It is currently Mon May 29, 2017 6:50 pm

All times are UTC





Post new topic Reply to topic  [ 9 posts ] 
Author Message
 Post subject: weber manifold "what if ?"
PostPosted: Tue Mar 28, 2017 8:50 pm 
Offline

Joined: Wed Mar 09, 2016 12:47 am
Posts: 17
Location: USA
model: 906 Paso
year: 1990
So, now that my bike is completed and running well I can turn my mind to more theoretical and unimportant things. I've often wondered why the weber manifold is split instead of open inside to make a common plenum for both barrels of the carb to feed. Seems to me that it would eliminate the need for synchronization if they were both playing for the same team. Has anyone ever experimented with removal of the webbing inside the manifold to create a common plenum chamber? What part of the carburetion black magic equation am I missing?


Top
 Profile  
Reply with quote  
 Post subject: Re: weber manifold "what if ?"
PostPosted: Tue Mar 28, 2017 10:30 pm 
Offline

Joined: Fri May 01, 2015 9:31 pm
Posts: 53
Location: Australia
model: 907 I.E.
year: 1991
I've heard that runner length can impact the speed of the column of air and thus its momentum into the cylinder, impacting on its filling of the cylinder. Or at least people I know who design for OEM have told me. It's an aerodynamic trade off thing.

but that said I'm with you, i would have thought there is merit in creating a crossover to account for carb imbalance. It would mean that you can't run a different tune on different cylinders though. on the later injected bikes they do run a slightly richer mixture on the rear cylinder for additional cooling. I've played with the tuning on my 907 (myECU) and with my laser thermometer I don't notice a major difference running the rear cylinder 2% richer.

Someone out there must have tried it. what was the outcome?


Top
 Profile  
Reply with quote  
 Post subject: Re: weber manifold "what if ?"
PostPosted: Wed Mar 29, 2017 12:27 am 
Offline

Joined: Wed Mar 09, 2016 12:47 am
Posts: 17
Location: USA
model: 906 Paso
year: 1990
Just spoke with my brother who used to be a nascar cyinder head guy and now makes custom race exhausts and also happens to be a bit of a mechanical genius. He reminded me that even in a well built engine every cylinder has it's own unique air/fuel requirements. For example, he said, that in a nascar v8 the number 7 cylinder is almost always leaner than the others. considering the very different conditions that the 2 ducati cylinders must operate in because of their different orientation, it's easy to see why they would have different tuning requirements.


Top
 Profile  
Reply with quote  
 Post subject: Re: weber manifold "what if ?"
PostPosted: Wed Mar 29, 2017 11:24 pm 
Offline
paso grand pooh-bah
User avatar

Joined: Wed Dec 05, 2007 12:35 am
Posts: 1502
Location: Newzealand
model: 906 Paso
year: 1990
Yeah but .... the whole idea of individual manifold runners is that there is no interference from adjacent cylinders. The sort of manifold you speak of is called an interference manifold and usually any 2 ( or more ) barrel carb usually have a primary and secondary barrels to provide adequate velocity at low and high revs respectively , which is its advantage. It would probly be less of an issue on something with an even firing order .... like the old British 360 deg twins . I also think you would have a shit of a job getting the weber to behave without some serious #ucking about :D

_________________
I wish I was young again............Id be heaps smarter than last time


Top
 Profile  
Reply with quote  
 Post subject: Re: weber manifold "what if ?"
PostPosted: Wed Mar 29, 2017 11:52 pm 
Offline

Joined: Fri May 01, 2015 9:31 pm
Posts: 53
Location: Australia
model: 907 I.E.
year: 1991
Mc tool wrote:
Yeah but .... the whole idea of individual manifold runners is that there is no interference from adjacent cylinders. The sort of manifold you speak of is called an interference manifold and usually any 2 ( or more ) barrel carb usually have a primary and secondary barrels to provide adequate velocity at low and high revs respectively , which is its advantage. It would probly be less of an issue on something with an even firing order .... like the old British 360 deg twins . I also think you would have a shit of a job getting the weber to behave without some serious #ucking about :D


I accept everything you said and certainly wouldn't suggest an interference manifold would provide "performance" benefits... but it occurred to me that my injected 907 for which i can customise the tune for each cylinder runs nominally the same tune. Of course the injection and spark is timed for each cylinder and with the Ducati not having an even firing order then looking at a pulse view you would think that the manifold would potentially go from an overly lean to a rich mixture with each engine cycle (if, as in injection, the amount of fuel is fixed by the computer) but in a carb the vacuum would pull more fuel out of the carb... (and assuming the carb can fuel instantly in response to vaccum)

So why would you do it? Potentially you could use smaller primaries because both carbs are providing capacity for the engine... so if you had air speed issues (such as poor throttle response at low rpm/low accelerator) then the response of two "smaller" carbs (primaries) might give you more consistent response than a single larger carb (in the carb per cylinder model). At WOT you would still be limited by the secondaries can provide (and assuming they are the limiting factor).

As a counter argument though... i've played with intake runner length on old valiants and with everything else being the same (engine, carb) I got better drivability (throttle response) with a long runner intake manifold than i did with the short runner intake manifold.


Top
 Profile  
Reply with quote  
 Post subject: Re: weber manifold "what if ?"
PostPosted: Thu Mar 30, 2017 4:31 am 
Offline
paso grand pooh-bah
User avatar

Joined: Wed Dec 05, 2007 12:35 am
Posts: 1502
Location: Newzealand
model: 906 Paso
year: 1990
yep , check out this Mopar cross ram manifold for dedication to the long intake runner :)


Attachments:
Chryslercrossram_01_700.jpg
Chryslercrossram_01_700.jpg [ 118.63 KiB | Viewed 368 times ]

_________________
I wish I was young again............Id be heaps smarter than last time
Top
 Profile  
Reply with quote  
 Post subject: Re: weber manifold "what if ?"
PostPosted: Sat Apr 01, 2017 4:05 pm 
Offline
paso grand pooh-bah
User avatar

Joined: Sun Aug 01, 2004 12:00 am
Posts: 1090
Location: siena, italy
model: 750 Paso
year: 1988
You Just Need to buy another mainfold and cut a part of the diaphram between folds to see what happens.

_________________
have a nice ride, Nicola

Black "DUKE" 751582
ex...Red "smooth" 753349 :-(


Top
 Profile  
Reply with quote  
 Post subject: Re: weber manifold "what if ?"
PostPosted: Wed Apr 05, 2017 6:26 pm 
Offline
User avatar

Joined: Wed Oct 06, 2004 12:00 am
Posts: 315
Location: Nottingham, UK
model: 906 Paso
year: 1989
I doubt 'opening up' the manifold would aid performance in any way. Imagine that this were so, then air speed thru each throttle body would be reduced, leading to fuel mixture problems due to the reduced vacuum.
Incidently, running the rear cylinder richer (cooler) is most likely due to it being heated by the radiator whereas the horizontal cylinder is not.


Top
 Profile  
Reply with quote  
 Post subject: Re: weber manifold "what if ?"
PostPosted: Thu Apr 06, 2017 6:33 am 
Offline

Joined: Fri May 01, 2015 9:31 pm
Posts: 53
Location: Australia
model: 907 I.E.
year: 1991
JWilliam wrote:
Imagine that this were so, then air speed thru each throttle body would be reduced, leading to fuel mixture problems due to the reduced vacuum.
Incidently, running the rear cylinder richer (cooler) is most likely due to it being heated by the radiator whereas the horizontal cylinder is not.


Yes, I'd agree you would probably end up looking for smaller primaries to better match the operating situation. I was just wondering if that an interference manifold and suitable carbies would address the flat spot issue.


Top
 Profile  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 9 posts ] 

All times are UTC


Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to: