18, Sturovo, Dann. Thanks. Can't look only at compressor. Must include turbine. I'm just trying to get experience and theory to come together in my head.

 

Ah yeah. You have the perfect world thermodynamic heat rise from PV=NRT (or a more accurate model of your choice), and then you have how much heat you actually put into the air while compressing it and divide the first number by the 2nd number to get your efficiency.

 

Haha, I like your qualifier "or a more accurate model of your choice". Honestly, I prefer the Pressure-Density-Temperature models for gas capacitance. I am kinda particular here because I had a simulation class taught by a professor who(at the time) was at the forefront of using compressible flow in engineering applications. He has been teaching this class for over 40 years now. There are more accurate models, as you suggested.

Ignoring the exhaust side(bad idea if doing a total system simulation), Temperature is the main indicator of efficiency here, I would estimate that will get you within 5% of reality. We are talking about 2nd law thermodynamic efficiency, not 1st law. Screw 1st law efficiency, its not realistic, by definition.

 

Efficiency is easily defined as the theoretical temperature rise of the compressor divided by the real temperature rise. The real rise Is always higher than the calculated rise

 

I have more of an electrical background but I think v=ir kind of applies here?
Super simplified and probably wrong but we need to make this conversation more analytical:

boost = (restriction of engine+restriction of turbine)*(cfm intake)
cfm intake= (cfm through turbine)*(compressor amplification factor)

So the cfm through the turbine and its relationship to the compressor makes the power (cfm intake)? Boost is just a byproduct.
Shouldn't we focus on the turbine/compressor ratio and it's relationship to the engine restrictions?
Just a stream of thought...

 

Reading 18psi's comments, one thing needs to be said regarding power difference with turbo swap.

Much of this depends in the restriction currently being placed on the engine by the turbo. Going from that TD04 to the vf39 you see a large power increase because Subaru heads flow very well and the turbo was a large restriction. However i would expect that going from a GT35 to a GT42 and running both at 10 psi you'd see little, if any, difference because the GT35 isn't a restriction.

Make sense?

 

You're changing what he said though

What he said was that going from a small turbo to a large turbo all else being equal doesn't produce significant power increase, unless you increase the flow of the thing its pressurizing, in this case the engine, or raise the pressure being produced by the turbo. Which is flat out wrong.

I used my example because
a) it was about as "controlled" of a test as I can possibly think of. Nothing else changed aside from the turbo itself.
b) I actually did it and saw the results in real life
c) doing something very similar on a miata BP, like switching from a gt2554 to a gt2860 or 2871 all else being equal, the car will make more power at the same boost level. It won't be 1-2hp, it will be 20-30+. I guess to me that's significant, but other people may define the word differently.

Again, maybe i'm missing something.

 

As a matter of perspective on how much difference. Re the Subaru swap:
What would you say you would have to turn the boost on the VF39 down to in order to keep the max HP the same as stock? If from 10 to 8, then that would be one thing. If from 10 to 5, then another answer altogether.

If the first case, then we would say, "it takes 25% more boost on a small turbo for the same power". If the second case, we would say, "it takes 2X the boost on a small turbo for the same power"

Maybe on the next swap, you could try that, or you may already have the data.

Another correllary is that the situation sounds a bit like the net power from a supercharger. Some of the power stroke is negated by some of the exhaust stroke. Therefore, for the same net power (torque), the rods are more highly stressed with the smaller turbo (or restrictive exhaust) than with the larger.

 

yes of course, just because a turbo is rated at 53 lb/min at 3PR, doesn't mean that the turbo flows that amount at 30psi.

All a compressor map can do is tell you what a turbo is capable of.

No, the volume airflow is a factor of your displacement, VE, and RPM.

A typical 1800cc miata will have a Volume Air Flow (VAF) of roughly 63CFM at 2000RPM and 220CFM at 7000RPM. This only changes slightly with VE (heat, restrictions, etc).

The volume of air that actually enters the cylinders at a given RPM and VE is the same regardless of boost level.

The turbo increases the air density. When you increase the pressure over atmospheric, you're simply increasing the amount of air your motor can displace in the same amount of time. 53 lb/min is roughly 725CFM. That is 3 times the amount of airflow that your motor will inhale. Why would you put that turbo on your car? because if you want to stuff 3 times the amount of air, into a space, then you need to be able to draw in and compress three times the amount of air required. You need around 660CFM at 7000RPM with a PR of 3 (220CFM x 3).

is this miata.net?

 

Lol.

What he said.

Props for miata.net comment.

Dann

 

 

What Brain said is true. I don't think we have reached m.net level of poo-slinging yet, though.

The whole conversation it seems that many people are equating volumetric flow with mass flow, which is totally wrong. As Brain said, a turbo is a density booster. The throttle can be viewed as a density regulator as well, especially on a N/A engine. 220CFM at 30psig is a hell of a lot more air than 250CFM at atmospheric pressure.

Has anyone here done engine simulations? I have done quite a few on Lotus Engine.

It's free software(for a single cylinder version), just download the guide and most people here will be able to figure it out. Don't expect Lotus to accurately predict real world performance if you do not know all of the flow data for the head at different static pressures, and all of the flow coefficients for everything in the intake and exhaust, but it is great for learning concepts. I highly recommend doing this, it turns out being lots of fun.

 

Depending on a motors volumetric efficiency the turbine efficiency changes in turn changing the compressor efficiency. Since one of the factors in calculating efficiency is measuring the heat rejection on the turbos cooling mechanisms? Obviously the turbine and compressor share cooling bits.

So in an example where a similar turbo with different compressor but same turbine is compared. As long as the cooling system in the turbo maintains, the bigger compressor will compress more air? More dense air = more powa. Did i get that right? That sounds a bit redundant but whatever.

 

Not really changing... just adding an aspect of the discussion that's been completely ignored.

Absolutely. You can look at it this way:

You connect a hose that has a capacity of 1 GPM to a faucet that is adjusted to output 3 GPM. The output will be 1 GPM, so if you upgrade to a 3 GPM hose, you'll see a huge increase. If you start with the 3 GPM hose and upgrade to a 5 GPM hose, you'll see no difference.


Good example. I am not smart enough to understand the other examples and maths being bandied about, so I should probably stop commenting

 

this would be great if we were talking about peashooter exhausts...

 

Still incorrect. The hose was measured 1gph at xpsi if your using a 3gph pump the pressure and thus flow will be higher. Not the full 3gph obviously buy like 1.5 or 2. Bad example.

 

The relationship still stands. If the turbo is a large restriction, then changing to a larger turbo with no other changes will make a considerable difference in power. But if the turbo is not a large restriction, changing to a larger turbo will make less/no difference.

 

You are.
Sure, it made 20-30 hp more for a whole bunch or reasons, none of which oppose the points I made.
- Less restrictive turbo
- More stable boost control
- Lower air temperatures
- GT2554 was probably at the edge of the efficiency range

 

you will get 1.5 or 2gph at higher psi, yes. If you had a boost controller attached to the hose you'd get the same gph at xpsi measured earlier.

 

Elaborate please