The 'Maverick' Classic Porsche Forum and Classic Porsche Club for Porsche 356, Aircooled Porsche 911, Porsche 912, Porsche 914 & Porsche Replicas. 'We join for the cars but stay for the people'
Who runs velocity stacks on their 40/44/45/48's etc and why?
I have seen this fitted to quite a few cars and wanted to know of what benefit it gives. I know running them "open " increases the chance of crap being drawn in and gives off alot more induction roar. So why do it apart from the fact it looks the biz?
I had these on a GP buggy running an 1835 and 40's and made up some mesh covers for the tops and didn't have any issues , in any weather. Actually liked the extra induction noise but then thats me. As i never put K&N's on i couldn't tell if there was a difference.
Quite complicated if you really get involved with the design of intake lengths / velocity stack lengths.... have a read of this thread I did a quick google for:
I run velocity stacks that fit under the filters on my Dellorto DRLA 40's, its basically a 914 engine. About 50mm in height, supposed to reduce turbulence of the air entering the carb or summat like.
Nige wrote:I run velocity stacks that fit under the filters on my Dellorto DRLA 40's, its basically a 914 engine. About 50mm in height, supposed to reduce turbulence of the air entering the carb or summat like.
I seem to have a similar setup in my 914 - certainly have oiled-filters on short stacks on dells, but lengths and sizes? Dunno - I leave it alone if it works, and it all makes a good growly induction noise from about 75% throttle upwards.
I hope the following makes sense! your carburettors have a restriction in the carb throat (venturi). A 40mm carb has an inlet of 40mm, but the venturi reduces this diameter down to say 32 mm, below the venturi, the carb opens back up to 40mm say before it meets the butterflys. The carburettor works such that as air flows through this restriction, under neath, a vacuum is created. This vacuum draws fuel from the main jets into the airstream and off into the engine.
velocity stacks increase the length of the carb throat before it meets the venturi. The airflow becomes more 'laminar' also called 'Reynolds flow' ie less turbulent allowing the carbs to operate more efficiently, and for the fuel to be drawn in from beneath the venturis in a more constant manner than if the air 'tumbles' down the carb throat. They help airflow through the carbs at high revs.
It also helps responsiveness when you crack the carbs open from closed throttle, so technically should make the car feel a little bit 'crisper' in throttle response.
Oh, and they sound great too!
I have them on my '14, but run them inside K&N filters. The advantage of filters is that 1) it stops some numb nuts dropping something through the engine grill into the carb throats. It stops water getting in when it rains, and the car is standing.
Hope that helps! Whether you can really tell a difference, I'm not sure, but every little helps!
Wow thanks for that , understand now thanks to your describtion. So only draw back is crap getting in the carb and possible wet getting in.Plus being louder induction roar ( if thats your bag) and possibly crisper throttle response.
Now what size would you go if NOT running inside K&N?? I know they come in many sizes
I actually think that you summed it up rather better than my epic waffle!
Danny,
I run 40mm Velocity stacks on mine, as any taller and then the top of the airfilter would block the tops off! As for having different length 'stacks', I think that the longer they are, the better they are at high revs but maybe not so good at low revs. Some old drag racers have massive velocity stacks but then they are getting the nuts revved off them! I read about some chap that tuned each cylinder by having slightly different stacks on each carb throat (I think it was a V8 engine)!
I think that the only way you could tell is by swapping them around whilst running the engine on a dyno, whilst checking airflow etc. I think it's a case of trying some out, but on a reletaively small engine, I'm not sure what difference it would make
I actually think that you summed it up rather better than my epic waffle!
Danny,
I run 40mm Velocity stacks on mine, as any taller and then the top of the airfilter would block the tops off! As for having different length 'stacks', I think that the longer they are, the better they are at high revs but maybe not so good at low revs. Some old drag racers have massive velocity stacks but then they are getting the nuts revved off them! I read about some chap that tuned each cylinder by having slightly different stacks on each carb throat (I think it was a V8 engine)!
I think that the only way you could tell is by swapping them around whilst running the engine on a dyno, whilst checking airflow etc. I think it's a case of trying some out, but on a reletaively small engine, I'm not sure what difference it would make
