Seeing as how Kevin gave this thread a bit of a jump start, I'll add some recent learnings and observations to sticking turbo's onto engine
.
Most of a turbo charger's response will come from achieving 2 simple things: Keeping every single aspect of turbo plumbing volume down while maintaining high outright flows!
To a degree, a case in point is the Nissan Skyline GTR RB26DETT engines. From the original RB26 in the R32 GTRs to the last of them in the R34 GTR, the engine stayed basically the same. Same bore, stroke, conrod length, compression ratio etc!
And while the power always stayed at the Japanese agreed 208KW, the torque curve of the R34 is substantially broader (much more low RPM torque) than the torque curve of the R32 and R33's.
What is obvious is the changes in exhaust manifold and turbine housing design. The RB26 engines all used Garrett T/GT25 based turbo's and exhaust manifolds to suit. But when you look at the R34 turbine housings and and exhaust manifolds, while they used T25 flange patterns, the R34 turbine housings actually use smaller turbine housing inlets (about 12% smaller looking at the R34 GTR turbo on my desk) with the outlet of the exhaust manifolds made to suit!
If you give this some critical thought, you can see the true benefit of this.
According to Hugh McInnes's book on turbo charging (yeah it's old, but still has plenty of relevant information), exhaust gas exhaust gas speed out of an average exhaust port will need to be increased in velocity by between 3 and 4 times! But if you look at the average merge collector in a turbo exhaust manifold, you will see that there is a sudden increase in cross sectional area where the exhaust ports all join together.
That increase in cross sectional area will cause the exhaust gasses to slow and loose energy. That energy loss could have been applied too the turbine wheel and then too the compressor wheel!
Think about this, a twin turbo 60* V6 or a twin turbo inline 6 will have 240 crankshaft degrees between exhaust events within each exhaust manifold! So with such a large gap between firing events, why would you want an exhaust manifold and turbine housing design that allows the exhaust gasses to slow down via a large increase in cross section area around the collector and turbine housing inlet, only to then try and get them to speed back up through as they are pushed into the turbine housing's nozzle???
If you look at turbo'd Audi's of the last 15 years or so, you wouldn't!
A/R (Area/Radius; Garrett) Ratios/Nozzle Cross Sectional Area (cm2; Mitsubishi):
As mentioned, the job of the turbine housing is to increase the exhaust gas speed so the it is fast enough to spin the turbine fast enough to make the (comparatively) small compressor wheel spin fast enough so that it can pump more air than what the engine would otherwise use.
IE: Generate boost pressure!
The smaller the turbine housing's A/R ratio or cross sectional area number, the more the nozzle narrows around the inside of the turbine housing, the greater the increase in exhaust gas speed.
The trouble is, with small nozzle area, while they will increase the gas speeds of relatively low flow levels of exhaust gasses, as outright exhaust gas flow increases, the turbine housing will start to choke and the engine will start too push against ever increasing amounts of back pressure. This will result in a huge increase in exhaust gas temperature (and all of its associated problems) and pumping losses!
The waste gate, however, provides a path of least resistance for the exhaust gasses!
And so long as the wastegate's flow isn't exceeded, can provide MUCH need relief for the pre turbine exhaust gasses while still maintaining high average exhaust gas speeds.
In a nut shell and very simplistic, use a small A/R ratio/small cross sectional area turbine housing and a decently large wastegate. Audi have done the exact same for 15+ years and their low RPM torque numbers are legendary!
Exhaust Runner Diameter:
Until recently, I would have suggested making sure your exhaust runners were the same diameter as your exhaust ports. Most people will swear black and blue that making sure you exhaust ports flow really well is an absolute must, when adding a turbo or 2 too your engine! And to a degree, they are right. But maintaining as high an average exhaust gas speed is critical to achieving excellent turbine wheel (and so boost pressure) response!
Again, why allow the exhaust gasses to speed up and slow down only to then speed them up again as they are pushed into the turbine housing?
Blatantly pilfered from the
http://www.toymods.org.au/forums/tech-conversions/ of which I am a member,
http://www.toymods.org.au/forums/tech-c ... ifold.html has an excellent example of the effectiveness of providing well flowing, small diameter exhaust runners and how they effect the torque and power characteristics of an engine!