VENTURI

[Roger Lee Hooks Sr]

I have notice a long venturi on some boaters engine. What is the purpose or advantage of a long venturi ?

 

[Andy Brown]

Long vs short venturi will increase low and mid range torque. Improve launch on riggers. Will alter the mixture strength curve to be richer at top rpm.

I have had rigger/engine combos that would not pull props of the beach well. The addition of a longer venturi solved the problem every time.

 

[David Bryant]

interesting, thanks Andy

 

[Jeff Lutz]

Yes, this info is in numerous two stroke tuner books. Tidewater use to sell velocity stacks in the evo gold head/black case days. I tested several. With a local friend I tested a straight bore velocity stack about 1" long and a tapered bore velocity stack about 1" long on a cmb mod 80. The tapered bore did best, we could run a shorter pipe and launch more prop and the setup ended up faster on the top end because of this, on this particular setup. Jeff Lutz

 

[Andy Brown]

Yes, this info is in numerous two stroke tuner books. Tidewater use to sell velocity stacks in the evo gold head/black case days. I tested several. With a local friend I tested a straight bore velocity stack about 1" long and a tapered bore velocity stack about 1" long on a cmb mod 80. The tapered bore did best, we could run a shorter pipe and launch more prop and the setup ended up faster on the top end because of this, on this particular setup. Jeff Lutz

Jeff knows the history, but for the benefit of others, Yours Truly designed the velocity stacks that Tidewater sold for the 1994 CMB Evo engines. They where available for the 45,67/80 and 90 engine. That particular engine had a very short intake tract because the carb was machined in one piece with the backplate. So the stack made a big difference on that engine. CMB later copied part of the mods I was doing to the 80 Evo engine and it came stock from the factory with the Velocity stack.

The top end richness is not necessarily a bad thing. Often it is a benefit. I mentioned it because that is one effect of the longer vs the shorter. There are several ways to modify the mixture strength curve. The length of the Venturi is one of them.

 

[dr gary turner]

I had one of the stacks of Todewaters on a gold head CMB 90.

One day Iwas running it in a hydro and all of a sudden the motor picked up a lot of rpm.

When I brought the boat In the stack had broken off

So my impression is a long velocity stack increases Torque but reduces rpm

 

[Jeff Lutz]

Yes, this info is in numerous two stroke tuner books. Tidewater use to sell velocity stacks in the evo gold head/black case days. I tested several. With a local friend I tested a straight bore velocity stack about 1" long and a tapered bore velocity stack about 1" long on a cmb mod 80. The tapered bore did best, we could run a shorter pipe and launch more prop and the setup ended up faster on the top end because of this, on this particular setup. Jeff Lutz

Jeff knows the history, but for the benefit of others, Yours Truly designed the velocity stacks that Tidewater sold for the 1994 CMB Evo engines. They where available for the 45,67/80 and 90 engine. That particular engine had a very short intake tract because the carb was machined in one piece with the backplate. So the stack made a big difference on that engine. CMB later copied part of the mods I was doing to the 80 Evo engine and it came stock from the factory with the Velocity stack.

The top end richness is not necessarily a bad thing. Often it is a benefit. I mentioned it because that is one effect of the longer vs the shorter. There are several ways to modify the mixture strength curve. The length of the Venturi is one of them.

Yes, this info is in numerous two stroke tuner books. Tidewater use to sell velocity stacks in the evo gold head/black case days. I tested several. With a local friend I tested a straight bore velocity stack about 1" long and a tapered bore velocity stack about 1" long on a cmb mod 80. The tapered bore did best, we could run a shorter pipe and launch more prop and the setup ended up faster on the top end because of this, on this particular setup. Jeff Lutz

Jeff knows the history, but for the benefit of others, Yours Truly designed the velocity stacks that Tidewater sold for the 1994 CMB Evo engines. They where available for the 45,67/80 and 90 engine. That particular engine had a very short intake tract because the carb was machined in one piece with the backplate. So the stack made a big difference on that engine. CMB later copied part of the mods I was doing to the 80 Evo engine and it came stock from the factory with the Velocity stack.

The top end richness is not necessarily a bad thing. Often it is a benefit. I mentioned it because that is one effect of the longer vs the shorter. There are several ways to modify the mixture strength curve. The length of the Venturi is one of them.

Yes, this info is in numerous two stroke tuner books. Tidewater use to sell velocity stacks in the evo gold head/black case days. I tested several. With a local friend I tested a straight bore velocity stack about 1" long and a tapered bore velocity stack about 1" long on a cmb mod 80. The tapered bore did best, we could run a shorter pipe and launch more prop and the setup ended up faster on the top end because of this, on this particular setup. Jeff Lutz

Jeff knows the history, but for the benefit of others, Yours Truly designed the velocity stacks that Tidewater sold for the 1994 CMB Evo engines. They where available for the 45,67/80 and 90 engine. That particular engine had a very short intake tract because the carb was machined in one piece with the backplate. So the stack made a big difference on that engine. CMB later copied part of the mods I was doing to the 80 Evo engine and it came stock from the factory with the Velocity stack.

The top end richness is not necessarily a bad thing. Often it is a benefit. I mentioned it because that is one effect of the longer vs the shorter. There are several ways to modify the mixture strength curve. The length of the Venturi is one of them.

Yes I knew you designed those stacks, I wanted to point out the tapered versus straight. in 1987 a machinist in my local club built some carbs with long stacks for three guys all running coyote 21's and cmb valvola's , they were also able to launch better props and ran well with them.

 

[Andy Brown]

I had one of the stacks of Todewaters on a gold head CMB 90.

One day Iwas running it in a hydro and all of a sudden the motor picked up a lot of rpm.

When I brought the boat In the stack had broken off

So my impression is a long velocity stack increases Torque but reduces rpm

Hi Doc

It picked up rpm because it was a little on the rich side. If the mixture was perfect it would have leaned and sagged or stopped.

I too had one break off. The engine leaned out and died.

The longer stack will allow a larger bore, so in the end the top rpm and power can be better.

 

[Andy Brown]

Yes, this info is in numerous two stroke tuner books. Tidewater use to sell velocity stacks in the evo gold head/black case days. I tested several. With a local friend I tested a straight bore velocity stack about 1" long and a tapered bore velocity stack about 1" long on a cmb mod 80. The tapered bore did best, we could run a shorter pipe and launch more prop and the setup ended up faster on the top end because of this, on this particular setup. Jeff Lutz

Jeff knows the history, but for the benefit of others, Yours Truly designed the velocity stacks that Tidewater sold for the 1994 CMB Evo engines. They where available for the 45,67/80 and 90 engine. That particular engine had a very short intake tract because the carb was machined in one piece with the backplate. So the stack made a big difference on that engine. CMB later copied part of the mods I was doing to the 80 Evo engine and it came stock from the factory with the Velocity stack.

The top end richness is not necessarily a bad thing. Often it is a benefit. I mentioned it because that is one effect of the longer vs the shorter. There are several ways to modify the mixture strength curve. The length of the Venturi is one of them.

Yes, this info is in numerous two stroke tuner books. Tidewater use to sell velocity stacks in the evo gold head/black case days. I tested several. With a local friend I tested a straight bore velocity stack about 1" long and a tapered bore velocity stack about 1" long on a cmb mod 80. The tapered bore did best, we could run a shorter pipe and launch more prop and the setup ended up faster on the top end because of this, on this particular setup. Jeff Lutz

Jeff knows the history, but for the benefit of others, Yours Truly designed the velocity stacks that Tidewater sold for the 1994 CMB Evo engines. They where available for the 45,67/80 and 90 engine. That particular engine had a very short intake tract because the carb was machined in one piece with the backplate. So the stack made a big difference on that engine. CMB later copied part of the mods I was doing to the 80 Evo engine and it came stock from the factory with the Velocity stack.

The top end richness is not necessarily a bad thing. Often it is a benefit. I mentioned it because that is one effect of the longer vs the shorter. There are several ways to modify the mixture strength curve. The length of the Venturi is one of them.

Yes, this info is in numerous two stroke tuner books. Tidewater use to sell velocity stacks in the evo gold head/black case days. I tested several. With a local friend I tested a straight bore velocity stack about 1" long and a tapered bore velocity stack about 1" long on a cmb mod 80. The tapered bore did best, we could run a shorter pipe and launch more prop and the setup ended up faster on the top end because of this, on this particular setup. Jeff Lutz

Jeff knows the history, but for the benefit of others, Yours Truly designed the velocity stacks that Tidewater sold for the 1994 CMB Evo engines. They where available for the 45,67/80 and 90 engine. That particular engine had a very short intake tract because the carb was machined in one piece with the backplate. So the stack made a big difference on that engine. CMB later copied part of the mods I was doing to the 80 Evo engine and it came stock from the factory with the Velocity stack.

The top end richness is not necessarily a bad thing. Often it is a benefit. I mentioned it because that is one effect of the longer vs the shorter. There are several ways to modify the mixture strength curve. The length of the Venturi is one of them.

Yes I knew you designed those stacks, I wanted to point out the tapered versus straight. in 1987 a machinist in my local club built some carbs with long stacks for three guys all running coyote 21's and cmb valvola's , they were also able to launch better props and ran well with them.

Yes Jeff, we built some tapered units at Tidewater also, but I think just for one size engine. I think the .45. I still have a few of them around here.

 

[Jeff Lutz]

Yes, this info is in numerous two stroke tuner books. Tidewater use to sell velocity stacks in the evo gold head/black case days. I tested several. With a local friend I tested a straight bore velocity stack about 1" long and a tapered bore velocity stack about 1" long on a cmb mod 80. The tapered bore did best, we could run a shorter pipe and launch more prop and the setup ended up faster on the top end because of this, on this particular setup. Jeff Lutz

Jeff knows the history, but for the benefit of others, Yours Truly designed the velocity stacks that Tidewater sold for the 1994 CMB Evo engines. They where available for the 45,67/80 and 90 engine. That particular engine had a very short intake tract because the carb was machined in one piece with the backplate. So the stack made a big difference on that engine. CMB later copied part of the mods I was doing to the 80 Evo engine and it came stock from the factory with the Velocity stack.

The top end richness is not necessarily a bad thing. Often it is a benefit. I mentioned it because that is one effect of the longer vs the shorter. There are several ways to modify the mixture strength curve. The length of the Venturi is one of them.

Yes, this info is in numerous two stroke tuner books. Tidewater use to sell velocity stacks in the evo gold head/black case days. I tested several. With a local friend I tested a straight bore velocity stack about 1" long and a tapered bore velocity stack about 1" long on a cmb mod 80. The tapered bore did best, we could run a shorter pipe and launch more prop and the setup ended up faster on the top end because of this, on this particular setup. Jeff Lutz

Jeff knows the history, but for the benefit of others, Yours Truly designed the velocity stacks that Tidewater sold for the 1994 CMB Evo engines. They where available for the 45,67/80 and 90 engine. That particular engine had a very short intake tract because the carb was machined in one piece with the backplate. So the stack made a big difference on that engine. CMB later copied part of the mods I was doing to the 80 Evo engine and it came stock from the factory with the Velocity stack.

The top end richness is not necessarily a bad thing. Often it is a benefit. I mentioned it because that is one effect of the longer vs the shorter. There are several ways to modify the mixture strength curve. The length of the Venturi is one of them.

Yes, this info is in numerous two stroke tuner books. Tidewater use to sell velocity stacks in the evo gold head/black case days. I tested several. With a local friend I tested a straight bore velocity stack about 1" long and a tapered bore velocity stack about 1" long on a cmb mod 80. The tapered bore did best, we could run a shorter pipe and launch more prop and the setup ended up faster on the top end because of this, on this particular setup. Jeff Lutz

Jeff knows the history, but for the benefit of others, Yours Truly designed the velocity stacks that Tidewater sold for the 1994 CMB Evo engines. They where available for the 45,67/80 and 90 engine. That particular engine had a very short intake tract because the carb was machined in one piece with the backplate. So the stack made a big difference on that engine. CMB later copied part of the mods I was doing to the 80 Evo engine and it came stock from the factory with the Velocity stack.

The top end richness is not necessarily a bad thing. Often it is a benefit. I mentioned it because that is one effect of the longer vs the shorter. There are several ways to modify the mixture strength curve. The length of the Venturi is one of them.

Yes I knew you designed those stacks, I wanted to point out the tapered versus straight. in 1987 a machinist in my local club built some carbs with long stacks for three guys all running coyote 21's and cmb valvola's , they were also able to launch better props and ran well with them.

Yes Jeff, we built some tapered units at Tidewater also, but I think just for one size engine. I think the .45. I still have a few of them around here.

. We tested tapered on a mod 80 but it may have been made by our local machinist. John Lalonde also tested straight /versus tapered on some big cmb engines.

 

[Christian Lucas]

Hi,

only a side talke. I had a good friendship,with the German Tunnelboat F1 Worldchampion driver Michael,Werner. He told me that one of the most important things of the Mercury Outbord motor was the lengh of the venturi, spezial,with the reedvalve system. At low rpm the system has very high torque to rev up the propeller and at high speed the incoming airwave was so strong that the reedvavle was held open all time that will give a high filling in the pre section crankcase . In the endresult the higher crankcase filling will give the tunepipe the chance to press back a lot of powerfull load . I am not a combustion motor expert but i think he wants to tell me that i have not only to look for highest power ,better to have a good powerfull range .

Happy Amps Christian

 

[Jeff Lutz]

I had one of the stacks of Todewaters on a gold head CMB 90.

One day Iwas running it in a hydro and all of a sudden the motor picked up a lot of rpm.

When I brought the boat In the stack had broken off

So my impression is a long velocity stack increases Torque but reduces rpm

When I saw your name I thought you were going to talk about the k&B stacks that were on a oring for floating. You use to love those!

 

[dwilfong]

Take note of the Speed Master carbs that Dale made. He explained why thy worked as good as thy did. The venture angle has a lot to do with how the carb reacts. Strait angle from tip to base at 17.5 deg worked the best. Adjust the length to the bore of the carb.1.5 times as long as the bore with a .100 lead to the bore.

Crap here we go with my big mouth again......................................LOL

 

[dr gary turner]

Jeff the K&B with the O rings would really let the Venturi reasonate.

They would sound like a 4 barrel carb opening up.

I would love to have such a Venturi on a .550 or .570 carb for a 101. A twin with those carbs would sound AWSOME. Better performance maybe

 

[richard jones]

Jeff the K&B with the O rings would really let the Venturi reasonate.

They would sound like a 4 barrel carb opening up.

I would love to have such a Venturi on a .550 or .570 carb for a 101. A twin with those carbs would sound AWSOME. Better performance maybe

Hi Doc, been running old style CMB, Gray carb, with .550 bore, and the long Venture, made 3 new spray bars, .99 th, 110. th and .119, been running the .110 spray bar, Andy Brown did the mods on the engine, running 10 1/2 length on pipe, when I pull the trigger, the boat is gone, just wish my hands would stop shaking so much, sometimes I JUST CLOSE MY EYES AND LISTEN FOR THAT UGLY SOUND !!!!!!!!!!!

MOBY

 

[dr gary turner]

"SOUNDS" AWSOME

Glad to see that you are running. Wish you could make the NATS in Huntsville but understand the issue

How long is the trumpet? Would like to see how you attached the trumpet. I run the same carbs on my 101 twin

 

[David Bryant]

Yes, this info is in numerous two stroke tuner books. Tidewater use to sell velocity stacks in the evo gold head/black case days. I tested several. With a local friend I tested a straight bore velocity stack about 1" long and a tapered bore velocity stack about 1" long on a cmb mod 80. The tapered bore did best, we could run a shorter pipe and launch more prop and the setup ended up faster on the top end because of this, on this particular setup. Jeff Lutz

Yours Truly designed the velocity stacks that Tidewater sold for the 1994 CMB Evo engines

was this one of the venturi's you designed? I purchased this one back in 1988 from Tidewater (for Picco P67/P80), they were very popular at the time.......

 

[dwilfong]

Here is a blast from the past....................

 

[Daniel's Racing]

H.E.W.

Howard E Weeks

Going back a couple years.

 

[Andy Brown]

Yes, this info is in numerous two stroke tuner books. Tidewater use to sell velocity stacks in the evo gold head/black case days. I tested several. With a local friend I tested a straight bore velocity stack about 1" long and a tapered bore velocity stack about 1" long on a cmb mod 80. The tapered bore did best, we could run a shorter pipe and launch more prop and the setup ended up faster on the top end because of this, on this particular setup. Jeff Lutz

Yours Truly designed the velocity stacks that Tidewater sold for the 1994 CMB Evo engines

was this one of the venturi's you designed? I purchased this one back in 1988 from Tidewater (for Picco P67/P80), they were very popular at the time.......

David,

I only designed the venturis for the 1994 CMB.

H.E.W.

Howard E Weeks

Going back a couple years.

Howard was a Master r/c boater. He did not compete, but his knowledge and skills where astounding.

I met Howard in 1973 when he worked at the Camera/Hobbyshop at Las Olas and Andrews, downtown Ft.Lauderdale.

Howard taught me a great deal about boats, engines and especially props. His modified JG props were works of art and were truly "game changers" with respect to performance.