Turbo A/R rate?

[sidewaymambo]

Hi,

After search forum, I notice some interesting thing about turbo; does this sounds right and make sence to you?

When they install on a 95' R33 GTS-T

A/R50.....2800RPM kick in
A/R63.....3500RPM Kick in
A/R70.....4000RPM kick in
A/R80.....4800RPM Kick in

thanks!!

[R33]

That sounds correct, I think. But it all depends on what you term as "kick in". If you mean the time the boost meter starts moving, the rpm figure should be a bit earlier I think. That is because on my AR64 turbos, it starts climbing at around 2500rpm even in 5th gear. And I think (and I stand corrected on this) it also depends on the cams setting. FYI, my cams are 264 in and 272 exhaust.

[Gonthrax]

Originally posted by R33
And I think (and I stand corrected on this) it also depends on the cams setting. FYI, my cams are 264 in and 272 exhaust.

Aye, your exhaust cam setting plays a part in spoolup, also your whole exhaust and intake system play part, the more they flow the quicker you'll spoolup. A polish job will cut the RPM for spool up down a bit too.

[R33]

Yap, that should be correct. I think that's why Top Secret polished and ported the head in my car.

[chrisr_nz]

Anyone got a good URL to explain about or have an explanation about a turbo's A/F rating? I don't really understand it ...

Cheers!

Chris

[Gonthrax]

Originally posted by chrisr_nz
Anyone got a good URL to explain about or have an explanation about a turbo's A/F rating? I don't really understand it ...

Cheers!

Chris

Here ya go, lets just hope no one has posted a link by the time I get done typing this, or my browser crashes
The exhaust and compressor housings on turbo chargers use a "scroll" design. For example, the exhaust housing's scroll is where the exhaust gasses enter the housing and are directed at the turbine. It's basically a smooth, tubular chamber that surrounds the turbine with a slot all the way around that acts as a nozzle to direct the exhaust gasses at the turbine. It's called a scroll because it slowly gets smaller in diameter as a goes around the turbine. This pressurizes the gasses, forcing them out of the slot/nozzle at a fast rate. In turbo-terms, the scroll is measured by the cross-sectional area of the scroll's "tube" (A) and the distance from the center of the "tube" to the turbine shaft (R). The values by themselves are not meaningful to the user and for the most part, R does not change much for different housings, but by dividing R into A, you get the A/R ratio. So, the A/R ratio of the exhaust housing refers to the size and shape of the scroll that is cast into the housing. It basically determines how restrictive the housing will be, versus how quickly the turbine will spin up. A lower A/R ratio (smaller scroll area, A) results in a more restrictive housing. This restriction speeds up the exhaust gasses and increases the amount that the gasses will expand. It's the speed and expansion of the gasses that causes the turbine to spin. So with a low A/R ratio, the turbine will spin up quicker, but as engine output and rpms increase, the restriction of the housing begins to build up too much back pressure on the engine, which reduces performance. A good rule of thumb for when there is too much back pressure is when the pressure in the exhaust manifold is more the half of the pressure in the cylinder. So basically, a larger A/R ratio will improve your engine's top end, while losing some mid range power and increasing turbo lag. A smaller A/R ratio will help the bottom and mid-range, but may effect the top end.
On the compressor side, the housing also features a scroll design, but it has the opposite function. The air leaving the compressor turbine has a lot of speed, but not much pressure. The scroll on the compressor housing starts small and gets larger as it approaches the compressor outlet. This collects the air and builds up air pressure. So, the compressor housing is designed to convert the speed-energy of the air coming off of the compressor turbine into pressure-energy, which is much more useful to an engine.

[chrisr_nz]

Gonthrax,

What can I say dude - that was EXACTLY the reply I was looking for!

I'm glad you managed to work out that I meant A/R though hehehe ...

Thanks HEAPS!

Chris

[Gonthrax]

Originally posted by chrisr_nz
Gonthrax,

What can I say dude - that was EXACTLY the reply I was looking for!

I'm glad you managed to work out that I meant A/R though hehehe ...

Thanks HEAPS!

Chris

No problem, I aim to please

Welcome to AF also

[chrisr_nz]

Gonthrax,

Firstly, cheers for the welcome ... can't believe how much stuff I've found on here that I've been looking for for AGES.

Secondly ... after your cool explanation on A/R, what's the deal with the letter after the compressor wheel size? I think the 20 in 20G indicates the physical size of the wheel (if not, correct me!) and the G indicates the angle/cut of the blades which effects the boost curve or how hard the turbo "kicks".

Got an explanation for this too?

Thanks,
Chris

[Gonthrax]

Originally posted by chrisr_nz
Gonthrax,

Firstly, cheers for the welcome ... can't believe how much stuff I've found on here that I've been looking for for AGES.

Secondly ... after your cool explanation on A/R, what's the deal with the letter after the compressor wheel size? I think the 20 in 20G indicates the physical size of the wheel (if not, correct me!) and the G indicates the angle/cut of the blades which effects the boost curve or how hard the turbo "kicks".

Got an explanation for this too?

Thanks,
Chris

Well I beleave your right on this one. The "13G" in the model name refers to the compressor wheel. The "13" is the size and the "G" is the style. The 13G wheel has an exducer (or base) diameter or 2.000" and an inducer diameter (air intake opening) of 1.580". "B"-, "C"-, and "T"-style compressor wheels have all blade tips at the same height. "G"-style wheels have blade tips at two heights, alternating high and low. Blades are always evenly spaced, but the number of and pitch of the blades can change between models. 9B, 13G, and 15G wheels have 12 blades while 18T wheels have only 6 blades.

[chrisr_nz]

Where do you get your info from? Don't want to bug you with questions too much and I can't seem to be able to find most of the info I need on the web ...

Thanks

[Gonthrax]

Originally posted by chrisr_nz
Where do you get your info from? Don't want to bug you with questions too much and I can't seem to be able to find most of the info I need on the web ...

Thanks

Well, when in doubt, a well phrased search on Google should find somthing. Some of it is from my personal experience though.

[P Plater]

Another Turbo Question that I've always wondered.

What does it mean when you have a T03 or a T04 turbo as opposed to a T3 or T4. Is it just an abbreviation or are they completely different things?

As far as I know, the T[number] designation refers to the size of the turbo. What "size" is it actually reffering to? And what does the 0 infront of the number mean?

Also, when you have a turbo with a different sized compressor wheel to the actual turbine, is this is expressed as like T3/4, and if so which number refers to the turbo and which refers to the compressor?

Any Help Appreciated.
Brian

[Gonthrax]

The T4 is a smaller size then a T04, I'm not sure exactialy how a T4 differs from a T04.
The Txx is refering to the size of the housings. Garret makes several different kinds of compressor and exhaust housings. There are two main measurements for a housing. The first is the overall size of the housing. There are several different sizes. Most of them start with "T", then have a number or two. Generally, the larger the number, the larger the housing. So, for instance, a T2 compressor housing is smaller than a T3. A T04 housing is very large, and the T60 and T70 housings are truly monstorous. (Usually only used on diesel truck engines.) What size housing you want will depend on how much airflow you want to run. A larger housing is capable of passing more air easier than a smaller one. To pick accurately, you have to know A) how much air your engine sucks in per combustion stroke (the displacement of the engine gives you a good estimate of this) and B) how much boost pressure you want to run, because the higher the boost level, the more air you're flowing. When it comes to housings, bigger is usually better - to a point. The price paid for a large compressor housing is very slow spool up. Generally you want to have the smallest possible housing that will still flow enough air at max boost to remain decently efficient.

Hope that cleared it up some, I'm sure someone around knows a bit more about the subject then me

[Turb]

Gonthrax,

Is there a way of identifying a turbo just by looking at it?

I have 2 turbos and i'm not sure which one is better..i think they are both T25's (one came from an RB20DET engine and the other one i'm not sure, some says it's T3 but i don't know how a T3 looks like and it looks exactly the same as the other one) but they look different when it comes to the compressor side scroll tube (the A in A/R). when you look at it they are the same diameter but the other one has a taper and when the tube reaches the compressor wheel the size is reduced in half while the other one doesn't change in size.

to make it simple..the compressor housing is the same size but the compressor wheel on one of them is bigger. so is bigger the better when it comes to compressor wheels?

The turbine side is exactly the same.

thanks for the help.