tell me what you know about blow through carbs

[mb911]

We are looking at doing a project on a 72 where it would have an old school turbo setup. I am looking for any info I can so I am prepared for pitfalls. Thanks in advance

[Jim2]

Porsche or something else?

I built a number of Toyota and Datsuns using 32/36 DGV/DFVs. Some side drafts too but no down draught carbs. I recall fussy jetting running one barrel per cylinder versus plenum distribution manifolds. Some carbs were difficult to seal off choke or enrichment components. Also discovered that float bowls would drain faster than neelde and seat could accomodate. Drill out needle and seat too much and you overcome float bouancy, especially when cornering. These used rising rate FPRs. Generally the blow-through applications worked, but some variations were limited or just fussy.

[copbait73]

I prefer the pressure carb route over pull thru.

I agree wth Jim the progressive two barrels were the best.

[mb911]

2.4 porsche 6 would be the test mule We are thinking carbs but could be swayed any way as long as it looks close to stock

[copbait73]

Look closely at this pic from the late 70s.



This 2.0L had W/G, mechanical blow off and water CAC.

With this experience behind me I'm doing the 73.5 2.4L CIS.
http://forums.pelicanparts.com/showthread.php?t=478216

[copbait73]

Originally Posted by mb911

2.4 porsche 6 would be the test mule We are thinking carbs but could be swayed any way as long as it looks close to stock

I say 2.4T MFI, 7.5:1 C.R. Most have attractive price but they are getting harder to find with original MFI.

It's my experience if you fab up your own manometer system so you can tweak the injection air screws every 1000 miles most MFI are OK. Saves spending thousands of dollars having them rebuilt.

[Quicksilver]

There are 2 basic methods of doing the pressure on a blowthrough setup.
- Pressurize all the openings of the carb. (This may be easier to do)
- Pressurize a box that completely contains the carb. (This is more complete)

Pressurizing "all the openings" is popular with a lot of carbs because all of the openings are contained inside the circular aircleaner opening and you can do it with just the basic tube(s) going to the intake. The only other places to seal are butterfly and choke shafts. There are lots of attempts to seal the shafts with o-rings, etc but the easy way is to drill holes into the side of the shaft bores and run the boost pressure into the holes. It will leak a bit of air but no mixture will come out.

The only other port into the carb is the gas and that is pretty easy. You just need to know the pressure requirements of the carb and put on a pressure regulator that is referenced to the boost pressure. For example: If the carb needs 5psi to feed the carb then you need a regulator to always feed boost pressure + 5psi.

The first internal worry is hollow floats because boost pressure can collapse the floats so it is common to switch to foam floats if available.

I would seriously recommend some sort of "throttle off" pressure-relief/dump-valve. When you let off the throttle under boost the pressure spike upstream of the carb can be a problem. Also releasing the pressure can help keep the turbos spooled up.

Blow through used to open up the selection choice of turbos because a lot of diesels turbos don't have seals that would deal with intake vacuum. There are so many choices nowadays that is doesn't make a difference.

If I was looking at doing this I would try talking to Gale Banks to see if I could pickup some useful tips.