Does anybody know if our engine is a "variable valve timed" engine? Most variable valve timed engines have four valves per cylinder, with the extra intake and exhaust valves opening in the upper rpm ranges. Our engine only has three valves, two intake and one exhaust, I think. Seems that we don't have the variable valve technology, given that. 215 horsepower really isn't that much for a 3.2 liter, so this kinda confirms it for me. Anyone know for sure?

 

Our engines are single overhead cam engines. Variable valve timed engines require dual cams. One for the exhaust valvetrain and the other for intake so that the intake and exhaust valves can be contolled independantly. The benifit of variable valve timing is that the intake and exhaust valve overlap can be changed to produce better combustion and evacuation at higher rpm ranges. So I would have to say that we have fixed valve timing. Keep in mind that this engine has been around for something like 12 years (maybe longer, I'm not sure) with only some very minor changes.

 

Not all VVT engines have to be dual cam. Several manufacturers have prototypes with no camshafts what-so-ever. The valves are solenoid operated.. Imagine this- changing your 'cam' profile by programming a PCM..

 

I too have heard of some work that Honda and Jag are doing in that area. I can see very easily how selenoids would work. But I don't see any way that a solid single cam with fixed lobes for both intake and exhaust valves could be made to work. That's not saying that it can't.... I just don't see how it could. But I sure would like to have a ability to change perameters through the PCM. That would really be cool.

 

I believe the single camshaft slides based on RPM value to different profiles on the single shaft that change the valve timings. But I'm not sure, I haven't invested much research on single camshaft operation.

I'm nearly positive that the Crossfire's engine does not have VVT.

 

With the dual cams the cams slide to change valve timing. As I said above.... If the intake and exhuast lobes are on the same cam you can't slide the cam to change valve timing and overlap at the same time. the overlap will always be the same. Just the timing will be changed.

 

I don't see why... The timing would change with the speed of the rotation of the camshaft. The profile would change if the cam slid. Why couldn't the valves overlap with a single shaft?

 

They do overlap. But to change the compression charactoristics the valve overlap (the time that both intake and exhaust are open together) needs to be variable. That's the whole thing that makes "Variable valve timing" give increased performance. You need to be able to control the intake and exhaust valves timing independantly.

 

Yeah, I understand that. I just don't understand why you can't do that with a single camshaft. The lobes in the cam would adjust the overlap profile based on how far the shaft slides. So even with only a single camshaft the timing could be just as adjustable.

The only thing double cams do is allow the valves to be moved at different rates, I would think.

 

You really aren't too bright are you!!! I don't mind explaining something once or even twice but here goes for the about the 4th time..... The valve overlaps need to be independant so the ECU can change the overlap to optimize compression in any air or DA (density altitude) conditions. If you were always operating in the exact same enviroment I supose you could use one cam and just grind it for one condition. But this is the real world and if it's going to work right the 2 valvetrains need to be controlled seperately. Damn.... I hope you got it this time. If not I really don't care. McDonalds isn't oing to have you designing cars... Thank God! :?

 

Yes, but you can have an infinite number of progressive valve timings on one shaft because the shaft has an infinite number of degrees to slide upon. For each infinitely small point on the camshaft there is a slightly different profile.

Your last post just made it seem like all SOHC cars will explode if they drive up a mountain. So tell me how this Crossfire you drive alters its valve timing for multiple conditions, if you don't mind.

The ECU takes care of intake volume. That would alter when ... I believe it's called Atmosphere Density, not D.A. ... changes because the amount per part of oxygen would become more or less dense. But valve timing and overlap is directly related to RPM value and nothing else. If valves were independent of engine cycles then the valves would collide with the cylinder head. The valves don't change, just like the engine RPM doesn't change, when elevation or humidity or oxygen levels change.

 

I'm an aviator and we call it DA you dip S----

And how much do you want to pay for this motor??? You want to take something that's relativly simple and complicate the crap out of it. When are you getting you engineering degree??? Never mind I already have the answer to that.... NEVER!!!

 

Maybe we're thinking of different things in the case of DA and AD... regardless.

 

I almost hate to side with James...;-) but the fact that you have only a single cam doesn't make CVT impossible. There are a few types around that combine a "fixed" and moveable lobe on a common shaft. How the actual adjustment is accomplished varies, but the end result is the same.
But it is also a fact that this is definitely not the way to keep costs down :-)

 

< You really aren't too bright are you!!! >

That's about where I stopped reading and stopped caring.

 

For those who are still reading....here are two nice web sites that explain the whole gizmo to those who do not have a masters degree in automotive engineering ;-)

http://www.aatap.com/

and

http://auto.howstuffworks.com/camshaft6.htm

And no I have not asked if the "smart valves" could possibly be fitted for the Xfire engine.
I think BMW had a really good explanation on their site for VANOS but I think this will do :-D

 

I didn't say that a single cam was not possible. My only point is that it seems to me that it would be much easier and less costly to use 2 cams... Or even solinoid rather than trying to grind a single cam with the variables that are needed to really accomplish the power improvements desired. I'm a firm believer that not very many things are impossible but there are a huge number of things that are cost prohibitive or impractical.

 

Some manufactures do use a single cam concept for variable valve timing.

A 2nd lobe for a single valve is normally deactivated- allowing the normal torque response at low RPMs. At high RPMs this 2nd lobe is activated hydraulicly to open this valve at a different timing configuration. This is accomplished through the lifter. Very similar in concept to the DC HEMI that deactivates cylinders to increase fuel mileage. This is also contolled by lifters that can be activated or deactivated.


So there you have it- VVT can be accomplished with DOHC/SOHC/cam in block/No cam at all.

 

Cool... Thanks, Jamikest..

 

Which is exactly what both James and I had tried to point out...but for some reason that doesn't count I guess :-)