Big Turbo's, why?

[Ultra Violent]

Will someone explain to me why a K29 turbo produces more BHP than a K27, for a given boost pressure. Since more air + more fuel = BHP,I don't understand how you can flow more air at a constant boost pressure...

[dazren]

Think elementary physics.

Same pressure within a larger diameter pipe would mean a higher volume of air flow.

DAZ
(Guessing, now someone can prove me wrong as usually happens)

[iguana]

Nowt wrong with ya physics Daz, thats a good explanation, also a small turbine may well push- for example 1.0 bar but that may be the most that little thing can spin at and may start to spin quicker than it is designed to do. 9m can explain this bit better than me I'm sure, but things like stalling & pulsing probabally should be inserted here.

This is obuiously not efficent whereas the big daddy turbine will push that presure easily and will have more in reserve.

Corse the big bugger will have more lag and when it comes on BOOOOOOOOST!!!!! with 2wd & in the wet it will try & sling you off the road but hey no one ever said old 911turbis were sophisticated engineering, just old fashioned grunt.

[GuyR]

Sorry Darren, not quite right...

Whilst the turbo has a larger tube, the pressure is measured in the intake plenum, which has an unchanged size.

The actual reason is due to compressor efficiency, which is shown on a compressor map.

When you run a small turbo at medium/high boost, it starts to lose effiency and ends up heating up the air, at which point the higher boost is offset by increased temperature. This is often seen when pressure is turned up on small turbos and there is no power gain.

The bigger turbo has better efficiency at the same boost, therefore lower temperatures and therefore more power. The bigger turbo is also able to deliver more power at higher boost. The offset is that the larger turbo takes longer to spool-up and hence has more 'lag'.

The reason I know this is I had 4 different sizes of turbos on my Skyline GTR.

Guy

[iguana]

Good link-

www.andial.com/content/faqquest/quest3.htm

[Ultra Violent]

Ah, Mr Spotty Face and his law of unquestioned sense...

Ok, so if I freeze the nuts off my intake, i'm laughing... Anyone see where this is going.....

[scottster]

You're going to start freezing your nuts to improve your blowing performance?

[pdV6]

GuyR said:

When you run a small turbo at medium/high boost, it starts to lose effiency and ends up heating up the air.

All gasses and liquids heat up when compressed, hence the need for intercoolers.

[GuyR]

Cooling intake charge, yep thats why uprated turbo engines use bigger intercoolers...

It's also why drag-racers, often spray nitrous or CO2 onto the intercoolers before a race, to lower the inlet charge, which often gets especially bad when siting on the start-line due to heat-soak. Using a fire extinguisher is the easiest external method, unless you plumb in a proper external spray-bar.

Guy

[GreenV8S]

If they're running methanol then you will often see them spraying antifreeze around the throttle assembly to stop it icing up before a run!