Why does bigger compressor = laggier performance?

[timjustindodd]

I just got my BNR S3 in. In general the car behaves MUCH like stock... very little side effects, spools up quickly and all, but there is still a noticeable difference in how long it actually takes to achieve 15 psi.

Obviously, the BNR uses the stock hot side, so there shouldn't be any physical difference in the actual spool up times.

My question is (and my friend and I were debating this)... if you have a bigger compressor housing, does the compressor wheel need to spin faster or slower to achieve the same number of boost?

My friend thought it wouldn't need to spin as fast since it's larger and pushes more air, I thought it would need to spin faster to overcome it's initial compressor surge threshold and then would get into the target zone after that. He also thinks it's "laggier" duh to being a larger wheel, more mass requires more energy to spool it up. I sort of thought since it's such a small amount of rotational mass, it wouldn't actually cause much of a lag.

Sooooo yeah, what actually causes the slower response of the turbo with a larger compressor housing when the hot side is still stock?

[SLOWHATCH]

Could be the tune?

[MS3-a(sore)ASS-rex]

Stock hotside doesn't mean it uses the stock turbine wheel or that the internal cross section of the housing itself is the same. Iirc they use a GT28 wheel...

[silvapain]

I believe it's equivalent to a GT2871. It uses a ported stock turbine housing.

[timjustindodd]

Interesting! I did not realize it was ported!

BUT, just for argument sake with my friend. If it DID have the stock side, but a larger compressor... which would achieve the same boost quicker (say 15 psi)? A larger one or a smaller one?

[chaser27]

[QUOTE=timjustindodd;2247672]more mass requires more energy to spool it up.
QUOTE]

Careful with how you word that. You can actually use less energy to spool up a turbine with more mass. It all depends on how it is distributed. It is the Moment of Inertia that you are interested in.

Imagine two discs. one has its mass evenly distributed around the rotating axis, the other has its mass located around the perimeter. The second disc is going to require more energy to "spool" even though they both have the same mass.

That being said. I haven't read enough about BT's to know the differences between a KO4 and BNR S3 to answer your question. But in general a larger turbine will not have to rotate as quickly to move the same volume of air.

[kimura]

Originally Posted by timjustindodd I just got my BNR S3 in. In general the car behaves MUCH like stock... very little side effects, spools up quickly and all, but there is still a noticeable difference in how long it actually takes to achieve 15 psi

just a guess but maybe you are simply hitting your load targets with less boost. stock compressor housing is a flow restriction. I'm guessing that upgrading to the S3 allows you to push the same mass of air at lower boost. boost is just a pressure differential

[Ziggo]

Its exaggerated unless you tune to help it spool, mostly by playing with the VVT to make sure that as soon as you punch it and the turbo should be spooling, VVT is at its maximum. You can also pull timing in the spool region to feed more exhaust energy into the turbo. The stock maps are setup to improve spool for the K04 and require a little adjusting to fit the BNRS3, with a few tweaks you will hardly notice the difference

As to why. As was said, the moment of inertia is larger for a larger turbo, requiring more force for the same amount of time, or the same force for a longer period of time to accelerate it.

Zigatapatalka

[Tabasco69]

You need to keep in mind a larger compressor moves a higher volume of air relative to boost psi I.e. a 3071 will cram more cfm at 15 psi the a 2871 would at the same 15 psi, spool is majorily effected by the a/r of the turbine housing, secondary are turbine wheel, compressor wheel and combination of both.
People who are familiar with the ego platform know of the black,red and green revisions of the stock Mitsubishi heavy industries TC which is a oe package with various wheel and ported housing combination total sleeper from outward appearance bnr is much the same for the ko4

[Ziggo]

If you have two cars, completely identical except that the compressor side on one is larger, but for whatever reason they both see the same BATs, then they will both flow the same mass for a given boost.

What really sets the boost vs flow characteristics is the size of the hotside of the turbo, the volumetric efficiency of the head and to some extent the back pressure from the turbo back. The efficiency increase (BAT decrease) from the larger compressor is not a huge driver, and the same improvements could be found with a meth system or better Intercooling. Phate's testing proved with e85 mixing in particular mass flow is not very dependent on BATs.

Zigatapatalka