[Kingtal0n]

Originally Posted by PoorMans180SX

I think when most people use the term "efficient" they're talking about the flow rate per wheel size. That's the advantage of new technology.

Exactly why I keep saying efficiency hasn't changed much-
1. is there are more than 1 type of efficiency?
2. the term 'efficiency' with respect to pumps and turbos has always been adiabatic efficiency
2.5 Adiabatic efficiency is what fluid mechanics book will discuss for all pages when it says 'efficiency' this I have taken to be the correct use for the term (but not it's only use)
3. Be aware of the difference flow rate per wheel size, and adiabatic efficiency.

All compressor wheels have roughly the same efficiency island (around 76% usually) potential thus they are all equally efficient for more than 20 years

Flow rate is a setting for fluid velocity through some area and when temperature and type of fluid is included the density is known which gives mass fluid rate, thus 'corrected' numbers you see on compressor wheel maps are highly variable depending on input temperature, making temperature a key component of adiabatic efficiency discussion AND flow rate in mass or velocity.

The limitation of a wheel to higher or lower velocity is part of it's construction. Flow rate per wheel speed size is a function of integrated engineering approaches and has little to do with adiabatic efficiency, which hasn't changed.
Adiabatic efficiency varies with wheel speed and pressure, Which are flow rate volume and pump head respectively.

TO give an example why this distinction is important, consider two identical turbochargers.
Two turbos are identical except one cannot exceed 40,000rpm, and other can spin 80,000rpm.

Do we say the one that can spin a higher is more efficient? Not generally. Just because it can spin faster does not make it more efficient unless the speed limitation has something to do with friction. There are multiple ways to ruin a shaft and disrupt energy transfer which is the point of this discussion. If we say the limitation is due to metallurgy; i.e. the wheel is made of some cast material which would disintegrate after 40,000rpm, then it has nothing to do with efficiency, It might be more well constructed or something like that instead of being more efficient.
Although there is more to flow rate than wheel speed, it was a easy way to show why 'efficiency' can be mis-used when the device in question is dependent on so many variables.

It is important distinction because,
No matter how much 'better' turbos get the need to intercool 200, 400, 800(or any number)hp worth of air isn't changing because adiabatic efficiency is always similar (no free lunch)

These smaller turbos are outflowing old-school turbos of quite a bit larger size, both on the compressor and turbine side of things. You can't argue that the inertial advantage of a smaller rotor group isn't significant when it comes to response time.

Load and time spent under load is an important aspect to this discussion. With a load dyno you can hold a engine at low rpm and load it for as long as it takes to build to some huge boost number and then say look I made XYZ at 2500rpm or something stupid.
So when we start looking at dyno graphs and trying to compare response or spool character, it must be done using the same exact dyno situation and dyno electronics and dyno roller weight and vehicle tires/drivetrain/etc and gear ratios. Otherwise the load will be inconsistent and the turbo will look better or worse on paper because if the load is reduced the engine will accelerate more quickly and the turbo will appear to spool slower. This is why 1st gear spool is always terrible looking on paper and why a 5th gear overdrive dynopull can make the turbo look like is spool really fast on paper.

All my dynos are always done on dynojet. This keeps load from the roller weight consistent at least. And dynojet calculates power from roller mass so there is no fooling it. To compare cars more equally.

I recommend anybody interested in 'turbo spool' consider these things carefully.

Now- about the response and fast spool behavior, I don't see that it makes much difference for daily drivers or drag cars anymore, maybe some other racing applications where exact gear ratio situations (coming out of a specific corner) require fine tuning that last 500rpm of spool from the turbine... but in most daily driving situations you can just downshift or avoid being at a very low rpm in the first place. A stock turbo can blow the tires off of first gear, response is unwanted after some point. The onset of boost should be tire sparing and keep the car from spinning so a slower rate of boost building is desirable and fine-tunable in the best situation. IMO daily driving favors a larger turbine lazier spool when the power at 2L is around 400rwhp+ because it ensures lower EGT and wider tuning window, safer on gasoline especially.

This is definitely an interesting discussion to me. I don't know if you can really factor in budget though. ....Seems kind of a moot point.

Ah, you got that right. Turbo can be free still can cost 5k for a setup.
In future issues, potentially blown turbos, engine failure -> turbo replacement, stuff like that happens and its nice to be using a more affordable unit.

I think it really just comes down to what the car is for and what kind of torque curve you want. I like a really snappy turbo for street cars, it's much more fun to have that kick in the ass torque whenever you need it IMO. This only really happens with small turbos or a medium turbo with a divided exhaust housing.

it looks like a 'small' turbo these days is 500hp and they seem to do well with response and tolerate a wide range of abuse. 500 is apparently 'low' power now