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Fri Nov 28, 2014 6:48 am

My webers can with velocity stacks. Is it ok to put the air filter around the velocity stack or are velocity stacks intended to be used without a filter?

Fri Nov 28, 2014 7:20 am

YES Use the short stacks,or just make sure u have some clearance between the filter tops/lids :wink:
They will give you better throttle responce :wink:

Fri Nov 28, 2014 9:50 am

I've always wondered about this too.. I've run dual 40IDFs since 2002, and the shop that built the engine back then, left the stacks off..

I put short stacks on a couple of years ago, but i've always been concerned because there's only maybe 1/2 an inch clearance between the top of the stack and the air filter top.. is that enough space?

Fri Nov 28, 2014 12:41 pm

1" clearance to filter top can be enough for adequate airflow, with more radical cams I like to have at least 2" clearance. (so reversion cloud is not upset)

Fri Nov 28, 2014 2:50 pm

tattooed_pariah wrote: I've always wondered about this too.. I've run dual 40IDFs since 2002, and the shop that built the engine back then, left the stacks off..

I put short stacks on a couple of years ago, but i've always been concerned because there's only maybe 1/2 an inch clearance between the top of the stack and the air filter top.. is that enough space?

For a 40mm throat intake the area is PI()*R^2 1260mm^2

All you need is an equal or greater area between the end of the stack and the top of the filter. Think of the wall area of a cylinder: PI()*d*h

For 40mm circ = 125.6mm to get 1260mm^2 area you need 10mm (3/8")

That is the minimum to maintain equal flow.

My combo is 1 5/8" stacks inside 2 1/2" filters. For 7/8" clearance above the stack intake. (22.225mm) This area is about 2800mm^2 and over double the area of a 40mm open pipe.

No worries at all.

For your kid's science project get the vacuum cleaner, a measuring tape, and a good AC ammeter. Turn the vacuum on and read the ammeter to get open pipe free flow power. Block the intake with a flat plate and read the power used. Now back the intake off the wall and see how far away the wall has an effect on the vacuum. The distacne from the wall where there is no change in flow is the same ratio as for intake stacks inside air cleaners or close to fender wells, hoods, or firewalls. You don't need much.

Sat Nov 29, 2014 2:37 am

Actually, 40IDFs don't come with velocity stacks, you have to purchase them separately. 44IDFs do.

Also, as the point of velocity stacks is to smooth airflow into the carb, you need a bit more than 10mm - the more, the better.

Sat Nov 29, 2014 6:31 am

Quokka42 wrote: Actually, 40IDFs don't come with velocity stacks, you have to purchase them separately. 44IDFs do.

Also, as the point of velocity stacks is to smooth airflow into the carb, you need a bit more than 10mm - the more, the better.

Not always about size, you'd think it was, was isn't always the case, I'll post up a bit of R&D later :)

Sat Nov 29, 2014 6:35 am

http://www.shoptalkforums.com/viewtopic.php?f=15&t=122047

Sat Nov 29, 2014 6:56 am

Are there stacks available that fit 36-40mm carbs properly? Not a 48mm stack on a 36mm carb...

Edit: never mind, I see AC.net has stacks for 36-40mm

Sat Nov 29, 2014 6:19 pm

The "top bore" of 36/40 idf/drla carbs is 38mm, but sometimes the lids are tapered out to 40mm+

similarly with larger 40,44,45IDF/drla the top bore is 42mm, but some lids are tapered to 44mm+

So there are 4 sizes??? apparently so.
And some are up to 7 degree taper and some are straight
and some are tall and some are short.

So there are.........about 30 different choices

Sun Nov 30, 2014 11:32 am

yes they need filters, yes they need more clearance,yes they need a betterflowing aircleaner top. yes the stacks need to be shaped differently so you can change jets and so they dont eat at the filter.yes the bottom,s need to be different so any water(h2o) that finds it's way to the filter dosent get funneled in the carbs but can drain out the outside via lip inside and a smaller one outside the filter to hold it in place. this would be easy to do with biller housing and a cnc programer with somebrains,knolage,forsight,no engineering skills. and or pressure cast in either thermoplastic or aluminum,stainless,guakmolie dip. the stacks can also be angled inward so they have better clearance with the small 4x7 filters and "C" shaped for good flow and jet axsessability.

Mon Dec 01, 2014 10:58 am

Quote: For your kid's science project get the vacuum cleaner, a measuring tape, and a good AC ammeter. Turn the vacuum on and read the ammeter to get open pipe free flow power. Block the intake with a flat plate and read the power used. Now back the intake off the wall and see how far away the wall has an effect on the vacuum. The distacne from the wall where there is no change in flow is the same ratio as for intake stacks inside air cleaners or close to fender wells, hoods, or firewalls. You don't need much.

here is a skill testing question for you.

I built this intake for my 1904cc with 36dells since I switched exhaust the engine falls on it's face when we get above 4000 rpm, it is running out of air.
So what would be the optimal size or shape of tubing?

it has 2.5" oval tubing feeding both carbs, and the stock 2" oval air inlet plus a scoop under the car with a 2.5" tube feeding the other side of the air cleaner.

Air cleaner too small? air intake too small? tubes to the carbs too small?
I basically went with twice as much space as the stock type 3 oil bath was using for the Solex carbs.

Mon Dec 01, 2014 11:11 am

W1K1 wrote: I built this intake for my 1904cc with 36dells since I switched exhaust the engine falls on it's face when we get above 4000 rpm, it is running out of air.
Is it running out of air or fuel? How did you come to your conclusion?

Mon Dec 01, 2014 11:19 am

W1K1 wrote: Quote: For your kid's science project get the vacuum cleaner, a measuring tape, and a good AC ammeter. Turn the vacuum on and read the ammeter to get open pipe free flow power. Block the intake with a flat plate and read the power used. Now back the intake off the wall and see how far away the wall has an effect on the vacuum. The distacne from the wall where there is no change in flow is the same ratio as for intake stacks inside air cleaners or close to fender wells, hoods, or firewalls. You don't need much.

here is a skill testing question for you.

I built this intake for my 1904cc with 36dells since I switched exhaust the engine falls on it's face when we get above 4000 rpm, it is running out of air.
So what would be the optimal size or shape of tubing?

it has 2.5" oval tubing feeding both carbs, and the stock 2" oval air inlet plus a scoop under the car with a 2.5" tube feeding the other side of the air cleaner.

Air cleaner too small? air intake too small? tubes to the carbs too small?
I basically went with twice as much space as the stock type 3 oil bath was using for the Solex carbs.

W1K1, Got another one of those handy dandy systems laying around? Im doing a FI to carb type 3 conversion with Dell 36s the weekend after this coming...Snappy looking! What size stacks are you using?
PS. I think you gave me details the last time I asked but how/where does the left and right air intake off the aluminum fan housing discharge to? Ill need to route some hoses for him to the right places. :D

Mon Dec 01, 2014 12:19 pm

That looks good. I was going to ad another fresh air outlet on the left side of the engine bay to feed my 42 DCNF's, but plans changed ;)

So, you changed the exhaust, now it doesn't breathe? What was the exhaust before and after?

Mon Dec 01, 2014 1:29 pm

stock heat exchangers and over the top exhaust changed to vintage speed type 3 with 1.1/2" J tubes. I had to rejet to accomodate the better breathing exhaust.

the stacks/trumpets are the short ones from Dellorto UK
http://www.dellorto.co.uk/merchandise/products.asp?CategoryID=16&PartsectionID=41

Quote: Is it running out of air or fuel? How did you come to your conclusion?

It runs fine without the air intake assembly and just the stubby filters on, with the intake assembly on, the LM2 goes super rich when it falls down while accelerating.

Quote: PS. I think you gave me details the last time I asked but how/where does the left and right air intake off the aluminum fan housing discharge to? Ill need to route some hoses for him to the right places.
to the heat exchangers if you have them, otherwise plug/cap them
http://i169.photobucket.com/albums/u236/mr_bojangles500/100_0242.jpg

Mon Dec 01, 2014 3:24 pm

I wonder if those heater port caps on shrouds would work? Ill order some and have a look. Thanks for the tip.

Mon Dec 01, 2014 7:37 pm

W1K1 wrote: Quote: For your kid's science project get the vacuum cleaner, a measuring tape, and a good AC ammeter. Turn the vacuum on and read the ammeter to get open pipe free flow power. Block the intake with a flat plate and read the power used. Now back the intake off the wall and see how far away the wall has an effect on the vacuum. The distacne from the wall where there is no change in flow is the same ratio as for intake stacks inside air cleaners or close to fender wells, hoods, or firewalls. You don't need much.

here is a skill testing question for you.

I built this intake for my 1904cc with 36dells since I switched exhaust the engine falls on it's face when we get above 4000 rpm, it is running out of air.
So what would be the optimal size or shape of tubing?

it has 2.5" oval tubing feeding both carbs, and the stock 2" oval air inlet plus a scoop under the car with a 2.5" tube feeding the other side of the air cleaner.

Air cleaner too small? air intake too small? tubes to the carbs too small?
I basically went with twice as much space as the stock type 3 oil bath was using for the Solex carbs.

Measure vacuum at different points in the system. The vacuum will be higher between the restriction and the engine.

Where is the fresh air inlet? Does it go rich when the car is not moving or only when the car is driven? Could the intake air be in a negative pressure area at speed? Is the fresh air intake configuration stock?

Mon Dec 01, 2014 7:39 pm

Quote:
I wonder if those heater port caps on shrouds would work? Ill order some and have a look. Thanks for the tip.

If you're just plugging them, the rubber pipe cap with gear clamp in the plumbing section of your local hardware store will do the job

Tue Dec 02, 2014 12:03 am

W1K1 wrote: Quote:
I wonder if those heater port caps on shrouds would work? Ill order some and have a look. Thanks for the tip.

If you're just plugging them, the rubber pipe cap with gear clamp in the plumbing section of your local hardware store will do the job

Thats a good suggestion.