What if the carbs use different shaped intake manifolds? - Ducati.ms - The Ultimate Ducati Forum
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post #1 of 8 (permalink) Old Apr 18th, 2019, 12:30 pm Thread Starter
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What if the carbs use different shaped intake manifolds?

Hey ,guys, my 900cc 2v engine has a new keihin fcr 40 carbs from yamaha yfz450. They need a custom manifolds since they are designed for vertical montage. This requires different shape of the manifolds one from each other. The manifold for the horizontal cylinder will be angled by 90 degrees while the vertical one will be almost a straight tube - 180 degrees. I've seen this combination on the old ducatis


where the manifolds are also different. Unfortunately the vertical Keihins can't be put on a short manifolds.

So my question is how does the different angled manifolds reflect on the engine performance /I will keep the air volume the same in both of them/.
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post #2 of 8 (permalink) Old Apr 21st, 2019, 5:03 am
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The length and shape will affect the resonant frequency and sharp bends will cause lots of turbulence and effect flow. Generally you want both manifolds the same for even fuelling but obviously you can't. All you can do is keep the bends smooth and maybe you can put a bend or two in the vertical to get it similar to the horizontal.

If you cant fix it with a hammer, its probably electrical.
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post #3 of 8 (permalink) Old Apr 21st, 2019, 6:20 am
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you should be able to run it on some sort of angle. i'd poke it across from the lh front, below the other carb ish. angle that one up a bit.

manifolds will be what they will.

or just get some carbs that fit where they should. fitting something fucked up just because you have them isn't starting in the best place.

know all wanker
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post #4 of 8 (permalink) Old Apr 21st, 2019, 7:26 am
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That 90* bend you show is probably death to building decent power.
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post #5 of 8 (permalink) Old Apr 21st, 2019, 10:14 am
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OP using side draft carbs where was originally a factory downdraft application, forcing radical changes.

A redesign of manifold configuration will end up with endless experimentation and redo's....then he's got to discern between manifold and carburetion performance issues going forward. This is why factories employ engineering departments.

Ya wanna work on it forever? Have fun. Or ride? If so...return to factory spec stuff.

I do respect, however....his bike, his time, his efforts, his choice.

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post #6 of 8 (permalink) Old Apr 21st, 2019, 10:44 am
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Lol... great advice from all, But he is head long into this mess and why stop now? I would not expect Great results but I have been surprised before and you seem to have already spent the money so what have you got to lose but your time at this point. You will want to find a dyno nearby because you have made a different enough change that standard jetting (including needles) will probably need changing.

Bends like that in manifolds are not a good idea for power but the long manifolds are good for midrange power so you are building a midrange motor 4000-7500rpm. Save your porting money as you would gain little from it where you will make power. If that is fine to you I would start by re-making the horizontal manifold to hold that side draft at the steepest angle you can get away with try and NOT run it flat as it is sacrificing the manifold shape too much.

Start with the stock oem manifold for the carby and see if you can get the carb to work there and if not slowly flatten things out, I bet you can get a better angle. The vertical will be easier to manifold but also try and keep bends smooth and at a minimum. know that different shape manifolds will fuel different so you will need different jetting for each probably, have the Dyno pull air fuel from each exhaust head pipe NOT the tail pipe after a merge.

I would expect the carbs to work but I would also expect a lot of R&D time/money spent, if you are doing it for fun why not and be sure to let us see those dyno runs after so we can learn from your adventure.

Just for those who read this later in a search: some of the reason a certain carb is used as there is much data on how to make it work so time and money spent on R&D can be less. some times the R&D is fun and you have access to parts you want to try. often though when you try something "new" you find it costs more than buying what is a expensive part but easier install. I have seen a number of combinations that were run and often they work fine with enough work put into them but they are seldom cheaper in the end. Some of us still do like to ask the question "why Not?"
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post #7 of 8 (permalink) Old Apr 22nd, 2019, 8:14 am
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Iíve used flexible exhaust pipe sections to form the prototype for intake or exhaust manifolds in the past. It wonít bend a whole lot but you donít want tight bends anyway. It will hold the shape youíve configured to use as a pattern to make a manifold from suitable material. Get a size as close as you can to what youíll be using.

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post #8 of 8 (permalink) Old Apr 26th, 2019, 11:40 am Thread Starter
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Thank you for the advices people. I don't know why I saw them with such a delay. I already gave my wax models for casting. So whatever happens happens. I still have a lot of work to the outlook of the bike ,so there is still time till the dyno tests. I hope I will be able to finish it soon and put the results here. It will be a good experience.
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