Major confusion about horsepower and boost
 

Hey y'all, I have a question I can't seem to find the answer to.

Why do some engines make much more power with less boost and same displacement as others?

For example, my Veloster makes somewhere btw. 200-220 on 23lbs of boost (NA motor is 1.6l 138hp), and my turbo NB makes the same power on 10lbs of boost (but the stock motor is 1.8l 100-115 hp!). The BP4W on 23lbs of boost would be making crazy power!

What's the story here?

Flow. More air + more fuel = more power. We made 300 hp on a built 2L BP Rotrex at 8 psi, but only 225hp at 10psi on a stock motor with the same blower. One head flowed much better than the other.

Boost pressure does not equal air density.

A larger turbo can move more air to reach a given boost pressure and generate less heat while doing so, and a denser air charge. This means that a given pressure does not result in a set density of oxygen. A smaller turbo operating above its ‘happy place’ will generate more heat compared to a larger turbo operating within its efficient range, resulting in less dense air and lower power potential.

Boost is a measurement of the restriction of flow through the entire combustion track.
You cannot compare an OEM system to an aftermarket system as the OEM is designed with many more constraints (read restrictions) than the aftermarket system.
OEM systems have to remain emissions compliant throughout most of its operating range.
The AFR readings that the Veloster will produce well into boost would destroy the BP.

On restrictions: You take your 10 psi stock BP engine that makes 220 and install a SM CNC ported head (keeping compression and cam timing the same) you will now make 10-15% more power at the same boost level.
This applies to both the exhaust tract and the intake as well. A less restrictive intake, a less restrictive exhaust both increase the mass of the airflow and the power without increasing boost.
Just merely sourcing the inlet for the system from the least heated area of the engine compartment is also worth density and power.

On OEM systems: these have to meet criteria that is not usually considered when building a high-performance system.

It has to be quiet, and you end up with highly restrictive exhaust systems in most cases.

It has to not exceed a torque limit to reduce driveline warranties. Look at the torque curves of most modern turbo cars. They reach a plateau and stay there for 3500 rpm span.
Almost looks like an electrical motor; the torque "curve" in no longer curvy. They bump up the low rpm torque via VVT and other tricks (variable length intakes etc..) and they chop of the normal peak using ignition timing, egr flow, and other "hobbling" tricks.

It has to not piss the EPA off. Cars have gotten insanely clean in the last 40 years. The HC and CO emissions from the past are 95% gone. The only big issue now is oxides of nitrogen (NOX). These occur above a specific temp in the combustion chamber.
Somewhere over 2500 degrees F you get serious production of NOX and the best way to combat it is to artificially restrain temps below this threshold. You can do this by adding an inert gas (EGR), reducing timing, or adding excess fuel.
Back 20 years ago most factory turbo applications went pig rich under boost. Adding fuel significantly increases HC and CO production so in a modern car (your Veloster) it probably doesn't go stupid rich and its fighting NOX production via timing and precise fuel control.

So your Veloster is doing a bunch of strange stuff to meet these constraints and thus requires much higher boost to get the same job done as your BP.

Without the constraints listed above the 1.6L Gamma would make 300ish at 23psi IMO

BTW stock boost limit for the Veloster is 18 psi.

Very few people use the phrase "HP per LB of boost" and that is the true measure of a systems efficiency...

It might be worth noting that some manufacturers like to add atmospheric pressure (14.5) to their "psi" claims and in-car gauges to puff up their chest for consumers...

The other answer; horsepower is equal to airflow and boost pressure can be seen as a measure of restriction on intake air and somewhat mutually exclusive from engine airflow. For example, a 2860 @ 10psi will make more power than a 2560 @10psi because it takes bigger "bites of air" per revolution. For the 2560 to make the same power (airflow) it may reach 14 psi because it has smaller blades that have to spin faster, generating more heat to flow the same amount of air. Either system could best the other by having more free intake/exhaust tracts or cooling the air charge more efficiently.

Any piping (exhaust or intake) will increase in pressure easier with a decrease in its own total volume, and increase in flow easier with an increase in total volume, limited only by what feeds it - turbo, intake tract, engine output or atmospheric pressure. A 2860 @ 10psi certainly can make less power than a 2560 @ 10psi if the intake piping is small enough to restrict the engine from pulling in enough air for the same target.

Anything that feeds air into or lets air out of an engine benefits from less restriction because more bigger, better air means more bigger, better bang. It's just a pump pulling in air to mix with gas to blow up so it can spin.

I googled a random dyno chart for a stock turbo Veloster off the Internet, and it shows the answer (blue is stock):

https://cimg5.ibsrv.net/gimg/www.mia...86b8aae3b0.jpg

Look at the blue torque curve, see how it peaks at 3000 RPM at almost 200 ft-lbs and then declines to barely half of that by redline, and how the power is basically flat from 4500 all the way up? The only way that happens is when the tune trims boost as the RPMs climb. That Veloster may be making 23 psi at 3000 RPM, but it's probably down in the single digits by 6500. Horsepower is torque multiplied by RPM, and that OEM turbo is too small to flow well at high RPMs. They're cranking in a ton of boost at low RPMs because that gives a big "kick in the pants" that feels fast and sells cars, and 90% of the population isn't willing to wind an engine out anyway. If the Veloster could manage that 190-ish lb-ft of peak torque at 6500 it would be making 235 at the wheels instead of 150.

So the short answer is that your NB has a larger turbo that flows better at high RPMs, combined with more raw displacement in the engine. It's probably also got cams tuned for a higher natural peak torque (which will give more power). If you're using miataturbo-standard parts (2.5 or 3 inch exhaust, metal core cats, etc) then you've probably also got a much freer-flowing exhaust than the OEM system on that Hyundai.

The red line on that curve is the reflashed ECU from whatever company it is that I swiped the chart from. :). It's basically the same shape as the OEM curve, just 20-30% higher, suggesting to me that they basically just dialed up the boost and timing a bit. They're pushing the turbo and hardware harder than the OEM tune does because they aren't concerned about warranty claims or emissions regulations. Your tune on the NB is probably similarly aggressive.

--Ian

This is my BP4W on 22 PSI a few years back.