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Old 01-26-2009, 11:14 PM   #21 (permalink)
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The thing about factory cars are they are full of compromises to make it work for everyone. Adding a spacer is not going to hurt anything except maybe 5000+ rpm top end power. A longer intake length will typically give you more power at lower rpms. To properly see what size intake you need is a lot of math and it is easier using a program like Engine Analyzer Pro to change the numbers and see how the engine responds.

I will be adding a spacer to my Astro this summer to improve it's towing ability. I doubt I will see a mileage increase with this van, but it is pretty well known adding a spacer to a tbi engine gives it better response at lower rpms while sacrificing top end power. Perfect for a vehicle primarily used for towing and hauling stuff.

I have the spacers made up for my Metro but that car is sitting in my garage waiting for warmer weather so I can swap heads and put the spacer on. I am not sure if I will be able to do an A-B-A type test with it but I will try and come up with something to see how it changes things. My main goal with it is to be able to shift gears at a lower rpm. If it can drop my shift points by 200 rpm without lugging the engine then it will be a good mod I think even if it doesn't directly improve mileage.

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Old 01-26-2009, 11:18 PM   #22 (permalink)
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Quote:
Originally Posted by akashic View Post
The internals of cars are a new thing to me, so please excuse me while I try to apply my limited experience with engine-talk...

I'm getting that the idea of this mod wrt FE is to lower the resonance of the intake and shift the peak torque -- and simultaneously the island of minimum BSFC -- to lower RPMs. Is that right?

YEP, YOU GOT THE IDEA.


I guess maybe what I was asking was if a car already tuned for more low-end torque would benefit less from the same spacer than one tuned for more high-end torque. At least with given thickness of spacer, the car with the shorter intake would increase it's intake-length percentage-wise more than one that's built with a longer intake. If that's actually how it works (and I didn't completely munge that explanation), then it seems sort of mathematically obvious that a lower-tuned engine would benefit less from the same thickness of spacer.

And to go with that thought, a car tuned for high-end performance has the most interest in a retuning mod like this, to get the BSFC curve into an accessible range for a hypermiler's driving style. Yes? No? (wtf?)

IN THEORY, YES, A CAR WITH LESS LOW-END TORQUE WILL BENEFIT MORE, IF THAT LACK OF TORQUE IS DUE TO THE AIRFLOW AND RESONANCE PROFILES ASSOCIATED WITH IT'S INTAKE STROKE.


btw, I understand a little about "tuning" from the acoustical world. It occurs to me that little hacks like this can accidentally work to your advantage if you happen to disrupt the response curve in such a way as to make a little bump in the "right" part of the curve. You might end up with a bizarre profile with several low peaks, but if you only care about 1900 rpm or whatever, then it might just work for you. So I suppose as you said, this is something that you just hack at until something works, maybe even without a predictable pattern.
Christ: "I believe, generally, that anything you can do to maximize the power output of your engine without increasing the fuel it consumes to do so will give you better FE." (in that specific RPM range, relative to the power produced)

Conversely, any mod which creates more torque almost always will change the linear power profile of the engine (torque curve), although not necessarily "across the board". That said, even if you do something that adds power at 2k RPM, and you're typically at 2300 RPM, you'll see a benefit from it. If you add torque at 3k RPM, and you're typically at 1900 RPM, you'll still probably see a benefit from it, but not nearly as drastically.

The above statement was kind of generalized, with certain assumptions:
  • You're an ecomodder, so you're looking for efficiency gains
  • You're not looking to make more power where you can't use it.
  • You ARE looking to make more torque where your engine "sits" during your drive
  • and you're looking to do it without introducing more fuel.

Basically, the only way to increase your engine's output without changing it's fuel intake is to change the way it uses the fuel it already uses. More specifically, to change the dynamic losses associated with using that fuel.
  • Increasing VE (assuming you haven't added fuel) will nearly always net you more torque.
  • Reducing frictional losses will net better torque for a given RPM range.
  • Ensuring that you're actually USING the fuel you're using.

Those are just a few ideas that might help clarify the thoughts I was attempting to evoke with that statement.

Thanks for catching it!

In addition - If you're ever thinking about changing TB's to a smaller size, since you can't exactly "port match" and you don't want to create a wake in the intake tract (this is not opinion... you really don't.) use a spacer that will smooth the transition of airflow from the smaller throttle body to the walls of the plenum. It'll probably also prevent the high-pitched whistle that you'll probably hear.
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Old 01-26-2009, 11:26 PM   #23 (permalink)
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One more thought - if you can get your hands on either "Delrin" or thermal plastic compounds, make a template from a normal intake manifold gasket (this only works for inline engines.) and cut it out of the delrin or thermal plastic. They're both natural insulators, which will keep intake temps down (debate on whether this is a good thing or not can be found all over the place on this forum), but they also can be used as "intake spacers" which will increase the EFFECTIVE intake runner length. (The length from the end of the "tube" to the intake valve head.) There are more gains to be seen here than by using a TB spacer in most cases.

Given a resonance chart, you can fine tune your intake runner length to provide the "harmonic bump" at exactly the RPM level you normally operate at, thus locating the KeyWest of BSFC right at that point. (I say this because it doesn't move the peak down, perse, but it does create another, smaller, peak.)

Harmonic bumps have extraordinary effects, since they cause a pressurized pulse to enter the engine for a VERY SHORT AND SPECIFIC duration. The higher your frequency, the shorter that period of time is.

If you google "intake Harmonics" you may find a calculator that will help you decide your optimum intake runner length.
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Old 01-27-2009, 12:36 AM   #24 (permalink)
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Found a calculator. But it said that for me to be in a harmonic order at the rpm I go down the highway at that I would need an intake runner 35 inches long.

http://www.bgsoflex.com/intakeln.html

Im not even sure how intake runner lengths are measured. But from numbers that I have read that seems really high.

Of course that was a generic calculator and they could change depending on the motor.
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Old 01-27-2009, 01:06 AM   #25 (permalink)
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Glad you found that - now you know that working for primary harmonics is essentially worthless.

Now - if you double the frequency of the harmonics, you're still getting that boost, but slightly less of it, and in a slightly shorter RPM range. I haven't found a calc that will actually show you more than 4 harmonic ranges, but this falls into the "lots of valleys, and a specific peak" area.

It says that you would need a 35 inch runner to maintain ideal harmonics at a specific RPM (lets say 2100, since that's what my Prix cruises 65mph at (approx))

35 inches represents the approximate length based on the volume of the engine (and the assumed volume of the intake runner) where the frequency will be in 1:1 time with the engine's intake strokes. This means it's hitting the runner's end during the power stroke, and hitting the intake valve during the intake stroke, or traveling 1/4 the engine's RPM per the harmonic level.

So - if you need a 35 inch runner to maintain ideal harmonics at 2100 RPM, then a runner 17.5 inches will also work. Why? You have double the resonant frequency... the air is working at 2:1 with the intake valve. This means that it hits the intake valve during the period between compression and power, then again when the intake valve opens.

17.5 is still very long. Lets try 1/3 of the ideal frequency 35/3 = 11.666 is still too long.

.25(35) = 8.75 inches
.125(35) = 4.375 inches

You can't go much past 8 pulses... you really lose alot of the kinetic energy of the air even at 8 pulses.

The idea is compromise. You can't ideally have 35 inch runners, so you sacrifice some of the pressure wave in an attempt to salvage what power you can from a shorter tube. Remember, that every time you make the wave hit a surface, you're losing some of it's kinetic energy (ability to stuff air into the cylinder). This is why harmonics only work on a VERY short RPM range, and are generally not messed with.

One example of harmonic tuning on engines is with the 3.2 liter Yamaha V6 in the Ford Taurus SHO models. It has vacuum assisted valves which open or close based on engine RPM to allow shorter or longer runners to be used. At higher RPM, the shorter runners are used. There are two very specific RPM ranges at which this technology allows for a "bump" in the torque output.

Ideally, you would build an intake manifold which utilized "adjustable" runners, whose length would be controlled by the engine's RPM.
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Old 01-27-2009, 12:48 PM   #26 (permalink)
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Ive heard about the valves in the intake to change the length of the runners. And what you were saying about the harmonics makes sense.

So ideally what you would want to do is make it so your runner length would end up so that at the rpm where you are cruising down the highway you would have a harmonic effect that would help get air into the cylinder. But would this really help economy? If you had more air in the cylinder and the same amount of fuel then you would end up with a lean condition. Therefore the o2 sensors would add more fuel to be closer to stoich.

Im pretty sure that I have the same motor as you Christ have you ever looked into the runner length on that motor? Or is yours the older style intake manifold?
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Old 01-27-2009, 01:02 PM   #27 (permalink)
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I haven't with this motor, I did with an older Cavalier Z24 engine (non-stock 2.8 hybrid) but we were using a butt dyno and just adding tubing to see where we could get a decent bump.

IIRC, we ended up using that 17 inches of piping, which, given the limited space, ended up being more restrictive at mid-high RPM b/c of the number of complex curves that were necessary.

I haven't looked into it any further, we only did that as an experiment for a school report when I was in 7th Grade.

I've thought about a few ways to make an intake manifold that will constantly adjust it's runner length based on RPM, but nothing has come to fruition yet. I believe that technology is something better served for use with engines that have truly variable valve trains, run by solenoids and a computer, with no cam. Said engines will probably also have variable compression and stroke characteristics... and it won't be me that invents them. I'm not that smart.
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Old 01-27-2009, 01:11 PM   #28 (permalink)
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Wow, this is really cool. While I know that engineering fields tend to parallel one another, I never would have expected my musical background to help me understand cars!

That said, I want to back up to one of these ongoing puzzles that seems to confuse everyone trying to wrap their words around it... the "power-efficiency" vs. "fuel-efficiency" morass. Maybe you can help get me unstuck here...

I'm starting to understand how tuning the intake response can help you generate more power in targeted RPM ranges. But here's where I'm stuck -- Am I understanding right that you're improving localized power-efficiency by allowing more air-fuel mix to be rammed in at each stroke? If that's the case, then it seems to me that this kind of efficiency is the opposite of what we're after -- we'd have the engine using resonance to become a more mechanically-efficient fuel-gulper. Like a champion hotdog eater, timing the dip, chew, and swallow to "efficiently" cram more in before the buzzer. But me, I'm a hypermiler -- I'm on a diet! How can hotdog-eating-practice possibly help me eat less?

I might buy it if the resonance principle will somehow also allow the engine to run off of a leaner mix. i.e. "The intake can now cram more air-fuel in, but doesn't because it doesn't need to, due to the cascading resonance effect." But so far that's a few layers of system dynamics beyond what I understand about car engines. Maybe if you can make an analogy to loudspeakers?
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Old 01-27-2009, 01:28 PM   #29 (permalink)
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It's technically the same volume of air in a closed circuit entering under more pressure.

Without proper tuning, and given some circumstances, it can cause your engine to lean out a bit, and your ECU WILL compensate for it, provided you're at a throttle position that requires it.

Since harmonics tuning works at any RPM range, and at any throttle position (other than closed) at any RPM range (with a lesser effect for lesser throttle, due to lesser amounts of air coming in, and less of a compound effect) the idea is that at cruise, you're at less than 20% throttle, and the resultant harmonics is enough to accelerate the same amount of air coming into the combustion chamber, therefore, it just increases the engine's VE at a specific point, rather than actually increasing the power. (Less pumping losses, not more power.)

It's kinda like when the electric company gives you a break at night b/c you're off peak. You're not actually using any less electricity, but you're paying less for it.

Your engine isn't working as hard to draw in the same amount of air, but it's making the same amount of power it was before... this results in an excess, which shows up as more torque.

Loudspeaker analogy - You have a 10" Sub in a vented box, and a 10" in a sealed box, both of optimal size and construction for the same sub, all things equal.

The 10" vented sub will have a specific frequency where it just hits SO MUCH harder than the sealed one. This is b/c of resonance. With a specifically tuned port (intake runner) the sub (piston) at some point in the harmonic range will be moving outward just as the influx of pressure is moving into the box, and will rebound inward as the influx of pressure is moving out of the box.

What this means is that while the sealed box has good all-around sound (like a normal, untuned engine), the harmonically tuned vented box will often have a smoother tone, hit harder, and create much cleaner notes on a given range ("bump" at certain freq).

The reason for this - when the influx of air is moving in at the same time the sub is moving out, the sub doesn't have to create a low-pressure area in the box to move. This means that it's using less kinetic energy (from electro-magnetic energy) to move in the outward (bound) direction. Using less kinetic energy to do the same job means that you're more efficient at it.

(Just for you Akashic!)
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Old 01-28-2009, 03:29 PM   #30 (permalink)
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I think I might try this when it warms up. I've read some reports from users online that said it helped their stock 4.0L Jeep setup. Looks like the one on the 4.0L Jeep will be really easy to build too, Just a square with a big hole in the middle.

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