Can a plenum be too large?

Obviously the individual runners have to be sized and shaped in certain ways, but does it matter what size or shape the plenum is?

Related question...
Do the runners HAVE to terminate at the plenum or can they actually extend into it more than just a trivial distance?

Question related to THAT question... (The numbers are arbitrary and I chose cylinders for the question to keep things simple.)
Assume you have a plenum shaped as a cylinder 20 inches in diameter and you have a cylindrical runner, 2" in diameter; that pierces the wall of the plenum and proceeds inside the plenum towards the opposite side of it's entry.
Is there a formula, rule of thumb or outright guesstimate as to how far the entry to that runner must be from the wall that it is approaching?

 

1.2583628473940 inches exactly

 

I've got relatively little knowledge on this, but here's my opinion:

The plenum should be sized according to the runners. In otherwords, if you have a high rpm engine with large runners, you'll want a large plenum to complement them so the engine runs well in it's rpm range. I could be wrong, but I always go by the theory that you should match parts.

Extending the runners into the plenum sounds like a bad idea. "Turbulence" and "dead spots" are words that come to mind. A good performance plenum will have a large cross-sectional area to avoid starving cylinders.

Also, are you talking fuel injected or carbureted? This might make a difference in the responses you get.

 

There is no such thing as too large a plenum from a power production standpoint. However, a large plenum does slow down response time.

There was a good article on this in the recent "Engine Masters" magazine. They showed where you could lengthen and taper the runners and gain the advantage of a larger plenum and the advantage of a longer runner, without the disadvantages of each.

There's nothing wrong with extending the runners into the plenum, as long as they don't restrict flow to other runners. A few years back, one of the EFI companies (Edelbrock or DFI, don't remember) offered a BBC manifold that the runners extended roughly 6" into a large plenum. Looked funny with the lid off, but worked well.

Mike

 

Fuel injected, and centrifugally supercharged.




That's kind of what I have in mind. Essentially, I was thinking, hey... What if I bought an LT1 manifold and hacked away everything except the "valley pan" and the plate that is the mating surface for the heads; then attached tubing to the openings in the plate that were sized and shaped to be, as much as possible, an extension of the intake port. I could even put a shape on the end like an air horn, to smooth the transition. Once I had the tubing in place and properly supported, I could just enclose the whole thing in a box. Then I could simply cut a hole in the front and attach a throttle body.

I would just need to know two things:
1) How long should the runners be?
2) How much clearance does the end of the runner need for airflow?

The second I could probably just eyeball. The first is probably going to need some math.

 

Remember that monster in the Bugs Bunny cartoon?

Bugs got caught in a flood and was fished out of the river by the evil scientist, who wanted to do experiments on him.

After the usual stuff the scientist got annoyed and released that monster from the closet. The one that was all hair with a big pair of white sneakers.

Remember how his head looked? Like an exaggerated loaf of bread with bulges on the left and right side of his head?

That's kind of like what the box would look like. There would be a box to enclose the runners, but at the top corners on the right and left side, where the runners are "pointing", there would be 1/2 of an aluminum tube. Picture a piece of 6 or 8 inch tubing sawed in half longitudinally, with each half being placed on each of the two upper "corners" of the box and the runners pointing directly at the inner curve.

 

Here are pictures of an intake with runners extending into the plenum chamber, which is what both the C5-R and C6-R intake manifolds do. This is the Harrop C5-R intake manifold, BTW.

http://www.ls1tech.com/forums/showpo...9&postcount=93

The "rule of thumb" I've heard used most often is double the total volume of all intake runners, from the plenum to the back of the intake valves. The other one that I've heard used somewhat less often is roughly the same volume of the displacement of the engine.

We're designing an intake for the LS7 with a CNC'd base that will be sort of a hybrid between a Hogan's intake and the Harrop intake above, with individual throttle bodies in the runners, but with a "traditional" common plenum. The runners will not extend into the plenum, or at least that's the plan for now.

 

I'm confused...

It LOOKS like there is a cover that goes over all that and presumably that cover would have a throttle body, but you already have throttle bodies in each runner...

Does this top cover I'm assuming mount the MAF maybe? Or am I completely on the wrong track?

 

For the runner length, if it were me, I'd aim for something the same length or slightly shorter than a stock LS6 intake. They seem to make good power and torque right where you need it. Simplistic method, but tried and true.

As far as spacing around the entrance, in order to match the flow area of the runner, the nearest wall should be at least D/4 away from the entrance. So, if the runner is 2" diameter, you would need the nearest obstruction to be at least .5" away. The valve/port geometric relationship is about the same. It's no coincidence that most ports with 2" valves stall out around .500" lift.

Mike

 

It's a combination of runner length and cross section.

Bret

 

That monster's name is Gossamer




And here is a human ricer who I think resembles Gossamer. Look at this kid's head!!! But hey, at least he won 3rd place at the Honduh wild dropfest!!!

 

Damn...
If he looks that unhappy after winning a trophy; I'd hate to see him mood when something actually goes wrong.

 

Yep.

Nope.

Yep... not sure what was so hard to follow.

A MAF sensor on a race car?!?

 

Yes, a MAF on a race car.

Seriously though, if there are TBs in each runner, then why bother to put a cover on it? Air filter? But you wouldn't put an air filter on a race car, would you?

 

Kinsler makes a C5-R manifold with with 8 butterflies and velocity tubes.... no plenum that I am aware of. I don't quite see the need for putting a cover on that type of intake, except to contain a filter. The concept of a plenum in that case escapes me.