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ENG: H-series Plenum on F22A6 runners

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    ENG: H-series Plenum on F22A6 runners

    I was tooling around the garage a couple of weekends ago and came to the conclusion I needed to take something apart... Why would we do that? More power of course!

    * Our hapless victim awaits...

    Tools needed:

    10MM socket and wrench

    12MM socket and wrench

    Various socket extensions

    #2 Philips head screw driver

    Flathead screw driver

    Various pliers and channel locks

    Intake plenum gasket (depending on the condition of your old one)

    IACV O ring (probably a good idea since it won't be young, and it might not seal properly)

    Shop manual of your choice (I made a goof on the coolant lines, that only Helm's could solve)

    This swap is actually very straightforward if you have an F22A6 manifold already on your car. Obviously, it won't work with the F22A1 or F22A4 manifolds. The process could be substituted if you have an F22A6 manifold that you are going to install, and can hunt down an H22/H23 plenum. All of my research indicateds that they are identical.

    Coolant line (depending on the condition of your old ones)

    1) Unhook your battery

    2) Remove your intake. If you have the stock airbox on the car, it is only necessary to remove the rubber tube.

    3) Unplug the IACV, TPS, FPR and all other vacuum lines from the plenum. Make sure you keep track of which is which. Unhook the throttle cable. If you have an auto, you will also need to disconnect the kickdown cable that runs to the tranny.

    4) Remove the coolant lines. If you are very careful, you should be able to get the coolant lines off of the IACV and FIV without spilling much at all. I lost about a Nyquil cup full. I then stuck them under something in the upright position. You will have to remove both of the coolant lines running to the throttle body. One line runs into the body just under the throttle plate and one runs out toward the wheel well. Again, if you keep the lines upright, you should lose very little coolant. I went ahead and drained them into a cup, because I already knew that I was going to have to fabricate at least one new hose. Remove only the TB end of the return hose in order to make it easier on reinstallation.

    *The coolant pipe circled in yellow is the coolant inlet for the TB. It is connected to the steel tube that also contains the breather tube for the intake, and then a separate hose runs down to the very top of the thermostat housing. The red line is the coolant outlet, which runs to the back of the intake manifold.

    5)Once everything is disconnected from the plenum, simply unbolt the 5 screws/nuts holding it down and remove vertically. Be as gentle as possible here, because this is where the plenum gasket is most likely to break. You may not be able to save it, but you can at least try. If it breaks, simply go get a new one. It is also a good idea, as it will lessen the chance of debris falling into the runners if it does break.

    6) Replacing the plenum is pretty much the opposite of removal, at least until it is bolted on. Obviously, I took the opportunity to clean everything I could while the plenum was off of the car.

    7) I first re-connected the coolant lines because they were the hardest to gain access to and are the most complex part of the install.

    Since the H22 throttle body doesn't have a separate coolant channel to keep the plate warm, simply take some coolant hose and loop it from the thermostat housing to the back of the IM.

    *Here you can see in the left bubble where the coolant comes out of the thermostat housing. In the right side of the left bubble you can see the metal tube that is attached to the breather.

    *In the right bubble you can see where I ran the coolant line down to the back of the IM where the drain line from the TB used to go. This is where leaving the old drain line attached to the IM comes in handy. To avoid crawling under the car to gain a view, simply trace the hose to its origin, pull it off and place the looped hose in its place. Don't forget the hose clamp!

    Now that the TB is bypassed, the rest of the coolant hoses need to be hooked back up.

    The FIV on the H series is located on the bottom of the TB instead of on the front of the plenum like the F22 (the Air Boost Valve is located in the Accord FIV position on USDM H22s. JDM H22s have a block off plate there instead.)

    You will want to remove the H series IACV from the plenum and put the F22 IACV in its place. Now is probably a good time to replace the IACV O ring. make sure to lube it with motor oil prior to install. They are interchangeable, but you will have to use both IACV bolts from the Accord as the H series IACV has one long bolt and one short bolt.

    The reason you don't want to use the H series IACV is that the inlet sits so close to the intake runner, it is impossible to not kink the coolant hose.

    *You can see how the inlet circled in yellow points straight down into the runner. A hose will not fit inbetween here and the runner unless you are very creative. I just used the F22 IACV.

    NOTE: The nipples circled in red were the outlets for the FIV and IACV respectively. The FIV inlet, circled in blue, will simply be rerouted to the H FIV as the hose is long enough. The outlet from the IACV (left red circle) will stay the same. It is the FIV outlet (the right red circle) that we will be making.

    Since everything else lines up, simply take about an 18" length of coolant tube and run it from the new FIV outlet to the IACV inlet (you will want to gauge the necessary length of the hose based on your preferences).

    Mine looked about like this when I was done:

    *You can see how the side facing entrance nipple made it much easier to get a hose on without kinking it.

    That pretty much finishes the coolant hoses. It is a good idea to open the bleeder bolt and top off the cooling system when you are done to avoid any unwanted complications.

    8) You will reuse your stock throttle cable. My H22 throttle body was a manual and my car is an auto, so you may need to swap the throttle plates. Just make sure that everything is on tight and note how the return spring came off and make sure it goes back on the same way.

    There was one complication with the F22 throttle plate that I didn't notice at first. The little stop tab on the F22 throttle plate (at least on mine) was located about 1-2mm closer to the stop screw. When I got done and started the car, it was idling at 2500 RPM. Once I backed the stop screw off, everything was fine. You may want to check this before putting the plenum on the car.

    You may also need to move the kickdown cable bracket from the F22 TB over the the H. Hook your throttle cable/s back up to the TB.

    *Every time I have tried to use an F22 throttle cable and an H series IM bracket, the little nut on the cable wants to occupy the same space as the bracket. Simply take the F22 bracket off and screw it onto the H plenum. This lowers the mounting point just enough for it to fit perfectly.

    Readjust the throttle cable to spec.

    10) The vacuum lines pretty much go back to the same relative area that they were before. The most important is the MAP sensor which hooks to a metered orifice in the TB. It still goes on the nipple closest to the throttle plate.

    11) The two hoses that used to mount right next to the map on the F22 TB simply go right around to the back top of the plenum. The location should be fairly obvious.

    *Here you can see the new vacuum hose connection points near the TB.

    12) The cruise control and the IAB will each occupy a nipple on the side of the plenum opposite the TB. If your car is a manual, you can also cap off one of the side nipples and run the IAB to the nipple on the back driver side of the plenum (pretty much the same as the F22). If you have an auto, you will need to hook the vacuum line for the engine mount control solenoid to the back nipple.

    13) The Fuel Pressure Regulator hose will run to the big vacuum nipple right on the front of the plenum.

    14)Reinstall your intake. If you are using the stock tube like me, it WILL fit on the H series TB, you will just have to work it because it is a tight fit.

    **Product Review: I was amazed by how much of a difference it made.

    I started with a stock F22A4 and after adding intake/full exhaust was rather disappointed.

    I first swapped the A6 manifold. It improved low end torque significantly, but
    didn't really contribute to the pull in the top end. It did pull to almost 6K though instead of falling off at 5.2K.

    Then I installed the cam, and it was a very significant change. Well more than ALL previous stuff combined!

    The H series plenum made almost as much of a difference as the A6 cam and manifold. It sounds meaner, pulls harder at all speeds/RPM's and pulls right into the rev limit at 6700 RPM.

    All in all, highly recommended and well worth the effort.
    Last edited by owequitit; 04-10-2006, 11:03 PM.
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    Please do not comment on the lines in the pics! I had very limited programs and will upload updated versions ASAP.
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      There are 3 parts to the intake manifold. I don't have time to upload the diagram and post it at the moment, so here is a link:

      Intake Manifold Diagram

      You will notice that there are 3 parts to the Intake manifold.

      There is the upper part with the throttle body. This is the part with PGM-FI on the top. It is called a plenum, basically derived from "common chamber" in Latin. This is also the only part I replaced.

      Below that is the IAB (Intake Air Bypass) assembly. These are the butterflies that open up at high RPM for better breathing.

      Then at the very bottom of the intake manifold are the "runners" that run the air from the plenum and IAB assembly into the individual cylinders.

      The IAB assembly is the same as on an H22/H23, so the H22/H23 plenum will bolt right on.

      H23 Intake Manifold Diagram

      Here is a diagram of the H23 plenum (it is the same as the H22). You can see that part #10 in this diagram is substantially larger than part #10 in the first diagram.

      Why do we want to do this? A larger intake plenum allows for more efficient breathing, especially at high RPM when the engine is pulling in a lot of air). The more air we can flow and add fuel to, the more power we make. It basically serves as a reservoir of air. The air in here is available to the cylinders at all times.

      It also has a larger throttle body which also flows more air. This mainly causes more power in the top end because of the increased flow, but it also improves throttle response and bottom end torque because it is easier for the engine to pull in air. It also gives a meaner intake tone, because the engine is expending less energy on sucking in air, and can readily receive the air it needs.

      The F22A6 intake manifold has shorter runners than the H23 manifold (an H22 manifold will not bolt up, as far as I know) so that again provides a little high RPM increase. You don't lose torque though, because the runner length is the same as it was.

      This modification basically allows you to create a better flowing intake manifold without a lot of effort (for those who have an A6 manifold on their car), and everything is OEM, so there aren't any reliability issues. It also resulted in a substantial increase in power, throttle response, and possibly gas mileage. The last tank gave me the typical mileage, but I was flogging the snot out of it, so it will probably go up slightly if I back off.

      If you have an F22A1 or A4, the A6 manifold swap has already been documented in the DIY forum.
      Last edited by owequitit; 02-24-2006, 01:26 PM.
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        dayam great job on the DIY. your engine is very clean. any power gains at all? i have a f22a1 which doesn't have the secondaries so i can't do this. great ingenuity though!
        - 92 LX coupe
        - 96 EX wagon (sold)


          Adding the F22A6 manifold/cam is pretty much a guaranteed 10HP upgrade over an F22A4, and 15 over the F22A1. Well, the exhaust may count for some, but that is about what I saw over my existing setup, and the exhaust was already done.

          I don't have any hard numbers, but I can say that it provided about as much improvement as the A6 mani and cam did combined. I think the cam may be important though.

          It used to not make a difference that the car shifted at 5.9k to 6.1k because it didn't really make a difference if you revved it higher (even with the A6 mani and cam). Now it continues to get stronger all the way to about 6.3K and continues pulling right into the rev limit at 6,700. It will also climb large mountain grades at higher speeds and accelerate up them much better at speeds at or above 90MPH.

          In my experience, the order of modification that I would recommend is this:

          A6 camshaft and manifold. Just the manifold increases the powerband a bit, because it will pull 500RPM further up the dial, but it really doesn't make much of a difference in terms of how hard. The cam is responsible for MOST of the increase. You could conceivably just add the cam, but since you can probably get both off of a suitable donor, I don't know why you would. It would probably be a bigger benefit to add these first. We know they make a measureable difference on a completely stock car.

          The plenum and TB. This would probably allow you to take much better advantage of a larger cam grind as well. It was very clear to me that the IM was somewhat of a restriction. My car kept up with a car that had full exhaust, cold air, ECU w/launch control, 260 or 272 regrind and was fully stripped w/ a 5 speed. I had an exhaust, A6 IM/plenum and A6 cam as well as 17's and all features of an EX... He was still running the A4 manifold, so I think that was his restriction. Based on my setup it provided about the same as all previous mods, but is only possible with the A6 or H23 swap...

          Exhaust (easiest to install, most people like the look/sound, improves gas mileage slightly). If more people made a well designed header with a 2.5" collector, or if you have an F22A1 this would probably jump up the list one spot. The A4 manifold may provide a negligible increase over the A6 manifold, but is worth 5HP over the A1 manifold. It is better designed than the A6 mani, but I don't have any measurement knowledge on it, so will assume they are the same. Most aftermarket headers feature a slightly better collector design than the cast manifolds, but they still choke with a 2" collector. DC Sports is negligible over an A4 mani, and the A4 mani is MUCH cheaper and durable. You could probably have it extrude honed for the $$ difference and beat the DC. The Kamikaze's feature the 2.5" collector, but the collector design is so so and they basically dump right into a down pipe. Not terribly different from an F23 exhaust manifold, which sucks. Theoretically, they should pull better in the top end, but with the collectors on there, I am not sure. Again no experience.

          Catalytic converter: whether or not the large gain I noticed was because of the converter itself or the fact that it was the last link in the exhaust chain, I am not sure. But I didn't notice much with either the catback or the header. When I swapped the cat it was pretty noticeable.

          Maybe do the whole exhaust at once...but most people who would be looking at doing this probably already have.

          If you add a K & N panel filter to your stock airbox, take a 2.5" Holesaw and cut a larger hole in the side; you can run a nice mandrel bent tube right in there (in addition to the lower 1.5" tube). If you put the other end into the fender, you have a quiet and cheap OEM appearing cold air intake that should outflow the engine. If you keep the A6 manifold, a 2" tube in addition to the lower 1.5" tube will equal about the same flow area as a CB7 TB.

          You could conceivably add 15-25 very documentable crank HP with just OEM bolt ons, that you could probably spend less than $200 on total (maybe $300 for all extra hardware etc) if you look hard enough. There will be no fitment issues, durability issues, or emmisions issues with it. I can't imagine the average test station being able to pick up on an H series plenum vs an F series plenum and the F22A6 as well as the H's had to pass emmisions anyway, so you would most likely blow clean. That works out to a 12-20% increase in crank HP, potentially for less than the cost of a cold air intake.

          I wish the nearest dyno wasn't 1.5 hours away, and that I didn't break my GTECH. It would have been easier to get numbers on the plenum install that way. All the other mods WERE documented though.

          But I digress...
          Last edited by owequitit; 02-27-2006, 12:19 PM.
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            Originally posted by owequitit
            If you add a K & N panel filter to your stock airbox, take a 2.5" Holesaw and cut a larger hole in the side; you can run a nice mandrel bent tube right in there (in addition to the lower 1.5" tube). If you put the other end into the fender, you have a quiet and cheap OEM appearing cold air intake that should outflow the engine. If you keep the A6 manifold, a 2" tube in addition to the lower 1.5" tube will equal about the same flow area as a CB7 TB.
            I did a 2.5" out of the side, made it air tight with cocking and with these cold temps it hardended right up...

            then i'm gonna run another 2.5 to my bumper and cut out one of the fog light temp and build a ram air induction through that.....good write up tho, wheres all the proof for the numbers since id be interested in doing the cam swap w/ my h23 manifold that i just ported .....i can load pics of the manifold from top to bottom if u need them....
            Last edited by ehulst; 02-24-2006, 07:25 PM.
            93 Accord- SOLD
            94 Civic- F22a1 Swap, current DD


              The pics would be helpful.

              Everything on the car was measured with a GTECH. Not a dyno, I know, but if you take everything in context, you can get a good idea.

              The cam and manifold were good for 10HP over the 90-91 EX as certified by Honda. Some may get more, some may get less.

              The 90-91 EX was good for 5 HP over the F22A1 as documented by Honda, and as a result of the exhaust manifold and system differences.

              If you start with an LX and add 15HP, that works out to a 12% increase. For the sake of brevity, I am basing everything on a perfectly standard engine.

              I saw roughly a 3 NET hp gain with the manifold, not a lot, but it did pull higher. The GTECH broke in between the cam install and the mani install, so I don't have an exact number on that, but I think based on the improvement, it is safe to say 6-7HP. I couldn't even feel a difference with the mani, but there was a BIG one with the cam. Again some will get more, some will get less. Say 9 HP net...

              The plenum "felt" like it added as much as those, so again I am guessing about 5HP.

              I will see if I can't spring for the drive down to Phoenix and a couple of dyno pulls.

              If you have a ported mani and exhaust system, you may want to consider a regrind too. I don't know what their durability is, but I haven't heard a lot of bad least about Delta.

              I saw your airbox, and was thinking about it when I posted that. I just saw the pics the other day, but couldn't remember who it was. Where did you get that tubing?
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                I will be starting construction on a high flow Carbon Fiber replacement airbox here in the near future too. I am hoping to increase filter area by 50% and still get it to mount in the stock location if I can.
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                  the tubing goes WAY back to when i had my 86, i did a custom Cold air system with it from a 93 Tegra, ebay junk then i just looked at the bends and found the right one to fit..people think i have an exhaust when i go past, but its really just the airbox making that loud noise, they are surprised when they see my stocker and no exhaust leaks ....i'll PM my pics page to u and u can sort through what u want ....
                  93 Accord- SOLD
                  94 Civic- F22a1 Swap, current DD


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                      i left the image stuff off but heres the 3 parts of the manifold....
                      93 Accord- SOLD
                      94 Civic- F22a1 Swap, current DD


                        Thanks a lot man. I will try to integrate them into the DIY.
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                              pics do not work for me either