What does 12.5 or 8.0 Refer to in Venturi inserts

[Todd Bellew]

What dimension is this refering to?
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Thank You,
Todd Bellew

[Tom Barth]

Todd,

Each venturi is the same except for a small hole near the nozzle. 12.5 = 1.25mm - 8.5 = 0.85mm. The hole is the progression port. It allows a small amount of fuel into the venturi just as the throttle is opened for acceleration. Similiar to the accelerator pump in a 4S carburetor.

Since there is little venturi vacuum at closed or very slightly opened throttle, it draws fuel from the idle port (downstream of nozzle) to flow directly into the venturi. The progression phase is the transition from idle to throttle valve control of engine speed.

I'm sure that had Rotax known that Dellorto offered two venturi inserts they would have made only one standard. I belive the purpose for having two different sizes is intended for motor scooters with automatic transmissions. It gives manufacturers more adjustment when selecting a carb for each particular scooter. My guess anyway.
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Tom Barth
http://www.greenflagkarting.com/

Power is good...more power is better... too much power is just right!

[Todd Bellew]

Thanks Tom, good info.
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Thank You,
Todd Bellew

[Richard Gray]

Todd, Tom - I believe that the 12.5 and 8.5 designations refer to the progression port and idle port sizes. We have two sizes of progression ports, 12 and 8; and only one size idle port, 5. The first number, 12 or 8 in our example, refers to the size of the progression port and the second number, 5 (the same for both available/allowed venturi), refers to the size of the idle port. The 12 designation would correspond to a 1.2mm diameter port/hole/jet while the 8 would be a .8mm port/hole/jet and finally the 5 woulld be a .5mm port/whole/jet.

If we acknowledge that the area of the port/hole/jet is the controlling factor for flow volume, then the 12 designated port will flow 2.25 times more fuel than the 8 designated one. Bear in mind though that we are talking the progression port here which has its signficant impact only during the initial throttle opening.

Hope this clears it up somewhat.
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Richard


"Running into the sun but I'm running behind"

[John Savage]

This may also be of interest http://www.karting.co.uk/Notice/Message.asp?FID=2&MsgID=0000463682
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Dellorto, Mikuni and Keihin Jetting & Kart Set-up Software

www.rotaxjetting.com www.kartsetupsoftware.com

[Harj Singh]

I found this, courtesy of arsenal racing online:

Rotax Max Carb Venturi Inserts (8.5 and 12.5) Courtesy of KartingAsia.com

The Dell'Orto VHSB 34(QD/QS) carburetor as fitted on the Rotax Max FR 125 engine has been delivered with two different venturi inserts by Rotax. These are either the "12.5" or the "8.5" venturi inserts. All new engines should be delivered with a carb that has the 8.5 venturi insert. This article aims to clarify the difference between these venturis and what it does to performance.

There has been a bit of confusion with what the different venturi inserts do with regards to performance because some have mistakenly assumed that the different between the 8.5 and 12.5 markings refer to a 4mm difference in the choke size, and thus influence the airflow to the engine. This is where the theories of "12.5 for long fast tracks and 8.5 for short twisty tracks" has come from. If the difference between the venturis actually was the choke size this theory could hold water but that is not what the difference between the venturi inserts is.

The 12.5 and 8.5 markings on the venturi insert refer to the size of two small holes - the progression port and the idle port (or progression hole and idle hole). The number before the dot (12 and refer to the progression port. "12" means a size 120 progression hole is in the venturi while "8" means that the progression hole is size 80. This measurement uses uses the same sizing practice as the jets inside the carb - the measurement is in 100ths of a milimeter. This means that the diameter of the hole is either 1.2mm or 0.8mm. The number after the dot which for Rotax Max users is always "5" means that the idle port is size 50, or 0.5mm. Please note that this is not me taking a guess as to what the numbers mean, Stefano Prandini from Dell'Orto S.p.A. has confirmed this.

View from the top in which you can see the different size progression ports:

Rotax Carburetor Venturis Just to repeat - the only difference between a venturi insert stamped with "12.5" or "8.5" is the size of the progression hole.

Performance Difference? Ok now that we've cleared up exactly what difference is between the two venturi inserts the question is is there a difference between the two on the track? First lets try to understand what the progression port is supposed to do. In short it deals with the transition from the idle circuit(throttle closed) to the main circuit(throttle open). The progression port is located directly upstream(towards engine) of the main spray nozzle. In the picture above the direction of air and fuel-flow is from left to right. The role of the progression port comes into play as the throttle opens just a little bit.

Here's the explanation from the Dell'Orto Manual:
"As the throttle valve lifts the idle circuit air/fuel mixture decreases and the progression port begins to supply emulsified fuel. The air now comes from the main air intake. The progression port (circuit) is important to the system because of the increased amount of mixture required by the engine as the throttle leaves the closed position. The transition circuit is activated when the driver begins to open the throttle and the vacuum decreases in the idle circuit. The mixture from the idle circuit decreases and it is necessary to introduce another circuit which can supply the fuel required for the transition from idle to the main circuit. This is called the progression. At about 1/4th throttle the vacuum begins to become constant and stops drawing the mixture from the idle port. At this point the vacuum is sufficient to draw fuel from the progression port fed by the idle jet in the float chamber."
"When the valve is lifted slightly (up to about 1/4 throttle) the vacuum generated by the inducted airflow begins to be consistent, and stops drawing fuel from the idle nozzle. Under these conditions, the vacuum is sufficient; however, to draw fuel from the progression port, which is always fed by the idle jet placed in the float chamber."

What does all that then mean when it comes to performance? When the throttle is opened just a tad the fuel does not come through the emulsion tube yet, the progression port is there to ensure that some fuel is fed into the engine at this point - without it the mixture would be too lean with only a tony amount of fuel coming out of the idle port. So the port only effects a small part of the throttle opening phase, which for most drivers only last a fraction of a second. It probably has a tiny effect on initial throttle response but other than that will all else remaining the same there should be very little performance difference. Rotax probably switched from the 12.5 to the 8.5 in order to improve something, which was likely to give a slightly crispier response on initial throttle application - which might improve performance a tiny bit by making initial throttle response better. If you're on an 8.5 venturi insert it's my opinion that you should forget about it and focus on other areas. The venturi insert simply doesn't enter into the performance equation in any considerable amount. When I tested both back-to-back in the same carburetor I could not feel any difference when driving and my laptimes were identical. That being said others have found that there is a significant difference between the two with regards to performance so best is to test it yourself and see if you find any difference. To summarize - the different size venturi inserts are only different in the size of the progression port, and this only effects the fuel delivery up to 1/4 throttle opening.

Finally one more picture, from the bottom of the venturi insert:

[Todd Bellew]

From above post, in the last paragraph which I assume wasn't part of the manual section.

If you're on an 8.5 venturi insert it's my opinion that you should forget about it and focus on other areas. The venturi insert simply doesn't enter into the performance equation in any considerable amount.


I'm not so sure.
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Thank You,
Todd Bellew

[Tom Barth]

You are correct Todd!
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Tom Barth
http://www.greenflagkarting.com/

Power is good...more power is better... too much power is just right!

[Andre Eriksen]

[Chris McGinley]

[Ray Chiappe]

I am not an expert on this. But the Max-Jet tells me to lower the main jet size by 10 numbers approx. I road raced with the 12.5 & jetted per the Max-jet and it ran great. So I would think that the 12.5 is letting more feul in through the whole oper. range??

[Tom Barth]

Hi Ray,

I don't believe the idle and progression circuits supply fuel after the venturi vacuum takes over to supply fuel for mid and WOT. If so, calculating area of main circuit jetting (main jet, atomizer) would not be 100% accurate.
_________________
Tom Barth
http://www.greenflagkarting.com/

Power is good...more power is better... too much power is just right!

[Todd Bellew]

Ray,
Very likely, actually most likely. The progression port is a "sub port of the "idle" circuit or low speed circuit. Like all float carbs the low speed jet does contribute fuel over the whole range. Of course, it's most influencial at closed or near closed settings.

The low speed circuit gets air from the small holes in the bell. WHen the throttle is closed all the air must moce through these holes. There is awesome vaccuum and you get great fuel atomization. Look how small the hole is for it. Once you open the throttle or BIG hole, air starts moving through it since it's easier.

So even at wide open throttle where most all the air moves through the venturi that hole in the bell for the slow speed circuit is still there and still open. SOME air does move through and draws SOME fuel out. So even at wide open there is small contributions from the low speed circuit.

And how you know this is, if no fuel moved through the slow speed circuit you have a horrible bog waiting for it to re prime every time you came off full throttle. But, since some is always being drawn it's always there ready for a shut throttle.

Once I worked with a team that had MAxter ICC engines. On the grid the builder showed me where he wanted the revs. With a full blast clean out you needed to see 14 1. In not you leaned the Air screw. WIth a short turn with great jetting it would go from stumbling at 13 8 or 9 and then feeely rev and have to lift it with a 1/4 turn.
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Thank You,
Todd Bellew

[Tom Barth]

Not sure fuel is supplied via the idle port after throttle is lifted beyond 1/4 opening.

"when the valve is lifted slightly (up to about 1/4 throttle) the vacuum generated by the inducted airflow begins to be consistent and stops drawing fuel from the idle nozzle." Dellorto Reference Manual.
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Tom Barth
http://www.greenflagkarting.com/

Power is good...more power is better... too much power is just right!

[Tom Barth]

Not sure fuel is supplied via the idle port after throttle is lifted.

"when the valve is lifted slightly (up to about 1/4 throttle) the vacuum generated by the inducted airflow begins to be consistent and stops drawing fuel from the idle nozzle." Dellorto Reference Manual.
_________________
Tom Barth
http://www.greenflagkarting.com/

Power is good...more power is better... too much power is just right!