ghost1000

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[QUOTE=peejay;11783817]6000rpm is only 67% faster than 2000rpm? So if I am driving at 2000rpm and am going, say, 50mph, then if I accelerate to 6000rpm I will be only going 87mph?



I'm saying if you are driving 100mph at 2000rpm and go to 6000rpm you will be going 300mph. 3 times faster or a 200mph increase, or a 67% change in speed.

If you,re driving 100mph at 6000rpm and go to 10,000rpm you will be going 166mph which is 1.66 times faster or 40% change in speed.

50mph *3=150mph not 87mph.
50mph is 33% of 150mph which means there was a 67% change in speed.

madbouncy

Using stock gearing for an FD and in first gear.

6000rpm = 31.2 mph
10000rpm = 51.8 mph

(52.8 - 31.2) / 31.2 / 100 = 67 % increase using 31.2 as your reference frame and 51.8 as the speed you're going to

(52.8 - 31.2) / 51.8 / 100 = 40 % decrease using 51.8 as your reference frame and 31.2 as the speed you're going to

They are both changes in speed, just depends which way you consider the speed to be going.

It is funny that with all your arguing about whether a car is going 67% faster or 40% slower you ignored peejay's post about compressor efficiency and the compressor map. Nobody is saying you can't have a situation where you can run higher boost as you near redline and make more power, all we are saying is that to do that, you're sacrificing low end power. So if your goal is just to make big power at redline, then you'll do fine with your method as long as you size your turbo correctly and don't mind the lack of low end. However, when you keep saying that boost is falling off and that you're trying to fix that, you are dealing with a completely different problem then what your experiment is going to fix. Upping the boost at a time when the turbo can't keep up is just going to generate more heat and that will reduce the amount of power plus be more likely to detonate, and that's assuming you don't over speed the turbo.

madbouncy

If having more time to fill the combustion chamber is what creates higher VE, shouldn't engines be making peak torque at idle and slowly declining as RPMs increase?

peejay

So, it's like, you can do the math but you still don't understand at all.

6000rpm is 200% more than 2000rpm. See how that works?

peejay

Actually, here's an interesting thread on that very subject, that I found while looking for the rotor housing heat map chart.

https://www.rx7club.com/single-turbo...d-dyno-956766/

There is good discussion on the downsides of big turbos vs. the somewhat unorthodox direction that this particular car was taken, and the rationale behind that (response more important than power). And pay attention to this:

Note explicitly: Just because the turbo holds boost, doesn't mean it makes power. Boost is not power, mass flow that can be effectively burned is power, boost is only a tool for increasing that.

And here is another interesting point from that same thread:

ghost1000

iTrader: (1)

I'll read the article tonight. This is the kind of thinking I'm looking for. Getting all the data making a hypothosys and seeing what blows up. This is how I have fun with my cars. More often than not I'm right in my unorthodox ways but I'm always thinking, learning trying to make things better


Check out the pressure map on the Bw362s. This should be a fun turbo that lasts. I picked it because it looks like a turbo I can grow into. My first setup power goal is 350whp , I want to keep the revs hi to get the most from my gearing. Being faster with less power.

ghost1000

iTrader: (1)

I'll read the article tonight. This is the kind of thinking I'm looking for. Getting all the data making a hypothosys and seeing what blows up. This is how I have fun with my cars. More often than not I'm right in my unorthodox ways but I'm always thinking, learning trying to make things better


Check out the pressure map on the Bw362s. This should be a fun turbo that lasts. I picked it because it looks like a turbo I can grow into. My first setup power goal is 350whp , I want to keep the revs hi to get the most from my gearing. Being faster with less power.

They are talking about the limits of a small turbo. The exhaust wheel being too small should be visable on the pressure map not a problem for my turbo. Small turbo shows exhaust back pressure vs boost. I think there're several possible reasons for boost drop off ill let you know when I find my limit. Like the quote said manifold pressure doesn't = combustion pressure and that's what I believe I'm fighting, why use the same manifold psi a low rpm as you would at hi rpm, at hi rpm with the right turbo manifold psi should be increased with out the risk of detination. If the exhaust side of the turbo can flow it u should be able to push a bit more but running 1 set psi I can easily see why it would drop off.

ghost1000

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My understanding of VE is the amount of air fuel that fills the combustion chamber, 1.3 leter displacement with 1.3leters air fuel is 100%VE. As rpms increase you achieve more power because of more combustion strokes per minute. The port timing has a lot to do with this but I'm no expert on that subject.

peejay

Boost is not mass flow. Just turning up the boost at higher RPM does not increase the mass flow unless the turbo can effectively work there (work, not just make boost). It will increase the heat load on the engine, though, which is Bad. Which is why, as noted, it can be good to decrease boost at higher RPM if you can do it.

C. Ludwig

iTrader: (27)

You can make a turbo engine make power at higher revs but, like anything else, it's a proper combination. In addition to the autox car in the thread mentioned earlier, I've also assembled and tune a local turbo drag car. These two are absolute polar opposites in their purpose and build.

The autox car is documented well enough. The drag car has a half-bridge engine and an old school T72 turbo. It runs a p-trim hotside wheel with a 1.32 housing. It makes peak power at 10.5k rpm and that is where we shift it. The shift point is actually a little low in terms of optimum performance but it's a self imposed limit to stress the engine a little less and to do our best to keep the flywheel and clutch inside the bell housing. The stock FD trans also HATES when you try to shift at those revs. So any desire to build a high rev combo and use a stock trans is going to be frustrating.

A high rev turbo combo is about a lot more than a loose turbo hotside and the idea of keeping boost high at higher revs. You first need the porting. Stock and street port engines, NA or turbo, don't make power over about 7500 rpm. Doesn't matter if you keep adding boost. This is what peejay is talking about. To your own admission, intake pressure doesn't equal chamber filling. Don't fill the chamber, you don't make power, regardless of how hard you spin it.

You also lose turbine drive when the VE falls. So, again, there is more to making manifold pressure at high revs than just spinning the engine harder.

To properly make power at high revs, you need to improve VE at those revs. This takes porting, proper intake manifold, and a completely different header than you would use on a low rev turbo engine. Again, it's a complete system of parts.

I really question why you'd want to do what you're saying. I'd first ask you what you intended use for the car is. If it's any form of competition outside of drag racing, a broad torque curve is much preferred to peak power anyway. At the 350 HP level you stated, it's easy enough to have your cake, 350 HP, and eat it too, broad torque curve. If you're using a stock tranny, it will thank you for shifting it below 8500 as well.

ghost1000

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I'm far more into drg racing than anything else. This car is not for competition, its for learning and pushing the limits of what I know and ocasionally drag racing or sliding around a parking lot.

My thoughts are to keep VE up at hi rpm by raising boost but I am aware of the other factors that's why I have been fishing for others experience and warnings before I try this. Thanks I also noticed boost controller offering a over boost feture to push a bit more up top but it makes more sense to me to control boost by rpm and not settle for 1 pressure for the entire rpm band.

ghost1000

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And I'm still not buying 1 manifold pressure settings is best for all engine speeds and I'm aware I will hit a wall with ports and turbo at some point so this is the experiment.

1st test
Stock ports over sized turbo 1 pressure setting. Find boost drop off Piont.

2nd test
Same setup but increasing manifold psi at higher rpm to fight. Boost drop off.

There has to be some gain here even if its not as huge as I hope. Even if its 100rpm its a win. If its a 1000 rpm its a big win.

I'm also interested in hearing more about engine internals for hi rpm. I know oil pressure is a biggie and also I've heard of people clipping the corners of there apex seals. More info please

C. Ludwig

iTrader: (27)

Boost drops off for one, or both, of two reasons. Either you lose turbine drive because you're way passed the meat of the VE curve of the engine or you're way outside the ability of the compressor to supply the needed mass of air. You can't change either of those by simply dialing up the boost controller.

peejay

You can never increase the VE by increasing boost. VE is a fixed function based on ports, intake manifold geometry, and exhaust manifold geometry/backpressure.

VE is volumetric efficiency, it is independent of the density of what is going into the engine.

ghost1000

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My understanding of VE is displacement vs atmosphere.
1.3L capacity with 1.3L air @ atmosphere is 100% VE

If the air is compressed 14.7 psi that's 2* atmosphere or 2*VE or 100% increase or 50% diffrence lol

With the blades closed its under vacuum which is lower VE vacuum is inch of water and I can't calculate that but its lower VE.

ghost1000

iTrader: (1)

You mentioned the meat of the volumetric curve which is a big player. I've noticed when a N/A motor goes turbo the power curve is very different , it is flatter and the rpm number that was peak hp NA is now lower with the turbo, this is where i might be able to recover something.

peejay

The VE stays the same whether the air is at 14.7psi absolute or 5psi or 30psi or 500psi. Volume does not take density into account. You can have 100% VE of a total vacuum (although it'd be hard to tell )

That is why I stress the difference between VE and mass flow, and why turbos are rated in mass flow instead of volume flow.

You can also have 14psi boost but if the turbo is whipping it up so hard that it's super hot, the mass flow is barely any better than if you were naturally aspirated. Only now, the engine is breathing super hot air so it needs to have the timing pulled way back, so you're making less power. (This really can happen with Roots superchargers!) 14.7psi boost is only twice as much air as atmospheric IF you can intercool it down to ambient temperature. Thus why I made the point of the compressor efficiency on that K27 911 map I posted earlier. By 6000rpm the turbo is mostly producing heat and not actual flow.

C. Ludwig

iTrader: (27)

^ What he said.

Where are you seeing drastically different peak power RPM values? A stock or street port NA engine is going to peak somewhere between 7000 and 8000 rpm. Slap a turbo on the same engine and it's going to fall in that same range.

A turbo is only going to accentuate the NA VE. All other things being equal, and they never truly are, but if they were, take an NA engine and slap 1 bar on boost on it and the torque curve will be the same, just doubled in value. Cosworth did this years ago in their turbo F1 engine development by employing a huge reserve of compressed air and using it to feed the engine. A turbo can't make this happen, but it's still the physics behind what is happening.

Past the torque peak, you start losing the ability to drive the turbine and that's what you'll need to do what you want to do. Only way to carry boost at higher RPM is to make mechanical changes to the engine that shifts the VE/torque curve to the right. When you do that, you're right back to where you started in regard to your thought of making horsepower well past the torque peak by carrying boost past the torque peak. You're caught in a loop.

One way to do what you're saying, building more boost at higher revs, is to utilize a belt driven compressor, such as a Vortech supercharger. The problem is, and there is just no way around this, you'll still end up trying to force air into an engine that doesn't want to accept it. The port and manifold combo is still saying no.

arghx

iTrader: (3)

Just a little FYI, there are three volumentric efficiency metrics that I use in an engine dyno environment:

Volumetric efficiency referenced to ambient conditions = AIR MASS / ( Engine Disp x engine Speed x .017361 x ambient pressure / ( ambient temperature + 459.67) x 1.3259 x hydrocarbon fraction in exhaust )

Volumetric efficiency referenced to intake manifold conditions = AIR MASS / ( Engine Disp x engine speed x .017361 x ( ambient pressure + manifold pressure) / ( manifold air temperature + 459.67 ) x 1.3259 x hydrocarbon fraction in exhaust )

Volumetric efficiency referenced to SAE standard conditions = AIR MASS / ( Engine Disp x engine speed x .017361 x 29.908 / (77.+459.67) x 1.3259)

All those various numbers are constants (ie, don't worry about them, they're there because physics). Air mass can be back calculated by using a fuel flow meter and air fuel ratio measurement.

Each metric is used for different purposes. OP is referring mostly to the first one, the one about ambient conditions. When I start boosting the engine my ambient-referenced VE can go to 200% or more. You're understanding how much air the engine is flowing relative to the conditions at this altitude, temperature, weather, etc. C. Ludwig and others are referring to VE referenced to intake manifold conditions which rarely gets up to/above 100%. The predominant elements here are for the most part engine speed and port timing (or valve timing on a piston engine). SAE standard conditions VE is like ambient conditions and focuses on sea-level standard conditions, disregarding lower barometric pressure due to weather or altitude or hot/cold ambient air around the engine.

There are two things I can comment on that you understand more on a well-instrumented engine dyno: friction and pumping work. The higher my engine speed, the more friction I have (wasted energy from moving the engine), and the more work my combustion has to do to make the torque/power I want. The higher my engine speed, the more pumping work wasted due to high backpressure and mass flow moving in and out of the cylinder, especially on a turbo engine with a restrictive turbine housing. You can't see the breakdown among these sources without combustion pressure sensors which cost big dollars. It's still there though

ghost1000

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arghx

Could you or would you be willing to post a dyno sheet and show the rpm where max VE is achieved related to turque and hp? In simple theory, when the engine is at peak VE it is at highest risk of detonation and it is after VE peaks and starts to fall it should be safe to start turning up the boost. My biggest fear is back pressure from the hot side adding to the risk of detonation. Being able to rev the engine higher b4 I shift means running lower gearing to accel faster and maintain the same top speed.

ludwig
the few examples I have seen of N/A to turbo had a lower peak hp rpm and I Believe this is do to the turbo back pressure, so yeah there is some change do to variables. In simple theory you are correct because once the engine is boosted it should have the same power curve do to the VE curve. atmospheric pressure is a constant and 10psi of boost is also a constant and it will follow the VE curve the same way. The thing about a turbo is it allows you to change the pressures the engine sees, so with the correct rpm based boost controller psi is not a constant and we can play. If it wasn't for a waste gate that is used to control boost and fight detonation I would not have a chance in hell of making this work


If the waste gate is still open after peak hp or peak VE then there is wasted energy that can be used to maintain VE which should allow you to rev higher. Do any of you see this with larger turbo on cars running pump gas?

arghx

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I don't have such information for a rotary, as I have never run a rotary on an engine dyno, only chassis dynos.

j9fd3s

iTrader: (3)

i am not sure you can say that. generally the torque peak is right around where VE peaks, and this should have the highest chamber pressures, but that doesn't necessarily correlate to detonation, although it could.

i have found that with NA engines they knock in the low rpm ranges, if you are aggressive with your tune, they knock in the 1800-2200rpm range, but then they are fine at the torque peak. it is true with a turbo you can change this, but to say knock follows the VE curve is too simplistic.

key word being should. at high rpm you still have to worry about timing and fuel...

so using rpm/gear based boost control has been done, but everyone is trying to go the other way. they are all trying to get more low rpm power... for example, i have heard about this FC in japan, and basically its design is to get turbo speed up to the maximum (100k) as fast as possible, and then just keep it there. as you might imagine, boost is like 20psi in the low rpms, and then falls off. i guess the car is tyre limited, and quite fast.

peejay

VE generally correlates to torque curve. Horsepower is a mathematical construct so that our puny minds can figure out the best torque per gearing.

Detonation... books are written on the subject, people discuss it at length...

BLUE TII

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I remember reading some Mazda info where they said around 3,000rpm is the knock prone zone for their rotary.

j9fd3s

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it depends on the hp/engine. my Rx8 pings when i let the clutch out too fast, so ~700rpm... the p port does it around 1800rpm.