xxxhp

Assume that you have two turbos that have identical compressors, but one of them has stage 5 turbine wheels & 1.06 turbine a/r, while the other one has bigger p-trim turbine wheels but smaller a/r of .63

What would be the difference in how those two turbos perform? what do you think would be the difference in spool time & top end power? also, which one will have less back pressure?

synapse

I've always been a fan of having the bigger wheel to begin with and just trimming it down with A/R to get better spool time. At least you have the option of going bigger A/R later, if you're build calls for it, or maybe you want to drag it instead of driving it on the street. Big wheel, big A/R will have the most top end power, and with the big wheel to start, you'll at least have that option always available to you with just a turbine housing swap. Bigger wheels will definitely have less backpressure, so long as the blades aren't overly restrictive compared to the smaller wheel.

Just be careful, the lower the backpressure, the higher the probability of boost creep.

Drew04GTO

iTrader: (1)

I've personally always been a fan of larger wheels due to lower driven pressures and increased turbine torque. As synapse mentioned if you start with a larger wheel you can always change housings to get what you want instead of having to get a new wheel/shaft installed. Just out of curiosity synapse why do you think you're more likely to see boost creep at lower pressures? I've found the exact opposite. Small a/r's and small turbine wheels generally require much larger wastegate flow to stop boost creep. My best friend's honda makes over 640whp on corn with a 38mm gate and no creep, using the largest turbine side he could find for his turbo. Would even bother to say that his wastegate placement is way less than optimal.

synapse

As some of you may know I design wastegates. And wastegates are pretty "dumb" devices. It is all about the pressure at the valve face and the actuating pressure that determines when they open. So, if the spring pre-load force is the same, and the pressure at the actuator is the same that is all pretty constant.

But you lower the backpressure and you have less overall force over area at the valve face to open the WG, given the same spring. Now, if the system is so efficient that backpressure actually drops as RPM increases, as a ratio to intake manifold pressure, then you really start to see creep. The solution is to start playing around with spring rates to have a faster lifting valve to compensate for that change in rate.

Each system and the combo it produces is different as well. With your buddy's Honda, he's probably running 25-30 psi to get that 640whp. Every turbo has what I like to call a "minimum boost happy place." You really can't identify creep if the turbo is running in its natural zone. Now try holding 6 psi with that Honda, and I think that you'd be hard pressed to do it with a 38mm gate. Creep is all relative, anyone trying to make 50 psi doesn't ever consider creep cause they are just trying to get more boost out of it. But someone trying to hold down 5 psi has a bigger challenge when it comes to wastegating.

engineermike

I think you need to understand how turbines are designed, then the answer to your question would be obvious.

Turbine wheels are very design-intensive. The modeling that goes into them is far more complicated than most realize. It's a tug-o-war between the CFD model to get the aero right and FEA to get the strength/longevity right. So, once you get a good turbine wheel ironed out, you now have to design a slightly larger one for more power, then larger still, and so on. The engineering time really gets out of hand. Then, someone figured out that you can change the A/R of the snail and it makes any given turbine wheel "act" like smaller or larger wheels. So, rather than designing 5 different turbine wheels for a range of needs, you can design one and just change the A/R to get more or less power output from it.

So, based on this, a large wheel with a very small A/R will work exactly the same as a small wheel with a very large A/R, assuming steady state.

Mike

Drew04GTO

iTrader: (1)

Never tried less than 10psi in that honda since it was way out of its efficientcy range below 15 psi or so. Had no creep at that low of a boost pressure, creep should only become more of an issue at higher exhaust mass flow rates, ie. more power. At 28 psi he's using every bit of available flow through that wastegate, but the gate itself isn't the problem of flow, it has a harder time making a 90 degree turn from the collector into a tube than getting past a valve/seat larger than the tube. I think as long as the turbine wheel and a/r have much more to do with boost creep than the wastegate does, most people just choose to run an exhaust side way too small since they arn't tolerant of "lag". Specific turbo on that honda is a Holset HX40 18.5cm exhaust housing, the housing itself is almost TWICE what most people run on DSMs, with nothing but a smallxsmall b16. The higher boost threshold atleast makes the car more driveable at lower speeds.


I would like to bring up something again that I have yet to hear anyone else reference, TURBINE TORQUE. Let me just pull some arbitrary numbers out of my *** for now, you'll still get the point. Lets assume there's a 2:1 pressure ratio exhaust/intake and running 15 psi of boost. Lets take the very tip of the turbine blades into consideration, .5"^3 on average for the first .75-1.0" of turbine blade. Lets consider the difference in torque between a GT42 and GT45 exhaust wheel. 3.2" inducer compared to 3.425. Just in that first .5"^3 on a GT42 there's ~528 oz/in of torque generated. Switching to a GT45 wheel that increases to ~582 oz/in. Each exhaust pulse is physically doing more work on the wheels/shaft due to mechanical advantage alone, assuming a/r is the same and mass flow is the same. The turbo might however have a higher threshold due to the larger intertial moments of the physically larger compressor/turbine wheel.