velocity stacks

[camaroboy383]

well guy's this has been somthin ive been thinkin about lately...ive been thinkin of experimenting with velocity stacks... now ive seen people run them and ive seen people (majority) not run them... i know nothing in relation to this topic as far as pro's and con's and thats why im popping the question.. so any insight is appriciated...

pro's,,con's,,gain's,,,losses,, effects on engine,, torque ect.

thanks a mill guy's

alden

 

[glenng]

well guy's this has been somthin ive been thinkin about lately...ive been thinkin of experimenting with velocity stacks... now ive seen people run them and ive seen people (majority) not run them... i know nothing in relation to this topic as far as pro's and con's and thats why im popping the question.. so any insight is appriciated...

pro's,,con's,,gain's,,,losses,, effects on engine,, torque ect.

thanks a mill guy's

alden

Alden, It's a good question. I only run .67 size sport & scale hydros and shove the motors as far forward as I can( to keep the balance point as far forward as I can). Like the boats to balance about 1/2" behind sponson transoms-thus no room for velocity stacks. all the .67 carbs seem to have a tapered or radiused intake anyway. I think I read somewhere that you should have about 1" of open space in front of the carb. Boats seem to run sloppy rich-so some turbulance at the intake may help atomize the fuel.

Glenn

 

[Chris Wood]

I can say with confidence that yes they do work.. several yrs ago we ran cmb 67 gold head evo motors.. we ran without one and put one on.. made 1 mph difference.. then we changed the lenght.. wow we picked up 5mph.. it is hard to do in hydros with the rear boom tube in the way.. so we have to work around it but yes they do help.

if you look back andy's first mac motors.. early early versions had a long velosity stack.. i know people that use them know and say the draw MUCH better than the current version andy was making.

chris

 

[Tim_Duggan]

General rule of thumb - 2 1/2 times the throat diameter clearance from anything. Sometimes this is unavoidable though.

The shape of the whole inlet tract is important, not just the velocity stack on it's own. The shape of the bell mouth and length of the trumpet from the actual inlet all play a part.

Do a search on "the venturi effect" and "port velocity". Should give you some ideas.

 

[camaroboy383]

glenn,, i completely understand about the tight quarters, and in those situations there is one simple answer i guess,, you just cant run one..lol.. hmmm,,now increasing turbulance,,, this is another question raised,, is it actually adding turbulance???? or becuase its a more directed flow,, does it decrease turbulance??? the velocity i can understand being increased as the flow is more directed to the mouth... opposed to being so disturbed at the mouth...

alden

 

[camaroboy383]

great were on the right track..valuable info quick like,, i love it...

inlet tract tuned length,, that would say alot of testing with length would need to be done...which i kinda figured..

shape of the bellmouth as tim stated,,excellent point by the way..

also the way it sits on the mouth,, inner velocity bore same as mouth o.d,,, or machine the stack so the i.d matches the mouth opening...

 

[dr gary turner]

Velocity stacks do work. We ran them several years ago. Mark Bullard built some carbs at MY suggestion with the stacks O-ringed in the carb. We ran them on our 80 Rossi motors. I set the straight away record the first time it was done with electric eyes with a carb with a stack.

By the stack being held in place with the O rings it would let the "trumpet bell"resonate. It would sound like a huge 4 barrel carb opening up when one reved the motor up. The O ring concept actually came from K&B. We just improved on the design.

We tried different lengths. The shorter stacks gave the motor more rpm and the longer ones produced more torque and less rpm. Gary

 

[Tim_Duggan]

Alden,

How we used to do it was attach the unfinished stack to the carb and then put the carb in the lathe to finish off so that it was a smooth, almost seamless join. The opposing side of the barrel in the carb would get a similar treatment on some carbs, particularly on the non-standard carb we used on the TT o/b's at the time which was 9mm bore and had a goofy oval shape to it. It would get a straight 7 degree cut back to the carb barrel to make it cone shaped. After modding, finding a needle was easier and the engine sound was different. Bottom end transition was much better too. Not all carbs need this, but that one certainly did.

 

[camaroboy383]

i beleive your talking about the 20j... jerry and i have spoken about that several times... i can see how machining the mouth and i.d of the stack will make for a almost seamless transition... the only issue with that will be i dont have a ton of carbs so testing lengths and shapes of stacks would require a ton of carbs... i would like to leave the mouths alone till i can find a length i know would work...

dr.turner,,, thank you for your insight and direction as to length...could you please elaborate more on the o-ring placement and fit to the lip of the mouth???

alden

 

[LH7018]

The shape and angle of the bell mouth opening are important also. See attached, not the best quality picture but you'll get the idea:

 

[Tim_Duggan]

i beleive your talking about the 20j... jerry and i have spoken about that several times... i can see how machining the mouth and i.d of the stack will make for a almost seamless transition... the only issue with that will be i dont have a ton of carbs so testing lengths and shapes of stacks would require a ton of carbs... i would like to leave the mouths alone till i can find a length i know would work...

alden

Not the 20J, it was a TT carb used on the inboard 21 marine engine and had a 9mm bore and a low end needle arrangement. I've never modded a 20J other than fixing a barrel misalignment due to the retaining screw.

On 21's I tend to leave the length alone - it can be tricky on them for air leaks and such, but on bigger engines it's easier. Usually just a spacer. Ask Jerry for a photo of the Picco 67EXR he got from me..... For inspiration on bell shape, look at the shape of the inlet on an aircraft jet engine - the end of the bell is curved back on the outside

 

[camaroboy383]

eliptical shape,, just like a jet engine.. man this is getting easier by the post... this is great ... obviously this is the desired cut... man i can hear the lathe cuttin already.. :lol: ..

gotcha tim on that tt .. the 20j really is a nice carb for the tt,,, surprised you never played with them... jerry sure knows how to bring them to life...

 

[Bill Gibson]

I wonder if you could get a piece of black delrin round stock (Mcmaster Carr) to experiment with shapes and sizes,then when you find the beast you want, make it out of aluminum...delrin is so sweet to work with and the size you need wouldnt be too expensive...you are right about directing the flow and its also about increasing the speed of the flow. Its kinda about creating an area of low pressure, then packing the air into the smaller area to speed it up consistently..I work for mosquito control and we design and build our own spray equipment in house and im always experimenting with diffrent nozzle shapes and sizes to pull liquid material from the chemical tank to the nozzle without using a chemical pump by using the venturi effect....kinda fun in that it gives you and idea of the crazy things that air does! GO FOR IT!

 

[Jerry Wyss]

eliptical shape,, just like a jet engine.. man this is getting easier by the post... this is great ... obviously this is the desired cut... man i can hear the lathe cuttin already.. :lol: ..

gotcha tim on that tt .. the 20j really is a nice carb for the tt,,, surprised you never played with them... jerry sure knows how to bring them to life...

you've put on a condom before?,, just gettin' it started? well there ya go

 

[Norm Doerr]

I wonder if you could get a piece of black delrin round stock (Mcmaster Carr) to experiment with shapes and sizes,then when you find the beast you want, make it out of aluminum...delrin is so sweet to work with and the size you need wouldnt be too expensive...you are right about directing the flow and its also about increasing the speed of the flow. Its kinda about creating an area of low pressure, then packing the air into the smaller area to speed it up consistently..I work for mosquito control and we design and build our own spray equipment in house and im always experimenting with diffrent nozzle shapes and sizes to pull liquid material from the chemical tank to the nozzle without using a chemical pump by using the venturi effect....kinda fun in that it gives you and idea of the crazy things that air does! GO FOR IT!

Look at all our carbs. Don't go aluminum stay with delrin all the way. A couple years ago I made a HUGE flat bellmouth made out of aluminum. It was way too heavy. I thought this thing was shiney and cool looking because you could see the thing fog up as air passed throught it. Marty told me it would fall off, and it did in the second heat at the Brandon race.

 

[Tim_Duggan]

Aluminium will stay put if you use an interference fit & use green loctite on 'em. The only way it will come off is if you heat it.

 

[camaroboy383]

i would imagine a press fit would be sufficient for testing..loctite for sure will make it stay put.. to be honest,, ive only seen a few and they were all aluminum... the delrin does make for super simple machining,, but the aluminum machines quite well also... think its gonna come down to what i have in my hand at the time...lol.. first im gonna play around with a cutter to see if i can get that eliptical shape with one swipe of the tool... im sure i will be messin up a couple pieces of tool steel.. :lol: ..

alden

 

[Brad Christy]

Alden,

You can cut it close with a couple of straight tapered cuts and smooth them out with a bearing scraper at low spindle RPM. Polish it out at high RPM with sandpaper and you're good to go.

Delrin works just fine. In the past, every carb I ever saw on any o fStu Barr's engines had delrin bellmouths. Aluminum, however, will work, too, but you have to "pin" them in place with an aluminum plug before finishing the inside work. After you press it in place, drill and ream a 1/8" or 3/16" hole (depending on carb size) all the way through, perpendicular to the direction of air flow, and press in an aluminum slug. Then you can finish the inside of the bellmouth and it will stay in place forever.

Thanks. Brad.

Titan Racing Components

BlackJack Hydros

 

[topfuel443]

Ive made several of them out of Delrin using a groud tool I made. They work great, and are easy to make and experiment with.

I LOVE maching delrin, so much easier.

~James

 

[Norm Doerr]

Aluminium will stay put if you use an interference fit & use green loctite on 'em. The only way it will come off is if you heat it.

That was not the problem The carb ears snappped off, bolts came loose.