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PostPosted: Mon Jul 19, 2004 7:37 pm 
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Joined: Tue Mar 04, 2003 6:43 pm
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Location: wellington, NZ
hey guys, realistically im admitting i aint no jack about turbos and i wana learn so i can make an informed desicion when buying a to4 very soon...
so could someone please explain what the differences between different trims, wheel sizes, a/r ratios etc
in fact i dont even no what the hell the a/r ratios are.

also
could someone give me some idea of what buildup in these sorts of things i would want in a to4 wanting around 400hp run on an fj20.

and yes i no i will have to run engine management, fuel system and external gate to match.. thats all taken care of.

really just need this knowledge on turbos to complete my beast

cheers
Michael


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PostPosted: Mon Jul 19, 2004 8:31 pm 
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Location: Karrinyup WA
well, the A/R ratio you see is the cross sectional area vs the radius of the compressor housing. A = area and R = radius.

the bigger this figure is, the cooler the boost produced will be relative to another housing of a lower A/R ratio.

for example, a 0.7 A/R compressor has a greater "area vs radius" calculation than does a 0.5 A/R compressor. now, if both the radii are the same for each compressor, then that means the one with the smaller A/R has a thinner compressor housing and will flow less outright than the larger A/R.

if the two compressors are off the exact same turbo (same wheels, same cores and same exhaust housings) then the one with the larger A/R compressor housing will produce cooler boost because although the pressure is there, it isn't being "compressed as hard" - for lack of a better term - due to there being more room inside the housing.

remember, boost isn't only what air the turbo can compress, it's also got to do with what prssure the engine can consume - ie a T25 compressor housing on a CA18 might make 9psi std with an exhaust housing to suit a 1.8L. put this same trim compressor on an A12 with an exhaust housing to match a 1.2L and it might make 14psi. same compressor, different results.

the best way to size a turbo is to look at the air your engine can consume, and then work backwards and decide on the exhaust housing (this will improve / kill spool up response), choose your core (plain bearing or roller bearing), choose your impellers and then choose your A/R for your compressor housing to match all of these put together.

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PostPosted: Tue Jul 20, 2004 7:25 pm 
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Joined: Fri May 16, 2003 7:47 pm
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Location: New Zealand
I only really know stuff about the A/R ratio's....

pro240c...
thats assuming your ignoring the wastegate tho right?.......otherwise they will both produce 9psi. ( assuming that the A12 can flow enough exhaust gas to produce 9psi with that exhaust housing )

dattohoe....
A bigger A/R on the front will allow the turbo to flow more air at the same boost pressure. tho it will make the turbo lag more ( theoretically....not sure if it'd be enough to notice while driving )....for the same reasons you get more lag with a massive intercooler when you don't need it.
and as pro240c said...work backwards....

heres a link with some real good examples of how to do some engine calculations ( not really on the turbo's.....but very interesting ).
http://auto.howstuffworks.com/bugatti1.htm
( reccomend you read the rest of the article too )
classic example of working backwards......not only did they deisgn the engine that way....they then designed the car around the engine!
lol

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PostPosted: Tue Jul 20, 2004 7:46 pm 
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datsunboy wrote:
pro240c...
thats assuming your ignoring the wastegate tho right?.......otherwise they will both produce 9psi. ( assuming that the A12 can flow enough exhaust gas to produce 9psi with that exhaust housing )


yeah that's right, that's why i said an exhaust housing to suit each capacity - which would more than likely include an internal wastegate - but the same compressor housing.

i should have been a bit more specific.... sorry. :oops:

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The Republic of Western Australia -
Propping up the Nation's Economy since 1901.


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