boost vs power
 

I'm trying to get my head around how fm2 system or comparable system makes 230rwhp at 12psi.

I'm thinking 12 psi = 0.83 bar or 1.83 bar absolute.
1.83 times more air compared to the 1 bar you get naturally aspirated.
So if a stock 1.8 makes say 100rwhp (I pulled that number out of my arse based on the 130bhp at the flywheel) would we not expect 183rwhp even in a perfect world with perfect 100% efficient intercooling, no power lost driving the turbine, perfect fueling etc.

I'm sure I've missed something obvious here so can someone put me right.

multiple ways to do this:

assume 1psi = 10hp

12 x 10 = 120

100 + 120 = 220hp.


assume 0.077 bhp/cid psi.

.077 x 109 x ( 12 + 14.7 ) = 224hp



in your math you need 130% more power, but also consider in 1.83PR there more than 83% more power. However, a 1.8L is going to dyno closer to 120rwhp so even then 120 x 1.83 = 220

Its not pressure, its mass flow and the ability to use it well.

A 1.8 running at 1 x atmo makes 100WHP. Lets attach a mass flow rate to that, semi-educated guess from me is about 10 LBs/Min of air. Meaning that the 1.8 engine will use 10Lbs/Mn of air with an intake pressure of 1 x Atmo.

At 1.83 Atmo, the 1.8 obviously processes more. Going by my guess on mass air flow, looks to be about 23 LBs/Mn for 230WHP. Few reasons this may be so:

An engine that has fairly crappy flow characteristics on the intake side or a lack of curtain area (which describes the BP pretty well) may see a huge gain in flow if you push air along. Especially if it also has some exhaust flow issues that result in a bit of pressure hanging around in the cylinder at the start of the start of the intake stroke. You need the boost pressure to overcome that residual pressure to get max cylinder filling. No idea if this really applies to the BP and if it does how much of an impact it is.

Plus, the turbo is the big ? here. A 2554 will heat the air up and kill mass flow even as its pressure goes up. So more pressure yet less flow. Not hard to do. A EFR71XX will run a LOT more pressure before that happens. In other words, 12PSI from a K03 is a much, much lower mass flow rate than 12PSI from a GT42R. Maybe the 42R makes like 30LB's/Mn @ 1.83 Bar where the K03 would be like 15. I totally pulled that out of my ass but you get the idea. So the BP is a restriction, but a large turbo overcomes that restriction by having way better mass flow through better density/more efficiency. We could probably get 400WHP out of a B6 at 12PSI if we used the right turbo.

Having said that, valves, heads, intakes, exhausts, are all restrictions, and restrictions usually mean that doubling pressure results in less than double the flow, not more than double. Which brings me to:

Which Dyno? Correction factors are the cushion of the butthurt. I love seeing Subaru and Evo guys apply like 30% correction factors to figure flywheel HP. Truth is, most correction % on dyno queen internet glory charts are pretty lame. I live in CO, I know altitude makes a big diff even with FI, but I wonder what the average 12PSI FMII makes on a Mustang dyno uncorrected at sea level on a 72 degree day with mild humidity on pump gas. This is not a knock on FM at all, but I bet it is not 230WHP.

Edit: One other thing: HP is RPM dependent. If we have crappy street cams that are designed to let grandma haul groceries comfortably (which we do on a stock BP), we probably run out of breath up top at 1 atmo. Boost will push that 'run-out-of-breath' point higher in the rev range, meaning that you will see increased HP from the increased flow at high RPM even if the peak flow number is not so great. Comparing TQ at a certain RPM rather than peak HP would eliminate this variable.

I'd bet it's around 220-230whp. They used to claim something like 265 but I see it's been changed to what I would consider a more realistic number. My FM2 put down 220 on a mild tune at 11.5-12psi.

The First Rule of Boost Pressure: Boost Pressure means fuck-all, other than a measure of how much restriction your motor poses to your turbo.

The big factor that you're missing is that the drivetrain losses are not a linear percentage of total power output; they do go up with the engine output, but not at the same rate. There are one or two folks who've run the same engine on both an engine dyno and a chassis dyno and reported this.

So given the (incorrect) simplifying assumption that power scales linearly with boost, if a stock 1.8 makes 140 crank and 110 at the wheels (which is about right for a 99), then it's losing 30 hp. If you put 12 pounds of boost on it (180 kpa), then let's assume the motor is now making 1.8*140 at the crank -- around 252. The drivetrain losses go up to, say, 35, so you'll now see 217 at the wheels.

This is all assuming a sea level dyno. Turbos behave in a very different way at altitude, and there isn't really a good way to extrapolate sea level performance based upon higher altitude performance.

---Ian

What HP does a miata with intake exhaust ecu tune etc make?

Depends on what's included in the "etc." Dann is this a serious question?

Yeah a turbo miata doesn't have only a turbo, it has a bunch of supporting hardware that would be considered legitimate mods on an N/a car.

You aren't comparing a stock miata with a 12psi miata, you need to compare a turbo miata without boost to a 12psi miata

Its late I hope that translates

While all that is true, the important part is that boost != power, flow == power. 1.83x more pressure does not mean 1.83x more flow.

Sure, but as long as the turbo is sized appropriately and the exhaust back pressure, intercooler, etc are up to snuff, it's usually fairly close.

Also, you generally see less than 1.83x more flow, which doesn't explain why you see more than than 1.83x more power at the wheels. :)

--Ian

This is everything you need to know. More air, more fuel, more power.

Good answers thanks! I think the dyno correction is the best point. I didn't even realise they corrected the dyno. Taking a measured value and turning it back into an estimate seems f***ing stupid.

The drivetrain losses being non linear is an interesting point I didn't think of it that way but it makes sense.

Same again about the pressure helping to scavenge the exhaust gasses (is there any overlap on stock cams?)

Not sure I completely agree on the flow vs pressure argument. Yes they are not going to be equal but the increase in flow is always going to be less then increase in pressure not more (unless I'm missing something obvious again).

The cool thing is that it doesn't matter. At all. :giggle:

Compressor efficiency would like to have a word with you

Yeah some more efficient then others but they are all less then 100% efficient.
All I was driving at was that 1.83 pressure ratio will always deliver less then 1.83x as much air by weight as N/A. Never more.

Well that's not true that's what sav is alluding to. Your assuming that the engine and exhaust turbine pose the same restriction at each pressure ratio but you'd be wrong

Dann

Yeap. And VE, and overlap, and burn rates, and intake harmonics, and exhaust harmonics...

It will range from "slightly less than it did when stock" to "slightly more than it did when stock," depending upon the quality of the parts involved, the quality of the tune, and the quantity, mass and polar moment of inertia of the etc.

It should be noted, of course, that a Miata with no intake, no exhaust, no ECU tune and no etc will make zero HP under all conditions.