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Discussion Starter #1
Hi,

Has anyone made up their own intake manifold (ie by grafting/welding a 105 manifold to a Twin Spark manifold) in order to convert a Twin Spark to run on twin Webers or individual throttle bodies?

If so, how did you block off the injector ports in Twin Spark portion of the intake manifold, and how did you make provision for a vacuum connection in the fabricated manifold for the brake booster (eg use one of the injector ports)?

I realise custom intake manifolds for this conversion are available for purchase, but I'd like to have a go building my own, if it's feasible.

Regards,


Nick
 

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Discussion Starter #3
Hi AvorioVeloce,

Yes... I have a friend who is very handy with a TIG welder, so if he/we can make up our own manifold by welding the carb side of a 105 intake manifold to the engine side of a Twin Spark manifold, that would be great. I'm hoping someone else has already done this and can provide some tips and perhaps a photo or two.

Regards,


Nick
 

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I did it back in about 1998 before you could buy such an animal. Just made up some plates that matched the webers and welded them on to the twin spark manifold stubs. Then welded up the injector ports and faced it all on a mill to take any buckling out of it. The brake vacuum fitting I can't remember what I did so it musn't have been a drama.

Helped a lot when you work in the right place and have all the right gear and skills.
 

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Discussion Starter #6
Hi hughqv,

Yes, I've seen that thread. Very impressive and inspiring. The photos of the intake manifold and accompanying text are both good, but I was hoping somone might have a photo of a fabricated manifold before it has been installed.

Hi Clayton105,

Thanks for the tip on milling the intake to counter any warpage from the welding process... a very good point. Did you have to take much off to get the mating surface flat (ie was there much warping)?

Do you think tapping/threading the injector ports and sealing them up with a large grub-style screw would do the job (ie as opposed to welding them up)?

My thinking is that I could install a one-way valve in the now threaded injector port near the firewall, and then use this for the brake booster vacuum connection.

Regards,


Nick
 

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It is a very long time ago, but I don't remember it being a huge amount.

I also remember that I did incorporate the factory rubber isolator mounts. I believe that in most cases, Mr Alfa did things for a purpose and you should stick closely to it unless you have a very good reason to deviate.

I welded and dressed the injector ports before beadblasting the manifold to disguise the fact that it was once a fuel injected engine. I also welded on the standard 105 thermostat assembly so that that all coolant hoses used were standard. The car had an fabricated aluminium cold air box (GTA style) with curved GTA trumpets etc. I basically wanted the engine bay to look like it came from the factory like that.
 

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TS inlet center bores are exactly the same .. just mashine 4 flanges clock them to accept rubber mounts and weld to TS inlet :cool:
note: TS inlet aluminium have high magnesium somponent and its little tricky to weld ... perfect clean and bit of preheat easy the job .... no warpage at all ;)
 

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There is a company here in the States that sells Weber mounting plates. See the ebay auction at: Weber DCOE or Dellorto DHLA mounting flange - PAIR | eBay

I know you have access to a welder, so if you also have a machinist you could obviously make your own plates to weld to the TS manifold. But this seems like a reasonably-priced ($37 US for the pair) solution for people without a laser cutter and milling machine in their garage.
 

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Out of curiosity, does anyone know where to get a manifold like this one? It looks like the angle is higher up than the Alfaholics equivalent, just like the GTAm?

 

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Building manifold

It`s not difficult to build your own. Some tips:
Use one of the single piece mounting plates offered on ebay. They have adequate spacing between the carbs for the stock type syncro linkage enen when using larger Webers that commonly won`t fit with stock Alfa manifold spacing. I used these building both the Montreal manifolds and the Duetto`s high port manifold. (see post 184 at:
http://www.alfabb.com/bb/forums/motorsports/146464-old-alfa-racer-pics-so-west-div-3.html.
The single pieces make carb alignment easy, and the center can be cut away after welding is completed to clear linkage if necessary.

If you have a mill to remove all of the excess castings from a FI manifold then runners can be custom built to improve entry and tune length to enhance RPMs as desired for engine application.
It is easy enough to buy prebent tubing of whatever diameter to weld in runners, or even to match large carb ports and taper to smaller diameter to match correct diameter runners at the head. (split large tubing and reweld the sectioned part.)

The Montreal manifold build is well covered starting about post 134 at:
http://www.alfabb.com/bb/forums/spider-1966-up/74007-spider-improvement-effort-5.html
 

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Oh I see. Devastating!! Does that mean the 155 head has a more 'inclined' port angle compared to the 75 head?
 

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Discussion Starter #16
As shown in the below photo, this is what I ended up doing with my custom Twin Spark intake manifold.

I had the injector holes threaded so that the three front holes are plugged with threaded bolts. The rear injector hole now takes the stock Alfa Romeo one-way valve to provide a vacuum source for the brake booster.

I cut the runner tubes from the underside of the Twin Spark plenum chamber, inverted them so they provided a slight "S" shaped upsweep, and had the runners TIG welded onto the Twin Spark intake manifold. I needed the slight upsweep (ie GTA style) so that the twin carbs will clear the clutch master cylinder on the RHD Alfetta pedal box I'm using (see photo).

I purchased a pair of 40mm DCOE laser cut flanges, and had these TIG welded onto the plenum runners. I'm using rubber "O" ring isolators between the carbs and the new intake manifold.
 

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If those are horizontal and it's on a 155 head, then the airflow makes a sharp angle at the head-manifold interface. That might not be the best way to do it. Do webbers have to be horizontal? If so, maybe the 155 is not the head to use with carbs.
 

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Hi Stefano;
I'm not an automotive engine flow engineer.

If those are horizontal and it's on a 155 head, then the airflow makes a sharp angle at the head-manifold interface. That might not be the best way to do it. Do webbers have to be horizontal? If so, maybe the 155 is not the head to use with carbs.

Can you elaborate? Not trying to sound rude, but if you have a special insight based on a science based response, I'd like to understand.
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Wes:

No I'm not a flow engineer either, but fluid dynamics was a core course for me once upon a time. I did say "might" which invites others to give their experience.

What I do know is that flowing fluids do not like sharp transitions. There have been exhaustive discussions about exhaust port angles when tubular headers are being designed. Those principles would apply directly to intake as well. The 155 intake angle is very steep and is one of the reasons this head is better than the 75. See the attached image. If you come in at a horizontal angle and then make a sharp transition into the ports, would the 155 be any better than the 75 at that point? I cannot believe it could be, as any benefit would be lost. And considering the shock wave due to the bouncing air, maybe worse (I do say maybe).

If someone has done it and can confirm this does not matter, I can believe that, but until then I have to assume a sharp angle change is a detriment to air flow.
 

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