'08 Viper: 8.4L, 600hp/560ft lbs

[Andy]

I wonder how VVT works with Pushrods. I assume the cam is still dead in the middle of the motor.

[CaptainHenreh]

I wonder how VVT works with Pushrods. I assume the cam is still dead in the middle of the motor.

I know for Chevy, the cam is advanced or retarded like a distributor:

GM's pushrod VVT uses a spline-type phaser to turn the camshaft relative to its drive sprocket. Given the single cam, intake and exhaust valve timing change at the same rate with a fixed relationship, but GM says the VVT helps improve fuel economy, reduce emissions and optimize the balance of low-end torque and high-rpm horsepower.

But all I can find on the viper is that it "VVT electronically adjusts when the exhaust valves are open and closed according to engine speed and load, allowing the engine to ÔÇ£breatheÔÇØ cleaner and more efficiently." Which doesn't really say shit.

Now, I can think of a couple of ways to do this...let's say you have a exhaust lobe with two profiles. It would just be a question of switching the profile from one pushing on the pushrod to another, like VTAK!. you could vary timing and lift this way, and with the different combustion chamber shape on an OHV engine, maybe you get more power by doing that on exhaust side. (Most DOHC seem to do it on the intake side).

But IANAE.

[CaptainHenreh]

Could you have some sort of variable rocker arm ratio controlled by a pivot of the pushrod? I read something about variable valve timing like that without having to have extra lobes on the camshaft. I'm no expert, just recalling what I read.

Yeah, but wouldn't that just affect lift? Still a good thought, though. That might be what they're doing. It's a bit annoying because Dodge is all like "ZOMG FIRST VVT PUSHROD!" and I'm like "Wait, Chevy has VVT in like, their minivans and shit...how is yours different?" And noone can teellll meeeee!!! It's a mystery!

[Andy]

Duh. Variable valve timing could be adjusted at the cam. I think the Ford Cobra motor did that too. I was thinking about the lift part. The Honda in me just assumes where there's VVT, there's also adjustable lift as well.

I can't imagine that you could package 20 pushrods on one side of a motor for the lilft part, run oil to the head for locking actuators without running into some real packaging problems. If they went to all that trouble of casting new blocks and heads, they might as well make the thing DOHC with 4 walking cane long camshafts. That would be awesome.

[Ryan T]

Only way I can think of is by using a Hotrocker. They have had some decent gains on a dyno by using this method. It's too complex for me to try to put into my own words, so I'm gonna copy/paste an explanation of how it works:

The Hotrocker mechanism for each head is composed of eight rocker arms with four fulcrum points, one shaft with four matching fulcrums, and pedestals with teeth to match the fulcrum teeth of the rockers. The pedestals attach the rockers and the shaft to the head. In addition, a twelve volt gear motor driven by an ECM through levers and a link, shift the shaft from one fulcrum to the other.

The shaft starts at engine idle in the right, minimum lift, position. The rocker contacts the shaft with the rocker tooth riding in the valley of the shaft. The shaft holds this position giving a ratio of 1.1:1. When the engine speeds up far enough above idle, perhaps 1400 RPM, the shaft is rotated counter clockwise a specified amount, placing the tooth of the shaft into the valley of the rocker arm. The ratio is now 1.3:1..

Note that the pedestals 1) all have racks matching the racks in the rockers, and 2) keep the rockers firmly located between pushrods and valve stems, and positioned correctly above the valves. The rotating of the shaft doesn't change the rocker or pushrod location.

As engine speed continues to increase, the shaft is rotated farther to place the shaft in the third position of 1.5:1. If speed continues to increase, the shaft will finally move to the fourth position giving a ratio of 1.7:1. (These ratios are for example only, higher and lower ratios can be specified.)

As speed decreases, the shaft is rotated clockwise to each lower ratio to match the dropping RPM. This produces four torque peaks for an engine, thus allowing it to always be operating on or near a torque peak.

The shaft is rotated from one ratio to another by a lever protruding from the side of the shaft. It is connected by a round link, which passes up through the rocker cover to a matching lever connected to the shaft of a twelve volt gear motor mounted on a bracket above the rocker cover.

An electronic control module in the cockpit reads engine RPM and sends shift power to the gear motor to shift up or down as RPM rises or falls. The ECM has a range of shift points the owner can select by turning a selection knob on the panel. LEDs display RPM and shift menues.



Looks like this when installed:

[CaptainHenreh]

Duh. Variable valve timing could be adjusted at the cam. I think the Ford Cobra motor did that too.

? Ford Cobra motor was DOHC?

I can't imagine that you could package 20 pushrods on one side of a motor for the lift part, run oil to the head for locking actuators without running into some real packaging problems. If they went to all that trouble of casting new blocks and heads, they might as well make the thing DOHC with 4 walking cane long camshafts. That would be awesome.

You would only need to run one pushrod per exhaust valve. Why would you need to run two? Also, part of the reason they can even fit a 90┬░ V10 in the first place is because it *is* OHV. Considering they made 400+ hp with a cam in the middle, I doubt they want to spend the money to make new cylinder heads, then redesign the entire engine compartment.

Maybe Jeff can shed some light? Paging Jeff. Please pick up the white courtesy phone. Thank you.


edit: Thayer, that's a pretty sweet method for OHV variable lift. Combine that with some kind of variable timing down low and you'd probably be set.

[Andy]

Ryan,

How many adjustments can that rocker make in a given time/rpm interval?

[Ryan T]

I believe the article said it has a maximum of 4 changes ranging from a ratio of 1.1:1 to 1.9:1.

[BLINGMW]

Only way I can think of is by using a Hotrocker.

that's pretty tricky! :thumbup:

[Andy]

I get the 4 stages but how often can the ECU make adjustments? Is it on the fly or set stages like my beloved VTAK.

Rex, I think chad's motor is DOHC and it came out of a Cobra. Granted I know nothing about Ford, but I think this is true.

I misread your idea about pushrod VTAK. Once again, the Honda in me was thinking of a cam that directly actuated the valves.

[Feersty]

Interesting discussion, although a lot of this is foreign to me. What are the rudimentary aspects of a pushrod?

[Mike]

Interesting discussion, although a lot of this is foreign to me. What are the rudimentary aspects of a pushrod?

google.com "how a pushrod engine works"

[CaptainHenreh]

[quote="Andy"]I get the 4 stages but how often can the ECU make adjustments? Is it on the fly or set stages like my beloved VTAK.

Rex, I think chad's motor is DOHC and it came out of a Cobra. Granted I know nothing about Ford, but I think this is true.

I misread your idea about pushrod VTAK. Once again, the Honda in me was thinking of a cam that directly actuated the valves.[/quote

I'm gonna go out on a limb here and say that the people using the HotRocker might not be totally keen on ECU's. My guess is that lift increases with RPM. Also, Mustang Cobras didn't have pushrod VVT. The pushrod cobras just had pushrods, and the quad cam cobras had, you know, cams and shit.

Also, feersty:

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[WRXtranceformed]

I'd think with an even bigger, heavier engine in order to balance the car out you'd have to add even MORE weight to the rear, further hurting the impact that all that power is going to have on its acceleration.

Boring out an engine isn't going to add any weight. Adding the vvt might add a couple of pounds, but I'd say no more than 10.

Sure, but then don't the internals have to be bigger as well? Maybe only marginally larger, but if you compare the piston heads and rods in my motor to stock ones, there is a noticeable difference.

[Andy]

Where the hell is chad or pete when you need them? I could've sworn chad's motor had a DOHC with variable valve timing on the intake side.

[Ryan T]

05+ GT's have variable camshaft timing, but the cobra engines before that didn't have any variable valve timing.

[HAULN-SS]

So - this might be off topic, but there arent any engines that have electronic actuators that move the valves back and forth instead of cams are there?

[Jeff]

I've said it once...and I'll say it again. Give me a 2001 RT/10 w/ the Cognac leather interior and I will be a happy man. I'll find someway to drop twin turbos on the bitch. (and some "D's"...as it were.)

[CaptainHenreh]

So - this might be off topic, but there arent any engines that have electronic actuators that move the valves back and forth instead of cams are there?

No.

Not yet. The first place we'll probably see it is in road tractors. BMW has been working with Valeo, Ricardo, and Lotus...but so far we haven't seen any real-car applications. The biggest hurdles are, obviously, the computer control, but also being able to set the valve gently back in its seat. Electronic solenoids aren't known for their fine control, and alot of them are either "on" or "off" which wears a valve pretty quick.

My guess is we'll see it on the road by 2010. The downside, of course, is that everything will be computer controlled. The throttle plate will be eliminated altogether, so the only control you'll have over the engine is nicely asking the computer for some more power, please. On the other hand, once people figure out the mapping software, you might see some extraordinary power gains from chips.

[HAULN-SS]

Yeah, I figured as much, and fuel economy would also go up a shitton. Need more power, increase valve lift a bit, or if you're just farting around, open them enough to keep from stalling - or shut off however many cylinders you want.