Results 1 to 6 of 6

Thread: Modified 912 intake manifolds for 1" cross-over tube! (pics)

  1. #1
    Premium Member (Donated) GravityKnight's Avatar
    Join Date
    Jul 2015
    Location
    Colorado
    Posts
    559

    Default Modified 912 intake manifolds for 1" cross-over tube! (pics)

    Been working on this for the last week or so when I can. I have a spare set of intake manifolds to experiment with. Got a wild hair to go bigger on the crossover tube. Flygas has a set you can buy with a large cross over tube cast right into them for ~$1000. I read about another guy making his own crossover setup that went between the manifolds and the carbs (on a pusher, most of us don't have room).

    Figured, I have a milling machine, and a welder, what's the worst that could happen!?!?!

    -Milled them out, welded in some alum bends on a ~45 deg angle roughly. Had to weld and tap new ports for my manifold pressure gauges. Bought some silicone 45 deg bends from HPS to form the crossover tube. Made an insert and welded on a chunk to the pipe so I could tap that for a fitting to run to the HACMAN leaner (basically like it was before). Carved out the insides and made everything pretty smooth and nice to not disrupt flow etc. Quite a bit of work involved in this project.

    -Results are positive. Previously, I had some weird roughness in the 44-4800rpm area, sometimes up to 5k depending on altitude when the throttle is pulled back to a slow cruise. This actually has nearly eliminated that! Which is weird, because that wasn't a side to side sync problem, that was a front to rear cylinder air fuel ratio problem best I could tell.... rotax's manifolds suck really bad for front to rear cylinder equal fuel flow. Anyway, it idles down real low and feels smooth. Still not going to idle it that low for any length of time, just nice when you come in slow, throttle way back for prop braking, and during the rollout on the ground you can bring it down slow and it feels good and has to be a little better on the gearbox. Engine is overall smoother and feels better in the sub 5k rpms.

    Interestingly enough I'm running a little leaner overall at WOT. Wideband o2 gauge and 4 egt verified. I have 158 main jets in the carbs now for the zipper, might bump up to some 160's I have laying here. Sort of scratching my head as to how it has leaned out a little just on the top end, but if it's getting more air in it, I'll give it more fuel and see what we get! Not necessarily a bad sign by any means.

    Fuel economy seems better in the cruise, especially low throttle cruise area. Only have ~1.5 hours on it since the mod, so not making any claims just yet. But I was leveled out today, throttle pulled way back, indicating 93-94 mph, leaned out (not crazy lean, ~1400 egts) burning 3.3gph... maybe less. Interesting.

    Pictures below. More reports to come soon. Thinking rotax really should have installed a larger tube from the factory, the engine just feels better. Might consider doing this mod for others if there is interest.

    Starting tool. A mill is required IMO because you are boring through the two holes already in the manifold in order to get the location right and not have a huge gap to weld up.



    4 different cuts to make a 1" ID hole



    Couple pictures of the tube stuck in the hole, nice snug fit




    I cut the tubes back to a 45 deg angle after this. Would be best to start with 45 deg bends to begin with, or around there, 90 is too much bend to clear stuff on the motor/fuel lines etc.




    Installing them... pain in the rear with all the crap I have on mine (rotax airbox, my custom ram-air intakes, radiator hoses and radiator etc.)



    Couple shots of the basically finished setup

    S6S 116 w/VGs|MGL/FDS/AoA
    114hp zipper912|homebuilt headers/muffler/ram-air
    75"WW STOL|22x8.50mains/21x8.00nose/custom fork
    YouTube Channel

  2. #2
    Premium Member (Donated)
    Join Date
    May 2014
    Posts
    161

    Default

    Gotta hand it to you Todd! That takes guts to mill into a hollow cast piece like that - good going! One the same note, I have heard of others using an adapter that threads into the existing port going to a larger crossover tube 1" or so as well (if memory serves me) and several have reported similar improvements. I recall the idea being - it's the larger space in the tube allowing more room for the "pulse" to move back and forth, but not necessarily requiring a larger port to do it. Hope that doesn't quash your mod factory idea

    Scotty

  3. #3
    Premium Member (Donated)
    Join Date
    Aug 2015
    Location
    Anderson, Texas
    Posts
    228

    Default

    Nice Todd, very impressive

  4. #4
    Premium Member (Donated) s19flyer's Avatar
    Join Date
    Feb 2008
    Location
    Texas
    Posts
    4,586

    Default

    Nice!

    Hal is doing this mod too. Don't remember the price but $300 sounds familiar.
    Glenn
    Avid Flyer, S6ES, S19, S20 In Progress
    http://www.mykitlog.com/s19flyer
    Remember, I am not immune to mistakes!
    What works for me may not work for you.

  5. #5
    Premium Member (Donated) GravityKnight's Avatar
    Join Date
    Jul 2015
    Location
    Colorado
    Posts
    559

    Default

    Thanks fellas!

    Nice!

    Hal is doing this mod too. Don't remember the price but $300 sounds familiar.
    Thanks Glenn. That seems reasonable. I'd probably do a set for someone for 300.



    I installed 160 main jets today and flew it around. Richened it up a little, not quite as much as I'd hoped for. Before this mod (and these intake manifolds, which are ported a little to try and match the porting I did on the last ones, so they are very similar but not exactly the same) I was seeing about 12:1 air fuel ratio at wide open, and I could lean from there if I wanted. 12.5:1 is peak power, but because the front and rear cylinders don't see the same mixture when WOT, I usually left it around there and played it safe. I was seeing 13:1 after this setup.. new jets I'm down around 12.5:1 which isn't bad, but that's at 7000ft. Kind of strange to be honest. Oh well, it's running good, and even with the leaner mixture my max EGT at WOT was just above 1400. Not sure how it would do at low elevation, suspect it would be too lean. Kind of scratching my head, and you can't get bigger than 160 jets I don't think. Though they can be opened up if you have the right equipment.
    S6S 116 w/VGs|MGL/FDS/AoA
    114hp zipper912|homebuilt headers/muffler/ram-air
    75"WW STOL|22x8.50mains/21x8.00nose/custom fork
    YouTube Channel

  6. #6
    Premium Member (Donated) scsirob's Avatar
    Join Date
    Aug 2008
    Location
    Netherlands
    Posts
    426

    Default

    Quote Originally Posted by GravityKnight View Post
    ... Kind of scratching my head, and you can't get bigger than 160 jets I don't think
    The BING carbs support much larger jets. Jabiru runs the 3300 with a single BING 94. These have 245 - 255 main jets, I even have a 280 somewhere. I believe Rotax are Bing 64? Very similar carbs.

    The number is directy related to the hole size. 160 = 1.60 mm diameter. If you want a 170, just drill with 1.70mm. Make sure to deburr and engrave.. Get a load of small ones and experiment.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •