are Gen IV)

It's periodically helpful to review the many Wikipedia definitions of an internet troll to see which ones fit certain situations.

1. Attention Seeking by asking intentionally naive questions?

1. Attention seeking by posting an intentionally outrageous argument?

2. Attention seeking by asking implausble questions?

I'll agree that I blew it on this topic. I posted a poorly worded ramble. Maybe I could have revised it a few times and made it better, or maybe it's the best I can do. Don't know. Nonetheless, I'm sorry for what I've done.

But please understand that I am not trolling. Surely you think I'm a clueless idiot, but I'm no troll.

Again, my apologies.

-phaeton