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

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    Premium Member (Donated) GravityKnight's Avatar
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    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

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    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

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    Nice Todd, very impressive

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    Premium Member (Donated) s19flyer's Avatar
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    Nice!

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

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    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

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    Premium Member (Donated) scsirob's Avatar
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    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.

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    So I drilled out two sets of jets. I bought 1.62mm, 1.63mm, 1.64mm etc. drill bits. Started with 1.63mm, too big, too rich. Tried the 1.62, still to big and rich, slightly better though, but one of two things is happening.... The bits measure out smaller than the labeled size I'm assuming because drill bits always make a larger hole, but they seem to be coming out a little large, even using a lathe to drive them through centered and controlled... or bing's sizes are not quite accurate on their jets. I'll verify this when the 162 jets I ordered show up friday, (thanks for the tip on other bing jets, found some 162's that should be just right without drilling!!) my guess is that even the 1.61mm bit won't go through the 162 bing jet. Weird.... tired of changing main jets to be honest so I just ordered what should work and hopefully that will get me by until I finish my BIG project and get rid of the bings altogether



    Some materials I've collected. I've been wanting to do this for a couple years. Gave it some thought a while back, and decided it was too much work, and I plan to sell this plane in the next year or two anyway. Then after fighting with how bad the bings and crappy un-even intake manifolds divide fuel from front to rear cylinders I decided I couldn't take it anymore. If it works, I'll have a system figured out for the next plane too.

    Part of the motivation was after heavily "clocking" the carburetors, I was able to get pretty even EGTs in cruise (Wide open is still a real bad split), and when I had the cruise split just right and EGT's within a few degrees across all cylinders I could get another 2-3mph cruise or more at the same fuel flow. Was indicating 105mph, at 4gph, level flight, with a finely pitched prop (rpm 5500+) with these tires and other crap hanging off my plane that is pretty damn good. Being able to lean all cylinders together without 2 of them getting too lean, made for some real efficiency improvements... full power would benefit greatly if I could bring them all together as well! But I can't with this crappy design.

    I figured I was nuts, and my ideas and designs in my head wouldn't work, until I saw that someone else had done almost the same thing I want to do! http://www.hangardelcielo.com/fotos_vi.html

    My design is different, and shouldn't require moving the ignition, coolant tank etc. I'll bring the injector back further under the radiator and keep everything where it is, I'm also not using the aero injector they have on their setup. The Ellison TBI unit is far superious in my opinion after doing research. Or the Rotec TBI (basically a copy of the Ellison). The spray bar with lots of progressively larger holes really helps with atomization. Not as good as fuel injection, but very simple, and superior to a float carb.



    I've been researching the aero injector, revflow carb, ellison and rotec units for the past few weeks. I like the Ellison and Rotec.

    Anyway, I'll start another thread when the building is underway. I'm sure it will turn out to be a HUGE pain in the behind to get this to work properly, as these spray bars often times require enlarging holes in the spray bar to fine tune the mixture. But I have the proper instruments to dial it in properly (wideband o2, 4 egts etc.). Should be fun. There are countless reports of people picking up 150-200 static rpm on lycomings and nice fuel savings at cruise etc. Just evening up EGT's would account for some towards both those.
    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

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    Any tips on the other EGT placement? I'm wanting to do that.
    I bought and did not install a Rotec. Got cold feet after reading some negative reviews. I still have it and would like something better than the Bing carbs, so if you tame the Rotec, I'd like to hear your tips.

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    Premium Member (Donated) GravityKnight's Avatar
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    Which model Rotec did you buy?
    If you decide to let it go, let me know, I have not bought one yet. I talked with Rotec though and they suggested any of the 40mm models would work fine as a single unit / 115hp+ hp
    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

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    Mine says TBI Mark II Fuel System. It looks similar to but is not identical to the one in your picture.

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