Quote:
Originally posted by Sunder
The GT3040 is mean to be capable of providing air for up to 600bhp... My target HP is more like 350... So why would the turbo be so close to the edge?
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realise that the 3 main things you need to watch for in a turbo are: air in, volume of air in, and air out.
you have to have a turbo that flows enough air to suit your needs. more volume of air helps by not having irregular feeding, as the conditions change on the motor from throttle angle and RPM, the turbine produces enough total air to keep feeding the motor. as the exhaust gas exits, it spins the turbo... restricting the exhaust thru the turbine spins it up faster, but too restrictive will cause backpressure and compressor surge.
in the case of a T3/TO4 hybrid, considering i dont know the A/R ratios of either the hot or cold side, nor the compressor trim, it's impossible to haphazard a guess... but with an output capable of 650hp, that's a lot of flow. but with that flow, is it tuned for your redline? your redline being ~9000rpm on the S2K, the turbine housing has to be large enough to flow all that exhaust gas at peak rpm, yet be small enough to spool the turbo efficiently.
also consider rotating mass... if the compressor side supports 650hp, and you only want 350hp, obviously you could get away with using a smaller turbo, thus allowing boost to come on sooner in the rpm band due to less rotational mass for your exhaust gas to have to push.
in your case, i would guess a 60 trim compressor section with at least a 60 A/R compressor housing, mated to a 1.06 A/R turbine housing, all T3 Garrett. this would provide you the power level you desire.
now i did notice that a difference in 2 psi changes compressor maps to a T4 O trim setup with an S3 compressor. it's always better to slightly upsize so you're not running excessively high temps in the turbine, and also from compressing. to give you slightly longer "legs", a TO4 S3 compressor mated to a T3 1.06A/R center and turbine section, resulting in a T3/TO4 hybrid, would probably yield you better spooling results and still give you enough headroom should you need it.
full boost at 4000+rpm is kinda useless in my opinion... it's nice compared to nothing, but it should come on at lower levels to be even more useable. a turbo too large for your application, or improperly sized and set up will result in boost coming on at almost worthless rpm levels. a properly matched system will provide full boost around 3000rpm or less.
now in the case of dramatic power levels, like 650hp... the turbo can only spool so fast with the amount of exhaust gas provided. so obviously a 650hp civic will reach full boost later than a 350hp civic... any turbine housing smaller to encourage faster spoolup will be a cause of backpressure at higher rpm due to much more exhaust gas from all the boost.
a very careful balance, as you can see, but still based on the 3 points to address on a turbo as i stated at the beginning.