Paxton/McCulloch Pressurised Air-box

Why is it thought necessary to have a pressurised air-box surrounding the carb fed from a Paxton (or McCulloch) centrifugal blower? And how do you get the throttle linkage in?
I don't think Studebaker Avantis had one - just a sealed tube from the blower to the carb throat.

 

They did it both ways.

My '28 has a Vs57 with a bonnet instead of a box that goes around the entire carb.

I think it looks a lot cleaner, and its easyer to get to the jets.

Main drawback, from what I've read and from what I've been told, is some leaking of raw fuel out of the carb past the throttle shaft.
Its probably happening a little bit, but I'm not seeing any evidence off it....

 

I cant find pics right now but i have a factory one for a packard that has the carb base (2bbl) in the box and the linkage is outside. if you want i can get a few pics for ya

 

Some had one setup, some the other. The box was simple and trouble free in service, but bulky. The carb bonnet took up less room, was cheaper, but you had to seal up the carb shafts and pressurize the float bowl.

The box or enclosure had holes for the throttle and choke. Not sure if they were sealed with grommets or not. They also had an overflow vent to drain off spilled fuel, under some conditions the carbs could flood.

 

Some pictures of a box Paxton setup.

 

Avanti R3's had a pressurised box. The R2 did not.

 

The pressurized box is to help keep the carb from seeping gas out of every seam (or shaft) as the supercharger pressurizes it.

 

If you have a blow through system you need a carb that is modified to handle the pressurised air that is being p***ed through it. A normally aspirated carb creates a vacuum within itself so wants to pull the outside atmosphere in. A pressurised carb wants to do the opposite.

You have to the seal throttle shafts and other items otherwise you'll get fuel/air mixture being pushed out between the gaps. You also have to vent the fuel bowl into the venturi rather than the atmosphere. This balances out the two. If you didn't you would be actually pushing fuel back down the main jets and into the fuel bowl.
There are other considerations but these are the main issues. Some factory set-ups have the open carb as you say but these will have modified carbs to handle the boost. The principle is the same for early blow through turbo carb set-ups.
Some carbs can be modified to handle boost and others can't.

The "in the box" trick allows you to run a standard carb with no modifications. This is because the carb is completely surrounded by pressurised air so functions as normal because everything is balanced. Throttle and chokes linkages are fed into the box but would have to sealed where they enter the box. The box is of course completely sealed otherwised you'd would lose boost.

 

Thank you, chaps.

On a Ford Flathead which carbs would be suitable for blow-through without a box?

Were boost-referenced fuel pumps available in the '60s?

 

The first McCulloch blowers offered to the public in 1953 were for Ford V8 cars. They used the stock carburetor with an L shaped rubber pipe from the blower to the carb. It looked like a radiator hose.

They modified the carb with bigger main jets pressurized float bowl and possibly seals on the throttle shaft (not sure if they had such seals then). They also pressurized the fuel pump.

Do a web search and you should find some old photos. There is a site called VS 57 or something similar that has a lot of info.

 

Nice Judd ,That paxton still lives at my house. RLS

 

I believe 97's are good because they are vented in the throat of the carb.

I don't believe its the pump that is boost referenced but the fuel regulator.

These are available....

http://www.burtonpower.com/parts-by-category/fuel-system/fuel-regulators-filters-carb/fsefpr009.html

They increase the fuel pressure as the boost increases to compensate. Top quality and period looking as well.

 

Thanks Enbloc.
You are, as usual, a mine of technical information.

 

What you got brewing?

 

The box is so that the float bowls are at the sam atmospheric pressure as the venturies. I read something on the theory on it a while back. Off all that I read (about 20 pages as I recall) the idea is that the entire carb be under the same pressure as opposed to just the throats being pressureized. it als mentuioned that you have to reference your fuel pressure regulator to the amount of boost to maintain flow to the carb. IE if you normally want 7 psi to the needle and you are running 10 psi boost you need to have your regulator bump up to 17 psi when that boost is achieved or you will not get fuel into the carb.

 

Incidentally if you want to make a pressurized carb box, an old pressure cooker should work well if you have room for it. You can pick them up for a few bucks at yard sales and thrift stores. The real old cast aluminum jobs have a fifties vibe to them.

 

The floatbowls are the same pressure as the throat of the carb, because thats where the F/B vents are.

So that doesnt change with bonnet or a press box.

And they put them there to have them under the airfilter in N/A applications.

So any blow through carb will need at least foam filled floats, because the hollow br*** ones will collapse.


The main differences are only, as far as I can see, that the little bit of pressure that will escape past the spindles is dry air with a box, instead of mixture with a bonnet.
And that you can probably go a lot higher with the boost pressure with a box, because if you have pressure on the outside of the carb as well, the boost cant rip it apart.

With a relative low boost setup like a Vs57 the bonnet works fine, and it looks much better than the ugly Paxton pressure box.
And for what it is ( just a simple box with a linkage in it and some fittings ), it is ridiculously expensive...

 

Well, there's always some crazy thing brewing in my mind, but this is as a result of meeting another crazy old person recently. I went looking for a Lancia sliding pillar front end but the first things I saw in his shed were three Studebakers, from a 1930s doctor's coupe to an Avanti. The Avanti didn't have a blower on it, but he showed me a couple of Paxton/McCulloch units on the bench, one of which was variable speed (VS-57) while the other wasn't. There was no sign of a pressure box on the Avanti which got me wondering. Incidentally, he also had a flathead in a Lancia Lambda special so I'm trying to get him to bring it out to join us on the hill climbs.
Everywhere you go there are crazy people and intersting things - I'm happy to say.

 

LOL! Thanks for the 351W that's in my buddies Fox Mistake now. You have the Paxton hooked up and running yet?

 

Biggles
With the box set up you have top use a boost indexed fuel pump also to bring up fuel pressure as pressure in the float bowl increases.

 

Here are a couple of old photo's from the '57 Ford Factory Y block 312 with the Paxton-McCulloch super chargers on them. There were a few differences if you look hard.
Normbc9

 

I'd be scarred to run it without a box on low pressure carbs such as 97's, because the fuel needs to be at a higher pressure than the incoming air and strombergs hate over 2 or 3lbs of pressure anyway

 

Original McCulloch blowers only put out 4 or 5 pounds pressure. The Kaiser version was 4 pounds the aftermarket or universal model 5 pounds. Don't know about the Ford or Studebaker model but they must have been about the same.

The McCulloch has a regulator that allows pressure to build up to a preset level then does not allow it to go higher. This is done by a movable pulley that controls the impeller speed.

 

You can - (and need to) - 'boost
reference' any normal stock or
aftermarket mechanical fuel pump
to use it in a blow-through
super-or-turbocharged application.
On most pumps it's simple to do.
Somewhere on the fuel pump
housing - on the "dry" - ie -
'non-fuel' side of the diaphragm
housing, there will be a small vent
hole that normally references the
diaphragm to atmospheric pressure.
To reference the pump to boost
pressure instead, you need to
dismantle the pump and drill and
tap the atmospheric vent hole for
a hose or line fitting. Re***emble
the pump, install it on the engine
and connect the new vent fitting
you've just added to a line going
to the pressure box, if you're
running one, or to the carb
pressure bonnet, if you're not -
and you're done.

Mart3406
===========

 

---------------
It doesn't matter whether you're
using a complete pressure box to
enclose carb - or just a pressure
bonnet to seal the top of the carb -
either way - YOU *NEED* TO
BOOST REFERENCE THE FUEL
PUMP ON *ANY* BLOW-THROUGH
SYSTEM.

Mart3406
=========

 

Yep..

 

Judd Still in a box waiting for a nice mustang -falcon to come along RLS

 

Correct on the boost reference pressure to the fuel pump. McCulloch also recommended adding an electric pump for more volume if necessary.