Datsun Roadster Forum-311s.org

Greetings, thought I'd post regarding the dyno results from Garage 4age on YouTube where they tested how much space you need to have in front of the carb (in this case velocity stack) opening before you choke things off (Intake runner clearance - dyno tested) Knowing this makes a difference if you're adding stacks or aftermarket filters on your SU or sidedraft (Mikuni, Weber, Dellorto) carbs (stacks a very good idea BTW), as there's not a lot of space. Turns out, you don't really need much room... he got all the way down to 20mm before he started to see changes, and even 10mm was not a huge difference and mainly at high rpm... 3mm killed it. He has a very nice set up with stacks. This result may help some of you as you're looking at filter set ups. I've also read that anything that touches or interferes with the edges of the stack is very disruptive for airflow, so unfortunately those slip on screen filters that attach to the stacks are probably not good, though I have no objective dyno data I can point to.

Sorry, couldn't seem to post the link, but you can search YouTube.

Hi Greg,...there are a bunch of videos on that channel, regarding stacks & dynos.....would you happen to have the YouTube link you're referring to ?
Thanks, Mike

Thanks for posting Gregg
very informative

exceptional, thanks (Garage 4 ) and Greg for posting & continued work and tracking of these details, much appreciated.

Good point Greg, I’ve experimented over the years and my final setup is a stock SU baseplate, minus the oem stub stack and a hogged out cover with only the mounting bridge left on it. Cheapo Velocity stacks 2.75 in great for low/mid range and sufficient top. Because the stacks are wider 2.75 I cut a rain gutter mesh to 3.5 in width and wrapped with my wife’s retired nylons for now. Yeah, it’s not going to keep dust and crap out down to microns but it’s better than just open stacks and there is no restrictions in breathing! Sounds better too. Stock filters are so so restrictive!

This I what I ended up doing for my Dellortos. For the EFI I just use similar looking dual foam socks on the TB stacks now as they don't have the same openings as a carb. Not sure this would work on a Mikuni due to how the plate bolts to the carb (the upper "bolt" hole is not threaded on a Dellorto). This set up allowed me to remove/install the filter and stacks as a unit to overcome the space limitations between the inner fender and the carbs when using stacks. As usual, the pics are right side up when selected...

Still got these carbs along with many extras BTW, in case anyone's interested... Anybody own a Lotus?

Video suggested they were using the wrong stacks…

And of course I have to throw one in there that makes everyone go, "EEEwwwww".
I read that full radius velocity stacks had a benefit (though that may be only a Harley thing)

So I kluged my version of that + the blow-by re-routing hose ......

rwmann wrote: Tue Mar 11, 2025 7:55 am Video suggested they were using the wrong stacks…

Not sure what you mean? One of the things I admire about these guys is that they systematically test everything and try to identify confounding factors in their experiments. You can be sure the set up they have is most likely the best for that engine or what they want it to do and where. Believe me, they have tried so many stacks (including size, shape, "vortex" modifiers, length, angle), so many cam timing changes, ignition timing changes, injector positions, exhaust headers and complete systems... you name it (and even noting oil temps as a factor to control in their testing). These guys do what I wish I could do and would do if I had a dyno. I'm impressed by their experimental method, repeated testing to confirm results and just reporting the results while not trying to sell you their product (though you can buy t-shirts!).

I really value this kind of site. For example, try to find any objective DYNO data on something like adding an anti-reversion mod to the exhaust... I think it may be a great idea, but both Daryl and I have not been able to find any dyno data, especially for 4 cylinder engines like ours. Daryl sent me a good article on merge collectors that WAS tested on the dyno before and after, but of course for a highly developed V8 (worked good BTW). Garage 4age tests on 4 cylinder NA engines in 1600-2000cc displacement. Yes, they have a 4 valve head and dual cams and can independently advance or retard them, but they only make one change at a time, run it and post the whole dyno graph so you can see where and how much things change. They also answer questions and do tests based on viewer input and "is it myth or is it real"... like "You MUST have xyz space in front of your velocity stack...".

I should probably buy a t-shirt!

Great stuff... I received new piper cross filters about 2 months ago, but have been side-tracked with a different roadster project this year.... I still need to get them mounted up in a way to get them over the stacks due to space limitations.

FergO2k wrote: Tue Mar 11, 2025 1:52 pm I read that full radius velocity stacks had a benefit (though that may be only a Harley thing)

They do, they allow air to be pulled from behind them, adding volume and smoothness. The main thing velocity stacks/air horns/trumpets is is to straighten the air. Flow into the carb is to be smooth, and linear (until it hits the choke). The are stacks out there that are too short to do much. Disrupted air isn't great. It gets more complicated when driving, how the air flows in the engine compartment, disrupts air and air crossing the stack isn't great either.

There are good ones that aid performance, and others, that don't do much.

Gregs672000 wrote: Mon Mar 10, 2025 8:32 pm As usual, the pics are right side up when selected...

I find that before I attach a pic, open it on your computer and rotate it 90 degrees. Close and re-open the pic and rotate it to the correct orientation and close. Then upload.. works every time for me.

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.