Lotus Elan

Secondaty throttle

PostPost by: lotusbzz » Thu Jan 31, 2019 6:22 pm

Secondary throttle or not secondary throttle? this is the question

I know that after reading the following statements somebody will name me as the worst blasphemus of this Lotus Elan forum , but anybody ever tried to make the TwinCam engine working without that nasty flange with cross pipe?
Is this secondary throttle really necessary?
1.jpg and

Throttle what? It's just a piece of aluminium that connects the both intakes !
I do not need any vacuum, I've just fit the Spyder's headlight pop-up.
2.jpg and

… I remove them!
…. or better do not ?

Any advice for me?
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PostPost by: david.g.chapman » Thu Jan 31, 2019 6:47 pm

The missing section in your photo contains a balance pipe. Without this the engine will probably run incorrectly, although I can't remember exactly what is affected.
No doubt someone else will chime in with more information before too long...

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PostPost by: oldelanman » Thu Jan 31, 2019 7:15 pm

Secondary throttles are part of the Stromberg federal emission set-up. What you have is the standard European Stromberg mounting system, no secondary throttles.
The balance tube is there to even out manifold vacuum fluctuations which result from the combined effect of the firing order and the siamesed inlet runners and give smoother running at low engine speed. The adaptor plates with the large O rings and Thackeray washers provide a flexible mounting which prevents fuel frothing in the carbs. Both of these are necessary and, in my opinion, should not be removed. Others may have a different view.
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PostPost by: RogerFrench » Thu Jan 31, 2019 8:52 pm

Exactly what the other Roger said. Not a good idea to remove them.
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PostPost by: 10kph » Thu Jan 31, 2019 9:16 pm

Engines with multiple SU or Stromberg carbs require a balance pipe between the carbs to smooth out low throttle openings. Look at healey 3000 , E types etc
Lotus used the bulky aluminium casting to also receive the crossover pipes from the exhaust side to provide quicker warm up and lower emisions.
The carbs look better without the balance adaptor but it probably helps airfow into the engine by having a longer inlet port. Ie more torque.
The balance pipe only needs to be 1/2 inch diameter so how about machining two alloy adaptor plates connected out of sight undeneath.
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PostPost by: 69S4 » Fri Feb 01, 2019 8:27 am

There's two versions of that balance pipe arrangement (for the non federal cars), the 'high' (like Lotusbzz's) and the earlier low version where the pipe goes (roughly) along the middle of the casting.

In Miles Wilkin's Twin Cam book he says that whatever the marketing reason for the change (better idle, less icing in winter) the real reason was that neat fuel pooled in the pipe (= fire hazard) with the low version.

If that was a real concern I'd be a little worried that putting in an underslung pipe would be even worse. I suspect that if they could have got away with not using the cross pipe system at all they'd have dumped it at that stage.
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PostPost by: Esprit2 » Sat Feb 02, 2019 8:58 pm

lotusbzz,
The integral Stromberg manifold connects the 1-2 intake ports to one carb, and the 3-4 ports to the other carb. Then the 1-3-4-2 firing order results in each carb receiving two consecutive intake pulses ('sucks') in a row, followed by a two-pulse rest period.

Fuel is heavier than air, it has more inertia, and it takes it longer to respond to an intake pulse and get going. As a result, the first port in a pair will receive a relatively lean mixture. By the time the second pulse hits, the fuel will be moving better, so the second port gets a richer mixture than the first port. Then the rest period hits, it all comes to a stop, then starts over again when the next two-pulse hits that carb again.

The firing order is 1-3-4-2, and it starts with '1' simply because that's the way our simple minds work. With the engine running continuously, you can jump into the sequence anywhere, and

1-3-4-2-1-3-4-2-1-3-4-2-1-3-4-2
can just as well be...
2-1, 3-4, 2-1, 3-4, 2-1, 3-4, 2-1, 3-4

or..., if L = Lean and R = Rich...
L-R, L-R, L-R, L-R... on and on
2-1, 3-4, 2-1, 3-4...

When you check the sparkplugs, have you ever noticed that two cylinders consistently appear to be running richer (black & sooty), while the other two appear leaner (cleaner, tan colored)? All the above is why.

The balance tube connects the small plenum area that occurs where the two manifold runners join just before mating with the carb. And the balance tube adapter manifold enhances that plenum area a bit.
The balance tube allows for a little cross-talking between the 1-2 and 3-4 pairs. Not a 'full flow', but enough vacuum reaches the resting carb from the active carb to keep 'some' flow going in the resting carb. That keeps the heavy fuel flowing a little bit. Not fully up to speed, but also not starting from stopped each time the next pair of intake pulses arrives.

The balance tube has to be large enough in diameter to flow enough between the carbs to help, but small enough in volume to be able to reverse directions quickly... almost instantaly. There's complicated black art involved in balancing carb size, engine size (ie, suck pulse strength), plenum volume, balance tube diameter/ length/ volume, tube location (height), and a bunch of stuff. It looks simple, but it's very complex. For the Twin Cam, Lotus' High Balance Tube mounting adaptor seems to be the best.

Go ahead, eliminate the mounting adaptor manifold with it's high balance tube. Then notice how crappy the engine runs without it. Try re-tuning the carbs to improve the situation, and you'll find there's always a conflict between the two ports in a pair. Get #2 rich enough to run half-way decent, and #1 is way too rich... fowled-plug rich. Lean out #1 to where it wants to be, and #2 is so lean it barely runs/ burns valves.
Same with the 3-4 pair.

The Balance Tube Mounting Adaptor doesn't totally eliminate the problem... that's an unrealistic expectation. But it DOES improve the situation to the point that the engine runs fairly well, if not a 'little' rough at idle. There's still a difference in plug colors in each pair... but to a far less degree.

* Acceptably manageable with the balance tube adaptor.
* Problematic without it.

*~*~*~*~*
The Secondary Throttle is an entirely different subject. All Zenith-Stromberg Twin Cams (and 907s) have the Balance Tube, but only the Federal Emissions ZS Twink has the Secondary Throttle. The Federal emissions-lean mixture is difficult to ignite, and burns poorly. To improve that situation, the Secondary Throttle diverts the intake charge from the carbs up and out of the manifold, over the engine to a hot spot 'oven' on the exhaust manifold, where the mixture is heated to aid fuel evaporation; then over the engine and back into the intake manifold. Since drivability is almost non-existent to begin with, this monkey-business 'helps' and is worth while... but it comes with a complexity, a torturous inlet trac, and throttle lag. It's not good, but it is the lesser of two evils.

lotusbzz, your engine does not have the Secondary Throttle... at least not as shown in the photos. If it did, then the way to remove it would be to first defeat the emissions set up, change the carbs' needles/ jets/ diaphragm springs (copy Lotus' non-emissions Z-S set-up), and adjust the carbs to enrichen the mixture so the engine runs well to begin with. Then delete the Secondary Throttle butterflies & shafts, plug the shaft holes, and plug the egress-ingress ports to-from the exhaust manifold; but KEEP keep the Balance Tube part of the assembly.

As Roger (oldelanman) mentioned, the adaptor along with it's mounting O-rings and Thackeray washers, provides the equivalent of the 'soft mounts' that are used with Dellortos & Webers. With the carbs mounted out on the end of an arm (the manifold), engine vibration shakes them around and causes the fuel to froth. That seriously complicates jetting and mixture control. The 'soft mount' is critical to dampening the vibrations, minimizing fuel frothing and helping the engine run well. The 'soft mount' function alone is enough to justify keeping the Balance Tube Adaptor.

Your engine only has the Balance Tube Adaptor... do yourself a favor, and KEEP IT !!

Regards,
Tim Engel
Last edited by Esprit2 on Sun Feb 03, 2019 6:11 pm, edited 5 times in total.
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PostPost by: RogerFrench » Sun Feb 03, 2019 4:57 pm

It's almost blasphemous to follow Tim's erudition, but there is one other effect he hasn't mentioned, and an excuse to tell of Chapman's sorcery.
Let us imagine that No. 2 inlet valve is open, the piston descending on the intake stroke. The valve stays open for a little while on the following upward stroke, but before No.2 is finished, No.1 inlet opens. This effect is known as charge-robbing, or port-robbing, and is most noticeable on siamesed-port engines, which we don't have, and long-duration camshaft, which we might.
Balance pipes help mitigate that effect too.

Chapman's wizardry back in 1951 was first to de-siamese the inlet ports of the Austin Seven, then build a manifold that connected 1 to 4, and 2 to 3, so that the demands on each choke of a twin-choke carburetter would be evenly spaced and alternating. This was such a successful modification that it was banned by the organising motor club.
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PostPost by: rgh0 » Mon Feb 04, 2019 10:00 am

RogerFrench wrote:It's almost blasphemous to follow Tim's erudition, but there is one other effect he hasn't mentioned, and an excuse to tell of Chapman's sorcery.
Let us imagine that No. 2 inlet valve is open, the piston descending on the intake stroke. The valve stays open for a little while on the following upward stroke, but before No.2 is finished, No.1 inlet opens. This effect is known as charge-robbing, or port-robbing, and is most noticeable on siamesed-port engines, which we don't have, and long-duration camshaft, which we might.
Balance pipes help mitigate that effect too.

Chapman's wizardry back in 1951 was first to de-siamese the inlet ports of the Austin Seven, then build a manifold that connected 1 to 4, and 2 to 3, so that the demands on each choke of a twin-choke carburetter would be evenly spaced and alternating. This was such a successful modification that it was banned by the organising motor club.


Chapman actually copied the engine mods when Derek Jolly shipped an engine to the UK frm Australia for a 750 formula car that Chapman was building for him and Chapman opened the crate and examined the engine

I posted this a while ago but the myth that Chapman invented it lives on........

"By the way the famous 750 formula engine that Lotus used and caused the rules to change to ban desiamesing was developed by a guy called Derek Jolly who built it in Australia, had a lot of success and then took his "secret" engine to the UK to race there. He ended up with Lotus as did many itinerant racers did and Chapman used the idea. Derek ended up having a long friendship with Chapman and was the Lotus importer in Australia for many years. Mike Bennett who has the first Lotus formula car (a Lotus 12), plus a S3 7 and Elan in Adelaide has some fantastic photos and film that Derek took while working and racing with Lotus in England in the early days"

cheers
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PostPost by: RogerFrench » Mon Feb 04, 2019 2:47 pm

I knew about the Derek Jolly engine, but Peter Ross in 'Lotus the early years' writes that Michael Allen said that Chapman's block was already modified when they uncrated the Jolly engine. It was all a long time ago, so who knows now?
Certainly Jolly and Chapman remained on good terms, suggesting they may both have had the intention, though undoubtedly Jolly did it first.
I believe Clive Chapman still has the Lotus block.
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