What's the big deal about Dual Overhead Cams?

[ThePresence]

Firstly, does it matter that they're overhead? How does that increase performance? Secondly, how do dual cams increase performance? After all, one camshaft can open and close all those valves and rocker arms just the same........or cant it?

 

[vi edit]

Non technical answer: They generally have longer legs than a pushrod or SOHC engine. In a pushrod/SOHC engine, they tend to fall flat on their face at about 4,000 RPM's all the way to redline. A DOHC comes alive at that point and pulls pretty strongly all the rest of the way.

Drawback is less umph down low. But that is corrected by changing number of valves around EX: audi 5 valve per cylinder design.

 

[Demon-Xanth]

It allows for a 4 valve arrangement with less reciprocating mass. Though the downsides include a taller engine (or wider in the case of V engines). Pushrods CAN be used to get a 4 valve/cylinder arrangement, an example of this setup is the 24V Cummins turbo diesel that's in the Ram, though it makes for alot of reciprocating mass can can't rev as high.

IMHO, DOHC in most passenger cars, especially large cars, trucks, and SUVs, is largely marketing hype.

An example of this:
GM had a DOHC 3.4L V6, the pushrod 3.8 was smaller, about the same power, more torque, cheaper, and smoother. When it came time to release the Olds Intrigue they put the 3.8 in. The lack of the "DOHC" label hurt sales.

Next time you're in a car w/ a tachometer, watch how far it goes up with normal driving, with high reving cars you rarely go over 5000RPM, with torquey cars you rarely go over 4000RPM. DOHC's main benefit is over 5000RPM.

It's not bad tech, just overrated.

 

[lRageATMl]

Originally posted by: vi_edit
Non technical answer: They generally have longer legs than a pushrod or SOHC engine. In a pushrod/SOHC engine, they tend to fall flat on their face at about 4,000 RPM's all the way to redline. A DOHC comes alive at that point and pulls pretty strongly all the rest of the way.

Drawback is less umph down low. But that is corrected by changing number of valves around EX: audi 5 valve per cylinder design.

to add to it a litte (another non technical answer) they tend to rev much higher (I.E. my car can rev up to 6800-7000RPM while a SOCH GT can only rev up to 5500RPM) so the powerband is increased greatly (almost always in your powerband). the umph you loose down below can be compinsated with gearing.

 

[Demon-Xanth]

Originally posted by: vi_edit
Non technical answer: They generally have longer legs than a pushrod or SOHC engine. In a pushrod/SOHC engine, they tend to fall flat on their face at about 4,000 RPM's all the way to redline. A DOHC comes alive at that point and pulls pretty strongly all the rest of the way.

Drawback is less umph down low. But that is corrected by changing number of valves around EX: audi 5 valve per cylinder design.

The lack of low end is mainly due to cam timing to take advantage of the high end breathing. And about the 4000RPM "fall flat on thier face", tell that to my 4.7 in my Dakota that peaks at 5000RPM and a friend's 377 Chevy SB that gets shifted at 7500RPM. (it was driven on the street until he sold it to another friend who put it in a 10 second Nova).

And as far as gearing goes, I don't consider cruising at 4000RPM to be a good tradeoff.

 

[lRageATMl]

Originally posted by: Demon-Xanth
And as far as gearing goes, I don't consider cruising at 4000RPM to be a good tradeoff.

well I have 4.30's in my car...at about 80mph i'm at 3000RPM in 5th gear...not bad at all. Hell, my dad's lexus is at 3100RPM at 80MPH.

but on things for sure, I can take off like a bat out of hell ... now if my tires jsut wouldn't spin as much

 

[vi edit]

Demon, the engine I was thinking of in particular that "falls flat" at ~4,000 is the GM 3.8L. It just runs out of steam between 4000 and 5000 RPM's. Subaru's boxer 2.5L H4 also runs out around there as well.

 

[vi edit]

Oh, I had the 3.4L DOHC in a Monte Carlo. That thing was churning at about 1700 RPM's at 75 MPH

 

[yakko]

Originally posted by: Demon-Xanth
It allows for a 4 valve arrangement with less reciprocating mass.

That is wrong. Honda has a single cam 16 valve engine that was used in the Civics years back.

 

[Demon-Xanth]

A 4 valve SOHC as well as many 2 valve SOHC heads still have rocker arms, which adds mass. Many DOHC implementations do not have rocker arms, but rather have the cam directly actuate the valve.

 

[LAUST]

DOHC is awesome in certain applications... just remember as with 99.999% of ALL things in life, it has PRO's and they do have their CON's.

 

[slag]

I thought the intrigue had the 3.5 liter engine, not the 3.8.

In fact, i am 90% sure the 3.8 never made it to the intrigue.

The 3.8 does fall flat on its face, thats for sure. A simple rocker arm upgrade or a new cam makes all the difference though.

Joe

 

[vi edit]

I thought the intrigue had the 3.5 liter engine, not the 3.8.

In fact, i am 90% sure the 3.8 never made it to the intrigue.

They had the 3.8L in the first year they made the Intrigues. And I think as a base engine the second year with the 3.5L as an option. This would have been the 98 and 99 model years.

 

[CraigRT]

Non technical answer: They generally have longer legs than a pushrod or SOHC engine. In a pushrod/SOHC engine, they tend to fall flat on their face at about 4,000 RPM's all the way to redline. A DOHC comes alive at that point and pulls pretty strongly all the rest of the way

normally yeah

On the Neons, not so true.. my engine (SOHC Magnum 2.0) comes alive above 4,000 RPMs (true story) with the same HP rating as the first generation Neon DOHC's feels almost exactly the same in the power curve. including the high RPM where the Magnum engine rocks. between 4 and 6,000 RPM the new SOHC engine hauls. The DOHC engine is the same way, which is kinda strange.

 

[Howard]

They allow for lower reciprocating mass, meaning you accelerate quicker, and with more valves, you can cover more area in the head with them, leading to faster breathing (intake/exhaust), you can have cam-changing/phasing controls (e.g. VTEC), etc.

They also tend to be more efficient, as in HP/L.

 

[Thegonagle]

Low-end torque is a function of cam tuning, not the placement of the camshaft(s). The reason pushrod engines are tuned for low-end torque is because the design is incapable of running at as high an RPM as a DOHC, due to the high reciprocating mass of each of those push rods. DOHC engines are tuned for higher-end torque because they are capable of high RPM. As we all (should) know, an equal amount of torque at 6000 RPM rather than 3000 RPM calculates out to double the HP, so they can get a higher peak HP rating out of a DOHC design. A high peak HP rating sells more cars (and is more fun to drive).

Originally posted by: Demon-Xanth
A 4 valve SOHC as well as many 2 valve SOHC heads still have rocker arms, which adds mass. Many DOHC implementations do not have rocker arms, but rather have the cam directly actuate the valve.

This is true. Some examples that make this point:
[*]All of Honda's engines that rev higher than 7000 RPM are DOHC designs, while the Accord and Civic SOHC designs redline at or around 6500 RPM.
[*]Saturn's SOHC engine has a redline 1000 RPM lower than their DOHC engine, the exact same engine, except for the head.
There are of course other, probably better examples, but those are two I can think of off the top of my head.

BTW, to clarify what Yakko said, Honda, if I?m not mistaken, first brought a 4 valve per cylinder, SOHC design to the US for ?88, making it standard equipment in every Civic except the 4-speed manual base hatchback. This 4 valve per cylinder, SOHC design lives on to this day in every Civic, except the 99+ (US model) Civic Si.

 

[teddymines]

The primary reason DOHC is used is to support more than 2 valves per cylinder. A SOHC arrangement would make it difficult to pack more lobes on the camshaft to support this. More valves = less flow resistance = more power.

One drawback is the complexity associated with timing 4 cams so far away from the crankshaft. A timing chain or belt is required and tolerances must be tight.

Pushrod engines use rods to actuate a rocker arm. The extra mass requires bigger valve springs, which cut efficiency and limit top RPM.

 

[Mill]

Well with my SOHC you could tell that power fell off at abotu 5800RPM. I now have a forged crank so I COULD rev above that, I have ported and polished heads, but without changing my cams it is still senseless to rev over 5800RPM. FWIW the best times in my GT before the nitrous and SHM engine came when the auto would shift at 5800RPM.

With new cams(not regrinds) and a few other things I would like to go up to 6500 RPM.

 

[Angrymarshmello]

Nice memory Garfang

 

[OS]

My car has a DOHC motor. Midrange and low end is just fine, I can leave tire marks from a dead stop even though it's just a 4 banger econobox. :Q

 

[LAUST]

Originally posted by: OuterSquare
My car has a DOHC motor. Midrange and low end is just fine, I can leave tire marks from a dead stop even though it's just a 4 banger econobox. :Q

Dropping the clutch??

 

[Demon-Xanth]

You don't know what low end means until you get to the point when you wonder how to control it.... then that point is usually followed by "how can I get even more?". Then you find out a site called "Turbo Diesel Register" exists at which point www.pr0n.com starts getting 10 hits less per day.


Seriously though, low end power isn't usually advertized because people aren't familiar with LB/ft as they are with HP, though the best ways to see what the engine is like is with curves. Then you'll notice that Toyota's 180HP VVTI Celica and Mopar's 230HP 4.7 engine differ by more than 50HP, there's some 165LB/ft missing. (and the 4.7 is considered a high reving engine for it's class)

For an amusing comparison, compare the 300HP LT1, 405HP LT-5, and 405HP LS1 from the corvettes. The LT5 has 4 cams vs. the others having one, and they are all about 350 CID, MPFI, and placed in the same car (granted that there were some changes between the chassis)

 

[OS]

Originally posted by: LAUST

Originally posted by: OuterSquare
My car has a DOHC motor. Midrange and low end is just fine, I can leave tire marks from a dead stop even though it's just a 4 banger econobox. :Q

Dropping the clutch??

Powerbrake.

About a week ago this convertible 2000+ v6 mustang with 4 guys in it wanted a piece of me. It was hilarious cause I beat him by a car length.

 

[iamwiz82]

Originally posted by: OuterSquare
My car has a DOHC motor. Midrange and low end is just fine, I can leave tire marks from a dead stop even though it's just a 4 banger econobox. :Q

i'm calling BS on this one.

 

[OS]

Originally posted by: iamwiz82

Originally posted by: OuterSquare
My car has a DOHC motor. Midrange and low end is just fine, I can leave tire marks from a dead stop even though it's just a 4 banger econobox. :Q

i'm calling BS on this one.

ehh, whatever. Doesn't matter to me if you believe me or not.