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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 miata 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?
 
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More boost does not equal more power.

CFM is really what you should be measuring. PSI is pressure. CFM is Volume.

More air, more fuel....


You'd have to consider both of your turbos and how much air they are moving. I suspect your miata turbo flows more CFM and is very efficient for the engine it's paired with. The K03 on a veloster is relatively tiny and high strung. CFM is relatively low.

Consider this example... a car hitting you at 40mph would produce about 101,966 newtons of force. a semi truck hitting you at 40mph would produce about 2,000,000 newtons of force. To make the car have the same impact as the semi, the car would have to be going about 177mph.

replace the different vehicles with different turbos. replace the speed with psi. replace the weight with cfm. You can see why some turbos would have to work harder to make the same power. And that's before you start talking about efficiency ranges and what a given turbo can even do before it self destructs lol
 

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I will add to the above, many people do not truly understand torque and hp concepts. Take BMEP and piston surface area and a fair estimate of torque can be calculated. When the engine reaches the sweet spot whare BMEP is highest max torque will occur. A dyno will give that figure, it is a real time measurement of torque, we have that much torque to use right now. HP is work over time so the dyno is saying if this engine can make this much torque at this rpm then it will be capable of doing this much hp of work over time. There are engines with huge torque figures but low hp because they can not rev high enough to accumulate enough torque events to create high hp. Our Veloster turbos are set up to make lots of low rpm torque and if the turbo can not maintain that level of cylinder pressure at higher rpm hp will suffer. I also have a Miata 1.8, mine is an NA with no forced induction. Two factors about the 1.8, it is a fairly rev happy engine with a 7k red line and it was designed for from the start forced induction. I also have a Passat turbo. It has a direct injected 2.0, a fair amount more displacement than the Veloster yet hp is very close to the same. All about what type of power curve the designer wanted.
 

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Here is an analogy that may help. You have a very long nail to drive into a very hard piece of wood. Swing the hammer once and you have delivered x number of ft lbs. of energy, or torque in the case of our engine. But the nail is not fully driven, how many blows can we deliver, each with the same energy in a given time to set the nail. Torque= one blow of the hammer, HP= how many times can that force be applied.
 

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Here is an analogy that may help. You have a very long nail to drive into a very hard piece of wood. Swing the hammer once and you have delivered x number of ft lbs. of energy, or torque in the case of our engine. But the nail is not fully driven, how many blows can we deliver, each with the same energy in a given time to set the nail. Torque= one blow of the hammer, HP= how many times can that force be applied.
Oh wow, I've never thought about it like that. Love it!

BMEP would be a much better compare the power between your VT and Miata. Engineering Explained has a great youtube video on this. I can't even explain it off the top of my head.

HP/TQ = Accurate. Not precise. HS Graduate level math. Easy to use.
BMEP = Very Accurate. Very precise. Graduate level math. Hard to use.


For myself and probably most, it's an engineering equation that requires me to learn more math than I care to, just to understand. I'm not an engineer/mathematician or anyone that works for an OEM... HP/TQ is good enough for me!
 

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I believe turbo g4fj's make about 97hp "N/A" (no boost. Imagine disconnecting the hotpipe) or maybe it was 93... I don't feel like doing the math.
It's not setup the same as the non-turbo 1.6 plus there's a turbo to spin.

All else equal, at 23 psi, very rough math puts the Veloster motor at 250hp and the Miata 1.8 at 300hp.

If that's in the ballpark of what you were thinking then that's probably a big part of the answer.
 

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A little further explanation,
At 97hp and 115 that's about 6 HP per 0.1 liters for both. A 0.2 liter increase would be right around that 110-115 range NA. Boost will compound that difference (% power increase over your NA potential power). And at risk of being cliche, there's no replacement for displacement. Can't believe I just wrote that... 🤢
 

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No replacement for displacement, only partially true. If you increase displacement will you increase HP? probably. But lets talk efficiency. There are two ways to increase displacement. larger bore or longer stroke. The drawback with longer stroke is increased piston speed, and destructive loads on the reciprocating parts. That leaves increased bore, problem is that as bore increases it becomes ever harder to control detonation. On large bore engines the flame front, post ignition by the spark plug, has so far to travel that the pressure wave ahead of the advancing flame front can create spontaneous ignition at the outer bore. When these collide we have detonation. Detonation causes a very high pressure spike which among other things blasts away the pistons protective boundary layer. There are only two ways to make power in an ICE engine, BMEP and or RPM. At any given slice of engine development history there will be a maximum BMEP. BMEP continues to rise due to advanced combustion chamber design, fuel deliver and such. Prior to the the change to all four strokes 250cc naturally aspirated 2 stroke moto GP racers were making around 7 HP per cubic inch. That equates to over 2100 HP on a Ford 302, again this is naturally aspirated. Look at any family of engines, yes the larger displacement versions make more power, but look at HP per cubic inch, they rarely match the smaller displacement engines. Large displacement often equals more power but less efficiency, personally I have always liked the smaller displacement over achieving engines.
 
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