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Author Topic: Effect of removing weber air stacks?  (Read 1210 times)
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QikNip
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« on: September 24, 2018, 07:22:01 PM »

I have two DCO 50 Weber (two barrel) carburetors on my G/CPRO naturally aspirated engine and plan to install a closed system hood scoop feeding air into an air box that will enclose both carburetor throats. I've researched the duct size and it looks as though a round tube of 3-4" in diameter will be sufficient and an air box of approximately 5X that diameter's area will be to able to adequately slow / quiet the air.
My problem (aside from being way over my head in the science of all this) is a very tight engine compartment that severely limits the size of that air box. To gain space for it, I'm considering the removal of the four air stacks from the Webers. My question is whether or not that might have a deleterious effect on the air flow into the throats. (My worry is that the air stacks are intended to prevent the side-by-side throats from interfering with one another. Any thoughts on this one? huh
Rick
« Last Edit: September 25, 2018, 02:15:06 PM by QikNip » Logged

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« Reply #1 on: September 24, 2018, 09:14:16 PM »

Removing the Weber stacks could reduce the air flow through the carbs by as much as 15%. The Weber stacks are especially efficient at getting the air to smoothly flow into the carb. Just take one off and look at the ugly hole that is left. If you are really close for space shorten them or if they are late model Webers then they are the bolt on air horns and there are some after market horns that are very short.

Rex
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« Reply #2 on: September 25, 2018, 12:16:10 AM »

They are called velocity stacks for a good reason so you don't want to lose them. If you can't come straight out with them, do you you have enough room to make them 90 degree bends & then do the air box thing?
  Sid.
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QikNip
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« Reply #3 on: September 25, 2018, 07:18:09 AM »

Sid/Rex:
Thanks for the replies. They pretty much confirms what I'd suspected. As for leaving the current stacks in place, the air box's outer wall would be approximately 2" from the top of the stacks. I fear that close proximity would restrict air flow and undo everything I'm trying to accomplish. I'll look into shorter stacks, but my concern there is that Weber engineers probably knew what they were doing with the height they chose.

Anyone with any additional knowledge on the stack height issue, please chime in!
Rick
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jacksoni
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« Reply #4 on: September 25, 2018, 07:45:36 AM »

In addition to the above comments, taking off the stacks will change the harmonic tuning of the pipes which can affect the power and power curve. On my engine a 1 inch difference in stack length moved power curve 500 rpm. Your comment about the engineers is of course appropriate, but other features of the engine and its use affect the required length. And what you want to know is total length stack to valve and the rpm range you run in or want. A shorter length moves tuned curve up generally speaking. And the actual point of tune is approx. 1/2 stack diameter above the physical end of the pipe so if you are pushing the tune take that into consideration with your calculations.

I had similar concerns about the distance from the box wall to the stack and though it is hard to figure without either dyno testing or CFD or someone with lots of experience ( I used the latter mostly) and aimed at at least 1 diameter of the stack from the wall over it. Your 2" is probably adequate. ( see above 1/2 stack diameter comment). Much of the air flowing into the stack is coming from the sides anyway, not straight down.  As I recall airbox volume is recommended as being at least 2x engine displacement. Lots of varied opinions on that one.

Flow steal from adjacent firing cylinders does happen. Not sure how much it may affect things. Would need something like 02 sensors in each pipe, careful plug reading, etc to try to see and then possibly tune each cylinder( if possible) to compensate. I have seen a flat plate inserted between the stacks to try to fix this. Personally probably wouldn't go there without surely proving its necessity (and a power improvement therefrom).
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Jack Iliff
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« Reply #5 on: September 25, 2018, 07:50:29 AM »

Velocity stacks not only provide an optimally smooth path for the air to enter the carburetor, they are also a tuning device for the inlet tract.  By adjusting the length of the stack, the harmonics of the inlet tract can be matched to the desired operating rpm of the engine.  That is why various length horns are available--some pretty short and some rather long.  You definitely do not want to simply delete them. 
A 2 clearance at the end of the stack should not make any noticable difference in performance.
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« Reply #6 on: September 25, 2018, 07:51:10 AM »

Along the removing the stack thoughts and Rex's comment, have heard of folks removing the air cleaner from a 4 barrel on whatever engine they had, thinking the cleaner must be a restriction to flow. Generally slowed down. Then put the base plate back on but without the filter and top and things pick back up. Flow into the carb was smoothed out. Actually there is an Engine Masters (Hot rod/David Freiburger videos about various testing things on a dyno) video testing various filters and stacks on a carb. It might be interesting to look at. Its on YouTube.
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Jack Iliff
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« Reply #7 on: September 25, 2018, 09:18:51 AM »

Sometimes Most times little things have consequences - intended or not!  shocked


* Carb Inlet Tests.jpg (200.32 KB, 1219x940 - viewed 115 times.)
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« Reply #8 on: September 25, 2018, 10:41:09 AM »


Removing the Weber stacks could reduce the air flow through the carbs by as much as 15%. The Weber stacks are especially efficient at getting the air to smoothly flow into the carb. Just take one off and look at the ugly hole that is left. If you are really close for space shorten them or if they are late model Webers then they are the bolt on air horns and there are some after market horns that are very short.

Rex


I do a LOT of Weber flow testing.    15% is too much, BUT, expect to lose a solid 6/8% of FLOW.   AND, turbulent air in the carb may cost more in terms of hp.


In addition to the above comments, taking off the stacks will change the harmonic tuning of the pipes which can affect the power and power curve. On my engine a 1 inch difference in stack length moved power curve 500 rpm. Your comment about the engineers is of course appropriate, but other features of the engine and its use affect the required length. And what you want to know is total length stack to valve and the rpm range you run in or want. A shorter length moves tuned curve up generally speaking. And the actual point of tune is approx. 1/2 stack diameter above the physical end of the pipe so if you are pushing the tune take that into consideration with your calculations.

I had similar concerns about the distance from the box wall to the stack and though it is hard to figure without either dyno testing or CFD or someone with lots of experience ( I used the latter mostly) and aimed at at least 1 diameter of the stack from the wall over it. Your 2" is probably adequate. ( see above 1/2 stack diameter comment). Much of the air flowing into the stack is coming from the sides anyway, not straight down.  As I recall airbox volume is recommended as being at least 2x engine displacement. Lots of varied opinions on that one.

Flow steal from adjacent firing cylinders does happen. Not sure how much it may affect things. Would need something like 02 sensors in each pipe, careful plug reading, etc to try to see and then possibly tune each cylinder( if possible) to compensate. I have seen a flat plate inserted between the stacks to try to fix this. Personally probably wouldn't go there without surely proving its necessity (and a power improvement therefrom).


Weber engineers provided differing stack lengths for the reason of tuning.

My experience (ymmv) is that there is little "charge robbing" in independent runner systems, as long as the distance between stacks is 1 x the stack diameter, or more.


Velocity stacks not only provide an optimally smooth path for the air to enter the carburetor, they are also a tuning device for the inlet tract.  By adjusting the length of the stack, the harmonics of the inlet tract can be matched to the desired operating rpm of the engine.  That is why various length horns are available--some pretty short and some rather long.  You definitely do not want to simply delete them. 
A 2 clearance at the end of the stack should not make any noticable difference in performance.


Absolutely YES to all of the above!    Again clearance should be 1 x diameter MINIMUM.   1.5 x d,  or 2 x d is better, but space is always at a premium . . . . .


Sometimes Most times little things have consequences - intended or not!  shocked


Does paying attention to all the "little details" matter?   I dunno, but I think you should ask the guy who finished second . . . . . rolleyes

 cheers  cheers  cheers
Fordboy
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« Reply #9 on: September 25, 2018, 02:13:10 PM »

Gents:
These are all outstanding thoughts that I'll carefully consider as I proceed and I genuinely appreciate them. Given the possibility that it could bomb despite all the valuable input here, it strikes me that I'll need to fabricate a system that can be removed in its entirely easily - just in case.  cheers
Rick
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« Reply #10 on: September 26, 2018, 05:11:31 AM »

Gents:
Given the possibility that it could bomb despite all the valuable input here, .....
Rick
I am writing the 'excuses' manual in parallel to the build.  wink
John
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« Reply #11 on: September 26, 2018, 07:50:52 AM »

Gents:
Given the possibility that it could bomb despite all the valuable input here, .....
Rick
I am writing the 'excuses' manual in parallel to the build.  wink
John

John:
Brilliant idea. We probably all need to author one! cheers
Rick
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« Reply #12 on: September 30, 2018, 10:38:23 AM »

you might be interested in this discussion (from 2015 but recently revisited bringing it back to my mymind): http://speedtalk.com/forum/viewtopic.php?f=15&t=44252
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Jack Iliff
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« Reply #13 on: September 30, 2018, 11:19:28 AM »

Also, there are a lot of charts available about intake length tuning vs various factors. This is one based on  RPM and is what I had in mind when posting a couple days ago:



* intake_tuning_lengths_chart_477.jpg (55.85 KB, 800x542 - viewed 74 times.)
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Jack Iliff
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