Thinking of removing secondary throttle assembly

[RogerFrench]

Hi Roger
I was trying to reinforce what you had said as some of the subsequent posts appeared to be missing the point.

cheers
Rohan

Apologies, since it could only have been me, I do not want to confuse issues.

I'll get my coat and go home now....

[Esprit2]

The reason that Strombergs have a balance pipe is to keep the manifold vacuum steadier at idle speed. Without the balance pipe the period of non flow is signficant at idle spped and this causes the piston to drop and then lift with each suction cycle which gives a poor idle and throttle response of idle. The balance pipe enables a more constant vacuum signal to the piston and thus constant piston postion in both carbs by ensuring they both see the same vacuum vacuum signal during the flow and non flow parts of the cycle...

I agree as far as you go, but that's only part of the story.

Petrol and metal are both heavier than air, and have more inertia. Each carb gets two intake pulses in a row. Air, being lighter, responds right away and starts flowing. Fuel, being further down the chain of events and heavier, takes longer to get going, resulting in a lean initial mixture which transitions to more rich. Then the second pulse hits while there's still some established momentum, so it skips the lean start and is more rich throughout... rich to richer.

Next, that carb gets two rest pulses in a row, the established flow fades away, and the whole process starts all over again next time around... suck, suck, rest, rest, repeat.

Layered on top of that is the air valve's inertia. The effect is similar in that there's a delay between the start of the air flow and movement of the air valve, so the first pulse starts out lean and gets richer as the air valve gets moving. Also similarly, the second pulse catches the air valve before it has time to drop, so it's mixture starts out richer and goes richer yet... compared to the first pulse. Same as the fuel inertia issue, but different.

The combined result is the sum of the two inertial effects, the first pulse in a pair is always leaner than the second, and the carb can NOT be jetted/ tuned to provide the correct mixture to both cylinders in the pair. If the first one to draw has a proper mixture, then the second will be too rich. If the second is correct, the first will be too lean. Read the plugs. 2 & 3 are each the first in their carb pair, and will look similar... leaner. Likewise, 1 & 4 are each the second in their pair, and will look similar... more rich.

Since going lean can lead to burned valves and/or pistons, it's best to err on the side of avoiding lean. Jet/ adjust the carb to favor a good mixture in the first cylinder, and the second will just have to be too rich.

Adding a balance tube allows the carb to draw some AF-mixture from the other carb/ manifold runner during it's two rest pulses. It never fully shuts down, there's always some flow, the fuel has some momentum, and the process doesn't start from zero each time. Similarly, there will always be some flow through the carb, and the air valve won't drop down to a full rest position. Fluctuation of both the fuel flow and the air valve motion is reduced.

The balance tube doesn't totally eliminate the problem; but at least the carbs don't start from scratch with each first pulse, and the mixture disparity is reduced. Not eliminated, but reduced. The balance tube is very important to the 2x1 set-up with a 1-3-4-2 firing order... do NOT remove it. The problem could be fixed with a different firing order (1-3-2-4), but that would require re-engineering the engine.

With 2x2 side drafts (DCOE or DHLA), each throat draws once, then rests for three pulses. Each draw starts from a standstill and goes from lean to rich through the course of the draw. However, the condition is repeatable for each throat, and you can jet/tune for it such that the total amount of fuel delivered to each cylinder is correct in total.

A single 2bbl downdraft (DGV, DCD, etc) with progressive throttles dumps into a common plenum volume at the top of the manifold (a single-plane manifold). Each manifold runner draws on the carb equally, whether it's running on the primary only, or the secondary has opened too. Again, it's a uniform condition, and you can jet/tune for it.

It's just that 2x1 set-up that's a problem, and the balance tube helps mitigate it.

Design of the balance tube is a bit of black art. It's more than just an inter-connecting passageway thrown on the manifold; it's diameter, length and position are all important. The Twink's balance tube is high mounted. Some non-factory secondary manifolds have been made with a low-mounted balance tube, and the engine doesn't run well with them. If you go looking for a replacement Euro-style secondary manifold without the ports for the cross-over tubes or the butterflies, then don't get suckered into buying one with a low-mounted balance tube.

Regards,
Tim Engel
Lotus Owners Oftha North (LOON)

[RogerFrench]

Tim,
I some detail you have described that which I hinted at in my earlier post, which Rohan didn't like too much and indeed I think, in retrospect, clouds the issue. Not that you're wrong - indeed, I remember back in the 50s and 60s there were people building inlet manifolds with generous balance pipes, even to the extent of pipes that appeared to continue through the main runner and terminate in buffer ends.

I'll dispute a couple of points you make. When you talk about the air valve's inertia, you seem to imply that the lower the valve for a given air speed the leaner the mixture, whereas the opposite is true. As the valve lifts the venturi size increases, the depression reduces and the mixture leans, even though the needle is tapered. Also, when you write about high- and low-mounted balance pipes you say that low-mounted are non-factory. That's not so, the initial factory Stromberg manifolds have low-mounted pipes, and I understand the change to high pipes was made because of fears of neat petrol collecting in the low ones with possible disastrous results.

[rgh0]

Roger / Tim

You are both right in what you say but miss one key issue that I was trying to emphasis in that the balance tube is really only useful at low engne speed around idle and a little above. At high engine speeds the gap between the suck suck is so small that there is no appreciable net flow in the balance tube which is small in diameter and long in length. Lotus made balance tubes both high and low, people appear to prefer the high mounted European balance tube than the original low mounted USA emission balance tube. Why that is I dont know. I have read comments that fuel collects in the low mounted tube and can create problems due to that.

cheers
Rohan

PS I took the strombergs of my esprit a month after i bought it 30 years ago. I have converted 3 stromberg heads to Webers and have one left to go if I need to. I have a whole box of strombergs and 907 stromberg manifolds, I have sold all my Twink stromberg adaptors / balance tubes to people who persist in using them. I dont think I will every put them back on an engine of mine, Webers or Delortos just look much cooler in the engine bay .

[CBUEB1771]

I remember back in the 50s and 60s there were people building inlet manifolds with generous balance pipes, even to the extent of pipes that appeared to continue through the main runner and terminate in buffer ends.

Very true, this is how the inlet manifold for my Elite/Coventry Climax FWE with two SUs is designed.

[RogerFrench]

Rohan,
The low-mounted balance pipe wasn't for the US, it was standard on the first European market Stromberg Twin Cams, which came into being before the Federal engines.
I've just looked up the Twin Cam "bible", Miles Wilkins' book, and he confirms that, as well as the bit about the high-level pipe being "neat fuel was found to be settling in the low-level pipe". I know I'd read it somewhere!!

BTW, I didn't miss the key issue that the balance pipe is only effective at lower engine speeds - I just didn't mention it but am very happy to acknowledge the point!

[Esprit2]

Rohan,
I agree with you that the balance tubes importance dwindles as the rpm rise. I didn't miss your point. But at idle and low rpm, real-world conditions not foreign to driving the car in everyday traffic, the Stromberg-Twink does not run well without the balance tube. The top of the tach may be more fun, but that doesn't mean the bottom of the tach doesn't exist.

The point of the thread was JGeezer's intent to remove the entire secondary throttle assembly, which includes the balance tube. That's not wise in a number of ways, and my comments talk to the benefits that the balance tube brings to the table.

For operation outside the balance tube's effective speed range, all that doesn't apply... true. But that doesn't mean it's not of value in it's effective operating range.

If you're going to run a 2x1 set-up (Strombergs, SU's, whatever), then keep the balance tube. And while you're at it, also keep the Soft Mounts... or is that another arguement?

*~*~*
I'm a Dellorto guy more than a Weber guy. I too have converted all my Stromberg engines to 2x2 sidedrafts, using Dellorto DHLAs. I've got boxes of Stromberg carbs, manifolds and fiddley bits that I'm keeping to preserve the cars' originality should I ever decide to sell them. However, so far, I'm not proving to be much of a seller.

In a steady state condition, the constant depression carbs can be made to perform very well. They're not turkeys, and in many ways they're actually easier to drive. However, there's a reaction time associated with the air valve function that takes the crisp edge off of the throttle response. Fixed venturi carbs, like Dellortos and Webers, have a more immediate throttle response that makes the engine seem more alert and urgent. Crisp. Even if the engine doesn't make more power, it seems more engaged. I like that, and IMHO, getting that feeling in a sports car is worth the price of conversion.

Regards,
Tim Engel
Lotus Owners Oftha North (LOON)

[singerv]

On the same vein but a slightly different question(s). I have a US exhaust emmison control layout. I have been a advised to remove the butterflies in the manifold as this is a cause of many starting and idling issues/problems. Ok, but then do I "have to" blank the crossover tubes or can I leave them? What is the expected effect of doing each? I was going to epoxy the holes where the brass rod is removed, another option seems to be just remove the butterfly plates and leave the rod, may acutally help with atomization or may give it someplace to crash into and slow not sure which! The manifold has a top and bottom channel, with the butterfly in place any flow would be channeled by the plate, without the plate the flow from the top and bottom would meet, anyone see this as a issue or is the intake of such a magnitude that it really doesn't matter?


thanks,

Vince

[Esprit2]

I'll dispute a couple of points you make. When you talk about the air valve's inertia, you seem to imply that the lower the valve for a given air speed the leaner the mixture, whereas the opposite is true. As the valve lifts the venturi size increases, the depression reduces and the mixture leans, even though the needle is tapered.

Only during a period of transition. The whole point of a variable venturi carb (SU, Stromberg, etc), is to maintain a constant depression (constant vacuum) in the venturi area immediately surrounding the needle & jet. Constant Depression is what the CD in in Stromberg model names means... like CD-2SE.

Vacuum is fed from the throat to the chamber above the diaphragm, where it tries to lift the air valve (piston) against the pressure of a calibrated spring. The diaphragm/spring combination is a classic pressure regulator configuration.

In a steady flow condition, the regulator responds to current airflow to raise/lower the air valve to a point that results in a pre-determined vacuum. As the airflow increases or decreases, the air valve's position is automatically adjusted to maintain a constant vacuum in the throat surrounding the needle & jet.

With a constant vacuum present, then a lower air valve puts the needle deeper into the jet, partially closes off the orifice, and reduces the fuel flow. That doesn't necessarily mean "leaner", since it's responding to a lesser air flow condition by reducing the fuel flow proportionately.

Similarly, for more air flow, the piston rises to a point that produces the same pre-determined constant vacuum. The resulting higher air valve position withdraws the needle from the jet, opening the orifice, and allowing the same constant vacuum to draw more fuel. Again, that doesn't mean the mixture is richer, it just means that more fuel is being metered proportionately to more air flow. The needle design can be tapered such that it does change the air fuel ratio as the needle position changes, but that's a design option and not the functional definition of how the carb works.

The needle's changing position can result in leaner/richer mixture, depending upon the needle's taper design. There is a plethera of needle tapers available to meet different engines needs, as well as address economy, emissions and power preferences. But that's a different topic, and not a description of the carb's basic function.

What you're talking about only applies to the brief transitional period when the air flow actually changes, but the air valve has yet to move and establish a new position as required to maintain the constant depression.

As you say, the increased airflow results in increased velocity through the variable venturi in it's present position, and the resulting increased venturi effect produces a stronger transitional vacuum. The stronger vacuum draws more fuel through the orifice even though the air valve has not yet moved and withdrawn the needle from the jet.

This transitional enrichment is why the constant depression carbs don't have accelerator pumps... they don't need them. But the enrichment is due to increased transitional vacuum, NOT due to the needle's position.

When the air valve does finally react, rise and withdraw the needle from the jet, the resulting larger venturi throat does indeed cause the flow velocity to drop, creating a weaker vacuum... but only weaker relative to that very brief, transitional "increase" in vacuum. Now, the vacuum level has returned to the pre-determined constant depression the carb is designed for, the higher air valve has slightly withdrawn the needle from the jet, allowing porportionately more fuel to flow in response to the greater air flow.

You write:
> whereas the opposite is true. As the valve lifts the
> venturi size increases, the depression reduces and
> the mixture leans, even though the needle is tapered.

Not really. In that split second of time, the depression does reduce, but only relative to the temporarily transitional increase in vacuum that just occurred. And it's not so much a matter of richer as it is more fuel flow in response to more air flow. Your statement sorta applies to that split second in transition, but you write it as if it defines the overall operation of the carb. It doesn't.

When the Air Valve finally moves, then needle taper comes into play again. But as that occurs, the vacuum again returns to the pre-determined level. The higher needle flows more fuel in reponse to the greater air flow that caused the higher Air Valve position, but it's not about richer-leaner.

From a flow metering, weir stand point, the higher needle allows more fuel to flow in response to any given vacuum... a lower needle flows less. The increased fuel flow during transitional acceleration is due to the temporary increase in vacuum, just sucking on the soda straw harder. That's not about needle position.

A similar but opposite transitional lean condition exists when the air flow is reduced. Velocity through the variable venturi drops off, the vacuum gets weaker, and draws less fuel from the orifice. Again, that's a function of weaker transitional vacuum, and not needle position. Needle position hasn't changed yet, but it's about to as soon as the Air Valve reacts.

Normal Operation:
Lower air flow = lower air valve & needle = less fuel flow
Higher air flow = higher air valve & needle = more fuel flow.
More or less fuel is delivered in proportion to more or less air flow, and doesn't necessarily mean richer or leaner. With constant depression, a lower needle delivers less fuel, a higher needle delivers more fuel. It's not about lean-rich, it's about fuel delivery proportional to air flow.

Transitional Operation:
Air flow changes, but the Air Valve has not yet reacted.
Increased air flow = stronger vacuum = more fuel flow.
Reduced air flow = weaker vacuum = more fuel flow.

CD carbs are very good at metering consistancy under steady flow conditions (which is why they were used for early emissions engines), but they're slow to react to chanage (slow throttle response). Anything that averages out the air flow, reduces the spikes, and keeps the Air Valve's flutter to a minimum is a good thing. At low rpm, the combined effect of the 2x1 carb set-up and 1-3-4-2 timing promotes fluctuating air flow and Air Valve flutter. The balance tube maintains some level of air flow through the carb at all times, minimizes the Air Valves dance, and results in a more consistent mixture control.

To a large extent, this is becoming a case of two people in violent agreement. Unless you have something really relavent to say, I'd prefer to let this discussion die of it's own dead weight. It's gone way beyond JGeezer's original post.

Also, when you write about high- and low-mounted balance pipes you say that low-mounted are non-factory. That's not so, the initial factory Stromberg manifolds have low-mounted pipes, and I understand the change to high pipes was made because of fears of neat petrol collecting in the low ones with possible disastrous results.

I didn't say "low balance tubes are non-factory", which implies "all"... those are your words. I did say, "Some non-factory secondary manifolds have been made with a low-mounted balance tube, and the engine doesn't run well with them." There was a long discussion about that on a different forum some years ago.

I am aware of the early factory manifold with the low balance tube. But I have no experience with it, so didn't write about it.

Regards,
Tim Engel
Lotus Owners Oftha North