What's the rationale behind this?
 

I was reading a thread about shaft rockers on www.pro-touring.com recently and a gentleman on there made an interesting statement. Here's the quote:

http://www.pro-touring.com/forum/showthread.php?t=18904

What's the rationale behind this? Does it have something to do with offsetting the "load" that the ends of a camshaft sees in regards to the timing chain sprocket and distributor/oil pump drive? Also, wouldn't a setup like this require that the "inner" cam lobes be ground with less lobe lift to equal out the valve lift over all cylinders?

Anybody ever had any experience with a configuration like this?

Depends on the motor that he is talking about. It's not about loads but it's about what the different cylinders need. The rocker arm change also changes the lobe area and the valve duration. (just not seated duration)

In that instance the cam to match the inner cylinders needed more duration and more lobe area than the outer cylinders.

FWIW, you have to know what your doing but "require that the "inner" cam lobes be ground with less lobe lift to equal out the valve lift over all cylinders?" your going in the right direction here. You don't have to have equal lift, which I dont think he was talking about in that situation but if the cam is done correctly you don't need more lift on some cylinders vs. others. The constraints that characterize lift are still the same cylinder to cylinder.

Bret

Is this a common practice, and what kind of increase in power is available?

i tend to agree that you would have to know what combination of parts he was running and then some... having fabricated intakes and done port work on sportbike engines followed by flowbench analysis and dyno tuning, i would question how equal the air flow was to each cylinder such that the changes/differences mentioned would result in more power.

-just my 2 cents

How difficult is it to set up a flow bench so that the intake and heads are flowed together? Is this something that any flowbench can do or does it require a special type?

It may not just be about flow. Intake tract length has an effect on tuning pulses, and typically on a single carb (or TB) centered on a single plane manifold, there are short and long runners.

You may want to have the longer ones boost the bottom-mid area of the torque curve and the shorter ones the mid-top end, OR you may want both of them to produce torque (and power) in a fairly narrow rpm range (think Cup Daytona and Talladega 300 rpm band engines). For this you might need different valve events for the short and long intake lengths as well as different exhaust primary lengths.

Cup engines are treated as "eight individual engines running on a common crank" at least by some builders. That's carrying it to the extreme, but they are in it for the last few lb-ft available within the rules.

Is it a common practice outside of Cup, etc.? Those who use it probably aren't talking. I'm sure the cam manufacturers know, but they won't tell you what they are doing for others.

Old one, I think you nailed it.

SStroker, i agree with you on the runner length comments. thats pretty much what i was getting at. it is amazing how what sometimes seems like small differences can have noticable effects.

just like you said in your example, we do the same thing with our intakes for the bike motors. we convert a 4 carb motor to single inlet FI (dang restrictor rule) and slightly stagger the tuned runner lengths to help broaden the torque spikes. for the type of tracks we run, a car with a wider spread of power makes better lap times than one with a narrow powerband. as you know, what you design for all depends on what it is you need.

SoundandFury, to answer your question about flowbenches...no you dont need a 'special' flowbench. just one that is big enough to fit your jig and parts on. when i was developing our intake for this year i made a jig which could accomodate the cylinder head in any of the 4 locations. this allowed me to get the resultant flow of each runner and adjust accordingly. is setting it up difficult? no so much difficult as it is time consuming. involves measuring mouting holes accurately, making the adapter to accomodate your specific setup, and ensuring everything is sealed good. hope that helps. :)

i can send you a pic of the intake if you'd like...lemme know.