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 Post subject: Twin Turbos
Posted: Sun Sep 07, 2008 9:39 pm 
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I was just browsing the snort website and i noticed the twon turbo manifolds. This got me thinking, could you use two use two different size turbos to get the best of of both worlds,quick response and top end grunt. Would this work or would the manifolds symetircal design not benifit a small and a large turbo, if you get what i mean. cheers

 

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Posted: Mon Sep 08, 2008 4:41 pm 
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How do you want your answer. The short answer is no. The long answer is a bit more in depth.
First you must try to picture in your mind what is going on with 2 turbo's of different sizes feeding air to the same engine. Got that mental picture locked in? Good, now lets get explaining.

We should all know by now that turbo's aint turbo's. Small turbo's flow less air than large turbo's.
If a small turbo was constantly fed exhaust air on a large engine then it would simply "run out of puff". Its physically too small to support the amount of air that you want to flow through it. How does running out of puff feel like? Like someone has pulled your hand brake up all of a sudden and the car refuses to accelerate.

Well what about large turbo's? Oh yes they have the right flow characteristics for the larger engine. But what about on a smaller engine. They wouldn't make any boost 4000-6000rpm. But when they do it comes on all at once. Its not savage like nitrous (which some users say felt like waiting at the lights and being hit from behind by a road train travelling at 130 clicks) but it is still a big jump in power over a short space of time never the less.

What about combining the two? This is where sequential set ups come into play.

The only way a sequential turbo set up would work is when both turbo's are mounted on a common exhaust manifold. Usually back to back. In that manifold is an actuator. That actuator moves so that only one turbo is working at a time. At low rpm and/or boost levels the flap blocks off exhaust air to the big turbo. That way the small turbo can quickly get up to speed and make some boost down low.
As the rpm and/or boost levels rise then the actuator blocks off exhaust air to the small turbo and redirects it to the big turbo. Because the engine was already making power from the small turbo, the savageness of the big turbo was been reduced.
Think of it like a baton change over in a relay.


The snort manifold is a parallel twin manifold. Two manifolds, two turbo's. Both the same size. The objective of parallel set ups is where two smaller turbo's are used to compensate for one large turbo. both run the same boost pressure and both are perfectly matched.

Some guys in the US use more than 2 turbo's in parallel setups. Theres a vid floating around of a guy in a 57 chev powered by a gen III with, wait for it, 8 turbo's.

The only other setup you would be slightly interested in is the compound turbo setup but this is only for diesel engines. this is where the dump of one turbo feeds the exhaust housing of the next turbo. The turbo's can either be the same size or increasing I order as you go along. Most turbo's I've seen linked like this are 4 in some truck pulling prime movers in the USA.

Hope that helps somewhat.
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Posted: Mon Sep 08, 2008 6:15 pm 
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Yes, that helps alot. Thanks

 

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Posted: Mon Sep 08, 2008 8:09 pm 
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Short answer, yes.

Google Cat's ACERT system and you will learn something about the sequenical turbo charging.

Quote:
The only way a sequential turbo set up would work is when both turbo's are mounted on a common exhaust manifold. Usually back to back. In that manifold is an actuator. That actuator moves so that only one turbo is working at a time. At low rpm and/or boost levels the flap blocks off exhaust air to the big turbo. That way the small turbo can quickly get up to speed and make some boost down low.
As the rpm and/or boost levels rise then the actuator blocks off exhaust air to the small turbo and redirects it to the big turbo. Because the engine was already making power from the small turbo, the savageness of the big turbo was been reduced.


Right idea, but you would have a massive and unstoppable air leak from the non functional turbo.

Check out the ACERT, it is a proven worker.

There's a turbo diesel Astra in Europe with a similar setup and produces something like 700NM and 300HP from 1.9L.
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Posted: Mon Sep 08, 2008 10:30 pm 
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I'm only going with technology I've seen first hand. Think Mazda RX7 series 6, 7 and 8.
Also think toyota 2JZGTE.

Both are sequential twin turbo setups that function exactly as I have described. These have been around from the early 90's.

I see where you are coming from in terms of air leak but these systems have no known air leaks.

The cat ACERT System is primarily for diesel engines where the design dictates boost over flow as opposed to flow over boost for petrol turbo engines.
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Posted: Mon Sep 08, 2008 11:55 pm 
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If you want good throttle response ? Build a strong engine with 9.0 compression BUT you MUST have very good tune and boost control !!
You will find sequential turbo's to be pita to get right.. You won't be able to stop surging back and forth from one turbo to the other..
Nissan and Toyota have spent many hours getting it right.. Besides 99% of upgrades end up being single...Bigger turbo tech has changed the last few years as well as ECU programing..
Where there is big swings in ignition timing to aid spool and help low end torque to get engine up on converter etc.. Subaru have gone back to single turbo now also..
Diesels are often set up to run in Edit.. in series [one into the other, compound] where high boost is required..
Works well on low revving diesel engines, the heat build up would KILL a petrol engine, or the inside turbo...
There are other alternatives now with variable exhaust trim so turbine is in sweet spot much longer..

 

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Last edited by EBXR8380 on Thu Sep 11, 2008 11:45 pm, edited 1 time in total.

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Posted: Tue Sep 09, 2008 12:02 pm 
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Yea, it sounds like more trouble than its worth. A mates dad has a new Bmw 335 twin turbo. They way it drives is awesome, It doesn't kick you all of a sudden, as soon as you put your foot down, it pushes you into the seat all the way to the red line. After being in F6's and the 335, i really like that instant and constant thrust of the Bmw. That kind of feeling is what id be aiming for, but since the ford six doesnt rev as high as the BMW (6000ish verse 7500) maybe a single would be better, and easier, and cheaper :lol: . Thanks for your help, it made for some very interesting reading.

 

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Posted: Tue Sep 09, 2008 9:31 pm 
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Caterpillar have been building compound turbo setups for a few yrs now, well before ACERT technology was even thought of. they simply have 4 turbos on a V16 (way bigger than ur setup!) 2 are smaller for low down and the other 2 are large ones. each set of 1small and 1big run one bank of eight cylinders. when i first seen this it gave me a great idea to do the same with just 2 but would need ALOT of research!

 

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Posted: Tue Sep 09, 2008 10:17 pm 
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Yeah, thats exactly how i was thinking, but its sounds more complicated than it seems.

 

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Posted: Tue Sep 09, 2008 10:32 pm 
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you would have to think long and hard before you picked your turbo's.

But also the airflow requirements on our falcon motors spinning to 6000rpm would choke the small turbo pretty quickly,
Unless..... you start with a moderate turbo such as a GT35/40 or T04Z and then go bigger from there. *dang a light globe just lit up in my head*
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Posted: Wed Sep 10, 2008 6:01 pm 
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How big are we speaking? :D . The GT35 is the standard turbo on a XR6T isn't it? Could you go slightly smaller than that so the boost comes in earlier?

 

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Posted: Wed Sep 10, 2008 6:41 pm 
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Man i had twins on my old AUXR8 and it made a lot of power very quickly.
Be aware of what you are doing.

 

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Posted: Thu Sep 11, 2008 8:30 pm 
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Hey Xcabbi - fair call.

But as for the pressure over flow in a diesel engine, I'm gonna have to disagree there.

The Cummins Signature 600hp engines run 12.7 psi of boost and draw 900LPM into the turbo, off the top of my head.

But I do also agree that a twin setup is way more drama than it's worth, for this application.
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Posted: Thu Sep 11, 2008 9:54 pm 
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Sorry, got the volume wrong.

Checked my course notes, the Cummins Signature pulls 12.7 psi and at governed speed the engine draws 41kg of air per minute, not 900LPM.
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