DIY Throttle Body Spacer
 

This is an easy mod for most cars. I have been trying to think of stuff I have not done to my car and one thing I came up with was a spacer to put under the TBI. Of course nobody makes anything for a Metro and even if they did it is not worth spending much money on something so easy to make. Spacers can show a couple of mpg improvement on an engine using a carb or TBI injection. On multiport it can be hit or miss whether it works.

So you need two things to make one. A plastic cutting board and a TBI base gasket. I got my cutting board from RiteAid locally, it was a nearby store and it was exactly 1/2 inch thick. It cost 10 bucks.

Start by laying the gasket on the cutting board and drawing out the pattern on the board. Drill holes in the mounting bolt holes and some starter holes in the other areas. Use a dremel or a grinder with a deburring bit so it doesn't clog up. I couldn't find my dremel so I used an air grinder with a deburring bit.

http://metroxfi.com/wp-content/uploa...0188-thumb.jpg http://metroxfi.com/wp-content/uploa...0189-thumb.jpg

One thing to note. You don't have to cut all the holes out. Some of them are much larger than are actually needed. They are used for vacuum passages and as long as they are big enough to let the vacuum source through they don't need a huge hole. On the main bore I made sure it was nice and smooth and properly sized to the manifold. The other holes I just shaved the plastic down so the vacuum signal could get through but otherwise didn't bother cutting it totally out. You can cut them out but it makes it take way longer and you don't gain anything.

Typical reports are a much better bottom end response and slightly improved mileage. I will find out soon with mine and report back. If I feel like it, I will make a second one and stack them for a one inch spacer. I am not sure what would work best on this setup one inch or a half inch. I might try both ways if I get time and if my mpguino ever comes in.

LET ME KNOW!!!

All the Mazda 6 guys claim improved mpg, but for $80 I dont see the return on gas... but If I can make one like you for less than $10 then I will for shure do it...

I have MPFI though

I say go ahead and grab a board from a store or take the one out of your kitchen if you can get away with it :) If other people have reported increased mileage then you really won't have much to lose by giving it a shot. If nothing else the leftover part can be saved and put to use in your kitchen like it is supposed to be or used for fabricating other parts. The plastic is pretty easy to work with and if you warm it up it seems like it could be formed easily.

It took me probably 30 minutes to make the first one shown in the pictures. I Could probably make one in 5 minutes with a drill press. If I had a hole saw assortment and some routing bits to go along with it.

I am probably going to make one to go in my Astro van sometime or another as well. It will probably give me a more accurate number to show how much of an increase there is from it. The Metro is getting some other stuff done at the same time so I am hoping I can tell what it gives me but it might take a few months to sort out how much it helps that car.

Are you doing this to change the effective length of the intake runner or to make a thermal break for the throttle body?

This is the first I've heard of adding a throttle body spacer to improve efficiency - aside from your earlier PM. (Coyote asked if I wanted to do an A-B-A test on it, but unfortunately my cruise control situation hasn't been resolved, so I can't run reliable tests with the Flea.)

It might be worth posting a note about the theory of why this mod may help.

I think the idea is to lengthen the intake, giving it a lower resonance frequency, and moving the peak power band to a lower rpm. Should be perfect for the way WE drive.

tb spacer, what about uim spacer?

Indeed, this design is evident in the length of the runners on the honda crx hf, civic cx, and civic vx. However, because of the overall intake design I think for these cars a tb spacer wouldn't do much since the resonance is along the runners into the plenum and would not change with a tb spacer.

^ Well in the performance world throttle body spacers DO work to improve hp and torque all accross the rpm band.

So with that in mind, it does change something. How that translates to better fuel efficiency? Well, the engine is making more power because its more efficient and this mod does not add air (turbo/supercharger) or increase air consumption, it simply changes the way the air travels. This should in theory improve mpg's.

It also creates a larger plenum area, which means slightly more air available for higher RPM operation.

I'm also pretty sure that lengthening a tube while keeping the same diameter increases air velocity, though I might be wrong on that.

Anyway, if it does increase air velocity, that would mean better atomization, as there would be more turbulence in the air, which might make better power/economy.

AFAIK though, the biggest gains are seen in the higher RPM bands (mid-range torque and higher HP) due to the extra volume of air in the plenum.

As an add-on to this - Using thermal plastic to make an intake manifold will net you better HP and TQ as well, due to the temp differential (thermal plastic allows the incoming air to cool the intake manifold, as opposed to the cylinder head heating it... air temp decreases and you get better throttle response and HP)

Which just means less work for you. There are 3-4 bolts/nuts/studs/whatever holding the TB on, put a spacer behind it, and you're good to go. You'll need two TB gaskets though.

Subscribed for results - wanna see if "bigger is better" in this case.. If adding more than 1 spacer really does anything more for it.

This would be an easy/great first mod for wifey to try on her car, since she's into engine work anyway and wants to learn to fabricate stuff.

But there must be an optimum length, probably related to engine displacement and desired rpm. This sounds more complex than 'slap a 1/2" spacer in it and go!'... come to think of it, probably related to cam timing too b/c the valve opening and closing would set up the pressure pulses. Kind of like expansion pipes on 2-stroke motorcycles, no? (just on the intake side, and with valves...)

I'm also confused by the claims of lower RPM gains because this article Best Carb Spacer Cheap Horse Power - Popular Hot Rodding Magazine emphasizes the gains in the upper rev range (although there are some mid-range gains too) like Christ states, but I'm not an engine builder and I'm more than a bit rusty at all of this.

This leads me back to the use of a TB spacer as a thermal break rather than as a method of tuning the resonant frequency in the intake tract.

Thermal break is partially true too, although most commercial spacers, AFAIK are aluminum...thermally conductive.

I'm not sure about the low-mid TQ, although adding length to runners does move the resonance effect (n/a turbocharging) down lower in the RPM range, which would create a peak effect of TQ being lower than the actual peak... and still move the actual peak lower in the range.

PS - Runner resonance length is calculated as "distance from valve to valve" the plenum counts as an equal distance for all the runners, since it provides an interference of sorts.

What this means: effectively, adding a TB spacer, you're increasing runner resonance length.

I also didn't specifically note that with TBI - the "plenum" isn't like the MPFI plenum at all.. TBI is more like carbs, in that the plenum is an area that can actually be equally divided between runners without giving more or less volume for each runner's resonance length.

This would definitely attribute better fuel mileage to the TBI/Carb crowd, due to the same idea behind using it for anything else.. longer runner - lower TQ.

Obviously, if you're making more power on the same fuel, you're running more efficiently.

Well if it would actually increase the runner length then it should increase the torque. From what I have learned about runner length here, TPiS, says that a longer runner should result in more torque but slightly less hp. But for an ecomodder who never sees high rpm's this could be helpful. I am also into making things fast and I would not install a tb spacer for hp gains on my Camaro. No one has found gains, but they dont look at fe.

Am I correct to assume that this mod should benefit y'all's little racecars a lot more than a truck that's made for towing?

No, longer intake runners mean more torque. Early gm efi truck motors had some awkwardly long intake runner length, like 12 inches or something maybe more, so they would make more torque but the length of the runners made it so the air could not go through the intake fast enough to allow the engine to spin much higher then 5000 rpm.

So a truck made for towing should benefit more bc of the added torque. If adding at the throttle body has the same effect.

Im not sure if anyone really knows if adding a throttle body spacer really does anything, a carb spacer is different and there are definitely gains there. Some people claim gains from a tb spacer and others dont I think it just kind of depends on the motor.

Good idea on using the cutting board I think that once it warms up a bit i will try this and do some testing to see if it makes a difference on my car. I have used cutting boards on my car for a different purpose but it wasnt for fuel economy. I used them to make strut spacers, I havent gotten them yet bc it got cold but I think they should work.

The only HP claim from throttle body spacers is due to velocity increase... it's pretty much BS. The idea is that the TB spacer will create a "turbo like" velocity increase that means more air gets "rammed" into the cylinder, so that it's pre-pressurized, and gives an increase in HP.

The physics behind it assume that you're making the same space of air move a further distance than it normally would, as in an airplane wing. (The air over the wing is moving faster, which creates a low-pressure zone, allowing lift. It's the same volume, at a higher speed, when the plane's speed increases.) not that the air is constantly being consumed. (The airplane effect doesn't work unless it's in open-air, with a splitter. Air will only travel faster to meet the same airspace that it was separated from. If no separation occurs, no increase in velocity occurs.)

The physics also assume that the engine isn't under vacuum. A net velocity increase while under vacuum would require a longer AND thinner runner, and there is a point of diminishing returns.

The point is, you can get better power gains elsewhere. Such as extrude honing (sandy water). If you're looking for a quick torque gain, then use a TB spacer.

The effect is greater on direct runner setups (Refer to D16Y7 and D15B2 honda engine, which has a "up and down" throttle body, which resembles a carb.) than on large plenum or offset plenum setups. (Offset plenum - most MPFI cars, runners go into a "bottle" with a TB at one end. Large Plenum - Plenum contains more volume than the runner length does, from intake valve to end of runner.)

^^Id agree with everything you posted.

But the question is can it help with fe?

I think that it might bc if you could increase the torque a small amount then the car could push down the highway better and not need to down shift up hills with cruise on.

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.

I call this "wasted potential". Since you're going to be building it on your own, with almost no money invested, and we all know that it can't hurt, but it seems that noone can provide scientific evidence that it will help, maybe you could just try it for yourself?

What's the worst that could happen? It doesn't do anything, and you have a nice talk piece under the hood that netted you a few hours mechanical experience and a few hours of cutting, grinding, shaping experience.

See what I'm getting at? At the very worst, your gains equal experience and knowledge, rather than FE and torque. You've nothing to lose by just "giving it the old college try" as it were.

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?

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?)

This intuitively-appealing statement keeps reappearing in different forms and on different forums. Doesn't it need to be qualified as to HOW you're maximizing your power? If you're maximizing your power linearly across-the-board, then I think this statement might be true. But it generally doesn't work that way, right? You might amplify your power in a certain rpm range much more than in others. And you may even retune the system in such a way as to create resonance peaks in one range that reduce power in other ranges.

So any power increase isn't necessarily helpful. Hypermilers need efficiency increases in a very particular RPM range particular to the vehicle. If we can retune the system so that that particular range gets more power, then that should be helpful.

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.

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.

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.

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.

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.

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.

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?

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.

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? :)

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!)

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.

Thanks, Christ! That was a pretty good description of how ported speakers work. So I'll take some time to think on that helpful analogy and maybe revisit it later.

Meanwhile, though I think I get how the intake acoustics contribute to power, and I see the reasonableness of the argument that "more power with the same amount of fuel equals the same power with less fuel", I get stuck on a counter-argument: If you're getting your power increase by cramming more air in, then you're also cramming more fuel in. So your power-per-unit-fuel efficiency should be the same.

But it's starting to make sense now. This is how I'm explaining it to myself:

The air does double-duty in driving the engine -- 1. chemistry for combustion, and 2. mechanical compliance (spring action). Assuming the same volumes of air, and therefore the same fuel usage, the spring action is the variable that can either add free torque if the intake resonance is in phase with the combust cycle, or subtract torque if it's out of phase. So by tuning your intake to your preferred RPM, you can get free torque on the same amount of fuel.

I guess what was throwing me was the idea of ramming more air into the cylinder, which if I'm understanding right, is a misinterpretation. You're really pressurizing the same mass of air, ie. less volume, not more. So since stoich is mass-to-mass, you use the same amount of fuel to get the torque-bonus (or torque-loss, as the case may be).

You got exactly what I understand to be the principal behind harmonics and intake systems, and exactly what I was trying to convey. With a bonus audiophile analogy. Damn - you really worked me on this one. :P

FastPlastic - You lucky summuna.... here is your throttle body gasket:

http://info.rockauto.com/getimage/ge...tex/1G1212.jpg

Yup. Should be about to knock that out in 15-20min. Probably spend more time looking for longer bolts if I need them.

Coyote X - Did you end up replacing the mounting bolts?

You probably should use longer bolts if you plan on making more than 1mm of spacer. The typical flange is only 1/4 of engagement, and most OEM's use the "correct" length bolt for many things these days.

Also - you should have studs... most do, iirc.

This is definitely a simple mod that I think I will try.

Anyone know how you measure the intake manifold? Is it just from the throttle body to the engine?

I'm trying to get an idea of how tall this should be. I've seen several on the market for the Jeeps that look like there about 1". I guess another question I have is could you make it too tall? I would imagine the hood would be some what of the limit but when it comes to the harmonics you would just drop down a multiple or two depending on length. Then again I saw 35" was the optimum at 2100rpm. So I'm guessing that would be difficult to even remotely come near that with this spacer.

Are you guys only adding a spacer at the throttle body on mpfi, or adding a spacer between the head and the intake plenum? the latter is a part that is offered in the aftermarket on my dodge and other performance oriented cars.

Hello everyone, this is my first post. I did this with my 1989 plymouth horizon with a 2.2 TBI 5 speed.

If you want to temporarily try it out, you can go to a junkyard and find gaskets for your car. Stack 5 or 6 together and you'll have a pretty sizable spacer. Thankfully the ones in the dodge 2.2 are 1/8" so i only tried two.

I found it added to the low end. I have no idea what it did for mileage, those were nearing the end of my teenage years so I was more concerned with how many gears I could keep the tires smoking (1st-3rd if you were wondering). I did get it from ~14mpg to just over 30 (still driving like a jerk) with new plugs with a wider gap, oil change, timing advanced 5*, TB spacer, and a better air filter. On a 290k engine, god knows which one of those helped : /

This discussion reminds me of the Dodge-Plymouth SonoRamic intake setup used in the early 60's on big-block V-8 cars. IIRC, the California Highway Patrol had some of these. The intake runners in racing models were 15" long, in others (CHP) were 30" long, a 4-BBL carb on one side of the engine bay feeding the bank on the opposite side, and vice versa.
The carbs had to be set to run rich through most of the power band to avoid running too lean at the RPMs where the ram effect kicked in--not very good for FE .
Ray Mac