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LS3,LS7,L92...explain the geometry and need for an offset rocker..please

I've searched and the closest answer was a thread by JohnB. Still ambiguous.

I understand the heads will bolt on LS2 and LS1 blocks, bore sizing etc.

The MotorTrend article (http://www.motortrend.com/features/a...709_ls3_engine ) says they repositioned the pushrods in relation to the valves by 6mm. - how can that be?

I thought they repositioned the rocker stands/bolts??
What am I missing??

Thanks in advance.

joel

See5

iTrader: (53)

Basically the intake valve is so big (2.1" and 1.6" exh) that there is not room to put the rockers on centerline of the valve like the LS6 and all previous. The LS7 has the same problem and the LS3 and L92 share the ~same casting.



10/4 ?

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Yes.
When you view the intake pushrods are they "tilted" more? The intake valve and rocker are placed further from the exhaust valve yet the lifter cup remains in the same location (i.e. same lifter position in the block as LS1 and LS6/LS2??). The geometry of the pushrods to lifters/rockers is changed/skewed?
Thanks for the help See5.

KCS

iTrader: (20)

I'm not saying you're wrong, but I'm pretty sure the main purpose of the offset is to get a bigger, straighter rectangular port without the pushrod getting in the way.

Project GatTagO

iTrader: (7)

But when you enlarge the port, does the valve not have to be moved over in order to be centered in the port, thus the need for the offset rocker? And since the valve is moved over, than that also allows for the valve to be enlarged, and also requires the 4" bore to accommodate the whole deal. Right?

Andrew

KCS

iTrader: (20)

I don't think so. When it's englarged, it's opened up towards the center of the bore to get a straight port more like a Hemi, rather than a LS1 that angles in toward the center of the bore. I'm not 100% sure about the valve spacing, but I can find out tomorrow when I go to school. I think that it's the larger bore that allows the larger valve, not a change in valve spacing.

See5

iTrader: (53)

Let's just say it is a great design and to optimize the valve and port design they offset the intake rocker?
You can see the angle to the intake port.

Bink

Okay, but what does that do to the pushrod/lifter/rocker arm geometry?

My confusion is in understanding the changes and compensations for the changes. If you compare photos it looks like the the valve spacing (bigger bore) and rocker locations have been changed. If these heads are compatible, and we know they are, with LS1/LS2 blocks then the lifter bores are the same/ original location. That seems to mean that the pushrods are at an offset/lateral angle...true??

I'm just trying to mentally "see" the overall picture.
Thanks for all the input, guys.!!

joel

See5

iTrader: (53)

Bink,
In order to maintain common lifter bore centers, rocker/push rod alignment to those lifter bores, while accommodating larger valves and ports, they offset the rocker tip to catch the valve center. The L92 valve will not clear LS1/6 bore because of the sheer size and space available in the head chamber.

GM certainly did not consult with me for design but, IMHO they have done wonders with packaging and compatibility for the LS "small blocks". Put a 5.3 or a LS7 in car and it is hard to tell by looking, what is in there.

$ho_tyme

iTrader: (3)

+1Thats my view on it.

Old SStroker

Don't believe everything you read in a magazine!

The rocker arm picture in the MT article (provided by GM, I'll bet) is a good one, but the added caption at the bottom (provided by MT) isn't so good. The intake valve was moved 6 mm in the head. The pushrods stayed where they were and at the same angle. The lobes on the cam are in the same places. The rocker mounting posts, at least in the LS7, are in line with the pushrods.

The offset rockers weigh the same as the straight LS1 arms and seem to use the exact same bearings, fulcrum, etc. They are a good design for stiffness and mass. The bearings are probably the weak point for higher spring and dynamic loads. Also, they are non-adjustable, of course. It appears that the same steel casting rocker body can be made into different ratios (1.7-1.8+) by where the pushrod cup is machined in the casting. For a production engine, even the LS7, it's an elegant and EXTREMELY economical rocker compared to aftermarket stuff. Of course, if you need adjustablility, you need the aftermarket parts.

As was mentioned, the intake was moved to accommodate it's size, the "square" ports, and to put it where it wants to be in the larger bores.

Jon

KCS

iTrader: (20)

So the valve placement was changed. I guess I stand corrected. Thanks for clearing that up.