stub stacks on SU's

[mperdue]

I've been looking at various stub stacks for SU's and I'm wondernig if anyone here understands the function of these things enough to know which design is best. I've attached a few representative pictures of the various designs I've seen (including a longer ram jet but I'm not really too concerned about that because it's difficult to use with an aftermarket filter such as a Ramflo or K&N). But anyway I'm wondering if the circular one is best because of the overall symmetry, or if the smaller thin cast ones have some advantage because of the smaller size and shallower mounting area. Or does nothing but the curvature and length really matter?

The first one is a cast stub stack made by K&N. A similar part is available from aptfast.com for 1.75" "HS6" style Z SU's. The Billet stub stacks are also from aptfast.com as well as the ram jet (4th picture). The round one (3rd pic) is from a Mini and won't fit a Z but there maybe some out there with 1.75" bore. The are all for use with aftermarket filters of course.

[240ZX]

I've been looking at various stub stacks for SU's and I'm wondernig if anyone here understands the function of these things enough to know which design is best. I've attached a few representative pictures of the various designs I've seen (including a longer ram jet but I'm not really too concerned about that because it's difficult to use with an aftermarket filter such as a Ramflo or K&N). But anyway I'm wondering if the circular one is best because of the overall symmetry, or if the smaller thin cast ones have some advantage because of the smaller size and shallower mounting area. Or does nothing but the curvature and length really matter?

The first one is a cast stub stack made by K&N. A similar part is available from aptfast.com for 1.75" "HS6" style Z SU's. The Billet stub stacks are also from aptfast.com as well as the ram jet (4th picture). The round one (3rd pic) is from a Mini and won't fit a Z but there maybe some out there with 1.75" bore. The are all for use with aftermarket filters of course.

Well, heres what I know. Basically, carb stacks provide better air-flow into the carb by reducing restrictions and providing increases in air velocity. Regarding which one is best, it all depends upon where, in the engines RPM, you want the benifits. Generally speaking, the shorter stacks provide the best effect at higher RPMs and the longer stacks provide the best effect at lower RPMs. FYI, I run the set of the stacks you have pictured, at the far right, with K&N filters. How much difference did it make? Couldn't tell you, as I have not done a comparison between running with and without them. I know from my engine induction experience, that any device that smooths out the air-flow into the carb is a benifit as opposed to having a sharp square edge that the air must flow over. Well, that what I know about stacks in a nut shell Hope it helps you to some degree!

[Arne]

I've heard that the stacks inside the factory air box are actually relatively good. (I have no personal knowledge of that, though.) So if I was planning to ditch the airbox in favor of aftermarket filters (I'm not), I'd look for something similar to the stock ones. Of the ones you pictured, the RamJet (4th pic) are closest.

[jmortensen]

When I ran SU's I cut the stock housing up and put the stock air horns inside K&N filters. When I switched to Mikunis I had stacks made very similarly to the 2nd pic from the left. I got a 3/4" aluminum plate, had 6 holes drilled in it, and used a carbide router bit with a 3/8" radius to cut the radius in the aluminum. Actually I had a machinist do it for me. It was fairly inexpensive, and I just didn't have the money or room for a full radius stack.

[mperdue]

I've heard that in general the ram jets aren't designed for use with aftermarket filters such as K&N because they stick out so far that it restricts air flow against the back plate of the filter. Ramflo filters don't have plates out in front but the conical and oval K&Ns do (see attached pic from MSA). I thought the jets were primarily intended to be used with filter socks or some other completely open filter such as a RamFlo.

But anyway my question primarily was whether the outer structure or flange area has any significant bearing on the air flow characteristics into the throat of the stub stack and carb. Meaning, assuming the inner horn shaped portion of the first 3 pictures are identical, does rest of the structure affect flow (one is just a small lip, the other is a diamond shaped raised structure and the other is a circular raised structure).

[Arne]

I'm no expert on this, but I shouldn't think the flange would have any measurable effect. The goal (as I understand it) is to straighten out the airflow and reduce turbulence. If so, all you need is the radius are.

[jmark]

I am going with the ITG air cleaners & TWM air horns (similar to ram jets). I'll try to get some pics tomorrow after installation.

[Aussie Zed]

The function of air horns is to reduce the entry loss coefficient from the large resevoir of air inside your engine bay into the pipe flow that exists inside your intake runners. My fluid dynamics textbook tells me that a square entrance will have a loss coefficient of 0.5 and from there the loss coefficient is a function related to the raduis of the bell mouth to the pipe (intake) diameter. The entry loss coefficient decreases with an increasingly large radius on a bell mouth but after about 0.15x(diameter of inlet) the intake loss is pretty small (~0.08). This doesn't mean 5 times better flow but it is a significant improvement.

Most of the bellmouth radii pictured look pretty small to me and I doubt that they have been designed by an engineer or someone with some experience with fluid flow. Choose one with a big bell mouth radius, the curved trumpet style ones have a large effective radius that prevents vorticies from forming and should be quite effective especially with a rolled lip.

Also, the tuned length of the intake runner will be changed from standard with a bell mouth that doesn't protrude out the same amount as the airbox does.

I knew all of this theory would come in handy one day!

[mperdue]

Aussie Zed,

You lost me a little there or you're getting your radii and diameters mixed up. Did you mean to say 1.15xDiameter, not 0.15xDiameter? You said that is the radius of the bell mouth - so at 0.15x1.75" (for Z SU's) that's 0.2625" radius or 0.525" diameter. That is smaller than the 1.75" intake! Or if you mean that is the difference between the bellhouse radius and intake radus that gives you 0.2625" larger radius, or 0.525" larger diameter for a total of 2.275" diameter. That's a ratio of ~1.3:1 (bell max ID to intake). I don't think the first 2 have that ratio of bell max ID to intake diameter but the third one is between 1.3 and 1.4. It's not nearly as long/deep as the air horn though.

The more I think about it leads me to believe the real answer is any of them are better than nothing and probably none would show a considerable difference in performance for my application. Thansk for the help.

[jmark]

ITG & air horns were installed yesterday. Likes pretty nice. Now I need a small filter for the valve cover. I think K & N makes one. Anybody got a part number? I'll get pics soon.

[Aussie Zed]

I was pretty unclear on what's what, sorry! See the attached pic. You want to maximise the radius on the bell mouth, r compared to the inlet diameter D. An r/D of 0.1 is OK but you wouldn't want to go much smaller, an r/D of 0.2 is twice as good again but the effect tapers off after about r/D of 0.25.

It is also worth noting that if the stub stacks stick out (as most of them do) the flow becomes what is called re-entrant, which can create more losses. This is due to the big sweep that the incoming air does, causing vorticies (swirling, energy eating air) depicted by the red ovals on the third picture in the inlet. The negative effect that this has is almost exactly equal to the positive effect of a good bellmouth, which is possibly why many people experience no gains from installing them. Just me guessing on this though.

The blue lines on the second pic show how it should be done, the red ovals on the third pic show the vorticies cause by the flow coming from the sides.

[jmark]

I got my ITG air cleaners & TWM air horns installed. Now I am waiting on a K & N filter for my valve cover vent hose. My only question now is what to do with the fuel bowl vent hoses. I'll get some pics soon.