Twin Stromberg Manifold - mounting plane

[Jolls]

Hi All,

I am tossing up between molesting the firewall of a the ute project to fit triple SUs or running with a set of twin strombergs.

Rob correctly identifed when I posted an image of the twin manifold that it is mounted in a plane parallel to the head.



Most twin manifolds are are raked so I am trying to determine what advantages/disadvantages the parallel plane option has when compared to the raked manifolds. My gut feel is that the differences are marginal and given the agricultrual nature of our old greys it would be hard to differentiate between the options.

thinking about the different setups I have identified a a few advantage/disadvantagew between the two options.

The parallel plane option, when on level ground like in a garage, setting or checking the float level would be impracticable. However, this could be overcome by jacking the vehicle until the fuel bowls are in the same "level plane"; the same as the raked plane manifolds. The carbies being in a raked positon, while on level ground, is also not aesthetically normal.

On the other side of the coin the advantages provided by the raked options are also its disadvantage. By levelling the carbies it creates different length runners. In a performance engine this may make a bit of difference; but probably not much between the two options in a "real world" application.

Have I missed something here?

[Errol62]

It may run leaner at the front but I don’t know really.


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[ardiesse]

In a raked manifold, the carburettors are level when the car's on the level.  This is how the carburettors are designed to operate.  On the other hand, the manifold you have would be a lot simpler to make.  I don't think you'd notice the difference in operation with the carburettors tilted slightly back.  It mainly depends on how the float level changes with tilting.  Easiest just to set the float level at the midpoint of the float bowl on the left side, with the car on level ground.

rob

[Harv]

I'm assuming "raked" means one carb sits higher than the other (but with the top of the carb parallel to the garage floor).

Its probably important that the mounting pads (the flat surfaces the carbs bolt to) are machined so that they are flat (parallel to the garage floor) when the engine is in place. The typical engine "tilt" from front to rear is 3 degrees. If the pads are tilted 3 degrees, there is a 2.5mm difference between the front and rear of the float bowl. With all the slooshing of the fuel you would think that 2.5mm would make little difference, though Dr Terry has seen some huge differences in mixture in small changes to float level.

In a boat, the engines are often tilted even more (to get the prop into the water). Marine engines often run to 14 degrees, which would make a 13mm difference in the carbs. This is why the marinised manifolds have a real "wedge" cut into the mounting pads in order to get the carbs parallel to the garage floor.

Once the mounting pads are flat, you could machine one higher and one lower so the carb tops are level. This would make it easier to fit a common air filter to both. With twin air filters it would look marginally neater, but maybe not worth the drama of setting up a mill to make two cuts (one at a higher height and one at a lower height).

Cheers,
Harv

[Jolls]

Clay, Rob and Harv,

Thanks for your thoughts. I follow along and appreciate the input.

I have been unable to find any other examples, in my interweb research, of a manifold similar to the one I have. I am not sure if that is because there are inherent faults in the design, it is a home made bodgie or it had a short production run for some specific purpose. If there is/are problems with the design I would prefer to know now, and search for a replacement manifold, as opposed to fighting the design to adequately tune the beast down the track.

Harv's response had me running up to the shed to check if the mounting flanges were parallel to the engine or if they had been cut to set the carbs parallel to the ground. Sure as eggs the flanges themselves are set at around 3 degrees.





So in this case the carbs will be also be parallel to the garage floor; however, not at the same level above the floor. Most other twin manifold designs have the carbs level with the garage foor and the same level above the ground. Having said that I am not convinced that having the carbs parallel to the floor make much of a difference apart for making it easier to set the float height. You could easily jack the car up to achieve a "level" to enable the floats to be set correctly if the carbys were tilted off the ground plane.

In a raked manifold, the carburettors are level when the car's on the level.  This is how the carburettors are designed to operate.  On the other hand, the manifold you have would be a lot simpler to make.  I don't think you'd notice the difference in operation with the carburettors tilted slightly back.  It mainly depends on how the float level changes with tilting.  Easiest just to set the float level at the midpoint of the float bowl on the left side, with the car on level ground.

I don't think that the carburettors are designed to operate in a level plane. I do agree that they were designed to be tuned in this plane. A car operates up hill and down dale so the caburettors are constantly changing orientation and are required to operate effectively in the changing conditions. So to my mind as long as they are in the same plane as each other there shouldn't be any difference.

I'm assuming "raked" means one carb sits higher than the other (but with the top of the carb parallel to the garage floor).

Its probably important that the mounting pads (the flat surfaces the carbs bolt to) are machined so that they are flat (parallel to the garage floor) when the engine is in place. The typical engine "tilt" from front to rear is 3 degrees. If the pads are tilted 3 degrees, there is a 2.5mm difference between the front and rear of the float bowl. With all the slooshing of the fuel you would think that 2.5mm would make little difference, though Dr Terry has seen some huge differences in mixture in small changes to float level.

In a boat, the engines are often tilted even more (to get the prop into the water). Marine engines often run to 14 degrees, which would make a 13mm difference in the carbs. This is why the marinised manifolds have a real "wedge" cut into the mounting pads in order to get the carbs parallel to the garage floor.

Once the mounting pads are flat, you could machine one higher and one lower so the carb tops are level. This would make it easier to fit a common air filter to both. With twin air filters it would look marginally neater, but maybe not worth the drama of setting up a mill to make two cuts (one at a higher height and one at a lower height).

Cheers,
Harv

It does make sense for the carbs to be parallel to the garage floor to enable the float levels to be set easily, and now that Ihave looked more closely that is how mine is set up.

The raked manifold makes much more sense in the context of using a common air cleaner or for twin aircleaners to be same level above the garage floor from a purely "bling" perspective.

My gut feel is, and I could very well be wrong, that as long as the fuel bowls are level with the ground plane when the float levels are set either setup should produce similar performance outputs. The raked design means that both carbs are at the same height above the garage floor whereas in the parallel design I have they are in the same plan but at different heights above the floor.

Cheers.

[ardiesse]

Well spotted with the mounting pads for the carburettors.  The three-degree tilt means the throttle shafts aren't collinear, so you'll have to set the linkages up with a relay rod, bellcranks and adjustable pullrods to the throttle levers.  The "ears" on the manifold would take the relay rod.

In a way, this arrangement has its advantages: you don't have to go through all the hassle of changing the throttle shafts over to the type you can gang together.

Rob

[Jolls]

Well spotted with the mounting pads for the carburettors.  The three-degree tilt means the throttle shafts aren't collinear, so you'll have to set the linkages up with a relay rod, bellcranks and adjustable pullrods to the throttle levers.  The "ears" on the manifold would take the relay rod.

In a way, this arrangement has its advantages: you don't have to go through all the hassle of changing the throttle shafts over to the type you can gang together.

Rob

I have been thinking about the setup and it is starting to make sense to me.  The manifold is dual plane so has a common plenum and the linkage setup allows for accurate adjustment of the throttle opening. This is similar in nature to the Speco manifolds. As a result it could be set up so that the carburettors are synchronised or as a progressive linkage to provide a phased fuel and air delivery system, with one carburettor providing the initial response, followed by the secondary for higher performance.

The flexibility of this setup (and the Speco one) is probably the advantage of the design.

I have read Harv's Stromberg guide a few of times. I understand the need for venturi restrictors and smaller main jets with multiple carburettors. However, I am now wondering if this could be overcome using a progressive linkage system (without overcomplicating things).

My understanding of the progressive linkages is limited; and I wonder if it is worth investigating further. I imagine, that if the advantages were so good there would be information more readily available. My basic understanding is that the twin carburettors are set up to provide both economy and performance. The fuel system operates of a single carburettor to a set point and then receives a "boost" in the "power band" when the secondary carburettor kicks in. The key considerations being when the secondary kick in and developing an opening profile on the secondary so that both carburettors reach WOT throttle at the same time. I imagine that a pregressive linkage system would negate the need for venturi restrictors and perhaps the need to change the mains for smaller jets.

So, has anyone played with a progressiive linkage? Is it worth investigating or is it such a PITA to setup that it is not worth considering?

The setup I have looked to have been thrown to the back of a garage many years ago. The venturis had not been restricted and the main jets had not been changed. It could well be that someone back in the day couldn't get it to run properly (because they hadn't read Harv's guide). It was also set up with a US and Australian carburettor so that may have been another issue.

Tomorrows job will be to rat though the twin carburettor box and check the linkages to see how they were setup.

[Harv]

The manifolds have a common plenum, but the carb location is far from centralised in a dual setup. I suspect plenum mixing is limited. Running progressive linkage could exacerbate what is already a pretty rough fuel distribution.

Easy way to get a feel would be to fit progressive, and to run a IR heat gun at the exhaust tubes (for headers). See if there is a difference in temperature caused by either lean/rich maldistribution, or by “mix is about even but some cylinders are starved”.

Cheers,
Harv

[Errol62]

Sam Crupi has made a progressive linkage on his EK wagon, however it is a triple Stromberg downdraught set up rather than twin. The central carb runs solo under light pedal, with the outer two coming on under increased pedal. Seems to work well, however I share Harv’s reservations on how it would work with your setup.


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[Jolls]

The manifolds have a common plenum, but the carb location is far from centralised in a dual setup. I suspect plenum mixing is limited. Running progressive linkage could exacerbate what is already a pretty rough fuel distribution.

Easy way to get a feel would be to fit progressive, and to run a IR heat gun at the exhaust tubes (for headers). See if there is a difference in temperature caused by either lean/rich maldistribution, or by “mix is about even but some cylinders are starved”.
Cheers,
Harv

Sam Crupi has made a progressive linkage on his EK wagon, however it is a triple Stromberg downdraught set up rather than twin. The central carb runs solo under light pedal, with the outer two coming on under increased pedal. Seems to work well, however I share Harv’s reservations on how it would work with your setup.

Thanks for your insights. I can see how the triple setup would be a better option to work with. My thought process was the the progression would only be enough to overcome the inital pressure issue, overcoming the need for the venturi restrictor, and from then operate as a "normal" twin setup. I'm defintiely not thinking of some wild setup where the secondary would open at 50% of greater throttle. I was hoping that someone may have already done this "experiement" and could advise if it was a folly or not, and if not, what sort of settings are workable.

I know that I had read somewhere about a progressive linkage being used in a multiple carburettor setup - it may have been Sam's.