To overcome, and also remedy each end of the discussion, would it not make sense to have the gas flow from all four cylinders to be split evenly to each turbo. This would decrease pulsation. Then have each turbo feeding the same four cylinder intakes through a plenum design. The smaller turbos will spool quicker because there is less enertia and mass to overcome. All said and done, the twin turbos would look great and it has sparked a great Discusion.
Sorry phunkdoktaspok I was intending to say that in my belief the argument that L18 had, was slightly more weighted, though you both have a valid point. With all things there is a payoff. you have to way up the positives of each, and decide on what you want to compromise.
But as we know, the decision has been made, and as enthusiasts I think we can find way to make twin turbos work on an A15.

i suggested a plenum type thing for the exhuast earlier but i think losses would outway benefits

remember 40 times per second at only 2500revs, surely thats not really "PULSING" its pretty damn consistant if you ask me, once every .025 of a second, not even a school boy can pump that fast

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because the wastegate determines the maximum boost reached...

to explain why a short turbo manifold works better than a J pipe is simple. You obviously want the exhaust pulse to encounter the turbine while it still has the most energy available to drive it. As exhaust gas exits the combustion chamber it is superheated and still expanding, and therefore accellerating. So peak exhaust pulse speed occurs somewhere in the early part of the exhaust manifold. After that it is dissipating energy in the pipes. So the closer you mount the turbo to the head, the more energy you can capture from the exhaust pulses before it is simply lost.

That's it, nothing to do with having to fill up the larger volume of pipe with exhaust gas before the turbo starts working.

Packets of gas with their pressure peaks and troughs (Pulses) actually cause the turbo to respond better at low engine speeds as the gas in the packet is still at high pressure.

At higher engine speeds when packets have become almost homogenous, pressure peaks and troughs tend to even out. If there is sufficient resistance due to boost pressure on the compressor wheel, turbine pressure will build to maximum packet pressure.

This is why exhaust valve timing is very important on high boost turbo's. The gas needs to be at sufficiently high pressure to overcome the compressor boost.

As L18-B110 said, the closer you mount the turbo to the head the better the response.

An interesting variation is the use of long runners in world rally car (WRC) and race car turbo exhaust system's. They use long runners to keep pulse packets seperate as long as possible.

The argument for short or long runners really depends on what you want out of the setup.

1200RC, If nothing else it will be very educational and may actually work well.

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True my wording was wrong and you are correct

How I should have written it is:
Why do both setups create the same overall boost level at the same rpm?

I have experienced this when I have swapped a j pipe for a short manifold.
Same turbo ( off a VG30DET ) same A12 engine same car same everything except manifold.
Wastegate hose disconnected ( so wastegate not controlling anything )
This turbo is off a 3 litre single turbo engine, so there is no way an A12 could outflow this turbo and max it out.
Short manifold starts to spool up a 1300rpm and by 3000rpm 10psi boost.
J pipe manifold starts spooling up at 2100rpm and by 3000rpm 10psi boost.
As rpm continues to increase so does boost levels the same.
( so no way wastegate is controlling it, especially when it cant be actuated )
And there is no way the turbo was maxed out as I have taken boost levels past the 30psi limit on the gauge.
Also the driving conditions were the same, I can say this as this wasnt just a 5 minute test, the car was a daily driver, so nothing like different weather conditions ( ie temp )or driving conditions ( ie one up hill and the other setup down hill ) could have been an influence.
Now if it was relied souly on these pulses, like I stated before the pulses would be weaker by the time they got to the turbine on the j pipe. Ture / False?
So how did I get the above results?

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So a j pipe manifold has a longer path before the turbine. And once this exhaust gas is created to quickly to pass through the turbine unnoticed , it will cause the turbine the begin to spool up. Now since there is a longer path, there is a larger volume that can fill this manifold. So its going to take longer for this pressure to build ( turbine begins to spool at 2100rpm) But once this pressure is built up the turbine will spool quicker.(ie 3000rpm 10psi boost)
( Remember the turbine is a restrictor, it will only flow as much volume of gases as the pressure of these gases forces on it.)

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Yes thats when you will get the most energy from these pulses,
But you seem to think you can pump as many litres of air that you can into an engine and it just becomes lost and you only end up with bursts of energy and no volume of gases!
I understand that the airs volume goes down when the oxygen is burnt but can you say that the waste gases have no volume?

Quick question L18;
How much blowby do your engines have?
( to me your gas volume is exiting the engine somewhere it shouldnt be!)

Cheers Steve

phunkdoctaspock, you are now taking everything I have said entirely out of context. All my comments, which from the start have been focused on poor low end response due to the pulsing problem, have been made in reference to a twin sequential turbo setup on a four cylinder motor.



Your experience with the setup you describe should have been a noticeable improvement in how the turbo spooled up if the short manifold was well designed.

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So why did you continue on about 2 or 3 cylinders operating 1 turbo.
In a sequential setup all cylinders are supplying both turbos.
Thats why I differentiated the differnt setups in my second post.
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And all my comments have been on why this pulsing problem is not the only contributing factor.
Did you take the time to read my posts?
Whether I described 2 cylinders or all cylinders, makes no difference, I showed that gas production and flow is a major contributing factor.
How is this taking it out of context?
Yes 2 turbos have frictional forces to over come.
Yes these frictional forces are going to be twice as much as a single turbo.
This being because the compressor/turbine axle and bearings are going to be roughly the same size in each turbo as it is with the single turbo.
But if your read what I wrote You can see my point, these pulses are not the only factor, gas production and flow is a contributor too.
So my point being whether 2 cylinders or more, a gas volume is still being produced, and when the turbine becomes a restriction for this volumes flow, flow will be increased from the pressure build up of this gases volume, with this making the turbine spool. So like I said once again, with a correctly sized turbo, just because it is supplied by only 2 cylinders isnt going to make it non operational.
Key Word " Sized " Small spools earlier, larger spools later.

Out of context? Not if you read it all.

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In your theory yes this would be true.
I cant say how well designed the short manifold was, but a description of it is;
4 runners of aproximate equal length, as short as permited to mount the turbo close to the head but still away from the carbie.
Not to much to go wrong and cause dramatic losses of overall boost, but yet for some reason it still created the same overall boost level for a given rpm as it did with a j pipe.
Like I said thats from my experiences, not something I read in a book.

Cheers Steve

sorry, my bad - you know very well I meant twin simultaneous (if you read any of my earlier posts as closely as that one ), which is what the whole topic is about anyway.

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well, there's an important point you neglected to mention in the previous description of the system - it's a draw through setup. What size and type carb are we talking about, and just how far away is it? I don't really care or want an answer to those questions, but if it didn't spool up earlier in the rev range with the short manifold you either got it very wrong or something else was the restriction in the system - like the carb or inlet manifold or exhaust system. Or maybe it was just terrible ignition timing off boost, etc etc

anyway back to the point...

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Because That

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I cant say I did, you see it gets confusing when you actually write one thing and mean another.
The whole topic is what is better twin or single, twin has 2 setups, so both are involved in the topic.

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Did you read my post? If you did you would have read;
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And for you information its a blowthrough setup with standard carbie, original primary jet and secondary jet drilled to the same size as a 3.3 commodore secondary jet.
My description of the manifold is meaning runners short as possible but still long enough to mount the turbo away from the carbie ( ie carbie on original inlet manifold, so the runners are just long enough to mount the turbo a little bit away from the carbie )
No problems down low and none up high either!

And plain and simply the last part of your post is a joke which shows you have trouble reading and understanding.

YES its agreed you are talking about pulsing problems down low when only using 2 cylinders.
BUT I am saying this is not the only contributing factor, Gas production and flow are a major factor. So whether it is 2 or 8 cylinders this gas production is still being produced and with a correctly sized turbo the problems of it being less on 2 cylinders then any more cylinders isnt an actual problem.
You get it Gas is still being made!
#### it is hard!
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