Designing a single carb manifold. Need flange dimensions.

[chuck c]

dcryder » Wed Apr 23, 2014 10:58 pm[/url]":3uspn0pn]
How much?

Don't hold me to it, I have to get a solid quote on it but I think about $250. That's made by a pro shop by certified welders.

 

[chuck c]

chuck c » Wed Apr 23, 2014 12:26 pm[/url]":lpeu1vy6]
1-1/4"

Correction: 1.5" They also have the tooling to put a "bead" on the ends of the tubes as you see on radiators outlets and such. That will help get a good seal.

 

[chuck c]

Drat! There's a major flaw in my design. Parts of the linkage on the Solex goes below the flange! I can't use that big flat plate. It must have a raised boss or something to sit on. Back to the drawing board.

 

[Ansimp]

Can you build your design around the two barrel to one barrel style adaptor with a mounting for the solex?

 

[chuck c]

That would make it easy to switch carbs just by using an adapter to match up to the top of the plenum. It also makes it cost more. I need to think about it but I really need to get my hands on both carbs to do it.

 

[chuck c]

Latest version showing the angle of the runners relative to the intakes, beads on the ends of the runners, and a little personalization.

 

[chuck c]

I got to measure the carb assembly today and get those dimensions into the model. The runners are 1.5" expanded to 1.6" at the ends. The plenum is about 3.25" hexagon. Now when I get the carb I can finalize the design. It doesn't look very good but it's functional. I may add a boxy heat shield to enclose the plenum for better appearance and to hold some warm air around it.

 

[joedrum]

i like your design a lot ... the way it done it will bring in very close runner lengths .. about as good as it gets ... i think for sure that you should incorporate the one to 2 trainsition piece and use the DFT carb for the heart of the project ... the oldwing motor is a very wide range rpm motor ...and the 2 stage weber fits this situation perfectly ...it is my opinion that the one barrel carb lack the ability to to cover the rpm zone that an oldwing has as well ... but thats just my opinion here ... and if it was me id just flat plate it as you had design and from there use 1 to 2 adapter from there ... i think this would raise the carb off the floor center the charge ..speed the air right at carb and linkage is no problem what so ever .... i been into this for quite some time and its looking like the beenies from this keep adding up .... i like the pic from post 19 chuck... that looks great to me and transition adaptor from there ... this will keep the carb sticking up to much ...by using the intake horns you are going to rather tall off the motor in the first place...... and use the room you have to its most .....

 

[chuck c]

The plan was to use press-in 8mm studs pointing up. I'll have to press the ones out of the carb. This way you can drop it down over the studs and get a washer and nut on from the top. BUT from looking at the Solex you can't get to the studs from the top anyway! It's going to make getting the carb on really difficult. That's no good. So now I'm back to my earlier idea of a large opening in the plenum and an adapter plate you attach to the carb first, then lower it onto studs pointing up. That means another gasket which I do not like but may have no choice.

The adapter would make it much easier to switch between carbs. I could start with the Solex and switch it to a Weber later without a total redesign or even taking it off the bike. 4 nuts and it's off. It would also make it easy for a DIY guy to mount any carb he wanted to try. I could sell a blank plate you can cut to fit your carb. All this drives up the price. However...... that's if it's made out of metal. PVC is an option. It would be much cheaper and would not have the cooling effect of metal making it less likely to freeze. I'd just have to be sure it could stand the heat.

 

[slabghost]

1.5" might be a bit big for runners. It has been done and works but so have 1" runners. The smaller runners will speed the charge over the larger runners.

 

[chuck c]

The other side of that argument is the longer runners are more restrictive than the original setup slowing throttle response, so bigger diameter helps counter that. The id of 1.5" tube is darned close to 34mm.

 

[dan filipi]

I've been very surprised at the manifold designs and runners that work. Your design looks good.

 

[slabghost]

chuck c » Tue May 06, 2014 7:16 am[/url]":tl2wzlyt]
The other side of that argument is the longer runners are more restrictive than the original setup slowing throttle response, so bigger diameter helps counter that. The id of 1.5" tube is darned close to 34mm.

Keep in mind the original set up was barely a 3" long elbow between carb and valve. Bigger diameter can slow the flow and throttle response. For our application the best is yet to be verified though. Go for it.

 

[chuck c]

Let's think about this. The total length of the air path is about the same as the OEM. The big difference is the venturi and throttle plate move from about 3" from the valve to maybe 9". That means the piston intake stroke has to pull air from about 3 times the distance. Air in a tube is like a spring. Pull one end and a ripple works its way to the other end until it's equal all along the path. The OEM carbs keep that very short so there's very little of that. I think the vacuum will be higher in the longer runner given the same ID. So by upping the ID a bit you drop that resistance back to more like the OEM. Does that make sense? It increases the volume of air the piston has to draw from. It also gives the fuel more time to vaporize evenly. Also I know we aren't dealing with just one stroke, there will be a constant flow and moving air has momentum. It doesn't take many variables for this to become a mind-boggling engineering problem that car companies have spent millions trying to optimize!

The only way to know would be to try different versions on the same bike and run it on a dyno each time. That ain't gonna happen. We do know that simple manifolds work. If I can get the runners as close to equal as possible in length and resistance to flow it should beat the problem of cylinders getting unequal charges and some running rich and others too lean. I think that's the biggest killer of performance and efficiency.

There's one thing I can check, that's the vacuum on each runner. If they are pretty close to the same, even if they are a bit different form OEM, it should work very well.

 

[dan filipi]

I checked vacuum at each runner and they were all the same because they all tie to the same center.
Vac reading was the same at the runners as the center vacuum.

 

[chuck c]

dan filipi » Tue May 06, 2014 10:49 am[/url]":302l4mo0]
I checked vacuum at each runner and they were all the same because they all tie to the same center.
Vac reading was the same at the runners as the center vacuum.

Hmmm. Yeah, that make sense. Vacuum should be the same everywhere in the manifold. Duh. Is there any other way to check for equal flow? Like, a bench of some kind.....

https://en.wikipedia.org/wiki/Air_flow_bench

 

[chuck c]

HEY! :doh: I'm an idiot! I use SolidWorks every day and it has a flow simulation software add-on! I bet I can get a free 30 day trial!

 

[ekvh]

Wouldn't vacuum slowly deteriorate due to additional friction losses along the walls? Not much, but a bit. You would have more ramming effect because you have more volume and therefore mass, pushing it in the cylinder. I wish there was something that could be done to change the nearly 180 degree turn it has to make. That must be the advantage to the Weber's like Randakk and others use. With the single carb its almost 270 degrees of turns. I think every time a charge negotiates a turn it changes shape.

 

[chuck c]

ekvh » Tue May 06, 2014 4:10 pm[/url]":3sv8zzox]
Wouldn't vacuum slowly deteriorate due to additional friction losses along the walls? Not much, but a bit. You would have more ramming effect because you have more volume and therefore mass, pushing it in the cylinder. I wish there was something that could be done to change the nearly 180 degree turn it has to make. That must be the advantage to the Weber's like Randakk and others use. With the single carb its almost 270 degrees of turns. I think every time a charge negotiates a turn it changes shape.

I makes sense to me that vacuum would be slightly stronger right next to the head than at the carb, but only a tiny bit. I can also believe the straighter the path, the less that would be. In the end I don't think it hurts that much. Maybe only on the top end. Maybe measuring and comparing vacuum right under the carb and at the head might be a simple indicator of how much drag the path causes, if it's even enough to measure with a standard mechanic's gauge. In that case, if it can be picked up with a vac gauge, you'd want it to be equal on all 4. I'm fixated on the idea that keeping all four cylinders fed the same is vital.

 

[Ansimp]

I don't think that intake manifold length is that critical , if you look at many straight six cylinder manifolds the centre 2 cylinders are right next to the carby and the rest just get further away ( longer runners). With all my Subaru experience ( and early VW ) I would say the longer the runners the more critical choke and manifold heating is. I have an EA81 in my 74 Kombi without any choke ( don't ask) and I start it by 2 pumps on accelerator pump and just let it run ( barely idles ) until I can feed is some throttle. It often stalls while it is warming up and I have to repeat the process, if I lived somewhere colder I would have to have a working choke to get it to run.