Space in front of velocity stacks

[Gregs672000]

To elaborate...

"Video suggested they were using the wrong stacks…"

Not that the engine doesn't make power, rather, if it was 1/2" clearance that **started** to limit power vs greater or unrestricted clearance, suggests to me that the (untested) runner length was the limiting factor, not bell lip radius (though could also be, also untested), or lip-to-fence clearance.

Cool, sounds like you have some understanding of such things. If you're interested, I think you would find numerous videos where they test all kinds of parameters and designs of stacks, including some that were FEET long! These guys are also very aware of how the entire thing is a system and that one change can be effected by something else. For example they have a great video on NA tuning an engine where they start with a particular head, then swap heads and cams and find a big sudden drop at a specific spot in the power curve. They try adjusting cam timing and ignition but it doesn't go away, just moves it. A change in the exhaust (looks like he adds a resonator) reduces it significantly. In the end he finds 35+ hp, all be it mostly at very high rpm. The 4age is a pretty cool engine... nearly 10k rpm! They test several different versions, heads, cams and displacement. I love this site.

[Gregs672000]

The more I watch, the more I like the Toyota 4age. Kinda surprised nobody has suggested a swap into a Roadster. Lots of aftermarket support, very adjustable, responds well to tuning and spins to 9500rpm... what more could you want (other than Nissan on the valve covers...)? Reminds me of the CA18, but with a lot more support.

[rwmann]

The 4AGE is a later-spec 1.6L Formula Atlantic engine, replacing the Cosworth BD-series, making 240-250 HP at 10-11,000 RPM.

[Gregs672000]

https://www.emeraldm3d.com/articles/emr ... th-intake/

Interesting article I found today... dyno results using various lengths of velocity stacks on a 180hp 1600 Ford Zetec. They tried several lengths from 40mm to 170mm (1.5" - 6.7"). In the end, the 170 gave the best, smoothest torque but unfortunately in our case the biggest stack one could reasonably fit on a U20 with sidedrafts is around 80mm (3"). The difference between the 80mm and the 170mm was a consistent 6-8ftlbs of torque between 3500-5500rpm. The current stack I have in is around 40mm (1.5"). An 80mm stack produced 2-9lbs additional torque over the 40mm. All of them produced the same torque at 6000rpm. There was some variations and dips, which were much more prominent with short stacks, at least on this engine. Unfortunately they don't give HP, and it's noted that the taller stacks tend to move peek torque down lower in the rpm range, which may be some indication that it might choke a little at higher rpm. But with all sizes giving the same torque at 6000rpm I'm feeling safe at the max size I can fit.

Of course, this was on a different motor than ours, but I think it's applicable. It especially illustrated how running your TBs, sidedraft carbs or SUs without a decent and appropriately long stack has a detrimental impact upon output and in places our smaller displacement engines need. However, it was also noted that the effect is only in play at 80% or greater throttle; the engine won't make more torque at part throttle, only when you mash it. But I sometimes like to do that.

Anyway, I decided to order a set of 75mm stacks. They'll be here in a month, coming from the EU.

P.S. It's been discussed that the lengths given here in this example are not a direct reference on "what size stack length" YOU "should" run... the entire intake tract is what you are tuning and the overall length varies per the head/intake system. The GENERAL trend however was the longer the tube the smoother and higher the torque (see article posted below), with the concern that too long will begin to reduce top end power. That was seen in some of the dyno runs I've seen but usually only approaching way over 6500-7000 rpm where most of our engines are not spending a bunch of time. I do hope to experiment between the two stacks I will have when I'm on the dyno. My plan (at this moment) is to tune with the longer stack as they tend to require a little reduction in ignition timing down low due to better cylinder filling, and my main goal is to finalize my timing map with a "safe" curve for this high compression engine.

[drieseck]

Good stuff! Much appreciated.

[GeoffM]

Thanks for posting this. The concept of "how close is too close" is something I often talk to civil engineers about when they're designing a sump, and how close to the bottom their intake pipe bell or pump suction bell sits off the floor. They're very similar shaped as a velocity stack and are there for the same reason...smoothing out the transition in an orifice flow, preventing barrier separation of the flow from the sidewall of the hole.
Hydraulic Institute, the folks who publish pump design standards, states that the minimum clearance between a suction bell and the floor of a sump is .3D. Thats basically 1/3 of the bell diameter. Anything closer, and you START to affect the velocity entering the bell, thereby reducing pressure in the pipe. So using this standard, with a 3" Velocity stack (75mm), you'd want a minimum of 1" to be safe. This is just a rule of thumb and conservative...there's lots of other things that can go into it.
Air and water work on the same physics principals, just one is compressible and one isnt.

[Gregs672000]

Thanks Geoff. I measured the distance between the stack for #1 and then the inner fender (the closest), measured the thickness of my foam filter and decided the largest stack that will still allow space for the filter to stay off the mouth etc was about 3". Bummer as I'd love to grad some of the torque that's there with a larger stack. Don't know if I'll be able to test this on the dyno given you're charged by the hour, but if I can I will run both sets and see what, if anything, it shows for our engines. I gather there are a number of factors at play, especially as the cam duration/overlap increases and how the exhaust interacts... they talk about pulse waves etc... exhaust primary and main pipe size, mufflers and resonators, and merge collection all factor in and can cause dips and peaks in the torque curve. It's all a SYSTEM, and the only way to really know is to test. That's why I go searching for these kinds or articles and value our collective experience.

[Gregs672000]

Here's another 4a-ge based test, stock motor... same result... "run the longest stack you can...".
https://m.youtube.com/watch?v=oddW4Ecy8m8

This was followed by an exhaust header and main pipe change... that resulted in a a considerable loss of power...
https://m.youtube.com/watch?v=rK9Cc8EvDWA

One sees why a dyno and objective data are really important. What sounds and looks like a good idea isn't necessarily so...

[Gregs672000]

My new stacks arrived today from the EU... Carbparts.EU. Didn't take very long (about 2 weeks?) and their communication was excellent, informing me when they shipped, when they arrived in the US, and when they were out for delivery. Quality is great, fit is perfect though as with all stacks one must look down the throats and make sure they line up perfectly as there's some adjustment afforded by the mounting holes. Other stacks I saw on the internet were for "40-45mm" which would leave a considerable lip between the stack and 40mm TB throat; these are for 40mm only. I have plenty of space in front of the stack and they do not force my filters to be up against the mouth. With the weather it may be a bit before I drive the car and I don't expect some giant difference, but we'll see what changes if any are noted in what the engine wants for for fuel. Some have also found a need to retune/refine their ignition timing a bit due to better filling of the cylinder at different rpms. BTW, I tried rotating etc all pics but they STILL came out upside down until you select them... I remain technically challenged