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 thoughts on Compound turbocharging? 
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 Post subject: thoughts on Compound turbocharging?
PostPosted: Sun Sep 18, 2016 1:07 pm 
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with there not being a lot of choice of turbos for the L series that bolt on. the ones that do tend to be too big and laggy, i was wondering if Compound charging might be an interesting way to go, say over a VNT set up.

i don't know how you'd get on with the later electronic pumps on the later L series though.

ive seen lots of people with cummings lumps sporting two turbos and from what i can tell it seems to work very well for them. i wonder if we could scale it down and use it on the L series.

easy way to get more boost without an expensive single turbo?

thoughts?

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 Post subject: Re: thoughts on Compound turbocharging?
PostPosted: Sun Sep 18, 2016 2:01 pm 
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The pump is not a problem as they can be mapped and hybrid injectors do wonders. The issues on the l series is the exhaust housing being to small. The compressor side I thought was good for near 200bhp. If they where parallel on the exhaust and compound on the intake that may work well enough.
Remember the Cummins are very large engines that flow lots of exhaust gasses and smoke is not normally a problem and neither is revs as 3-4k is absolute max if not less

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 Post subject: Re: thoughts on Compound turbocharging?
PostPosted: Sun Sep 18, 2016 7:41 pm 
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Not sure about the diesels but I'm looking at compound at the moment.

I see a build with someone in a zs diesel having a lot of trouble with large boost on a single turbo so not too sure how well it would work with compound boost.

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 Post subject: Re: thoughts on Compound turbocharging?
PostPosted: Sun Sep 18, 2016 8:08 pm 
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Vnt is the way to to be honest. Most people build a small arduino microprocessor to control it. It would need to monitor boost(map), rpm's, and throttle minimum, could have temps for engine and intake for safer control

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 Post subject: Re: thoughts on Compound turbocharging?
PostPosted: Sat Sep 24, 2016 5:24 pm 
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I think Kris has just launched his VNT controller to the market which is certainly worth considering.

Compound charging can work great but its is very very complicated and well lets face it the L series and the rest of the rover isn't!.

I'm a great fan of the KISS principal (Keep It Simple Stupid), which is why I went old skool medium sized turbo on mine. If I was doing it now I'd probably go VNT with some controller but then a maestro is old skool so perhaps not... :D

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 Post subject: Re: thoughts on Compound turbocharging?
PostPosted: Sun Sep 25, 2016 10:58 pm 
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Quote:
I think Kris has just launched his VNT controller to the market which is certainly worth considering.


Thanks - and yes - we most certainly have - details here:

http://www.dieselpowered.co.uk/vnt-turbo-controller.html

It's a fairly simple controller but quite sophisticated in that it doesn't act as a 'boost' controller, it's a bit backwards (but actually a positive thing) in that it works to meet your target boost with the least vane closure possible. It's a different paradigm to what you'd expect but most OEM boost control works on a similar model.

anyway, I have tuned vehicles with compound turbos before and they can work, but they MUST be well matched. The one I tuned, the company fitted a HX35 as a low pressure turbo and a small VNT (GT17 frame iirc) as the high pressure turbo - this did not work particularly well, and despite beating single turbo GTB2260 builds with the same engine in terms of overall horsepower, the curve was so much better on the lower power builds that they could comfortably win in quarter mile events (which they were built for). Then there were the reliaiblity concerns!

It does work though, but a vnt is far more forgiving for error - if its slightly too large it spools late, if it's too small you won't make your target power - but in any case you've some room for adjustment and most reasonably sized turbos will generally 'work' even if not 100%. A compound can be completely undrivable and very complicated to set up, with any change in setup requiring a mass of fabrication. This aspect gets expensive, and fast.

Another option is a larger wastegate turbo, with considerably higher boost threshold and lag, but quite reliable and relative power increase.

It's worth noting that lag/threshold gets an unfair hearing IMHO. When tuning VAGS with GTB2260's for example, they hit full boost about 27-3100 rpm) - which on paper looks quite bad, however if you plot the boost curve most are making similar boost on a fourth gear pull as their stock counterpart. The same goes for their torque curve which is very similar if not slightly lower. Yes the lag exists, but it isn't all that bad, as a general rule as long as you don't push too hard then a correctly sized turbo, even a wastegate one will give you back at the top anything you lose at the bottom.

Due to a change in plans (I have other work to do and a better GTB2056 turbo to use) I did think about selling the old exhaust, downpipe and turbo (fitted to ex manifold with adapter plate) with GTB1856 hybrid turbo I used to run on my old 45. I never bothered because despite most fabrication being done, there's still a bit of fabrication needed (oil drain sorting out and intake pipework). Might flog it one day if there's still an appetite for l-series tuning in the future.

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 Post subject: Re: thoughts on Compound turbocharging?
PostPosted: Tue Oct 25, 2016 2:39 pm 
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Reading this, isn't a VNT an unreliable choice for the L-series? As to make good power they are usually smokey.. and thats going to clog the vanes up real quick?


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 Post subject: Re: thoughts on Compound turbocharging?
PostPosted: Tue Oct 25, 2016 9:20 pm 
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Not really as the main reason a diesel smokes is overfuelling, and to stop this you can either back the fueling off or ad more air, a vnt turbo can normally supply more air faster than a normal wastegate turbo so smoke would be less on throttling up, as a note some older diesel cars like the old 1.9 fiat tipo td didnt have any waste gate or vnt and purely relied on the amount of exhaust gas coming out so to up the boost all you had to do was up the fueling it didnt smoke to much more as it had the extra air to compensate

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 Post subject: Re: thoughts on Compound turbocharging?
PostPosted: Tue Oct 25, 2016 11:49 pm 
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At the end of the day a car will only smoke as much as the mapping or mechanical fuel control allows - so you shouldn't avoid vnt because of smoke, but you should avoid smoke because of VNT.

All that is set in the tuning stage however, and providing you make reasonable decisions here you should be able to fit a VNT without disadvantage.

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 Post subject: Re: thoughts on Compound turbocharging?
PostPosted: Wed Oct 26, 2016 12:17 pm 
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Thanks those are the answer's i was looking for, but less smoke less poke as it were?

I'm looking at turbo's at the moment, but im thinking i can make more power with a waste-gated turbo, than a VNT.. can a VNT make 180bhp+ without smoke?


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 Post subject: Re: thoughts on Compound turbocharging?
PostPosted: Wed Oct 26, 2016 7:21 pm 
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Hmm yes it can, all modern high power diesels use VNT and dont smoke,
smoke does make power but you need air to reduce smoke and a vnt can spool faster and lower so will smoke less, a wastegated turbo will smoke on spool up untill there is good boost to balance the ratio, you could also play with injector pressures, nozzle spray patterns ect..

2 bar of boost on a vnt is in theory the same as 2 bar on a wastegate but normally a correct size vnt will flow better so will produce more power,

a wastegate'd turbo is great for high boost as can be stronger than a vnt, but if you want drivability then a vnt is better,

there are a lot off 2.0 and below diesels that can run over 200bhp on standard vnt turbo's and still have great low end power.

the only big draw back is that vnt turbo's do not like high exhaust temps, but then if you have high temps then something is not quite right.

there was a guy that fitted a basic vnt to his and got unmapped over 180bhp with the vnt controlled by a standard wastegate actuator, with a controller it would have been far better on economy and power.

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 Post subject: Re: thoughts on Compound turbocharging?
PostPosted: Thu Oct 27, 2016 10:04 pm 
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mclellan51886 wrote:
Thanks those are the answer's i was looking for, but less smoke less poke as it were?

I'm looking at turbo's at the moment, but im thinking i can make more power with a waste-gated turbo, than a VNT.. can a VNT make 180bhp+ without smoke?


The topic can be as simple as complicated as you like - but you don't need smoke to make poke - that's a bit of an old fashioned point of view, however running with a slight excess of fuel (which normally releases a bit of a haze) will generally outperform the same set up running smokeless.

Performance does not require smoke, however and it's perfectly possible to build a car to increase power without drastic levels of smoke. You just factor in an allowance of which you'll sacrifice a bit of the extra horsepower in return for cleaner burning.

It's actually the other way round if we want to be pedantic - it's people who sacrifice the egt and clean burn characteristics for a smidge extra power, but we can run around in circles.

VNT's are complicated and to run effectively (empthasise that last word) they need intelligent control - boost pressure isn't everything in the turbo world and the optimum control for torque and safe operation is not a control system whose output is tied to boost levels alone. Hence our digital vnt control which is mapped by torque demand, engine speed, and driving conditions.

A wastegate system is probably cheaper and almost certainly more simple, the downside is a higher boost threshold and increased turbo lag over it's vnt equivelent.

All turbos, even vnt's will have some lag but wastegate turbochargers will suffer more.

In the end, anyone doing a turbo fitment needs to consider the options and design a setup that theoretically meets their requirements. Sometimes the extra lag is acceptable tradeoff for simplicity, for others it's worth the effort to factor in drivability at sacrifice of a more involved build to gain more 'area under the curve' and prevent them from getting bored with the car if it turns out to be a bit annoying to drive around town at lower engine speeds with a big blower bolted on.

No rights or wrongs here, just what you can tolerate and what rewards you expect for what input.

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 Post subject: Re: thoughts on Compound turbocharging?
PostPosted: Fri Oct 28, 2016 2:24 pm 
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Thanks for the in-depth knowledge!...

Talking numbers, what sort of power can be made me a VNT on the l-series as happy day to day driver...

With a gt17, i'm looking at 180bhp, but full boost isn't until 2700rpm
GT20 on sdi's approx 190bhp... but it acts the same as a gt15, looking at rr trace, makes the same boost at the same revs at gt15 does, and maybe 200bhp plus with 200tdis


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 Post subject: Re: thoughts on Compound turbocharging?
PostPosted: Fri Oct 28, 2016 9:42 pm 
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sdi's will top out about 180 in any form, you won't see a lot more than this - so in some respect fuelling is a problem if you want to exceed these figures

The power output of a VNT varies depending on what gets fitted, they can be as small as any wastegate and as large as any wastegate

Not many people have fitted vnt's to their l-series, I had a GTB18x hybrid on my 45 for a while, and i've helped fit a GTB1756 to another members car and tuned it for him (using one of my controllers). Not had it dynoed though we don't expect massive numbers in any case due to the injector limitations - though we will push for some figures someday. it drives very well.

Unless you'll trade for a bit less drivability the GTB1756 is where i'd go for a VNT, should be good for the 190-230bhp bracket and lag won't be too bad. Even though it's called a GTB17, the turbine size is not a far off wastegate GT20 size which works in your favour.

Unless you're really desperate for dyno figures you can also just max out your sdi's on it, sacrifice a few hp but have a highly strung car without bathing the streets in smoke - they're a comfortable match.

I have a GTB2056 here as well which I'd like to try one day if my TIG welding ever gets good enough.

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 Post subject: Re: thoughts on Compound turbocharging?
PostPosted: Mon Oct 31, 2016 10:43 am 
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Thanks! A lot of info to go on there! not sure on the highly strung, i know the pump can flow around 78mg/s maxxed out, but i dont want those stresses on a day to day driver!


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 Post subject: Re: thoughts on Compound turbocharging?
PostPosted: Mon Oct 31, 2016 7:53 pm 
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I think that's overthinking it a bit really, forget the mg figures because once you fit sdi injectors that's thrown out anyway.

One of the bigger holdups with making decent power atm is a decent set of injectors that have more benefits than drawbacks when compared to longer injection durations with sdi's.

I won't go into a mass of detail on this, but to make x power you need y amount of fuel, every route will have good and bad points and making more power is going to increase stresses.

Pump failure as a direct result of long injection windows though is not something we see a lot of. I am not a massive fan of pushing small injectors hard, but weighing up the specifics of sdi's vs alternatives atm, if target power can be attained with sdi's then I would definitely stick there.

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