how much shaft play is too much shaft play?

[fatboyz39]

buy a new turbo tight arse lol....

[string]

yep that's exactly right

No it's exactly wrong. A larger turbo compresses air into the same space -> the charge pipes, and an ever thirsty positive displacement pump. For a given volume of air, the only way to change the amount of oxygen molecules is to decrease temperature, increase pressure, or increase volume.

Pushing a small turbo to 14psi for example, may push it well off it's efficiency islands. This means the air will come out at a high pressure, but high temperature (and thus lower oxygen content per unit volume), causing a power loss compared to a larger, more efficient compressor at the same pressure level.

Your boost gauge (if hooked up to the intake manifold) reads the average pressure in the manifold throughout all cylinder filling cycles. As your crank pulls the piston down on the intake stroke, it sweeps say 500cc. If your turbo were actually pushing more air, you'd see more boost, but the wastegate is controlling this thus you'd get the same amount of air regardless.

Blitz was on the right track: The wastegate controls pressure. The ammount of pressure determines the oxygen density of the air. The only way possible for more power to be generated at the same pressure and temperature is for a greater volume of that air to be in the cylinders at spark time.

Volume flow of air is proportional to the pressure difference across the path, and inversely proportional to the resistance of the path. The greater the resistance of the turbine, the higher pressure exhaust gas will be inside the turbo-manifold just past the head, leading to less air in the cylinders and more exhaust gas.

Exhaust reversion is the real killer of power. All this talk about bigger turbo's pushing more air because they have more space to compress into is absolute nonsense. When people say that a small turbo "chokes" an engine, you can almost take it literally. A small turbine at high flow rates creates a virtual capacity loss - you physically can't get any more exhaust gas through it so it stays in the cylinders. Not much oxygen in exhaust gas = less injector pulse width = less torque. This is why turbos that aren't well matched front and rear, can have unbearable 'lag', yet still drop off in the high end. The more efficient you get your turbine, the more power you'll get out of a well matched compressor. Unfortunately when it comes to 1.6L to 2.0L Honda motors, this means that if you are running 30psi, you won't be seeing it anywhere before 6000rpm.

I challenge you to run an array of different compressors off a high powered electric motor, with intake regulated boost control such as found on a roots supercharged system. I will put money on the fact that after accounting for compressor efficiency (intake temperature), you'll get near enough the same power at any boost level you choose (so long as the electric motor isn't being run from the alternator ).

[Limbo]

we were talking about the amount of air the turbo itself holds at a given pressure.

[string]

we were talking about the amount of air the turbo itself holds at a given pressure.

I'm pretty sure everyone is talking about making more power at the same pressure, which is all about the flow rate. I don't even know what your statement above means.

[mooshie]

I don't think you should play with your shaft more than once a day. Might damage it...

A better solution is to get someone else to play for you.







sorry, couldn't help it.

[TODA AU]

No it's exactly wrong. A larger turbo compresses air into the same space -> the charge pipes, and an ever thirsty positive displacement pump. For a given volume of air, the only way to change the amount of oxygen molecules is to decrease temperature, increase pressure, or increase volume.

A turbo is a centrifugal compressor.
It is not positive displacement pump by any means.
Further, the diffuser area & cover A/R play as much a part in it's ability to compress air efficiently as does the compressor itself.
In any case, the larger turbo spins slower than the smaller turbo.
In short it introduces less heat.
The reduction in heat affords an increase in density ratio at the same pressure with no other changes.

[string]

A turbo is a centrifugal compressor.
It is not positive displacement pump by any means.
Further, the diffuser area & cover A/R play as much a part in it's ability to compress air efficiently as does the compressor itself.
In any case, the larger turbo spins slower than the smaller turbo.
In short it introduces less heat.
The reduction in heat affords an increase in density ratio at the same pressure with no other changes.

You clearly missed my point. If you actually read my post completely you'd find I'd already mentioned compressor efficiency (2nd paragraph) thus completely negating the point of your post.

I assumed constant intake temperatures for the purpose of removing it as a variable. Of course compressor efficiency plays a part - it's not the only factor, nor is it the biggest factor. My post in it's entirety deals with exhaust reversion.

"A larger turbo compresses air into the same space -> the charge pipes, and an ever thirsty positive displacement pump (the ENGINE)."

[TODA AU]

You clearly missed my point. If you actually read my post completely you'd find I'd already mentioned compressor efficiency (2nd paragraph) thus completely negating the point of your post.

I assumed constant intake temperatures for the purpose of removing it as a variable. Of course compressor efficiency plays a part - it's not the only factor, nor is it the biggest factor. My post in it's entirety deals with exhaust reversion.

"A larger turbo compresses air into the same space -> the charge pipes, and an ever thirsty positive displacement pump (the ENGINE)."

LOL… Fair comment…
My bad for not taking the time to read everything you said.
I apologise for my previous post, in hindsight we're actually on the same page.

[AH_HUH]

another dumb question..

If 2 turbos both produce 13 psi and on the same engine... and peak power is made at the same RPM... should the power output be identical?

Cause the turbo Im looking at has shaft play and will proabably need a rebuild, but i blades are scrapping the housing or anything like that, but the play is quiet a bit.. Could this hinder the power output provided that it is still producing the target PSI?

hey dude, buy the turbo off my sti so i can upgrade. its a garret gt3540 with the smaller rear housing.