pressure vs power when setting boost psi
 

The car I recently purchase was set to 15 psi, and the car ran fine. The owner never had any problems, and the car was tracked. I was under the impression that 12 psi was safe limit for stock engine. This is a 1.6. I was told that it is based more on the power the engine is outputting (when setting boost) and the engine is good for about 250 rwhp. I though that it was based more on the internal pressure of the engine. Since setup could dictate different power outputs at a given psi of boost. I've detuned to 12psi, but what is the correct thought process? Thanks

I dont think there is a "set" boost level that is the limit for a certain engine. Many run different levels of pressure with completely different results. Not much help at all, I just dont want this to turn into another "how much power with how much boost" thread.

It's internal pressure. Boost pressure is just a measurement of the restriction in your intake manifold; cylinder pressure is what you want to stay down from. 12psi used to equate to around 220whp, and at those boost levels you can run a motor pretty much forever. Up it to 14-15 and start getting into the 240-250 range and you'll significantly shorten the lifespan of the motor (mine lasted around 29k miles before upchucking a rod).

Careful, now. The last guy who asked this question was indicted on capital murder charges recently. :teehee:

I agree about "how much power, how much boost"
 

I agree 100%. I am starting to tune my car, but datalogs won't tell you if your engine is about to go pop. 12psi seems to be an agreed upon "safe" limit. Just want to make sure that people agree on this. While I don't plan on rebuilding my engine anytime soon, I am all for running at a seemingly safe limit. If the engine happens to go belly up at that point, well I did take some precaution.

:giggle:

If you want the car to go 1 million miles go with less boost if you want to be a man crank it up.

if you want both build the engine and crank it up:)

+1 on 18psi. I ran mine until it died and then fully built a motor after that.

You can't have too much boost for starters. You might not have enough rod or piston though.

The relationship between boost pressure, compressor and turbine size and efficiency, volumetric efficiency, peak cylinder pressure, BMEP, PPP, EGT's, overlap, and many other variables that contribute to power output are somewhat misunderstood. No one thing can be changed without having a daisy chain effect on other subsystems down the line. There is no free lunch. It's not just "boost pressure" any more than it is just "internal pressure*"

*someone define this term that's been thrown out there.

I'm running at least 20g's.

internal pressure = peak cylinder pressure. This is why detonation is so bad: the peak pressures SKYROCKET.

My engine builder, dyno operator, turbo builder, and I think 15psi is safe in my car. My spark map is conservative with a shit-load of headroom, 11.5:1afr, in the center of the turbo's efficiency range, and lowered static compression motor. I know many with stock or modified STI and Evos with 14.7 (stock) and much more boost over 100k miles who track the cars once per month.

12psi from savinton's turbo is a shit-ton different from 15psi on my turbo

An ugly spark table is an ugly spark table, a happy spark table with a reasonable AFR is happiness.

Basically most people who have a proper understanding of how the various factors of a boosted motor influence each other know that there is no solid answer to what is "safe".
However, most everyone has agreed that 12 psi is about as far as you want to go on a stock motor to maintain a decent amount of reliability and longevity.

Detonation (more accurately, avoiding detonation) is pretty much the #1 factor to the lifespan of a boosted motor though imo. For example, right now I am at 10 or 11-ish psi. I would not go higher than that with my current setup, but if I installed water injection, that along with my big intercooler would probably make 15 psi "safe" i.e. fully resistant to detonation.

On a side note, the rough equivalent of power that is produced at 10 psi is the average breaking point for a 1.6's differential ring gear. The rough equivalent of power that is produced at 15 psi is the average breaking point for a 5 speed. So, while there is no exact psi that is the max "safe" amount for the motor, there is a rough "safe" psi for a given car. We don't all drive around with Torsens and 6 speeds.

-Ryan

-Ryan

:)

I like hearing my own name...

Most importantly man u can run 20 psi and only make 100 hp and the only that will breack is your heart as we pull on you. These pressure benchmarks assume everything is humming and you are within a nominal range of normal compared to people of a similiar setup. So the real answer here is find a compressor map and a turbine map for your shit figure out your timing and fueling and then we can tell u wether to be worried or not or even better u read them and figure it out for yourself wether your estimated hp/tq via cfm airflow and overall Air density is enough to breack things.

wtf? Why can no one understand that 12psi from a Savington's turbo, is nothing like 12psi from Pat's turbo?

Since the 1.6L has the same rods and stroke but with less bore, shouldn't a higher combustion chamber pressure result in the same pressure upon the rod and bearing?

not everyone can grasp what we are saying man u just have to be patient they either learn or fuck up forever. :magna:

Aren't we talking about raising the boost on a turbo rather than comparing one turbo to the other?

yes i think hustler is just emo on how many threads we get on "should i keep my 1556 turbo or get a 2456 i wana run the same psi whats the diff". u know what i mean

Yes and no. The issue is how much it can be raised by and Hustler is pointing out a huge point - that a big turbo's 12 psi effects the motor WAY differently than 12 psi from a small turbo, so a given psi could be fine or terrible for the motor.

Yeah I know. I mostly just ignore such posts. If I raged at everything stupid on the internet I wouldn't do anything else.

exactly^^^ , although other people have already said to look at the compressor map for his turbo. but the op might not know what they were getting at.
basically it's cfm's at a given psi. 12 psi might be safe with a t25 that flows (i'm just making up a number here i didn't look it up) say 275cfm. but a t3/t4 60-1 turbo at 12 psi might be pushing 400cfm, enough to pretzle your rods.
so i think the concensus is, if your running a small setup t25,t28,14b 12 psi is plenty, especially for trackday use. on the street 15psi would probably be fine since you would rarely be at that psi or for more than few seconds. unlike a trackday where your at wot for 20mins. straight 3-4 times a day. i personally only run 9-10psi at the track. it's plenty fast, i pass porsche's and vettes on a regular basis, and hang with modified porsche's and vettes on hoosiers with my ra1's. remember our car's strong point is lightweight. that let's me outbrake and turn faster than alot of ''badass'' cars. hell, i see fast spec miata's passing stock vettes and other's. with my extra 100whp i'm just that much faster. have fun.

Thanks for all the input. I put a post in the tuning section with a 4th gear pull. I'll also post pdf images of my timing map and fuel map. I sent the datalog to FM and was told that it was pretty good, and he made some comments on where to adjust. If anyone from here would look at them and give me some feedback I would appreciate it.

There aren't "different kinds of boost" in the way you guys are thinking.

Lets pick a magic rpm, say 4000 rpm.
X psi of boost from any turbo is going to produce the same amount of flow into the engine. The pressure differential between the intake plenum and the cylinder is the same so the flow should be the same. And it is.

The difference in power comes from:
-look at the compressor map and find the pressure multiplier and the CFM at that RPM. The higher the efficiency at that point, the cooler and denser that air is going to be. This means more power.
-newer compressors have much higher peak efficiencies. My old turbo peaked around 70 percent efficiency. My current turbo peaks around 80 percent. This means cooler and denser air and thus more power at the turbo's peak efficiency.
-newer compressors have much wider efficiency peaks. At values of CFM that would leave me around 50-60 percent efficiency on the old turbo, the current turbo is still around 80 percent. This is a HUGE difference in power. It also means that the intercooler doesn't heat soak as quickly since it has much less heat to dissipate.
-if you are running a very small turbine this can choke off flow at higher rpms but this is IMO a relatively minor influence compared to compressor size.

100% wrong. :bang:


Compressor efficiency is important, but not for the reasons you mention. Your points are valid, but you're missing the big picture. Cooler charge exiting the turbo is nice, but when we have an IC that's 90% efficient, the difference between 88*F post IC temps with an efficient comp. vs 91.13*F IC temps with an average efficiency comp. is trivial and NOT the reason why the engine with 3.13*F cooler temps made 20whp more.

The turbine is the main thing that dictates flow in the engine. It's the restriction. So why do we have a turbine? Because we need to supply power to drive the compressor. It's a struggle. The more power we want to make, the more air we need to move, the more power the compressor needs to move more air, the more restrictive the turbine becomes to supply said power and this restriction limits flow. :vash:

In your example, say the higher efficiency comp. is using 25hp where the lower efficiency comp. is using 30. The difference in power is not from the 3*F post IC temps, but from the fact that the turbine only needs to supply say 25 hp to the comp. instead of 30 so it opens up the wastegate some more and you operate at a lower delta P across the turbine to make 5 less HP than before, and the net result is less back pressure pre-turbine. This is where power is made.

First off, I'm measuring pressure at the intake manifold plenum, not the turbo. So I'm not wrong. The laws of physics are pretty straightforward- cavity > passage > cavity with a pressure differential between the two cavities = a certain amount of flow between them. Unless you change the characteristics of the passage between the plenum and the combustion chamber, this isn't going to change. The variables that determine how many moles of air gets into the combustion chamber are temperature and pressure. (edit: for a fixed shape of passage, obviously)

Also, intercooler isn't a magic bandaid. It requires a much larger volume of very hot air than warm air to produce a given volume of cool air. This means an inefficient turbo has to work harder to maintain a given pressure within the manifold. This means more power taken from the exhaust and more heat for the intercooler to eject into the ambient air.

I just reread your post I think we agree but we are expressing it differently.

Ok, I see what you were angry at in my original posting. I left the turbine out of it completely.

Yeap. That and you said that mass flow through the engine is constant for a given temp and pressure, regardless of turbo size. This is just obviously not true. I mean, if this were so, there would only be one size turbo. :eek5:

No, I was talking about flow into the combustion chamber, not through the engine. I just sort of took it for granted that any slack due to inefficiency would be taken out of the exhaust flow. I left the exhaust/turbine side out of it because for the most part we all run similar size and efficiency manifolds/turbines. The only big variable IMO is what is happening between the compressor inlet and the piston.

Is the mass flow rate going into the engine different than the mass flow rate going out? :confused: Conservation of mass? First law of thermodynamics?

Turbine size is what's critical. It's what ultimately dictates flow. You can have a T2 comp. or a T3 comp. Putting a bigger comp. doesn't magically make the engine flow any more air. It will only make more power if the new comp. is more efficient at the given range where it will be operated. But putting a larger turbine will ALWAYS make more power because that's reducing restriction, which is what mostly governs power.

Basically you're saying what common wisdom says, which is wrong. Read up.

what kind of boost am i using?

lol

The good kind.

Umm, no...

P=f/a

multiply that by a larger area (ie larger bore piston) and you get a larger force.

More explicitely, pressure is force over UNIT area, so for the same pressure, you get more force applied to a larger area.

Ex:

P=50lb/in^2

On an area of 40in^2, force will equal 2000lbs.

On an area of 30in^2, force will equal 1500lbs.

This is why it's harder to hold a larger umbrella in a stiff breeze, or harder to push a large suv through the air at 80mph than a Lambo. The second example has other factors, but you get to the idea.

So to answer your question, the 1.6L rod would actually be under less compressive force than a 1.8L rod for the same given peak cylinder pressure.

:bowdown:PHYSICS:bowdown:

The mass flow rate is the same, but the amount of work done by the piston during the exhaust stroke is greater. The orifice it's flowing through is "smaller" because the wastegate is closer to closed, to force more exhaust through the turbine.

Turbine sizing doesn't have that big an influence, at least in my limited experience. It still comes down to how much work is needed to turn the compressor wheel to produce a given amount of cooled, compressed air at the intake manifold. If you run a large turbine, less exhaust will go through the wastegate. If you run a small turbine, more exhaust will go through the wastegate. All you're really controlling with turbine sizing is the behavior at the edges- rate of low rpm spool-up versus high end power.

I said that with higher combustion chamber pressures in the 1.6, the stress on the bottom end would be the same.

You said that with the same combustion chamber pressures in the 1.6, the stress on the bottom end would be lower.

You just spent a page "correcting" me and came to the exact same conclusion I did!

First, you weren't very clear, but apparently we all do that from time to time. :fawk:

Second, I didn't come to any conclusion, I already knew this shit.

I did figure out after reading a few of your above posts you probably know enough about physics to not make such an elementary error.

The explanation was however worth it for newbs that may misread your post as I did, and come to a poor conclusion.

I'm glad you understand this now.


Uhhhhh....


Ok, I guess you don't quite get it. You acknowledge that:

Might as ask where said work to drive aforementioned compressor comes from?

Food for thought: Net work out of a turbine is the difference in enthalpy across the turbine. Enthalpy, h, is U + P*V where U is internal energy, P is pressure, and V is volume. (higher flow rate = more volume)

You need some certain delta h to produce some amount of work to drive the compressor. You have 3 ways to increase the difference the delta h across the turbine. Increase the internal energy of the working fluid (higher temperature), increase the delta P across the turbine (more restrictive turbine, or a free flowing downpipe, etc), or increase the volume (increase VE efficiency, increase engine RPMs, etc).

From a maximum power standpoint, operating the turbine at a higher mass flow rate and lower delta P to achieve the same work output will reduce back pressure pre-tubine and increase the engines respective VE, increasing HP. Turbine size is critically important.

I always understood it. You're just nitpicking.

I'm counting the wastegate outlet and the turbine outlet together as a single orifice of variable size. This is true for most manifold/turbo setups, at least as far as the engine is concerned. When you have a less efficient compressor, you close the wastegate more to increase the amount of pressure differential across the turbine wheel. This draws more work from the engine- it has to push the exhaust gasses through the smaller orifice.

:bowrofl::bowrofl::bowrofl:

You've said several things that were 100% wrong in this thread so far. So no, you didn't always understand it as you made a few false assertions that as I mentioned, would fall under 'common knowledge'.

You're responses are slowly starting to make more sense though. However, you must realize that "it drawing more work from the engine" is not exactly the big picture. IE-it doesn't just eat up 20 HP. Rather, the higher back pressure reduces VE, and you simply don't make those 20hp.

Look, it's difficult to express the operation of the running system from beginning to end using ordinary english words. To make it more easily understandable, I've tried to simplify it down by discarding as much as possible that doesn't have a big effect on miata setups.

You obviously feel I've oversimplified, but all of your complaints essentially amount to "you left out this or that detail." Nevermind that every model I used to describe the operation of the system in this thread CORRECTLY describes why it works. At the end of the day compressor design has a much bigger effect on our cars than turbine design. I know why all this happens the way it does, but I didn't feel like taking a long post and making it longer. It was boring enough already.

Each configuration of engine + turbine + compressor + plumbing produces a different set of behaviors across the rpm range of the engine. Going into excruciating detail about exactly how each setup effects the volumetric efficiency of the engine at each rpm point is unnecessary. Leave volumetric efficiency and all these other considerations out of it entirely. Just discuss compressor efficiency and go do something else with the extra time left over.

lol.
Detail: 100% wrong.

And
Detail: 100% wrong.
Detail: First law of thermodynamics violated.

I've tried to be polite, but fuck you.

No one here measures boost at the compressor outlet unless they are mentally retarded. Everyone except ITB people (who don't generally have turbos) measure boost after the throttle body but before the intake runners.

Stating that a given psi measured at the plenum produces a given amount of flow into the combustion chamber at a given rpm doesn't violate any thermodynamic laws. It's a simple statement that a pressure differential causes the air to flow from high pressure area into low pressure area. There's no physical law that cares whether that pressure was produced by a bottle of air, a centrifugal compressor or some sort of screw based compressor. It's just compressed air. There aren't any "big turbo particles" mixed in with it. I was making this point because I felt some people were saying that 12 psi of boost produced from a big turbo would somehow produce more stress on the internals than an identical amount of boost from a smaller turbo at the same RPM. If you didn't say this then congratulations, I wasn't talking to you.

nice av, power-queer.

wat

Pressure (symbol: p or sometimes P) is the force per unit area applied to an object in a direction perpendicular to the surface. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.

Mass flow rate is the mass of substance which passes through a given surface per unit time. Its unit is mass divided by time, so kilogram per second in SI units, and slug per second or pound per second in US customary units. It is usually represented by the symbol http://upload.wikimedia.org/math/a/8...4cc7946aef.png.



12psi @ 20 lb/min :ne: 12psi @ 40 lb/min

I'm not trying to be an ass, so sorry if I came across that way. I'm just debating with you, nothing more.


Ok.


You said pressure differential causes the air to flow. Hold onto that thought. This is key.


lol at the big turbo particles.


Uhhhhh... Bigger turbo's do cause more stress than smaller turbos for the same pressure. Duh.

And now I will somewhat explain why you are wrong.

The piston rising upward approaching TDC at the end of the exhaust stroke, the working fluid has some temperature and pressure that is no less than the pressure and temperature of the exhaust gases pre-turbine. (neglecting any scavenging effects)

Let's assume we're running a small turbo like a GT2545. We'll assume we're running 15 PSI of boost and seeing 30 PSI of back pressure pre-turbine. And let's also assume that we have an idealized valve train such that the exhaust valve is held wide open until the top of the exhaust stroke, at which point it instantly shuts and simultaneously the intake valve fully opens. And 9:1 compression.

At exactly TDC the exhaust valve slams shut while the intake valve simultaneously fully opens. Freeze time.

Now let's move forward in time 0.0000000001 seconds. What's happening? Air's going in the cylinder right? Wrong. The pressure in the combustion chamber is higher than that in the intake manifold. Like you said, flow occurs from high to low. In our 0.00000000001 second time frame the piston has not moved down much, so the gases have no place to expand other than through the intake valves and into the intake manifold. So you've got hot residual gases flowing into the intake manifold, heating up and diluting your fresh cool air/fuel mixture. How lame.

As time goes on, the piston drops, increasing the volume and pressure drops, and eventually it creates a vacuum such that flow reverses and the mixture of air/fuel/residual exhaust gases begins to fill the cylinder again.

Fact is, you can not do anything about the volume of the residual gases that will be mixed with the fresh charge. At best it's a combustion chambers' worth, but in the real world, it's more than that because we don't have a perfect valvetrain. With 9:1 compression, at best you will be mixing 1 part of say 1800*F residual exhaust gases at 30 PSI with 8 parts air/fuel at 15 PSI and say 100*F. Net result will be some air/fuel/residual gas mixture that's not as cool or dense as the charge exiting the IC. Actually, it will be MUCH less dense than the charge exiting the IC because that 1 part of 1800*F at 30 PSI residual gases are gonna expand and take up a lot of space. How much? I'll leave it at "a lot". I can do the math if you insist though.

This is why larger turbines are key.