Twin turbo setup Information

Hi All

I have recently bought a legacy with the twin turbo setup - does anyone have any information the how the mechanism works. A diagram would good. Generally the setup is good except when overtaking at highway speeds - the turbo lag between the primary and secondary occurs just as you are along side the car you are overtaking. Its quite an unnerving sensation to loose boost (and power) when you are on the wrong side of the road beside the car you are overtaking. The only way around this problem that I can see is to overtake in 3rd gear (as apposed to 4th like I would nominally do) - it certainly pulls but it feels little unkind to the engine to rev it right to the redline.

thanks for the info

Mike

Reply to
MIKE
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There is a valve in the exhaust system which can feed the primary turbo from all cylinders or share the flow between both turbos.

There is a value between the secondary turbo output and the intercooler, there is a differential pressure sensor monitoring the pressure across this valve.

At some point around 4300 rpm but probably depends on other things the ECU decides to open the exhaust valve to feed both turbos. The primary turbo looses a lot of boost. The secondary turbo spins up and when its output pressure matches the pressure in the intercooler the valve between them opens. Both turbos are then boosting and power output continues to rise.

You might be able to find a dyno chart on the web somewhere. If you do you will see that the engine peaks on the primary turbo at about 160bhp. Output drops to around 120bhp as the secondary turbo spins up then continues to around 280bph (for later MT versions).

That is the price you pay for having a small responsive turbo most of the time while still having vicious top end power. There were some changes made on later models to try to lessen the 'boost valley'. When you get to know your engine it isn't really a problem.

There is one other significant possibility. The engine is designed for 100 octane fuel. On lower octane fuel the engine will knock. The knock sensor will detect this and retard the ignition and open the primary wastegate to cut boost. If the engine is going to knock it will knock at around peak boost on the primary turbo which is about the same place the twin turbo 'boost valley' occurs.

The drop in power is more significant than the boost valley when this happens. No one really knows and it may have changed between models but my feeling is that the ECU remembers the engine knocked and limits boost to prevent it in the future, my feeling is it also slowly forgets otherwise the engine would never boost more than it did on the hottest day with the worst fuel it had ever seen. If you run on low octane fuel then every now and then when the ECU has forgotten you will put your foot down, and experience a big hole in power output. You try the same thing 1 minute later and it will be ok. You really need to run these engines on high octane fuel.

There is a site here

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with somestuff about the twin turbo Legacy. I don't think the guy's description ofhow the turbos work is quite right - maybe he had the vacuum pipes hookedup wrong on his car :).

Reply to
nospam

Yevy interesting. That explains why when driving a friend's 2.0 twin turbo it seems really hard to make it go faster than my 2.5 non-turbo. The 2.5 has 165 HP and I don't like to redline other people's cars (or my own, for that matter).

-- Bruce

Reply to
Bruce Hoult

Thanks for the information and the links they are very interesting. I had a thought while reading the information. Would connecting the output from the waste gate of the primary turbo to the input to the seconday turbo reduce the lag I'm experencing?

Once the primary turbo reaches its set boost pressure the wastegate opens to prevent it from overboosting so if you send the excess exhaust to the secondary turbo it may at the very least spool up the secondary turbo before the switchover. I don't think this would make the primary turbo over boost as the secondary has less back presure - and if it does you could seperate two exhaust paths so you end up with a twin exhaust pipes.

Thoughts?

Mike

Reply to
MIKE

Sounds reasonable but mechanically a bit impossible. I think the wastegate is an integral part of the turbo casting and wastes into the same hole the turbo exhausts to.

The exhaust valve could be used as a wastegate for the primary turbo with the desired result - maybe it already is.

I'm sure there is a lot of knowledge in Japan. maybe you can find something on the web if you have some way to translate. I haven't looked recently.

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nospam

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