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Discussion Starter #1
I'm In The Process Of Installing A GTA Fiberglass Airbox (repop) Onto My AutoX/Track Day Spider (for various reasons but mainly because I think it looks cool!), I Was Planning On Using GTA Curved Stacks With Piper Filter Socks (I already have them on my JrZ and was going to remove and use them). However, As Of Late I've Been Reading That The Curved Stacks Tend To Run Lean At High RPM So I'm Looking For An Alternative. I Want To Run Filters Of Some Sort But Fitment May Be Problematic. Anybody Solve This Problem Using The GTA Airbox?
 

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FWIW aftermarket curved stacks won't work anyway unless you significantly widen the airbox. The originals will however and there seems to be a cyclonic like effect with a lot of dirt particles ending up on the bottom of the airbox.

Filter socks inside the airbox will likely ruin flow.

You can put a lady's stocking in the intake part of the airbox or better use some filter foam at the intake side of the intake snout - not sure what you use for that on a Spider?
 

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Discussion Starter #3
You Are Right About The Fit. Cylinder #4 Stack Hits The Lid (the others clear). I Think I'm Going To Remove The Welded On Metal Snout On The Spider And Use The GTA Front Snorkel (and fab a mounting bracket). I'll Put A Foam Filter Of Some Sort Between The Snorkel & Airbox. I'll Also Install Some Shorter Stacks I Have.
 

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I made the curved stacks fit the GTA box and used it for a while before switching to the short straight stacks. I first ground off the large diameter that would hit the box but that wasn’t enough at first. I then sectioned maybe a half inch and rewelded the flange back onto the stacks. I solved the air filter issue by ordering a square k&n filter that fits in the front snorkel. Don’t have a photo of the complete assembly but here is a photo of the air box with the short stacks.
 

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If you are most concerned about performance then I suggest that you use an AR plenum and canister airbox. A race motor fitted with a modified one made 230 HP on Jim Steck's dyno. I have a modified one on my street motor and I made dyno runs with and without the filter. Details here: https://www.alfabb.com/bb/forums/spider-105-115-series-1966-1994/676728-79-spider-dyno-sequel.html#post8388106
How Was This Plenum Modified? Were No Stacks used At All? I Have An Original Plenum And Canister Airbox That I Previously Modified; Bored The Individual Ports For 45 DCOE's & Chamfered The Plenum Inlet As Well As "Swiss Cheesing" (added many holes) The Internal Air filter Support/Supply Inside The Canister (modified canister "buried" somewhere in the shop). I Would Expect This Setup To Be No Better (or worse) Than The GTA Airbox Without Stacks (although the original setup allows for an easier use of a filter).
 

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Not much modification to the plenum. Holes opened up to match the 45's and radiussed to eliminate sharp edges. The center tube fell out of the canister when I cut off the diverter valve/intake horn. After I welded in a short piece of 3" tube I fitted the UNI filter and I cut a disk of 3/8" plywood to make a snug fit in the rear of the tube. When pushed into place it holds the Uni Filter in the center.
 

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If you have never seen an original one of these boxes you may not know that the repros I have seen are missing a piece that was incorporated into some if not all the original boxes. Not sure of its purpose or necessity but I made and incorporated one into my repro box. It is a FRP piece that shields the holes for what I think are the pressure equalization ports on the carbs.

By the way some of the repros are very good copies dimensionally as I used a repro door to make a pattern in order to lay up a new door missing from an original box and the fit was perfect.

Ciao

Ken
 

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Looking at the second picture of the white repo airbox, it appears there is no radius on the inlets. Large radii, airbox or no airbox, are all important in order to avoid the totally unwanted sharp edge throat turbulence which will occur otherwise.
 

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Very brief lesson on stacks.

Short stacks may improve very high RPM torque, but at the expense of low and mid range value.

Middle length stack are actually short stacks, and mainly improve very high end. Operation at very high RPMs definitely requires special cams and dyno tuning.

Very lonnnnnngggg stacks, and/or long intake runners can help give a nice mid-range (read - usable on the surface of the earch) boost, but it's a rare enclosed engine bay that accommodate them.

You can get near maximum power from your particular street/trackday-use cams and other mods with no ram air scoops at all.

To add.... Short and medium stacks can also cause odd resonances down low/mid range that can disrupt your otherwise good jetting.
 

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Alfa Nord engines typically have a rich dip around 4500 rpm due to reversion. Fuel laden air is pushed back through the intake valve just before it closes. Some of that charge is then pulled back in when the intake valve next opens so it makes a double pass through the carb. Horns collect the charge so there is more available to richen the mixture. The result is a bigger rich dip and resulting torque dip.
 

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Stacks do have those results, discovered that with several of my cars. However, stacks (the open ends of stacks also usually have an air bell shape at the end of them) and air bells by themselves are different in design and effect, air bells acting as super short stacks, if at all, but most importantly smoothing the airflow entering the throat of the carb or FI intake duct. The length of a stack is what is used to ensure the incoming air mass, as it pulses due to the intake valve opening and closing during the cycle, stays well within the stack as a function of the cam profile and timing.

Had a car with more aggressive cam timing and profile, and the installed short stacks didn't have enough length to retain the incoming air/fuel charge, and it would spit out as much air/fuel at low rpms as was taken in depending on the back flow out the intake due to the valve not closing in time as the piston rose, the engine not able to "get on the cam" so to speak. Once stacks of a certain longer length were installed, the engine wouldn't hesitate to rev as it should, no air/fuel discharge back out the intake.
 

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Alfa Nord engines typically have a rich dip around 4500 rpm due to reversion. Fuel laden air is pushed back through the intake valve just before it closes. Some of that charge is then pulled back in when the intake valve next opens so it makes a double pass through the carb. Horns collect the charge so there is more available to richen the mixture. The result is a bigger rich dip and resulting torque dip.
You’re example is one possibility. There are many other versions.

The point is, horns can allow you to get a torque peak within a fairly narrow torque band, which is then maintained via a close ratio box mated to a rear end ratio chosen for the specific race course being used.

Street? Counter productive.
 

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I am repeating what Jim Steck told me. He lengthens the intake runner between the carb and the head for tuning and to get a torque boost without exacerbating the reversion.
The bottom line is that horns are unlikely to improve a well set up Nord motor and are more likely to make it worse..
Take a look at the torque curves in the thread that I linked. That is with Jim's head prep.
 

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I used exactly that airbox :) with full radius 3/4" high stacks that we made in fibreglass.

That was on a race track, on a 7800rpm engine, and it appeared to be the best solution alongside
long intake manifolds. Of course, here, no dyno. Bottom line was winning, and that we did.
Our track, is 2 miles long but narrow, so gobs of torque required, great fun.

Autodelta used their box with curved trumpets and that appeared to be the best. No filter.
My friend in the 'engine development section', said it was a good solution, but rebuilding the motor after every race was too...
After all, it was Italy's govt. money, no?

Regards, Alberto
 

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No horns

Here is a picture of the dyno sheet for a 2L Nord motor with Webers and a modified AR Plenum as described.
 

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Maybe I missed it..... but why would you worry about sock or other filters on a track day car (unless you running on dirt) 3/16 mesh at the intake will stop anything that can acutely harm the top end..
 
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