Originally posted by CTX-SLPR
Oh boy, ok, let me start with the VVT being a taper cam lobe, thats Honda's VTEC step that uses an electromagnetic solenoid at the end of the cam to physicially adjust the cam thrust changing the part of the lobe that rides on the follower at any given time. so the physical cam profile effects the lift, duration and phasing as however you want to grind the lobes within the limits of the followers ability to slide across the surface of the cam lobe.
The GM VVT actually has fixed cam lobes but uses a "phaser", no not the star trek thing, but a set of vanes in a housing where oil is directed at different pressures by a solenoid onto bothsides of the vanes allowing it to rotate inside of a 40deg range. Since all 4 cams have this feature they can be adjusted so that the LSA can be varried continously inside of the 40deg window of operation so you could theoretically advance one cam and retard the other to get as much overlap as possible but it would run like crap probably. I don't know for sure how Ford does it but its supposed to be all mechanical in the older Zetec motors.
As for the cylinder heads, most modern DOHC motors use multistage intakes so that you run on a smaller runner at lower rpms and high engine vacuum when the engine doesn't need to move a lot of air. The larger secondary passages open up like a quadrajet to allow more air into the motor and the injector richen it up accordingly. People do drive SB2 motors on the (Pro) street but they are high idling, low vacuum, rev happy machines that lack the sophisticated controls that help pump up the bottom end till the engines breathing its flow potential. If you look at a Honda S2000 motor, it revs to 9k stock but has horrible low end torque and can be quite a handful in stop and go traffic as you have to wind the engine up to keep it from stalling or have a really good foot for feathering the clutch.
As far as DOHC being a sales hype.... it can be but properly done you have way more potential out of them than a pushrod motor as far as breathing limitations, the hardware limitations on the otherhand vary from motor to motor and the parts are definately more expensive as you have 4 cams, 3 timing chains, 32 valves, 32 followers, high pressure oiling systems and don't forget that a 5.4L Ford DOHC Navigator or Lightning motor is actually overall bigger than an old school 427 SOHC FE motor!! The Cadillac Northstar 4.6 and the new 4.4 are actually very compact for DOHC motors so they are a bit more of a swap option though a Ford DOHC motor isn't terribly big, but still way too big for anything classic.
DOHC motors love boost, kind of like Hemi's, same reason, too much port flow means too little velocity at low engine speeds. Add a turbo or 2 or a blower and you have a wonderful package that winds it out and had the low end torque that you crave after driving pushrod or another boosted vehicle. Part of the reason why Ford can get Camaro killing power out of 281cid with a blower in the 03-04 cobras and you see impressive power to displacement numbers out of Honda's and such. My sig may quote Carrol Shelby but its true in the NA world, witness 6.0 LS2 to 5.7 LS1, but when you get into boost... well same thing applys its all about moving air and moving it effectively.
More mad scietist speaking, hope ya'll like this and Bruce, hope I've not made any critical errors,
So the cam on VTEC doesnt look like a normal cam w/ one lobe.. its has kind of a dual hump going on on the one lobe? So the cam lobes arent just like a hill shape, theyre more of a camel hump on each individual lobe? Or is it just a normal lobe, but oddly shaped?
Also on VTEC..there is solenoids that control the cam rotation? And these solenoids will speed up or slowdown to give longer duration?
What do you mean by followers?
So the VTEC is just an odd-shaped cam lobe, that has an electronic solenoid on the end that controls its movement, and it can speed up or slowdown the cam to change valve timing? I thought cams have the gears at the end of them, and are connected to the crank by the crankbelt(or whatever you call it)..so how can the cam speedup w/out changing the crank speed? Does the crank move that solenoid, instead of the cam directly?
On Chevy's..the crank rotates a housing, that has 2 solenoids that can be speed up w/ oil pressure, right? Its got the vanes that allow oil to pass through, and different vanes allow for different valve timing, correct? These vanes can be rearranged to change cam timing by up to 40 degrees, right?
Am I getting any of this?
What is Zetec?
NOW for the DOHC intake deal thingy..
You are saying that most DOHC motors have great flow in the runners(which are the tracks the air moves through to get into to combustion chamber,correct?)but since they have SUCH great flow,it does not give good low velocity at low engine speeds. Kinda like it flows a lot of air, but it isnt pushing/forcing it into the engine as it would be at higher rpms,whereas pushrod motors the headflow isnt as good,so it pushes the air in at lower rpm speeds better,right?
And you're saying that they are also not so great at low rpm speeds because they have progressive intake manifolds,that close at low rpms and open at higher rpms?
Would another reason be that because these engines rev so high, the cam itself isnt timed to give such great power at low rpm, but take advantage of the power uptop moreso?Is that safe to say?
Did I get enough questions for you?
If this is too much talk, can I contact you by phone, I SERIOUSLY want to know how this stuff works....and howstuffworks.com isnt as good as a person explaining it to me.PM me if thats cool or not.
Thanks for everything.
