Twice this week I've had people tell me they know a car (Miata in both cases) that's running 10-12 pounds of boost and makes 300ish horsepower. Now , my simple engineering mind says if the car makes 140HP stock, at 14.7psi, then it can't possibly make 300 without the boost adding about 1.5 more atmospheres. Have I got this wrong? My "logic" isn't allowing for things like a better exhaust, or an extra 100cc of displacement, but those claims seem really high to me. Educate me, please.
Apparently there's a basic calculation that gets you close.
HP_boost = ((boost_PSI +14.7)/14.7)*HP_stock
By that, they are 50HP high.
https://www.google.com/amp/s/www.motortrend.com/how-to/1305-boost-pressure-on-5-0l-applications/amp/
It's possible. Will it be livable?
It depends on the compressor compromise made. Just because I have 15psi of boost doesn't mean it's livable because of compressor sizing. I mean I can have quick boost and plateau or be real lazy but make power way up top.
What is it 10lbm/min to make 100hp? So, 300hp should be ~30lb/min... it could be possible as you are approximately double stock hp. But more likely I feel/think it is more like 250.
DeadSkunk (Warren) said:Twice this week I've had people tell me they know a car (Miata in both cases) that's running 10-12 pounds of boost and makes 300ish horsepower. Now , my simple engineering mind says if the car makes 140HP stock, at 14.7psi, then it can't possibly make 300 without the boost adding about 1.5 more atmospheres. Have I got this wrong? My "logic" isn't allowing for things like a better exhaust, or an extra 100cc of displacement, but those claims seem really high to me. Educate me, please.
Could be rounding error stacking up, too. If they are assuming a huge drivetrain loss factor, that'll artifically inflate the crank hp number. Especially if you are comparing wheel horsepower to their crank horsepower. "300-ish" in car lingo translates to anything in the 260hp+ range, so there could be some generous rounding going on. No one brags about a number that's rounded down...
But yes, your engineering logic is useful for determining the upper bound, assuming the NA potential of the motor was not increased. If, say, it was cammed when boosted, the NA potential may have increased and needs to be accounted for on top of the original rating
In reply to Mr_Asa :
That's exactly what I was thinking when I refuted both claims. I just did it in my head and said it couldn't possibly be more than 250, and that would be at the crank, not the wheels.
One thing often missed with forced induction is that boost pressure does not equate to mass air flow. Engines make power based on the MASS of fuel and air consumed. Pressure is just a measure of resistance to flow. Besides the complexities of adiabatic efficiency of the compressor and whether or not the setup is intercooled (read: intake air temp, which is computed with pressure to get density), the "missing" factor is cylinder head flow.
Engine A has a stock head and runs 15 psi boost. Engine B has a very nicely ported but not crazy head, a bit more cam, and a basically identical turbo setup running 8 psi. More often than not, engine B makes just as much if not more power than engine A because it's moving more air mass thru a much more efficient system.
This is why I don't get bolting snazzy, expensive turbo parts on stock engines and whining about boost limits. Take the time to make the thing move air.
Higher boost pressure doesn't directly correlate to more power output. All other thing equal, an engine with higher volumetric efficiency (thus more power) will have less boost pressure in the intake tract, not more.
In reply to cyow5 :
I'll have to ask what else they've done to the motors. In one case I know he sleeved the block and then had it bored out to be a 1.9 litre, so my mental math didn't allow for the extra displacement, but it's only a 5% increase. He's currently at 12psi and intends to go up another 10psi, at a minimum. I asked for a test drive , since it's my old Challenge NB !
DeadSkunk (Warren) said:In reply to cyow5 :
I'll have to ask what else they've done to the motors. In one case I know he sleeved the block and then had it bored out to be a 1.9 litre, so my mental math didn't allow for the extra displacement, but it's only a 5% increase.
Which generation is it? If it is ND, that's 180 crank but 150 wheel. 10psi would put the upper limit right at 300 crank, so you've also got to make sure you are talking apples and apples. Also, remember that adding a turbo means adding a tune and exhaust, things that would bump up the NA potential anyways. Still, turbos at heat, so ignoring the good AND the bad (tune, exhaust, inefficient boost), they kinda sorta cancel, so that's why the boost ratio idea works pretty well.
Edit, sorry, I see you said NB!
Edit again, 140hp + 10psi of boost gets to 230 crank but 200 wheel at 85%. If the tuner assumed more drivetrain loss like 20% instead of 15%, that'll get you an estimated crank hp of 250. Throw in a tune, exhaust, and a larger bore, that could get you close enough to generously round to 300, especially if that is uncorrected on a cold day. SAE corrrection alone can be 15 horsepower at that level.
Ecotec LNFs make 260 bhp using ~ 15 psi, or 290 bhp with a factory tune., out of 2.0 l engines.
The previous poster is correct, anything one can do to reduce restriction will allow more power on a given boost - things like fitting less restrictive CATs or exhaust systems. At a certain point the strength of the stock bottom end becomes the limiting factor.
BTW, the same engine can produce c. 375 bhp using c 24 psi but the bottom end needs strengthening if you go much over 400 bhp. There is a drag car that makes c 1900 bhp out of the same basic engine (in 2.2 l displacement).
In reply to wspohn :
It isn't really a question of specific output (hp/liter) but rather the ratio of output before and after boost
It depends on the size of the turbo and the CFM it flows. 14.7 psi from a small turbo and 14.7 from a big turbo will make vastly different numbers.
A few things to consider:
1) Factory advertised horsepower numbers are not pure engineering numbers, marketing weighs in on what they put on the spec sheet.
2) a 12 psi turbo Miata is not just a stock Miata with boost, some amount of mechanical intake and exhaust modification is required just to fit the turbo and most people go with a full low-restriction exhaust. Also, the factory ECU does not deal well with that much boost, so a 12 psi turbo Miata usually has a full aftermarket ECU. Applying those mechanical and electronic modifications to a naturally aspirated Miata will unlock more power, which needs to be taken into consideration when applying your pressure multiplication formula.
3) Calculating power from displacement involves a factor called VE (volumetric efficiency), which represents how efficiently the mechanical system can move gasses into and out of the combustion chamber. A simple multiplication like the one you are making assumes that VE remains the same, but this is definitely not true when adding a turbo to an engine.
4) Most people are measuring horsepower using chassis dynos rather than engine dynos, and then back-calculating to get "crank horsepower". A lot of people assume that drivetrain losses are a linear percentage, and that double the dyno-measured rwhp means double the drivetrain loss and double the crank hp. Drivetrain losses are not that simple, they include both fixed and linear components, and so tend to be lower than this calculation would lead you to expect. So it is quite possible to double the rear-wheel (useful) horsepower while not doubling the crank horsepower.
IME, most folks who are serious about measuring performance of turbo cars ignore crank hp and talk about the at-the-wheels numbers, because that involves the least guesswork. My stock Miata made 108 at the wheels, running 12-ish psi with a relatively simple (aka "dumb") piggyback ECU it made 220, and running 15 psi with a full ECU it made 250. 15 psi with a bigger turbo (and a built, lower-compression motor) made 270, and at 23 psi it made 340-350.
Nockenwelle said:Engine A has a stock head and runs 15 psi boost. Engine B has a very nicely ported but not crazy head, a bit more cam, and a basically identical turbo setup running 8 psi. More often than not, engine B makes just as much if not more power than engine A because it's moving more air mass thru a much more efficient system.
This.
For another reference point, a Saturn LL0 (124hp crank) will usually make around 220 at the wheels on 7psi.
Nockenwelle said:One thing often missed with forced induction is that boost pressure does not equate to mass air flow. Engines make power based on the MASS of fuel and air consumed. Pressure is just a measure of resistance to flow. Besides the complexities of adiabatic efficiency of the compressor and whether or not the setup is intercooled (read: intake air temp, which is computed with pressure to get density), the "missing" factor is cylinder head flow.
Engine A has a stock head and runs 15 psi boost. Engine B has a very nicely ported but not crazy head, a bit more cam, and a basically identical turbo setup running 8 psi. More often than not, engine B makes just as much if not more power than engine A because it's moving more air mass thru a much more efficient system.
This is why I don't get bolting snazzy, expensive turbo parts on stock engines and whining about boost limits. Take the time to make the thing move air.
Yes.
I've sold cams to people with turbo cars and they've come back saying, I was running X boost before the cams, and now I can only make Y boost. I'm making less power with your cams. Nope, you're flowing more air and the turbo can no longer maintain that level of pressure. If they still don't get it I give them the thumb over the hose analogy. Put your thumb over a running water hose, now take it off. Which one was flowing more?
In one instance I was running 20psi in a stock turbo, stock cam engine. After a cam change, and it was a mild one, the max I could run was about 12psi, and I picked up over a second in the quarter mile
In reply to DeadSkunk (Warren) :
My supercharged '96 Miata is pullied to make about 10 pounds at peak boost on stock internals. It has a header and a very mildly breathed-on head, plus extra fuel. It dynoed repeatedly at about 238-240 RWHP.
Don't question your understanding of boost pressure and HP in this case.
I bet they say they're very well endowed, too. Maybe that's why they need so much HP, to haul around all that extra mass?
DeadSkunk (Warren) said:Twice this week I've had people tell me they know a car (Miata in both cases) that's running 10-12 pounds of boost and makes 300ish horsepower. Now , my simple engineering mind says if the car makes 140HP stock, at 14.7psi, then it can't possibly make 300 without the boost adding about 1.5 more atmospheres. Have I got this wrong? My "logic" isn't allowing for things like a better exhaust, or an extra 100cc of displacement, but those claims seem really high to me. Educate me, please.
It may make 140hp at atmospheric pressure, but it's not making 70hp at 14 inches of vacuum. It may not even free rev to redline at 14 inches of vacuum.
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