Those are new to me, Peter - cool There's no way they'd fit under the drippers, though. Aftermarket LT-1 style cast covers aren't all that expensive, although every set I've bought I've had to machine a flange for the valve cover gasket. I haven't used 'em so I can't recommend 'em - but they're interesting for sure!

 

WOW, some folks need to read my avatar. nuff said

 

That's helpful

 

So Bill, doing this on a stock motor with stock heads will not increase performance? Only if you have a modified motor where the heads can benefit from the additional flow...

 

This is partly true and good info, but the duration does not change at all, the cam shaft lobes say when the valve opens and closes. The valve just travels a bit faster to cover the lift during the same time period of lift that the cam shaft lobe has ground into it.

 

There are a bunch of factors, sorting out in the two main factors we affect in changing the ratio. This is going to ramble a bit.

First, will the heads provide additional flow at the increased lift? If not then there will be zero benefit from the lift aspect of changing the ratio (leaving the whole stamped/roller tip/roller thing out of it) Back in the heyday of GEN I building with the cams at the time it was pretty common to run mixed 1.5/1.6 rockers depending on the weak side of the cam. But the flow tells the story here - and as always we need to look at the spring capabilities to make sure we don't hit coil bind with stock springs. I see stuff like this all the time - things like springs that allow more lift on the stock Vortec heads...when the head really won't flow at those lift numbers. You just end up with a nasty idle, poor low-speed driveability, poor fuel economy and in the end less power than a correctly-sized cam will deliver.

Second, an increase in duration will always produce more power, as we're leaving the valves open longer. But 2 degrees ain't much, and again we run into other factors such as the flow capabilities of the head and the compression of the engine. A relatively low-compression engine isn't going to see any benefit, and may actually see losses - which is why the first step with the 350/290 is to get the compression up as part of putting in a new cam. But this also closes up the lobe centerline angle, which has benefits as well.

These are general answers that don't fit every situation for sure. Engine builds are all about matching components to provide the best power within the limits of the Almighty Budget. What really screws things up, which is why I get wound up about "bolt-on magic", is when a bunch of changes are made without considering how they all fit together. I get a bunch of "it just doesn't run right" or "it just won't make power" engines in my garage and sigh as I pull them down and find all the mismatched parts.

FWIW, cam designers are brilliant, god-like beings to be able to sort through this stuff and build something uniquely adapted to the particular needs of a specific build. The fact that we have whole books full of cams tells us that it ain't a trivial exercise - and that we're living in high-performance heaven right now.

The 350/290 has a pretty decent cam (3896962) at .450/.460 lift, 222/222 duration and a 114 LC. As someone noted, this is similar to the L-46/L-82 cam which is very well suited to the stock heads. Putting 1.6 rockers on that cam changes the lift to .480/.490...which I am fairly confident is going to hit coild bind on the stock spring, and is beyond what the heads will flow at.

So, in summary - higher ratio rockers are goodness if there's capacity in the engine to benefit from them. I don't see this as the case with most engines making < 1.1 HP/CID or so, which includes the 350/290.

I hope I did okay addressing this question without opening the other steaming heap on the table

 

With a higher ratio, your valve is off its seat sooner and closes later because of that rocker ratio. The duration of the cam lobe doesn't change obviously, but the duration your valve is open from .050" to .050" is greater.

 

Sorry but no! The lift will be higher at .050 ok but the valve is only open the same amount of time set by the lobe.

 

So you're saying you disagree that you'll reach 0.050" lift sooner with a higher rocker ratio? Not tappet lift, valve lift.

"Seat to seat" was wrong for me to say, I was just coming to edit it, but from 0.050" to 0.050"?

The valve should reach 0.050" lift earlier on the cam lobe with a 1.6 ratio compared to a 1.5 ratio. Likewise, it should close later.

Please give me a bit more detail, but I cannot reason it to myself any other way.

 

We usually agree 100%, Manuel but we differ on this to some extent. Might just be terminology - let's discuss

Agreed that seat-to-seat duration doesn't change, but the ratio change produces an effective change in the lobe profile to be steeper - the faster valve movement you reference. This would show up as an increase in duration at .050...and act like an increase in effective duration as installed - i.e. the engine would run as if it had a cam with more duration. This is an interesting engineering distinction - but for the engine builder we can just consider that the net result is what appears to be an increase in duration.

Maybe we can agree that the "area under the curve" will increase and leave it at that

 

I drew a quick sketch of my thinking incase I wasn't explaining it clearly. Would you not agree with this?

 

This is just a terminology thing, but the .050" duration numbers on cam specs refers to tappet lift, not valve lift.
So while the valve will be open at least .050" for longer, you might as well be discussing .037", .156", or any other number you just dreamed up.

 

Well I had a pretty good idea of what the A in billa stands for, now I know for sure. Everybody has one, looks like your mine.

 

Your right that we do usally agree on things. On this one it maybe just an understanding of what one really means. If I understand what your saying is that the engine will react to the higher ratio rocker as if the duration was longer I would agree with you on that. But the wording is duration meaning when the valve starts to open till it closes will remain the same. The full profile of the valves path will surely be increased all within the same time period. Did we get closer?

 

Reported

 

Thanks, I screwed that up. But you would agree that increasing the rocker ratio effectively increases the duration the valve is open... except seat to seat

 

For any given valve lift other than zero, Absolutely.

 

We're good

 

The only problem with this is that weather it is .050, .020 or any other number this is not the full duration of the valve, only at a given place of the profile does the lift change because of the different ratio, these numbers of .050 and such are for checking purposes of the cam only and do not show the full profile of the valve movement.

 

I thought about drawing something, but I would just embarass myself

The whole profile of the lobe is different when you factor in lift - the valve is getting to a higher lift, faster - and that's an steeper (effective) ramp.