venturi

[BadaBing]

I have been spending the past 3 days sanding, painting and secondcoating my hull with fresh ablative paint, Pettit Ultima 40sr, (very impresed with it so far)
While doing so I have had lots of time to mullxideas over. Some months back Mr Neptume, in a thread about fuel imjection,, gave a wonderful explenation of why ourncarbs are actually a bit oversized and the busimess reasons for that.

In a later thread a conversation came.up about some a4's, produced fo r use in Canada which had a smaller venturi in the carb to bring the hp down to 25 hp.
This seems to contradict Dave's asertion that a slightly smaller.carb would be better.( Very important point here, I am nlt challqnging Dave's knowledge at all on this subject.) But if a smaller venturi reduced the hp to 25 there seems to be contradiction.

Back in my wild youth the first step we would take to bump power kn our cars or motorcycles was to add a bigger carb. For example a very close friend had a Corvair 6 cyl with a factory rig running 4 2 barel carbs. My own chevy 283 came with a 2 barrelxbut with an adapter plate and a 4 barrel carb it look on a new life.

Im womdering is anyone has experemented with a reworked venturi, larger throat, with a larger main jet or adjustable jet.

The fact is nomatter what we are never going to get much more speed under power due to our hull designs and trying to do so just eats up gobs of gas. However developing more umph at lower rpms (torque?) could be beneficial.

On my t30 Im very prop restricted But if I put in a new and longer shaft with a strut I could go to a larger prop then the indigo which may very well help me to have more lower due to the more.effeciant prop.

Of course all of this could.be the byproduct of dust and fumes that got past my mask.

Praise the Lord, its been warm.all.week and Im very close to splashing the boat!

[hanleyclifford]

The problem with increasing carburation on the A4 is that we simply do not have the vacuum to make use of it. We learned in our hot rod days that you cannot get more in unless you can get it out - hence exotic headers, dual exhaust etc. The A4 is constrained by a single exhaust discharge 1 1/2" in diameter, which most then dutifully further constrain to 1 1/4" (go figure). Edit: If you really want more low end "dig", the single most important thing you can do is go to 2:1 reduction drive - in which case you will "destroy" Absecon Inlet no matter what the tide is doing.

[Dave Neptune]

Whoa thar

The Stevedore came with a restrictor not a smaller venturi, the venturi is the same. Once the restrictor is out the A-4 is still a bit over-carbureted. It is the cam profile that restricts this engine not the 1 1/4 exhaust or the carb.
Take a good look at a VW zenith sometime it is a carb capable of about 40 HP. The HP restriction of the carb is "air flow" controlled by the "intake profile" not the size of the carb opening. The A-4 is just over one liter like the VW and has twice the carb area.
The Stevedore does well as a direct drive with the restrictor plate but is limited to about 18 HP. The A-4 at around 1800/2000 RPM's will make 18~20 HP when properly tuned in this configuration. However with a reduction gear the A-4 is able to get to higher RPM's to make more HP and the restriction must be removed to gain with a reduction unit.
Our same carbs are capable of making 70 HP on larger engines like the tractors they are also used on!! Just a slight change in internal metering.

Dave Neptune

[hanleyclifford]

The Stevedore came with a restrictor not a smaller venturi, the venturi is the same. Once the restrictor is out the A-4 is still a bit over-carbureted. It is the cam profile that restricts this engine not the 1 1/4 exhaust or the carb.
Take a good look at a VW zenith sometime it is a carb capable of about 40 HP. The HP restriction of the carb is "air flow" controlled by the "intake profile" not the size of the carb opening. The A-4 is just over one liter like the VW and has twice the carb area.
The Stevedore does well as a direct drive with the restrictor plate but is limited to about 18 HP. The A-4 at around 1800/2000 RPM's will make 18~20 HP when properly tuned in this configuration. However with a reduction gear the A-4 is able to get to higher RPM's to make more HP and the restriction must be removed to gain with a reduction unit.
Our same carbs are capable of making 70 HP on larger engines like the tractors they are also used on!! Just a slight change in internal metering.

Dave Neptune

You could put a roller Iskendarian set up on the A4 and still get very little extra HP unless you increased the exhaust.

[BadaBing]

I hope I did not open a can or wormbs here.
It really was a case of a mind left to wonder while the rest of me was buesy with hull work that required very little little thought.

My goal isnt more hp. Honestly I love how little.ful we.need tp make hull speed and have no need, desire or iinention of trying to push a huge wave ummder power to try to keep up with the powerboaters. But there are times when more low end power would be appreciated, like when trying to back off a mud.bar thatxwasnt "there" last week. It must be humerious to see me towing my T30 off a soft grounding with a 9.9 hp on an inflatable, sometime s it just takes a little more than the Tartqn and its dinky little pop can produce.

I did give some.thought to a v drive, mew.shaft, strut larger.prop.conversion but after an hour or so of head games sanity returns and I ask myself, "why"

[hanleyclifford]

Not to worry

The souping up discussion has been going on for my entire lifetime with few solid conclusions, but if you want to get off mud bars with gusto there is no disagreement about 2:1 conversion. I powered my way out of the mud at low tide in front of Utsch's Marina. Moyer Marine may have a unit available, and I also have a couple around the shop. Too late for this year but a great winter project for the fall.

[BadaBing]

Hanley,
Is there a 2-1 reduction gear that is not a v drive? please excuse my ignorance on this but I have thought all.ong that when talk turned to 2-1 it was for V Drives

Is thisna bolt on to the. exishing trans or does it replace the transmission?
Ho.great,,, another project to plan for next year

[hanleyclifford]

2:1 is available for "straight" drive but requires an additional 6 inches of engine length and that the engine be raised about an inch. Here's a shot from the Moyer catalogue, and one of my installation. You would also need a new coupler and of course a bigger prop.

[BadaBing]

Thanks
I think I have room for this and by adding the 2-1 and sliding the shaft back I might have room to add a strut and a bigger prop.
These are very pricy pricy items. With no core to swap its aboit 1100 plus the strut and new prop.
Top speed will remain the sameish. Guessing i might add a knot at wot for those troublesome against the tide inlet challanges. More gusto for a hole shot, or digging out.of the mud shot. But not very sure that benefit would equal the cost, probably over 2k when all said and done.

W ho knows hat madness might overtake me.next winter

[hanleyclifford]

Remember,

what the Boat wants, the Boat gets.

[romantic comedy]

I have been threatening to add a reduction gear to my Tartan 34 for many years. I even have the reduction gear. I need to guess at what prop I would use though. Problem is I can only fit a max of 12 inches. But the 12 X 6 hit hull speed at 1500 or so. I guess I could do it with a 12.

With the Indigo prop I have made headway into 35 knots and 3-4 foot seas. (only about 1 knot, but still went forward)

[hanleyclifford]

I'm running a 12x12 three blade - perfect.

[romantic comedy]

There is hope for me yet.
Hanley what is you cruise rpm and max rpm?

My T 34 is 1200 pounds. 25 feet at the WL.

[hanleyclifford]

Destiny goes about the same weight loaded for cruising 24' at waterline. I generally run about 2000 rpm but can get to 2700 easily if I need it. If you do the switch you will have a different boat.

[BadaBing]

I have been threatening to add a reduction gear to my Tartan 34 for many years. I even have the reduction gear. I need to guess at what prop I would use though. Problem is I can only fit a max of 12 inches. But the 12 X 6 hit hull speed at 1500 or so. I guess I could do it with a 12.

With the Indigo prop I have made headway into 35 knots and 3-4 foot seas. (only about 1 knot, but still went forward)

It is because of the prop size restriction that I think I would need to extend the shaft further outcpast the shaft log and existing cutlass, which would require the addation of a strut. Then we would be entering the supernatural world of prop optimization. Just about as clear as therotical physics.