Runner-Plenum Interface

[jjusb]

Have anyone studied or measured the effect from the increase in cross sectional area in the runner-plenum interface.

I was thinking of the differences of placing the runners at the bottom of a circular plenum, or protrude them into the plenum.

Obviously, placing the runners inside the plenum would provide a bigger change in cross sectional area, my question is how this increase, affects performance or tuning.

Are the amplitudes of the rarefaction waves smaller with the runners at the bottom? or simply the point of rarefaction is a little higher than the runner plenum interface (Making the effective lenght a little longer)?

[jjusb]

Have anyone studied or measured the effect from the increase in cross sectional area in the runner-plenum interface.

I was thinking of the differences of placing the runners at the bottom of a circular plenum, or protrude them into the plenum.

Obviously, placing the runners inside the plenum would provide a bigger change in cross sectional area, my question is how this increase, affects performance or tuning.

Are the amplitudes of the rarefaction waves smaller with the runners at the bottom? or simply the point of rarefaction is a little higher than the runner plenum interface (Making the effective lenght a little longer)?

[DaveC]

The most popular design uses velocity stacks. The bellmouth will need to stick into the plenum if the curve is more than 90 degrees, the ones I ordered look like 270 deg. I was going to machine them, but they are such a common item, its easier to buy them unless you just like machine work for its own sake.

I have not studied the effects of how far the stacks should protrude into the plenum. In my case, this is really dictated by packaging constraints, but intuitively I would think they would do better if they were closer to the bottom than the top of the plenum.

For turbo cars, flow is more important (maybe not w/ restricted motors, though?), so the optimal design for a turbo motor might be different. This is also something I have not looked into, as we have no plans to run a turbo.

Maybe Ben or Charlie or John, or someone who has done more extensive work on this can fill in some details...

[kozak]

IS A CURCULAR PLENUM THE BEST DESIGN or is a D shaped plenum better i have seen several with a d shape and the v-stacks protruding into the plenum. also if you use v-stacks whouldn;'t it be better to have them welded flush with the flat part of the D. and what do you mean by more than 90 deg.

[LCheung]

Why is flow more important for turbo cars than for non-turbo cars?

[Chris Boyden]

http://www.jenvey.com says it better than I

Quote:
What is the best Air horn ( / Trumpet / Stack / Bellmouth )?

The air horn serves three main purposes; 1) To convert the pressure difference between bore and entrance into air velocity with the minimum of energy loss. 2) To act as the interface between the induction system and the atmosphere, i.e. the point at which pressure waves change sign and direction. 3) To complete the system to the required overall length.
For ease of description the air horn may be considered in two parts; the 'flare' and the 'tube';
The main job of the flare is to spread the low pressure zone over the largest possible area - to reduce local pressure reduction - whilst guiding incoming air into the tube with minimum disruption or induced vortices. The flare should be shaped to encourage air to enter from the sides, but not from the rear, of the mouth. This is achieved by either finishing the mouth with a sharp edge when the arc is a little beyond 90 degrees from the air horn axis or by folding material back, parallel to the axis, when the arc is at, or just below, 90 degrees to the axis.
The main job of the tube is to accelerate the airflow smoothly and progressively. This is best achieved by an exponential shape - i.e. one where the radius of curvature is increasing constantly until the angle of the sides matches the next part of the system, usually the throttle body. At the intake end this should blend smoothly with the flare.
It should be noted that the requirements for fuel injection and carburation do not always coincide and the best horns for one may not suit the other.

End Quote

Our own flow bench testing showed that a flat plate with even just a 1/4" radius on a 1.5" diameter hole flowed better than a tube protruding with a 90 degree bellmouth radius of the same diameter. I don't have my notebook, so I don't remember the numbers. So we ended up fabricating the intake with the curved transitions at the plenum/runner interface.
A problem with this approach is that the bigger
the desired radius the thicker the interface. So it becomes lighter if you weld or attach to the bottom of the plenum then taper down. In '05, a plenum/runner interface was machined out of 1" plate, 1/2" radius on the inlet with tube stubs sticking out of the bottom, allowed us to do away with welding and just plug in the runners and bond them with epoxy, which is very fast compared to welding (outsourced AL welding).

I suggest testing different configurations on a flow bench with a head attached and a setup to control valve lift. The aerosocks sure do look like an easy way of making tapered runners out of fiberglass/carbon coupled with that heat shrink wrap tape. Sure beats metal spinning or sheet metal work on a mandrel for tapered runners.

I'd say that 90 degrees is an arc formed from the plane at the end of the tube stopping at the line parallel to the length of the tube.
180 would return the arc to the plane at the end if that makes sense.

[John Bucknell]

Pressure wave tuning usually only requires a 50-100% change in cross-section to produce a strong wave reflection. At the same time, ideal entrances to ducts want at least a 10% of duct diameter radius (preferrably 25%) on the bell-mouth for minimum flow loss. Velocity stacks are a way to quickly change your runner length inside a plenum (supposing you have a very large plenum), but have no inherent advantage from a flow or tuning point of view - in fact they tend to increase throttled volume....

[jjusb]

John, thanks a lot.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> Pressure wave tuning usually only requires a 50-100% change in cross-section to produce a strong wave reflection </div></BLOCKQUOTE>

Exactly the answer I was looking for.

[jack]

john, wat are you doing on fsae.com? aren't you supposed to be on your honeymoon? http://fsae.com/groupee_common/emoticons/icon_wink.gif

[DaveC]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Why is flow more important for turbo cars than for non-turbo cars? </div></BLOCKQUOTE>

I think I read that in "Theory of Engine Manifold Design" by Winterbourne / Pearson. I believe the idea was that flow volume and equal distribution are more important when designing for forced induction. Without turbo, manifold tuning is more important. For example, on fsae cars, you should be able to make more power with smaller diameter intake primary tubes, which would greatly reduce flow on a bench. I'm not saying manifold tuning is disregarded on a turbo vehicle, just that tuning may be compromised more in order to achieve even flow through the runers, etc... That may not hold true for turbo fsae cars because of the restrictor.

The intake on our car does not have a lot of space for a larger plenum, so the velocity stacks will protrude into the plenum however far they need to for the intake to fit between the motor and the frame. I'm keeping them as low as I can, but theres not a lot I can do unless I make the runners shorter/curved or the plenum smaller, which I dont want to do.

Chris, sounds like you need to start fishing for a sponsor to donate a TIG welder for your team. I couldnt imagine not having one around and trying to build a race car.

I guess I'll throw this in: what are your experiences with increasing the throttled volume via a large plenum? Obviously a larger volume will make the restrictor flow better, but when does it start to make throttle response an issue? We have 2 cars, one with a "small" plenum and long runners, the other is "large" with short runners, and I cant accurately compare them because the runner legnths are too different, but the large plenum doesnt seem like throttle response is an issue, and produces far more power.

[LSU Dave]

How are you all machining your bellmouths? I've thought of it, but can't think of a practical way.

[rjwoods77]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by LSU Dave:
How are you all machining your bellmouths? I've thought of it, but can't think of a practical way. </div></BLOCKQUOTE>

Get a radius tool (looks like a wood router bit) and use it with a tuntable on a manual or just on a CNC.

[DaveC]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">The main job of the tube is to accelerate the airflow smoothly and progressively. This is best achieved by an exponential shape - i.e. one where the radius of curvature is increasing constantly until the angle of the sides matches the next part of the system, usually the throttle body. </div></BLOCKQUOTE>

Could be tough to find the right curve, I'm not sure how less ideal a constant radius would be. Since my runners will stick into the plenum, I bought stacks that flare out to 76mm, and curve down to 36mm. They are 150mm long, so you can either weld them to tube for longer runners, or cut them down and the use the stack for the entire runner. You can find them at http://www.velocity-of-sound.com . They are Australian, so shipping is pricey, but the stacks are very reasonable, and their service was prompt and professional. I could have done it in-house, but it seemed like a lot of effort for no return. I dont think I could improve on the ones I bought without a lot of work. I just couldnt justify researching this topic, too much other stuff to work on.

[Dan G]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by DaveC:
http://www.velocity-of-sound.com </div></BLOCKQUOTE>
I've been looking for the link to that specific company for some time now, couldn't remember the name. Thanks!

[Chris Boyden]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Chris, sounds like you need to start fishing for a sponsor to donate a TIG welder for your team. I couldnt imagine not having one around and trying to build a race car. </div></BLOCKQUOTE>

Well, DaveC, you assume too much. Maybe you should start assuming less.

More specifically we need an AC TIG for Al.
We have an old bumblebee DC TIG for Steel.
For intakes and the like, I'd rather bond them with epoxy anyway, it's still faster and good enough. With welding Aluminum, you lose alot of strength in the HAZ anyway without post brine heat treatment, so you're not really gaining all that much in my opinion by welding AL. I've experienced too much risk with welding aluminum on these time sensitive projects, it sucks to ruin an assembly because of welding errors, All that work down the drain. Time is better spent testing.
For radiators and the like, well, a nice aluminum radiator that is welded is probably better than a bonded, plastic ends. But even those last 10 years on a daily driver. A 10 year old FSAE car that still runs is rare.

Yea, we just did what Rob said, a radius tool in '03 and '04 followed by a CNC surface profile in '05. You can also make v stacks by obtaining 6061-0 and make a mandrel, grease it up, and press short lengths of tubes into velocity stacks, but I doubt you can stretch them from 36 to 76 mm. That's a pretty large opening.

[John Bucknell]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by jack:
john, wat are you doing on fsae.com? aren't you supposed to be on your honeymoon? http://fsae.com/groupee_common/emoticons/icon_wink.gif </div></BLOCKQUOTE>

Honeymoon put on hold - stuck home sick with Mononucleosis (not fun).