ThetaTau87

That side converstaion I started yesterday about GT2871RS vs. RS2 turbos got me thinking. There are two main factors working to determine how quickly a turbo builds boost and at what RPM - Spool and lag.

Here's how I understand it.

The RPM at which a turbo is fully spooled is the point that the turbo has enough airflow from the engine to produce full boost. This is only partly related to the time it takes for the turbo to build boost when you change throttle position, commonly called lag. Spool RPM is most closely related to the size of the compressor and tubine. A bigger turbo will require more airflow (higher RPM) for a given displacement engine to reach full boost.

Lag on the other hand is more closely related to the mass of the compressor and turbine rotating assembly. Bearing friction also plays a big part becuase it contributes to the rotational inertia of the turbo. Lower mass and or less friction = less lag.

With ball bearing turbos the reduction in friction of the bearings allows a larger turbo to have less lag. Alowing big power to be achieved and still keeping lag similar to other smaller turbos with conventional bearings. Simply changing the bearing should not effect spool RPM. Spool would be determined by the size of the turbo and the design of the blades also by the peak boost determined by software settings. The turbine can be designed to be more efficient at lower RPM and spool lower or more efficeint at higher RPM with higher spool but more power. 2 identical turbos w/ the only difference being the bearing would have very similar spool RPM, but the ball bearing turbo should have less lag.

Simply looking at the dyno graph of an engine with a certain turbo will tell you what the spool RPM is but it will not tell you much about the lag characteristics of the turbo. Because a normal dyno pull is done at WOT all the way across the RPM band spool is the key factor not lag. If you did a pull on a dyno and started at part trottle at the RPM the turbo would be fully spooled and go to WOT then you could measure the time it takes to get to full boost and determine lag time.

These are my thoughts, please add your own.

450exc115

and I don't tend to put my thoughts down on paper/screen well..

FYI, bearing losses in a turbo are generally on the 5-10% of total input power. Fluid film bearings are in the 7-9% while ball bearings are 5-7% so you do gain some power from an identical turbine when switching from fluid to roller element bearings...

Turbines make up 60-80% of the rotational inertia of a turbo charger, the Alum compressor wheel on the other end take more power than anything..

Most of the problems with sizing a turbo is figuring out what you want it to do. Alot of the turbine function is driven by the nozzel size and the gas flow coming from the engine. Most older OEM turbo I have ever seen are one nozzel size and generally it's just the width of the turbine housing slit. Too big and the velocity of the exhaust gasses is to slow to really spin the turbine, tooo narrow and you reach max velocity too soon and the turbine runs out of power before getting over XXXX RPM..

The success of the RS2 turbo is that it was designed for our 2.2L engine. The down fall is they used the technology they had (fluid film bearings, compressor design using numerical calcs rather than CFD, and some what off the shelf KKK parts)

The GT2871RS turbo is very close to the RS2 in design, in fact it's a slightly smaller turbo but with the newer technologies (ball bearings, modern compressor wheel and modern turbine wheel) it should be close to the RS2 with better spool rates, more power in the lower RPMs but have less overall power due to flowing less at higher RPMs...

IF anyone hasn't read the book "maxium boost" then I suggest you pick a copy up.. While he does an excellent job with out getting super detailed he lays a pretty good foundation for understanding turbo and turbo systems.

ThetaTau87

It reconfirms why I want a GT2871RS and not an RS2. I want more power than chipped k24, but I want to give up as little on spool and lag as possible. I was thinking the GT2871RS with .63 A/R ratio would be the way to go.

It concerns me that the 034 stage 1 GT3071RS software only puts down 325 whp. This is roughly the same as an RS2 set up with aftermarket saftware. This is the power level I'd like to be at, but with the 2871 not 3071. I'd really like to see some dyno plots with the 2871 before I buy. 034 has said they are working on software for the 2871. I'm very interested to see the results.

I was not aware that the 2871 was actually smaller than the RS2. I assumed it was bigger, but had better spool due to the bearings and compressor/turbine design.

I should revisit my copy of "Maximum Boost." it's been a few years since I picked it up.

450exc115

Actually has a larger compressor than the comp RS2 but looking at the compresor map the RS2 flows more air, which means it might have a slightly larger blade profile..

Carfull with the HP numbers.. I've never seen a RS2 with 325 Wheel HP, I've heard of 370-380 crank HP but to get 325 whp you need to be in the 400s at the crank.

chefbob

VEMS

nate @ 034MOTORSPORT

This is designed for a stock engine, stock rods, etc. The turbo is by no means 'tapped out'.

We'll be releasing 'built motor' tunes in the near future.

ThetaTau87

I would like to get 300-325 at the wheels, but with the quicker spool of the 2871. I don't plan on making any internal engine modifications and 93 is what I run daily. Sometimes I'm forced to use 91 on long trips to other parts of the contry, but that would be rare.

UrS4boy

Agree on the hp numbers.

450exc115

Not going to happen with a GT2871. To get in the low 300s at the wheels you need high 300s at the crank if not low 400s. To do this with the GT your looking at pushing it to the limits and running high boost (30 psi most likely). That being said you may want to freshen the topend and increase the clamping pressure on the head gasket with some ARP studs. Then your looking at increased injectors (45-55lbs) possibly a fuel pump (044 unit), variable fuel pressure reg, definitly an exhaust manifold, a 4 bar Vmap, the proper ECU software, stage 3+ clutch and now your into the motor for 5K+. Which is fine if you want to spend the money..

For me as my S4 is a DD also, I'm just looking to do a RS2ish tune with a slightly larger compressor, injectors, software, and hopefully a EM but the plan is to keep the boost pressure down to 2.5 bar (stock pressure transducer) and keep a nice streetable clutch in there..

UrS4boy

RS2ish tune and 2.5 bar MAP do not go together very well. Buy a VMAP.