Silvia Club of NSW

hey all,
sorry for the n00b question, i know a fair bit about cars, but one thing i cant seem to understand, is turbo things. when people say 0.64 a/r what is this referring to? also with intercoolers when they say 600x300x76 does this mean 600mm high 300mm long and 76mm high?thanks all for your replies, great forum, im loving it

yep you're right about the coolers, but I think you meant 76mm thick


The a/r rating on a turbo is the Area / Radius ratio of the housing (turbine housing a/r is more important than compressor a/r normally)

Basically, the smaller the a/r the smaller the area the exhaust gases are forced through before they hit the turbine.

A smaller a/r turbo will cause more restriction in the exhaust, which will limit its maximum power - but due to the increased gas velocity hitting the turbine it will spool up faster.

One thing to remember is that you can only really compare the a/r ratios on the same sized turbo.
A .64 a/r T28 will flow much less air than say, a .64 a/r T3 turbo, since the a and r in the T3's a/r are larger than the T28's.

It's a good way to change a turbo's characteristics though.
eg, to make more power from the GTiR's T28 Nissan used a .86 a/r housing instead of the .64 on the T25G and T28 on the S13 and S14.

The GTiR turbo can flow more power, but it is laggier than the S14 T28.

_________________
The Engine Whisperer - fixer of things

ahhhhh ok thanks mate, yeah i mean't that about the intercooler, but what does the 0.86 stand for? so it is 0 area/.86 radius?

600x300x76 is 600mm length, 300mm width, and 76mm thickness. You just had it backwards.

ahh dammit, trying to think of what to type too far ahead so i didnt forget it hehe.thanks for the help peoples.good forum mods.

nah it means that the ratio between area and radius is .86

so if the radius is 1 then the area is .86

It's a bit complex to explain how it's actually measured on the turbo without diagrams and stuff, but the radius is basically the distance from the turbine center to the outer edge of the housing, and the area is the cross sectional area of the turbine housing "volute" at the point where the radius is measured.

Basically, a larger a/r has a fatter snail

This is an RB25 turbo's turbine inlet:



And this is a VG30 turbo's turbine inlet:



As you can see, the VG30 turbo's turbine housing has a larger area for the exhaust to flow through than the RB25's. These are otherwise very similar turbos.
The result is that the gas flowing into the VG30 turbo hits the turbine slower than it would in the RB25 turbo.
This means that if they were both tried on the same engine the VG30 turbo would need more revs before the exhaust was going fast enough to make it spool up and produce boost.

_________________
The Engine Whisperer - fixer of things