Help with turbo calcs...

[Hof]

Hi,

Well I'm still tryin to work out a suitable turbo.. Question is, some turbos come with the same compressor and trubine, just with a different turbine A/R..

Now when calculating a suitable turbo, you plot data on a compressor map, which is fine..

Thing is if that same turbo has two A/R ratios for the turbine how do you factor that into your choice, as the normal calculations help choose compressor, airflow etc, but not turbine A/R..

There are exhaust flow diagrams which show peak flow, but how do you work out if it will choke the engine??

Now I have read on the Garrett site that smaller A/R is more responsive, fun, boosts up quicker but with less high end and larger A/R is basically opposite..

Is it really this simple, as my gut feeling is NO..

Cheers,

Hof

[KiwiBacon]

Different size turbine housings (different A/R ratios) change the gas flow needed to spool up the turbo.

Smaller housing = boost sooner but higher backpressure and choking problems if you're way too small.
Bigger housing = boost later but lower backpressure.

Diesels run cooler exhaust than petrol engines and generally need smaller turbine A/R ratios for the same size compressor.

Tell us what engine and turbo combination you're looking at, that's always a good start.

[Dzltec]

To be honest, when a turbo has a few different a/r exhaust housings it is generally used when final tuning a car. There is a trade of as explained by kiwibacon, the best way to determine is on a dyno, measured turbine inlet pressure to see which one works the best.

Andy

[changaz]

as well as different a/r housings (both compressor and exhaust) you can also get different trim wheels, so one turbocharger can have anywhere up to 4 different variants, maybe more, and they all perform differently.
have a look on the garret website they have flow maps for their whole range of turbo's

[zagan]

Look at this website has some maths for working stuff out.

http://www.ackerman.tv/racer_math.html

You work out the engines in take air flow and the exhuast air flows.

You need more intake air flow than what is going out the exhuast other wise you can surge the turbo.

To work out the best turbo for an engine you'll need to work out the engine's exhuast pulses, as it's these pulses is whats turning your turbo's exhuast wheel.

A pulse comes from when ever the exhuast valve is opened, the rush of out going exhuast spins up the turbine wheel, this pulse also travels down the exhuast pipe till it hits a bigger air space in the exhuast this is normally your muffler but a cat and on the newer diesel's this could be a soot converter as well.

When a pulse hits a larger air space it expands and it spreads out in all directions so some of the pulse will return back up the exhuast pipe and back into the engine and sometimes back up into the in-take pipe, this is also called back pressure. Have too much and your engine won't work very well.

What your after in an exhuast is the pulse to exit just at the exit of the exhuast and you get very little back pressure happening, the car makers go for a smaller than needed exhuast for the extra back pressor to help the engine squzze in more clean air as this can help to keep to the envrio standards required, so most people simply make the exhuast larger and you can get away with it.

having a little bit of back pressure is also good but it has to hit the engine at the right time, this is with the pistion at TDC and the exhuast vavle at the closed overlap, when the exhuast valve opens you get a sucution happening with-in the cylinder above the pistion, this sucks out the burnt fuel/air (exhuast gas) and sucks in extra clean cold air from the intake, this is also why the size of an exhuast has to be correct as well.

Once you know what the air flow rates are for intake air and exhuast gases, you can then look up at the turbo maps, these are the turbos dyno charts.

The virtcal line on the left is the surge line if you cross into this the turbo will surge.

Surging is where the compressor wheel stops compressing/pushing air into the engine and it starts to spin the other way and it becomes the driving wheel, which makes the exhuast side the compressor wheel and sends the air backwards through your engine.

You don't this happening.

The numbers on the right hand side going virtcal are the RPM numbers, most turbos idle at 20,000rpm and will spin up to 300,000rpm at X engine rpm.

The horizontal line across the map are the engines RPMs so line one might be 500rpm engine speed you look at the side of the turbo mapfor what the engine speed is, not all turbo maps are like this though.

With any turbo the sweet spot is really around the 60% to 80% of the turbo map, really the middle part because nothing is perfect in this world.

Diesel motors need a turbo with a wide spread of boost from the turbo other wise you'll only get boost at certain rpms, if the sweet spot is small then the turbo will only boost at certain times.

Diesels have a tiny RPM usagable rpm range but you want the turbos sweet spot to be all over this range, for example a 4,500rpm diesel would only have 2,200rpm to play with over that and you start losing torque etc, so you'd be wanting 1,000rpm to 3,000rpm maybe 3,500rpm, so the turbo is on boost all the time.


Anyway that pretty much everything I know on this.

[KiwiBacon]

You need more intake air flow than what is going out the exhuast other wise you can surge the turbo.

Think about this a little. Unless your engine has a massive leak, all the air that goes in the intake has to come out the exhaust.
It's the difference in tempreature (hotter exhaust) which gives the exhaust bigger volume and the energy to drive the turbo.

We'll leave the rest for next time aye.