Need Clarification!

[doo doo]

Volume of air at PSI has nothing to do with density or heat, the concept of a larger body of air at the same PSI while correct, I don't think it's applicable, unless you are talking about increasing the amount of inlet volume eg pipe diameter and length, larger intercooler etc, or changing the engine size for the same turbo. Then it'll take a little longer to pressurise the whole system before the engine sees that pressure, and the air velocity for the same amount of air you had before will be slower. If it was too small before and created drag and inefficiency then you will get positive result. It doesn't affect the compressor efficiency of the turbo (how much the air is heated at flow rate and pressure) or the effect of the size of the ex housing a/r.

I like the physics equations mentioned, hopefully everyone will benefit in some way from the info..........

Edited May 21, 2010 by doo doo

[GTScotT]

My brain exploded. Alright, Ive seen to grasp the concept of what were saying here now with pressure and air volume etc. So basically a 400hp turbo will always run 400hp of air regardless of the pressure?

There are so many variations of the GT3076. I found some for $450, some for $700, some for $1000 and some for $2200.... I'm going to keep reading about specific turbos. I know I need a new manifold and a few other parts so I can get them before I get the turbo.

Luckily, we dont need to reinvent the wheel. Just as, we dont need to re-engineer the turbo either.

When I was at your stage, the dyno thread along with a couple of other threads on GT30s told me all I needed to know. Efficiency wise the 3076 is one of the best, but that doesnt necessarily mean its the best turbo for your RB25. Personally I was most interested in the HKS GTRS at first, the delivery was just about spot on and would get a near guaranteed 250rwkw. Then obviously being a perfectionist, knowing that the 3076 made 300kw and that the compressor was nicely matched to the motor, I couldnt settle for just 250.

The delivery of the 3076 just wasnt entirely what I was after, it seemed to progressive and lazy, this is only my personal opinion. Yet looking further into things I found that the delivery I wanted and the higher output were best compromised into the HKS 2835, which is seemingly alot more responsive and capable of roughly the 270-290rwkw mark with ease.

Next thing I knew a HKS 3037 cropped up for a steal and I thought hey, maybe I can get that GTRS feel AND 300kw if I match that to that RB30 I have in the garage! Bingo, this was going to work. Right till I went to get my crank balanced and it was a dud. So now Ive decided to cut my loses and be the regular wanker with a RB25 and that size turbo rather than the special type of wanker I would have been otherwise.

Point Im making here is, if I was to do it all again, Id grab a 3071 with an ex gate .63 rear and plumb a gate off the housing. Theres a chap that posted a thread with that on his mild RB25 with a .82 internal gate housing and got nearly spot on delivery to what I wanted and 292rwkw. You can find the thread in this section if you look hard enough. Hypergear can mod the housing for an external gate for $150 Im fairly sure, and theres a guy selling a suitable dump for $250 on NS. Then all u need is a 10mm spacer for the stock manifold and your off and racing.

You can also get an internal gate housing for the turbo to keep a stealth like appearance, yet for some reason the garrett internal gate housings (especially the smaller ARs) dont like holding boost to red line. Dont forget HKS do most of the R&D for these turbos in the first place, thats why they get rights to them for 6 years while garrett cant sell them till after that. You will also find HKS housings are unique to the garrett ones, my 3037 exhaust housing is nothing like a garrett one. FYI 3037 wheels = 3076 and 2835 is a modified 3071.

Viva la HKS for me and GL.

[SimonR32]

Volume of air at PSI has nothing to do with density or heat

Actually it has everything to do with density and heat

PV=nRT

[mad082]

A lot of good info here but I think some of it may be a bit full on for someone just starting to get a grasp on things, especially saying that to move 100 apples in 100 seconds, you need to move 100 apples a second... (haha, it's late dude, I know)

I think one of the easiest ways to explain it is this.

Grab a 600ml coke bottle. Put a hole in it just big enough for the nozzle of a blow-gun (off air compressor) & fill it with air.

Within a few seconds the bottle will be full of air, start to build pressure & will blow off the end or push the gun out.

Now grab a 20L oil drum & do the same thing. it will take a lot longer to build the same pressure...

Once full the pressure will be the same (e.g 5psi) but it takes a larger volume of air to reach that pressure

that is actually a rather useless analogy when talking turbo sizes. that is a good analogy if you were talking about different size intercooler pipes or exhausts, but as far as turbos goes it's useless, because in an engine the size of what the turbo has to fill with air remains the same if all you do is change the turbo. in your examply you have to round the wrong way with your example. you are changing the size of the wrong thing (in this case you are changing the size of what is being filled, not what is doing the filling).

[That_timothy]

Actually it has everything to do with density and heat

PV=nRT

+1 =D

[That_timothy]

Haha good stuff Tim, i actually understood that! Lol.

So by all respects, because a turbo is smaller it requires less volume of air to create say 15psi of pressure? Whereas a larger turbo will pass a higher volume of air, but not create the same pressure

cheers Martin. Yeah that's pretty much the gist of it, the other side to the coin with turbo's is that (as I understand it) they're self refferencing. Because you've got the turbine wheel spun by exhaust gasses, the more gas that's coming out your exhaust, the faster you can suck in air. So I imagine the reason huge turbo's are laggy is because it'll take ages with minimal airflow to build up sufficient boost pressure. This would also explain why when you see the psi readings on dyno sheets, they get really increasingly more steep, because it'd be a complicated differential equation relating the inlet speed to the exhaust, but it's easy to see that the more air you've already got, the more air you're going to get.

EDIT: Used the wrong word somewhere =P

Edited May 21, 2010 by That_timothy

[HtwCHACtkh]

Actually it has everything to do with density and heat

PV=nRT

What I think doo_doo is saying is that while everything is true. You also have to remember the flow is determined by the exhaust side, not the turbine side which takes care of the pressure. Opposite sides of the engine.

I think what mad082 said on the first page expands on this well.

[doo doo]

Actually it has everything to do with density and heat

PV=nRT

my bad. Just meaning to imply that the volume being pressurised isn't changing in this instance, as it is on some of the examples given.