single or twin turbo best?

the reason why you don't use 2 turbo's on a 4 cylinder:

splitting the exhaust gas will halve the exhaust enthalpy and more importantly the time constant between high pressure pulses in the exhaust is doubled. In layman's terms this means that each turbine has a much longer period between exhaust pulses (the flow is unsteady remember) and the turbine will slow down measurably in-between meaning that it has to be accelerated again with the next pulse even when running the engine at a constant speed. This will have a measurable effect upon the boost pressure generated so there will be alrge fluctuations here as well. This is of course the same with a normal single turbo set-up but the effect is far less pronounced. Two turbo's will also mean greater friction so more energy will be required to accelerate the turbo's at low speeds - two small turbo's will not have half the friction of a single large turbo, it doesn't work like that. And a sequential small and large turbo will certainly have greater friction, but are used on the 6 cyl engines (and above) to give greater driveability across the whole engine speed range.

Hope this helps,
tt

 

Hey its good to see that some members are more welcoming than others. And yet today I logon and there is another 3 PM's from ausgolfer ( mostly in jibberish ) I will take your advice D4RK1 and choose to ingnore them.

One thing I should say about myself is I like to discuss reasons for my posts. Its all to easy to say ( nope no good, won't work, or thats gonna work etc ) without giving reasons.
So if I debate anyones posts Dont think I am trying to proove you wrong. Its just I see things a different way and if you explain your views you might teach me something, Same goes for my posts. In the end we all benifit as we get all the information.

trendy tramp thats some good information you posted on the exhaust pulses. With that you are refering to constant twin turbo with 2 exhaust ports for one turbo and the other 2 for the other turbo.
My question is do you think these pulses would be evident when at only 1000rpm each exhaust valve is openning 8.33333 times a second? Increase the rpm and these pulses and following dead spots would become even more negligible.
If this were the only contributing factor I belive this would have a big impact on a turbos operation. But there is another major factor.
Gas Volume ( this is where turbo sizing comes into play)
Each combustion cycle produces a gas volume and this can easily pass through the turbine wheel unnoticed. Lets look at the turbine wheel as a restrictor and when the rpm of the engine is increased this gas volume is created to rapidly for the turbine to pass it unnoticed, as this gas volume is increased its flow past the turbine will become quicker and this will cause the turbine to spin.
So with a correctly sized turbo this problem can be overcome as there are 2 contributing factors.

On the sequential setup it can over come the friction easier. This is because 1 turbo is already being feed by all cylinders and when the second turbo is activated although the exhaust gases are now divided between 2 turbos the exhaust gas volume is now larger, as the engine is being forced air and fuel under pressure.

Like I said I am not trying to proove anything wrong I am just discussing the topic deeper.
I am always open for debate.

Cheers Doc

 

lol been watching this space for the past year and half rob havn't seen you at a jct29 meeting recently ?

 

Apologies to all, especially Dock. I was foolish in thinking that your statement "Jimbob I have been reading all the posts and its plain to see some people have the attitude of " thats just stupid" . I feel this is because they have no idea on this subject which to me makes their posts pointless." was in anyway rude or offensive.
Sorry for cluttering the thread more.
Welcome to the forum dock, no hard feelings.
Cheers.

 

Ausgolfer there are no hard feelings taken.
Its good to see that members especially fellow Australian members are adult enough to admit thier fault.
I look foward to learning from your posts and also you from mine.
Remember its a good idea to read posts carefully before you jump to conclusions.
But theres no harm done now as they have been rectified.

Cheers Doc.

 

Hi Paul,
Not had much time recently sorting the business out but I will make a concerted effort to get along soon. As for the car, I am in Lincs this weekend doing some more. Has to be done for Inters as it's on our trade stand

tt

 

Dock,

You mention exhaust pressure and the short time constant as the rpm rises but you should consider the exhaust enthalpy or internal energy. The temperature is important also here, not just pressure. Maintaining a high temperature will help to maintain exhaust gas energy. Longer periods between exhaust pulses will give more time for the manifold and turbine housing to cool, meaning a greater percentage of the next charge will be used to heat them again. And what may seem a very short time period for us is not a lot for an engine and certainly not a turbo. The turbocharger compressor and turbine wheels are designed to offer rapid acceleration by reducing their mass as much as possible - unfortunately this also means that they have very little inertia and because of the frictional forces at those high rotational speeds the drop-off in speed between pressure pulses can be substantial at lower speeds. You are absoltely correct though that at higher speeds the effect will become less and less pronounced - at 2000rpm the exhaust gas pulses will be spaced the same as a single turbo at 1000rpm.
Additionally, the friction properties will be increased slightly with 2 smaller turbo's. The compressor and turbine wheels will not have half the rotating mass of the single large type as they are not half the size. Additionally the shaft size is likely to be roughly the same [as well as bearing size] so again the friction will not be halved.
As well as this there are the practical aspecvts of plumbing-in the turbo's and you will also have 2 large heat sources within the engine bay that have to be fed oil to and plumbed-in. 2 sets of compressors runnng into rthe same intake have to be designed very well as you could get some odd effects. And there is no guarantee that the 2 turbo's will be running at the same speed which could lead to some very odd press-time effects in the intake system. Also because of the prescence of 2 intakes (1 additional volume to normal) there could be resonace tuning effects at certain freqency ranges within the system.

Aside from all that, it's a piece of cake

tt

 

Some good info! thanks, what about things like pulsars, arent they 2.0 twin turbo (although i heard they are not reliable) does anyone know if they are constant or sequential as it might be worth looking into thier system!?

 

i think pulsar GTI-R's are 2 litre single turbo from memory.

 

Ignore my previous post if this is correct!

 

Good stuff trendy tramp, shows that there is alot involved but yet nothing that cant be overcome if the conversion is thought out.
The GTI-R's that I have worked on are 2 litre SR20 single turbo but I cant say if they were different in other countries, the ones I have worked on have all been imports from Japan.
Its the same engine in the Silva's and the Bluebird's, I have never heard of any problems with these engines.

Cheers Doc

 

Yep the Pulsars are single turbos,

Doc, im pretty sure the Pulsar (GTi-R) was only ever released in Japan so the ones you have worked on or seen will be the same as what we have over here. All SR20's.

Well reading through this post has certainly blew the top of my head clean off, but seeing as i run a turbo'd motor ive gone over it all a fair few times so i can try to understand it all.

From what ive read it definatley seems a sequential setup is the more beneficial but more suited to the bigger boys!