Velocity Stacks on 40 DCNF's

abarth2litre

abarth2litre

1800CC SOHC X1/9
We purchased a pair horns from pierce manifolds with screens then removed the tops of the carbs, cut and ground off the choke tubes and installed the stacks. we think they look great and they sound louder but... they need readjustment so OFF to Auto Turismo for some fine tuning.
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Wait... I see a rusted nut!

HA!

It is indeed clean... but don't go looking at Peters in Ontario!

The BLING will knock you over... Bob Brown's MODIFIED stocker ain't no slouch either!
 
Dear New...

Ya know... I ain't no expert... but in my old rodding days the stacks and horns all played a part in part of the TUNE and performance of both carbs and injectors... like Hilborns and Enderles... and I would imagine the same for Webers... especially the DCOE types.

Overall RUNNER length from the carb inlet to the valve was generally equalized with the stacks and the horns not only directed the airflow, but had a big impact on the venturi velocity.

All rocket-science...

I wouldn't toss the horns if I were you... and take them to your tuner when ya go...

Looks great otherwise!
 
More rollback on the air horn lips makes them work even better, here's a picture of a genuine weber 40DCNF air horn on the left, and a TWM brand air horn on the right... the TWM is worth a few % more flow



SteveC
 
Where might a person find such horn bling?

I like the fully rolled over edges, where do you find them? Also Pierce manifold in Fresno Ca. sells wire mesh filters with rolled edges that attach to standard sharp edged horns. Might give similar results to the trick rolled edge horn you show in the right hand photo.
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My 6" Chrome Velocity Stacks...

I ran with K&N filters for years but last year I purcased these 6" velocity stacks and screens on Ebay for my DCOE's and I absolutely love them as they not only look good but the added length of the velocity stack helps with low end power.





'PeteX1/9
 
Very possible TWM dis-continued them years ago. TWM also made a nice injector throttle body in the DCOE form, but shorter. These were complete with fuel rails and etc. Expensive set up and nicely done.

I'll continue to suggest not using only the wire mesh screens as the only air filtration. This is an invitation for bad stuff getting sucked into the engine.

Make a proper air box and there will be further gains in air flow.

Bernice


karhawk said:
http://www.twminduction.com/AirHorn/AirHorn-FR.html
Click to expand...
 
For the DCNF type air horns you need to look in the throttle bodies section, strangely TWM don't list them in the carbs section?? They are not cheap, but they work the best of all types I've tried.

3004 throttle body is the DCNF clone

http://www.twminduction.com/AirHorn/AirHorn-FR.html

Mesh screens are rubbish for keeping out the fine particles that actually ruin your ring seal... and they are rubbish from a flow perspective too...

they completely destroy the effectiveness of the roll lip in smoothing out the air flow entry, as well as addding considerable flow resistance, I would never use them...

air boxes of sufficient volume and head room above the air horns is definitetly the way to more HP.

SteveC
 
V stacks screened and tuned

OK, lots of interesting commentary ..Bernice never disappoints! So first the tuning, the boyz at A.T.S. did the job and she's running much better then before, we now have power from the JUMP! The second that you crack the throttle the power comes on. We still have some coughing and sneezing which we will address in the winter. But there is a marked difference in the power from 1,500 and up. Dennis at Auto Turismo made a gasket for between the stack and the carb and we are very very happy. Sound? well put it this way, carrying on a conversation while driving is not an option. it is seriously louder then before. OK now to the debate screens or no screens while the screens don't stop the fine partials they would stop the proverbial leaf that could find it's way in?? The fine dirt the Rapunzel mentions ? Well this is a show car and street racer that spends a majority of its life above 4,000 rpm or on the trailer, so when it needs rebuilding ...it needs rebuilding. the screens seem to fit extremely well around and under the stack lips so only the mesh covers the opening . The guyz at Pierce did a good job As far as a air box for a couple of more HP?? Nah we are very pleased the way it is and our head designer stressed to keep the focus on torque over high HP so he ported it leaning towards low end, shifting, to high end. The look? if you could see the crowds around the engine bay at the shows, nuff said! So we want to keep it race lookin. Well!? that means screens or no screens. Again, we feel no difference in the power with or without screens. So what do you guyz and girlz (Bernice) think ??
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TWM still makes them :excited:

Steve's suggestion on a proper air box is correct, Every serious race engine has a properly designed air box and air cleaner assembly that is integrated into the design of the engine intake system and chassis's body work.

While the wire screens might look good, they are still IMO a relic from an entirely different era and should not be used alone.

As for cylinder head flow, there is a lot of mis-guided belief that more flow is always better. It's not true, air flow is always a trade off between total air flow -vs- flow velocity at lower flow rates. If the air flow velocity is low, torque at lower and mid rpms will suffer which seriously affects drive-ability. Or, bigger ports are not always better for a given application.

Velocity stacks work by reducing the abrupt transition between open air and a restricted air space. This is one of the reasons why a radii helps the transition and a larger radii is better than a smaller one.

Bernice

fiatfactory said:
For the DCNF type air horns you need to look in the throttle bodies section, strangely TWM don't list them in the carbs section?? They are not cheap, but they work the best of all types I've tried.

3004 throttle body is the DCNF clone

http://www.twminduction.com/AirHorn/AirHorn-FR.html

Mesh screens are rubbish for keeping out the fine particles that actually ruin your ring seal... and they are rubbish from a flow perspective too...

they completely destroy the effectiveness of the roll lip in smoothing out the air flow entry, as well as addding considerable flow resistance, I would never use them...

air boxes of sufficient volume and head room above the air horns is definitetly the way to more HP.

SteveC
Click to expand...
 
If the car spends a lot of the time at shows with the engine lid up, then the mesh screens are actually good insurance to stop some jealous sh#t dropping something down a carb when your back is turned...

Years ago my Dad owned a V8 alfa race car which had 8 lovely curved trumpets that stuck out of the engine cover... but when the car came to the pits, a set of plugs (kept together by nice chrome links) went straight into the trumpets...they were actually 50mm plumbing plugs!

Honestly the mesh screen are a serious impediment to flow... for a start the mesh wire removes open surface area from the air horn mouth...so basically you've added a restrictor to the air horn mouth... and secondly they disrupt the cyclonic (vortex) type of flow that you get at the air horn mouth (with full radii lips) which assists the smooth intake of air into the air horn mouth.

Remember back to high school... two identical bottles filled with liquid. Invert both of them at the same time but to one you give a slight "swirl" to assist the formation of a vortex at the bottle mouth. Of course the one with the vortex empties considerably faster.

The same principle applies at the mouth of an air horn.. we are doing all we can to encourage the formation of a vortex to allow air the enter the air horn mouth as smoothly and effectively as possible...

No gimicks or tricks, just simple physics.

SteveC
 
"No gimicks or tricks, just simple physics." - Agreed!

I started out with the standard single filter elements and found that the car just ran out of breath on the top end. Played with it for awhile and suspected that it did not get enough air. So I built an air box. The one shown is the second of 2 I built. It made a world of difference no more problems at all it will now keep pulling well beyond the tach's limit. A nice side effect is the sound is still there but not as loud. Sounds real nice without being annoying after awhile.

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I also initially made a set of air horns but instead just went with the Weber parts. I found the ones I made were just a bit too tall for the air box I ended up with and I did not want to impede the airflow. One other thing that hasn't been mentioned is the air horns also trap the stand-off helping throttle response. Any opinions on how much more could be gained by using the (Shorter) machined versions vs. the cast Weber versions?

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I dunno bout that air box

seems to restrictive on top clearance. the problem I was having with the air cleaners was the distance between the choke tubes and the flat solid top of the air cleaner was to tight that box seems the same to me. The guy who made my head showed me a pic of the 45 degree angle V stacks that faced the rear of the car now a box around them would work. Sorry but that sound is never annoying for me. When I want to unwind I listen to Zep or the sound of my X.
 
The clearance to the top of the air box is within the recommended requirement from a well known builder. I also did a CFD analysis to verify adequate flow. This is why I changed and went with the Weber parts instead of the ones I had made. More clearance would likely be better but I have the usable space maximized and definitely have no issues with the clearance. The "choke tubes" I believe you are referencing are Auxiliary Venturi's. They are what determines when the transition from the idle circuit to the main occurs. I got rid of the longer ones (2.5)and went with 4.5's way before I went with the air box. Not because of the clearance but to improve the transition point to better match the engine's characteristics. However if these were too close to the top of the air box or filter there could be drivability issues. It would likely make it almost impossible for the transition to occur.
 
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