RE: Velocity Stacks for 4 strokes from 4 sroke breakin  
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All Forums >> Glow Engines, Gas Engines, Fuel & Mfg Support Forums >> Glow Engines >> RE: Velocity Stacks for 4 strokes from 4 sroke breakin
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RE: Velocity Stacks for 4 strokes from 4 sroke breakin - 5/28/2008 3:11:54 PM   
w8ye



Posts: 17506
Joined: 12/11/2001
From: Taylortown, OH, USA
Status: online 		At certain engine speeds there is a pressure wave standing outside the entrance to a bare carburetor. Some of this pressure wave may get blown around the engine compartment.

The venturi contains this pressure wave and provides a slight fuel economy

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RE: Velocity Stacks for 4 strokes from 4 sroke breakin - 5/28/2008 3:14:21 PM   
goirish



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From: Litchfield, MI, USA
Status: offline 		Thanks W8ye

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RE: Velocity Stacks for 4 strokes from 4 sroke breakin - 5/28/2008 5:00:31 PM   
Campgems


 

Posts: 1931
Joined: 2/18/2006
From: Arroyo Grande, CA, USA
Status: offline 		The stack improves the flow of air through the carb ventrui. It straightens out the flow form the turblant entry on to the mouth.

The reason I use one though is that most four stokes sputter a bit of fuel out of the carb at lower RPM's. The Stack contains a lot of this spray.

I recently put an O.S 52 four stroke back into service with a stack from the new OS 56a. The fuel spit out of the carb has been reduced to almost nothing. A real improvement there. I'm not sure I gained all that much in the way of performance, but I didn't loose any. I'm cranking a 13x4APC with it and it is pulling it at 9700. Way more than what I'll fly at. Low end is very good and the transistion is very good also.

Don


(in reply to goirish)
       Post #: 28

RE: Velocity Stacks for 4 strokes from 4 sroke breakin - 5/28/2008 6:22:58 PM   
Sport_Pilot



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Joined: 1/21/2002
From: Acworth, GA, USA
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quote:

What I thought I would do is to just scale up the O.S 56 stack for the 70 and 91. I can't imagine that there is that much of a critical length. From my old hot rod days, it seems that the purpose was just to get the air flowing in a straight line prior to hitting the sprays in the carb. For us, I would imagine it would be the same and maybe a tad more to keep the whezzing contained for the next gulp of air instead of spraying in into the enging compartment.


There is a tuned length, both maniflold and exhaust headers are cut to a tuned length or mulitple of that length. Many modern passenger cars are also using this. The length is such that a sonic wave will help to push or pull the airflow through the manifold. The effect is not as strong as on the header, and even that is not as strong as a tuned pipe on a two stroke, but every little bit helps. The tuned length is based on RPM so the length on our model engines is about the same as a AA fuel dragster so it is impractical to make them that long. I suppose you can cut it to an exact part of that length but the sonic wave would bounce in and out so many times it would be very weak and of little help.

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RE: Velocity Stacks for 4 strokes from 4 sroke breakin - 5/28/2008 7:29:17 PM   
Campgems


 

Posts: 1931
Joined: 2/18/2006
From: Arroyo Grande, CA, USA
Status: offline
quote:

ORIGINAL: Sport_Pilot

quote:

What I thought I would do is to just scale up the O.S 56 stack for the 70 and 91. I can't imagine that there is that much of a critical length. From my old hot rod days, it seems that the purpose was just to get the air flowing in a straight line prior to hitting the sprays in the carb. For us, I would imagine it would be the same and maybe a tad more to keep the whezzing contained for the next gulp of air instead of spraying in into the enging compartment.


There is a tuned length, both maniflold and exhaust headers are cut to a tuned length or mulitple of that length. Many modern passenger cars are also using this. The length is such that a sonic wave will help to push or pull the airflow through the manifold. The effect is not as strong as on the header, and even that is not as strong as a tuned pipe on a two stroke, but every little bit helps. The tuned length is based on RPM so the length on our model engines is about the same as a AA fuel dragster so it is impractical to make them that long. I suppose you can cut it to an exact part of that length but the sonic wave would bounce in and out so many times it would be very weak and of little help.


That tuned length is important in a multi cylinder engine so the air flow is more consistant. IE if one cyl had a 12" manifold length and the next to fire only had a 2"length, The next one had 14" length, there would be an iregular pulse of flow through the intake, starving one cylinder, because of an overlap of demand. If all manifold lengths are the same, then the flow is more even. I can't see that with a single cylinder engine, that it would be an issue. The same is true for exhaust headers. THe design is to keep all lengths from the exhaust valve to the exhaust pipe the same so there is no pileup of exhaust. On a four stroke, getting the exhaust out of the cylinder is important, IE no back pressure. Take a look at the dragster engines. They have short exhaust pipes, one on each cylinder. No tuning effect, just a quick dump to atmosphere of the exhaust. I suspect that if it wasn't for the fire coming out of the exhaust the pipes would be much shorter, but they need to route that high temp exhaust away from the car and driver.

Two strokes are another animal though as there isn't a pump cycle to push the exhaust out like a four stroke. I'm only slightly fimiliar with the tuned pipes, mostly from my old motocycle days. If I remember correctly, there is a reflected pressure plus from the exhaust and the tuning caused the pulse to exit prior to the next exhaust and in effct creats a bit of vacuum in the pipe to assist the next exhaust cycle. It's been a long time, so this explaniantion may be wrong.

Don


(in reply to Sport_Pilot)
       Post #: 30

RE: Velocity Stacks for 4 strokes from 4 sroke breakin - 5/28/2008 8:32:24 PM   
Sport_Pilot



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Joined: 1/21/2002
From: Acworth, GA, USA
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quote:

That tuned length is important in a multi cylinder engine so the air flow is more consistant. IE if one cyl had a 12" manifold length and the next to fire only had a 2"length, The next one had 14" length, there would be an iregular pulse of flow through the intake, starving one cylinder, because of an overlap of demand.


While its important to be of equal length, that is not the reason. Once the manifold is full of fuel air mixture all cylinders will be exposed to the gas and vapor. The overlap is only a factor at low speeds causing loping with large overlaps, once the engine is up to speed there is no exhaust running through the manifold. They need to be of equal lenght if tuned to keep an equal boost from the sonic wave into the cylinders. That is why most modern engines have a plenum behind the fuel injector with equal length tuned runners cast or molded into the mainfold. Even economy minded engines do this these days.

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(in reply to Campgems)
       Post #: 31

RE: Velocity Stacks for 4 strokes from 4 sroke breakin - 5/28/2008 9:00:51 PM   
Sport_Pilot



Posts: 7728
Joined: 1/21/2002
From: Acworth, GA, USA
Status: offline
quote:

That tuned length is important in a multi cylinder engine so the air flow is more consistant. IE if one cyl had a 12" manifold length and the next to fire only had a 2"length, The next one had 14" length, there would be an iregular pulse of flow through the intake, starving one cylinder, because of an overlap of demand.


While its important to be of equal length, that is not the reason. Once the manifold is full of fuel air mixture all cylinders will be exposed to the gas and vapor. The overlap is only a factor at low speeds causing loping with large overlaps, once the engine is up to speed there is no exhaust running through the manifold. They need to be of equal lenght if tuned to keep an equal boost from the sonic wave into the cylinders. That is why most modern engines have a plenum behind the fuel injector with equal length tuned runners cast or molded into the mainfold. Even economy minded engines do this these days.


quote:

Take a look at the dragster engines. They have short exhaust pipes, one on each cylinder. No tuning effect, just a quick dump to atmosphere of the exhaust. I suspect that if it wasn't for the fire coming out of the exhaust the pipes would be much shorter, but they need to route that high temp exhaust away from the car and driver.


Dragster exhaust pipes don't need a common plenum or collector as an intake manifold does. The plenum has nothing to do with the tuning effect, it just provides a common chamber to hold the fuel air mixture. That doesn't mean the exhaust stacks are not tuned. They are longer than the intake manifold because they exhaust is traveling at a much higher velocity as is the sonic wave, thus it needs to be longer to keep them in tune. Funny cars need the exhaust to be longer to keep fumes away from the body and may be double length, but still tuned.

< Message edited by Sport_Pilot -- 5/28/2008 9:59:33 PM >

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