Dual Cams Vs. 16v

[scottzg]

Ok, so in some article on www.howstuffworks.com (im sure you can find which one, im too lazy) They said that the reason to have dual cams was because there was not enough space on a single camshaft to time all 16 valves on a 16v car. "Neat!" says I, "I effectively have dohc, and all those logos that say "dohc 16v" are redundant advertizing!"

But the liberty rs, the svx, and the ej20tt are twin cam flat 4s. So whats the deal? Is www.howstuffworks.com wrong?! What is dohc 16v on a flat 4 for?

My guess is the cams are staggered at high rpm or something. Imput?

[vrg3]

There's more to it... It's true that if you have more than one intake and exhaust valve it can be easier to use a separate cam for intake and exhaust, but also...

Valve overlap duration (the length of time that both intake and exhaust valves are open) is chosen when the cams are initially ground. That's true for both DOHC and SOHC valvetrains. The engineers do all their sums, draw all their diagrams, do all their tests, and come up with an appropriate amount of overlap taking into account the dual goals of power and emissions. But, if any of the premises change (i.e. if you change the engine's dynamics through modifications or if your goal is more towards power than emissions or vice versa) the ideal amount of overlap changes too. With SOHC the only way to change overlap duration is to change or regrind the camshaft. With DOHC you can just change the timing of the two cams separately. Of course, that appeals to enthusiasts but isn't directly of much use to OEMs.

But the OEMs do care about this one: the camshafts in a DOHC setup are each lighter than the shaft in a SOHC setup. That means less rotational inertia. In addition, they can be simpler because each camshaft can press directly on the valve to open it rather than having to press on a rocker arm that presses on the valve. I don't know if the quad cam EJ-series engines are set up that way or not. But anyway, the end result is that the valvetrain is lighter and has less play in it so redlines can be higher.

At least, that was my understanding.

[Legacy777]

vrg3 hit pretty much all the points.

Only other thing to add is that the rocker arm is just another link in the chain of things that can fail/flex/cause problems.

Honestly.....I think the major benefit of DOHC on our cars vs. the SOHC is the fact you can get higher redlines. If you want to build a screamer, a DOHC setup would be the way to go.

[morgie]

correct me if i'm wrong, but our EJ22T is 16v but SOHC ! :\ and valves are actionned by "hydrolic cylinders/pushers" ?! , there is 3 "lobs" on the cam per cylinders, 2 for intake, 1 for exaust.

Also, on a general note, (just a nice to know...) I've seen the internal of a 2.2l DOHC toyota engine (from a 94 celica), and while the first cam is driven by a strap with the engine crankshaft, the second cam is driven by a gear, connected to the first cam, inside the head, no "independant" cam timing possible there :\

[Legacy777]

Yes the EJ22 & EJ22T are 16v & SOHC.

They utilize rocker arms that push on hydraulic valve lifters that then push on the valve. I'm not sure about the lobes, I'd have to check my manuals.

[Dr Nick]

Just to be pedantic , all flat fours have at least 2 cams, one for each bank of cylinders! The turbo legacies here in the UK are generally referred to as "4-cam turbo's"!

I do obviously understand the point that they're not all DOHC - just feeling a bit cheeky today

[vrg3]

Yep... Subarus are the only twin-cam SOHC Japanese 4-bangers out there. :)

Morgie -- couldn't you remove the gear from its shaft and reposition it? That's of course a lot more work than just adjusting the position on a belt but it could be cheaper than regrinding camshafts...

[SubaruCO]

All the concern about cam adjustment is becoming less important due to almost every manufacturer offering variable valve timing. A hydraulically operated cam gear in the new K-series Honda engines (RSX-type S) can vary intake timing as much as 50 degrees either advanced or retarded. Not to say that aftermarket tuners won’t come up with a better solution or modification, but the user operated adjustable cam gears are slowly becoming obsolete. Also I’ve always understood that with a SOHC the power curve is set with the grind and can be moved along the RPM range with an adjustable cam gear but the area under the curve is static. With DOHC a tuner might be able to increase area under the power curve by adjusting overlap to work with other mods.

I keep wishing COBB would finish the series on their DOHC and SOHC engine buildups, http://cobbtuning.com/tech/index.html . They have some good pics and tech tidbits there.

[Scoobyslack]

Honestly.....I think the major benefit of DOHC on our cars vs. the SOHC is the fact you can get higher redlines. If you want to build a screamer, a DOHC setup would be the way to go.

which is why by the end of the year I will have a DOHC EJ22T

[JasonGrahn]

Also, on a general note, (just a nice to know...) I've seen the internal of a 2.2l DOHC toyota engine (from a 94 celica), and while the first cam is driven by a strap with the engine crankshaft, the second cam is driven by a gear, connected to the first cam, inside the head, no "independant" cam timing possible there :\

As does the SVX 3.3.