The airhorns slip through the airbox back and into the carbs.
The air horn tabs hold the airbox back on and retain the air horns.
However (on mine) the tabs would work perfectly if there air box were not there. With the airbox there, the tabs do not engage the air horns because the ring on the air horns is just slightly proud of the carburetor mating face and thus the airbox thickness separates the tab and the air horn ring allowing the air horns to rattle around.
Does anyone have a solution for this? I’ve got some ideas, but all of them seem like a lot of work for what has to be a simple problem that I’m sure isn’t new.
Kyle,
I have 152’s, with the early style metal air box back plate. Just put the engine back in the Plus2 on Friday. My tabs lock the air horns to the carbs as designed. A couple of questions, what gasket are you using between the carbs and the air box, and what air horns are you using? Normal formed metal air horns, or those turned out of aluminum? About the only change I make, is using nyloc style nuts to secure the tabs, rather than the normal nut and lock washer.
The ring around the air horn is barely proud of the mounting flange on the carb. So the tabs would need to either stick down into concentric slot between the airbox hole and the airhorn, or the airhorn ring needs to be taller.
My airbox back is fiberglass, so it’s thicker than a metal one. But this would still be an issue with a metal one, unless the metal were less than about .020" thick. I’m using paper gaskets which are quite thin.
I actually added o rings between my horns and carb (40 DCOE 151) body to ensure fuel couldn’t seep into the airbox. This causes the horn to protude further. I am not sure if 45 DCOE 152 are also prone to fuel seepage into the airbox (I think the screw fastenings for the venturi might avoid the issue) but they could be used to make the horns stand more proud and ensure no fuel seepage.
Kyle,
Your fiberglass back box is too thick compared to the metal back box. The only problem I see with the O ring suggestion is, it will space the air horns away from the auxiliary venturi. Try making a space to go between the tab and the ring on the air horn. With the grub screws on the 45’s the aux is pressed tight agains the fuel transfer port, which almost removes all fuel leaks at that point. 40’s do not have the grub screw, just a very weak spring that does not press the aux tight enough to prevent leaks. Fixing that problem is another story.
The smallest section oring I can find is 1/16. This is too thick to fit in the concentric ring open between the airhorn and the airbox and too thin to make up the gap height wise. I’d need a asymetrical square section oring. lol. Even McMaster doesnt have that! I could enlarge the holes in the airbox to allow the 1/16 section and stack two orings.
One thought is to make new tabs by turning a 3" piece of steel then cutting pie wedges out of it. I’d make them have a 1/8" deep inner lip to reach in to the air horn. Lots of work! But making chips is fun.
My other idea is to extend the rings on the air horns up. Welding seemed the best way. But knowing that I will then need to machine them, I figured brazing would be easier on the setup. Again, lots of work.
Knowing that anything that fails will end up injested by the motor, I will likely just punt and buy a set of Jenvey 60mm tall flanged airhorns. I have 40mm and 120mm on the shelf already from prior race projects, so I’m familiar with the pain associated with the cost! lol
Hard to imagine that I’m the first one to run into this issue.
May be of help,
Air Box metal thickness with thin layer of paint 1.3mm
Air Horn flange 4mm.
Air Horn rim protrusion through gasket, airbox approximately 1.5mm
I made up ?o? rings using 1.6mm Vitron round section, this fits nicely in the grove space, this size allows the air Horn to butt against the venture under slight compression.
This is all for 40 DCOE 18?s Series 3 Elan.
FWIW
If you go with flanged air horns, and you like to make chips, make a sleeve that replaces the insert portion of the original style horns. This would replace the needed stack up supplying pressure against the aux venturi. Even more expensive, would be to buy the extended aux venturi like used in the 40’s on some Alfas that did not use an air horn. The sleeves would also smooth the incoming air to the aux venturi. Your flanged air horns should have, at a minimum, a 8mm radius at the incoming end to smooth the air. You also need to have a minimum of 60mm distance from the end of the air horns to the wall of the outer air box cover. Anything less will screw up the air flow. This is especially true with the tapered air box covers.
Why do you think an insert is needed with flange style air horns in a 45 - 152? The aux/booster venturi is retained by a set screw-ish thing (pointed end).
I’d think all it’s doing is reducing the diameter of the inlet side of the car by .030"ish? Maybe that change in diameter would cause some turbulent flow. Might be worth something on a dyno that reads to the .1hp! No sure which way though! lol
Just a secondary measure of retaining the aux venturi. Sort of like having multiple return springs on the carbs. What is the bore diameter of the Jenvey flanged air horns? Same as the original sleeved units or the actual diameter of the carb bore? Fuel injection envy creeping in.
I ended up finding a set of Jenvey 60mm long flanged airhorns on fleaBay. They should be here tomorrow.
I tried mocking them up using other Jenvey airhorns I have and it looks like the airbox I have has to go too. lol
The old airbox is tapered and the #4 airhorn is very close to the outside wall (<1/4"). So it had to go either way. Also the old airbox isnt very tall and wont accommodate the larger bells on the Jenvey airhorns. The carb flange side of it was also warped and the seal sucked. Other than that, it’s awesome! lol
I’m back to the usual of “make it myself”. I’ve built airboxes from composite before. But with the limited space on the lower side, there is no room for the usual flange. Making one from composite that slips together in the middle will be too much effort for a one time thing.
My plan is fabricate the new airbox from TIG welded .060" 5052 Aluminum. I sheared and bent the box pieces on the brake last night. I rolled a tube for the 3.5" hose used. I will TIG it all together over the next couple of days. I’m basically copying the Kelvedon design with a removable top.
Should be fun. How do I get myself into these things? lol.
You may already be well aware, but along with the inlet tract on the head, the ram pipes / horns form a resonant system that helps / hinders airflow at certain engine speeds. Changing the length of the pipes will move the revs at which the engine generates maximum torque. A helpful tuning aid - if that’s what you intended.
This topic had me thoroughly confused for several messages until I realised that in America you are using the name air horn for something completely different than my knowledge.
In the UK an air horn is typically a device with a compressor and horn(s) intended to make a loud noise when you press the button at the centre of the steering wheel.
Whereas the tubes that feed air into a carburettor (which is what you are talking about) are usually called air trumpets in the UK
Just as a ‘for what its worth’ airflow into a carb trumpet is a complex thing.
Vizard did some studies, and a significant amount of air comes from behind the trumpet and over the lip. Equally the clearance from the trumpet mouth to the facing airbox wall is important. There are a number of threads on the subject, but the advice was to leave at least 1.6 inches from the mouth of the trumpet to the facing airbox wall.
Given Vizard’s work, it also looks a good idea to leave a decent clearance above and below the trumpet to allow air to flow from behind.
I’ve read Vizard’s books going back 30 years. I’ve also made quite a few airboxs before.
I left as much space as could be afforded. The ends of the airhorns are 1.5" from the opposing wall. I’ve worked on 500hp 3.5l NA motors were there was less than 1" from the opposing wall. I’m not saying that was optimal, but reality is what it is. That car won a lot of championships.
As you know, in an Elan there is precious little space under the airhorns. So that facet of the bell is necessarily close to the floor of the airbox. The rear facet of the #4 is also close to the rear wall. I could have shaped the box differently to get more space there, but it wasn’t worth it. These are very large diameter bells and thus likely less affected by the proximity of the walls than a stock Weber airhorn.
The old setup with the tapered (front to back and top to bottom) Elan airbox had the #4 airhorn less than 1/4" from the lower rear corner of the airbox. With that setup this car could spin the tires in all four gears.
This is a massive improvement and will give 99.9% of the available power from airbox shape.
