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I was reading your response in this thread https://www.ducati.ms/forums/57-supersport/719183-split-fcrs-both-downdraft-can-i-still-use-malossis.html and it raised a question for me.

I am about to start refreshing my 900SS engine. It came to me with a 944 kit installed by FBF, with stock carbs and knock-off end cans. I recently purchased some ported heads with ST2 cams, I have a set of racked FCR 41s, and a spiffy spaghetti system with high mounts. Your response was discussing the differences in bikes with racked vs. split FCRs, and you mentioned a customer bike with 950, ST2s, and splits not making the mid-range power of your 950 with 900 carby cams and racked fcr's.

I love the low down grunt and mid-range the SS gives, so from your experience it sounds like my plan will lead to less of what I like, correct? I have been thinking of moving down to FCR 39s as I understand these will provide better low down and mid-range. Thoughts?
 

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It is hard to directly compare because there are a number of variables as well as various reasons why we want what we want. These things often do not match up so we have to make a decision.


I have attached a few dyno run comparisons but they are not all the same bike and they have other variables so take it as one data point NOT the answer. If you compare them you will get an idea where I get my belief that there are situations that the rider would see more benefit with one vs the other. Just my experience and opinions so take it for what is worth off of the internet.

Lets look at
#1 a 800 monster motor I fitted in a carby monster 750 vs a monster S2r800.
The carby has long manifolds and those stock "junk" 38mm CV carbs and the S2r800 has 45mm throttle bodies and short manifolds much like individual carbs. The long manifold bike with the same motor makes much more mid range and low end power and the short manifold is better at high rpms. If you spend much time above 6500rpms the s2r will be the better bike. If not you will be slower than that carby. Once again application matters.

#2 Is my monster 950 with a rack of 39's an open airbox and a custom (well made ) exhaust. I have blueprinted the motor when I built it but it was made to be a street bike first so nothing crazy just a good clean build. The 944ss is a motor the customer says Bruce Meyers built as a 944 kitted motor only as well, no porting,cams or special tricks on either.On the down side for the 944ss is a custom exhaust that while well made is not made for this bike so the size of tubing I believe is much too large and it hurts the midrange.

In this case you can see the long manifold racks and smaller carbs were better everywhere than what on paper should be better in some peoples experience. I only know what I see.

#3 Added to the other two I put in another 950ss that has a proper exhaust split singles, 39mm fcr's but also porting and st2 cams. This is the green run and it like the monster 800 comparison shows that the short manifolds needs help to make them worth while. The porting and more aggressive cams give the boost up high that the 944ss does not have as well as a exhaust that works with you instead of against you.

I cannot help you decide because I have been there. some times we make decisions based on more of a look or emotion than outright performance and that is okay. I have tons of experience with 41mm and 39mm carbs and I know which I prefer on MOST of my builds. I do have a set of split single 41's waiting for my 985 motor and that should be a good fit. Otherwise I prefer the snappiness of the 39's. yes there might be another 1 or 2 hp with bigger carbs in some instances but I spend more time at part throttle and want a crispness they do just fine.

Long vs short?
Application.
daily driver or midrange mostly and I will take a rack every time. Better midrange and less maintenance plus you can use the high idle whip for start up.

Custom, race bike on a track where Hp is king or you simply love the look that splits give the bike.

honestly riding you will not know much different so if you have nothing to compare it to it really matters little. If I had a set in hand I would use what I had and be happy. If I was buying new I would start by asking What am I going to do with this bike? I have different bikes with different purposes and they all have a place just get you mind to be clear on what you are doing and what you want.
 

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Discussion Starter · #3 ·
Thanks for the info dyno runs!
 

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eric, have you noticed that 39 on a 900 or larger gives a flatter air/fuel curve than the 41, which have a leaner hump in the middle?
 

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It is hard to get direct comparisons as there are too many variables once again but what you asked is a good question so I went and looked at some of the runs I do have and saw little difference in air fuel between the 39's and 41's. It seems to be a matter of what circuit you can effect and what range you can adjust, there have been times where the main air jet was smaller than we wanted so drilling was required. I find the larger factor is the rest of the bike like the 944 with split 41's having too large an exhaust so the bike was wanting fuel that was MUCH richer than normal. A case of getting too attached to a part, it is a beautiful system but should be on a built 1000 not a mild built 944.
 

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Bruce had mentioned opening up the air jets and improving the curve, so I tried it. It did not work for me so there may have been a different configuration that made it work on his and not on mine, since you seem to have had similar results as mine I can guess I have some spare air jets that I will likely never use.

I do have a unit that someone made once upon a time, it is a aluminum block that ties the air jets together and has an adjustment knob. The customer who gave it to me swore it worked well but without dyno data I am skeptical. One of these days I will get the free time to install it on one of mine and play with it to see if it is snake oil or worth making another, free time....lol.

I wonder if the richness at the end of the runs is more about the long manifolds simply not passing enough air at high rpms and the mixture going rich. I can tell you when I built my 853 2-valve motor, but still ran the oem carbs and long manifolds you would hear a airbox resonance at high RPM's that sounded almost like the dry clutch groan. First few times I heard it I thought that the motor was seizing , after installing the split singles The noise was gone. I do not have dyno data from that time frame as it was one of BCM's dynos, I should have copied all runs when I left.
 

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the noise bit sounds interesting, but i guess it's choking (flow going supersonic maybe) and that's a pressure thing.

i wonder if pulling the air jets out would show any change. that would at least show if the 200 air jet is the biggest jet the carbs will respond to.
 

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Very good question on simply removing the air jets and measuring the change. I will try and remember the next time I am playing with a set , because inquiring minds want to know. Sadly I am not looking at any FCR bikes in the shop except my own and I get no time to play with my own stuff these days. I do have a growing list of things I want to try to get a better idea of the gains without overlapping mods, but as you are aware this stuff does not keep the lights on so it is only what little free time there is.

It is all fuel injection as far as the eye can see.
 
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