I recently read that dependent on the shape and in particular the stack offered by B&B Specialities, some velocity stacks do in fact lessen engine power. Has anyone had any first hand experience of this and can comment on this.

I was contemplating purchasing one from B&B to reduce the effect of the spitting from my carb, but will put that on hold for awhile.

Karol

If properly designed and matched for the application, the velocity stack will increase power and make for better transitions due to a more linear airflow into the carb or injector. If the stack does not increase power or actually decreases it... its a lousy design or mismatched for the application.

3W has one of the best ones for its engines... very noticeable difference with vs. without.

DP

All things being equal, there should be no difference in power, with or without. The venturi in the throat is much smaller than the inlet to the carb or the inlet to the velocity stack. If you put the stack on without changing the high speed mix, you will get a loss of power as the engine will be running richer because it's taking back in the mix that would otherwise have been blown out into the airstream. All the stack does is increase the mileage a bit and sucks back in the fuel that would have otherwise been blown around inside the cowl and making a mess. More so at idle speed.

Velocity stacks can in fact be "tuned" and produce pressure waves the same as tuned pipes. My experience in karting using them way back in the 60's and 70's(pretty much everyone stopped using them after that) is they provided minimal or no benefit, and yes in some cases, a negative impact was noted....but with that said, that was only in the application I was using them on, and in our low rpm airplane engines, I have no idea whether they have a benefit or not, and I've never found a need to try one. I think they are typically more cosmetic than functional, though they were certainly used a lot in race cars back in the days when carburators were used. I doubt the F1 guys were doing it just for cosmetics, but then again those were 4-stroke engines too.

My experience has been no noticable change unless the stack was mounted improperly then decreased performance. DO NOT use ram air, i.e. do not direct a slanted cut velocity stack into the air stream as that will usually be detrimental to performance.

Hello,
Someone else posted the info in this photo a while ago.
I cannot take credit for it.
John

This is very true. Intake vlocity stacks are usuallt tuned to the 2nd intake harmonic, as tuning to the first would create more power but it would have to be about 5 ft. long! I have used velocity stacks for tuning 2stroke MX bikes for years with the factory racing teams I have worked for. In fact almost every 2stroke competition mx bike has some kind of stack built into the mold of the intake boot from the airbox. I have spent considerable amount of time on the dyno with these 250cc mx 2strokes changing velocity stacks from 20mm, 25mm, 30mm etc all with variable results. Trick is to find the length that suits your intake duration and target peak TQ RPM, so effectively if you change props, you could go 'out of tune' with your intake tuned length.

But before I would even mess with tuned intakes on these engines, I would try and tackle a tuned exhaust. This would prove much more beneficial!

BK

Thanks for the great first-hand info BK. What I think your post reinforces is that velocity stacks may have a beneficial, or non-beneficial affect, and I know for myself, I wouldn't even bother to try one unless I was going to do a precise test using either a dyno, and with lack of that(the most likely case), different props to test at different RPM ranges....but I think you'd find on our engines that the benefit (if you got it tuned right) would be so slight, it might not even be accurately measurable on a tach.


Do you know the best case % gains you were getting with the MX engines?

Bill, the average HP gain with the properly tuned intake velocity stack would probably be around 3% (or 1.5 hp on a 50 hp engine). But there are other things to consider, like throttle response, width of power band (or useable power range), fuel consumption, etc.

Basically the higher the desired peak TQ rpm the shorter the stack had to be. But like I said, you could have a great performing stack that made big gains at a steady rpm that would absolutly ruin the throttle response. It is definitly a delicate balancing act.

BK

This is all good stuff and certaintly answeres my question and then some. My only reason for thinking about fitting a stack was to lessen the mess on my model caused by spitting from the carb. As long as it does not create any measurable reduction in power I plan on trying one.

Karol

Karolh, If your carb is spitting fuel there is something else going on.
negative pressure????
Come on guys help him out.

We've run stacks and as the previous posters have mentioned found no real benefit in using them, no matter how pretty they look. If it isn't tune for a certain application it won't help.

Some carbs spit more than others simply due to internal carb setup- The Walbro 27 for example on a 160 ZDZ spoits andth Tillotson spits very little .
proper needle adjustment does enter into this ---
Some engine layouts spit more than others -normally - advanced rotary timing setups at low rpm tend to spray back a bit
I run one of my ZDZ's with a stack (a 40) and another (a 50) without - can't really see any difference
Both are using a 201 Walbro-

T-one,

The needles seem to be set okay as the engine runs very well and has excellent transition throughout the entire rev range.

Maybe I'm just being a bit too finicky as the blow by from the carb does not really make an earth shattering mess, it just leaves a film of unburnt fuel on the side ot the cowl, and I would much prefer if it did'nt.

Karol

Put a stack on it and you won't have the mess. Simple.

Chrysler did a good deal of engineering work on tuned intake runners in the 1950s. The basic theory was developed on 4-strokes but it also works on 2-strokes if you take into account the differences in duration of the intake cycle. When the intake valve opens, a negative pressure wave travels at the speed of sound toward the carb until it reaches the open end of the velocity stack. A sound wave will reverse it's sign and now becomes a positive pressure wave when it expands into an open space (the end of the stack). The positive pressure wave travels back toward the engine at the speed of sound and can give a mild supercharging effect if the wave returns just before the intake valve closes. Longer intake timing requires a longer manifold runner. Higher rpm boost requires a shorter runner. A few modern high performance cars have intake runners that adjust their length with engine rpm to optimize the torque/power increase across the entire rpm range of the engine.

Didya ever try to change the spark plugs on one of those old Chizzlers?
Back in the 60's the high pro Plymouths would set on the ready line with bags of ice on those runners to keep em as cold as possible for the race .
Some of those setups were really potent -
Some were faster than 327 Chevs --some weren't
I put the trumpet on my ZDZ40 on the new 201 Walbro and added a big diameter header to the JMB can - the whole setup seems very good - it runs 7200 right from the one flip get go on the Mejzlic 21x8 -
now to try to sort out the best combo.

No, never worked on the cross-ram Chryslers. They were a little before my time.

Memory's a little foggy now but There was, I think, the Dodge Ramcharger with a carb over one bank that fed the cyls. on the other side with long curved runners. Carb on the other bank did the same. 318 I think.

no -larger engine the cross setup was primarily a performance setup

The primary reason for a velocity stack on a two cycle is to retain the fuel from carburetor stand-off. If you examine the carb in operation, you can see a mist of fuel outside the carb entrance. A stack captures this fuel. This can only be seen when there is dead air around the carb. One of the most obvious cases was on the McCulloch racing carts. They would wet a hand placed several inches from the carb.

Oh my goodness, did you just date yourself. Wow, does that take me back. McCulloch hasn't made a racing "kart" since the early 60's(I had one) and it wasn't many years after that their engines were obsoleted by the Italian rotary valve engines, Parilla's and Saetta's, plus a few others....then there was Yamaha....


I can't really agree with that statement. Maybe that was the case in the early 60's on McCulloch kart engines, but it certainly wasn't the reason on Formula 1 engines, nor more recently in the example mxwrench gave wtih MX engines.

DBryan,

That is not completely correct. Retaining fuel is a benefit from velocity stacks, but it is not the primary reason. Without going into a bunch of physics and junk, the purpose of a velocity stack is for two things, primarily to keep the column of air moving after the induction cycle stops, so that the next time it begins the air hasn't lost it's pressure. The second (and the reason stacks aree tulip shaped) is to funnel air in from a broader area without disruption of airflow (eddy currents, etc.) To expand on my first point: Air is actually a fluid medium, kinda similar to water (except it is compressable to a point). If you can imagine a bottle half full of water, tip it one way slightly, then tip it the other way slightly, what you will see is the water rushing from one end to the other and building up at the opposite end until you tilt the bottle the other way, and vise versa.

The comulmn of air kinda works this way in the intake tract. When the intake begins it moves a mass of air into the engine thru the intake manifold, carb, and port. Because this air has mass it also has velocity. When the intake valve closes (reed petal, piston port or rotary valve) it closes very quickly, and the air column that was flowing inward has nowhere to go so it bunches up and builds pressure right at the inlet point of the engine where the valve closed (like in the example of the water bottle). But just as in the case of the water bottle the pressure will quickly reverse direction and go the other way to try and even itself out to atmosphere. The idea behind tuned intake velocity stacks is to time this action so when the next intake event begins the air pressure behind the intake valve is at it's peak. This timing can be controlled by the length of the stack or port. The angle or shape of the intake velocity stack also has a play in this pressure by allowing a larger moment of air to effect the pressure build at the intake valve.

There is also a sound wave harmonic that is similar to a tuned exhaust at play here that also requires correct timing. You can begin to se how this can be quite difficult to calculate.

The reason the carb spitback is reduced with a velocity stack is because the incomming air charge is meant to be constantly under pressure (If the design calculated correctly) And directed only inward. When the intake is out of time, or the intake dwell is to large, or the engine is not running at the rpm that the intake was tuned for then the air column can crash against the closed intake valve and find its way backward thru the carb causing spitback. Usually intakes are tuned for peak TQ rpm, so any thing under that rpm the intake tuned length will be to short to be effective, alowing spitback to occur, and Vise versa, if the port is to long, to try and eliminate spitback then the intake length will be much to long to be effective for power production.

If your still with me congratulations! I tried to keep it simple!

BK

Nice write up !
yet another trick--is to increase the length of the passage from the rotary valve (whatever --to the carb.
This can reduce the sudden peak in the column of air which is flowing from the carb-to the engine.
I once did a lot of work involving low pressure pneumatic systems and developed some innovations using the overshoot and imbalances in double acting cycinders to trigger auto shutdown for safety purposes.
This plus working with fluidic circuitry, showed me the highly elastic nature of air can be both a problem and in some cases used to help tailor airflow .
Bill in 1960 I traded a beautiful Olds 88 4door rare torpedo back for a Moss cart with dual Mac 10's - I got it to go fast but it darn near killed me
I traded it to a race boat guy for a built up Chev engine for my 55 Austin Healey- with a 6 carb Offy log manifold which was wretched on the street -simply because of poor carb velocity at low revs -
Long way around to get to the issue at hand but I ended up using a three carb progressive setup to get best street racer performance . The spit back and other problems we see with th model engines are similar to problems seen with cars.

You've got me beat by about 3 years. I'll have to find a picture of me in my Mac kart witht Mac 30 and scan it in a post it for a few laughs........ and with a velocity stack on it too.

I never dreamed my little post could have started such a stream of comments, as the wealth of information that is available from this forum is simply awsome.

For me it's always like school's in session, and I'm all eyes and ears.

Karol