Jet-A-Vator carb grid.

[sser2]

Originally Posted by Isaac Zackary

Many late model carbureted cars did get grids under their carburetors for emissions purposes.

Yes, the key here is "for emission purposes". These grids are not flow restrictors or turbulence geherators. They are electric heaters that temporarily warm up the intake before water jacket warms up the intake manifold.

Venturi is the narrowest opening in the carburettor and the place where fuel is injected. Maximum speed of air flow through the Venturi facilitates fuel pulverization. Creating flow restriction elsewhere will reduce air speed at the Venturi and thus worsen the pulverization.

[Isaac Zackary]

Originally Posted by sser2

Yes, the key here is "for emission purposes". These grids are not flow restrictors or turbulence geherators. They are electric heaters that temporarily warm up the intake before water jacket warms up the intake manifold.

Venturi is the narrowest opening in the carburettor and the place where fuel is injected. Maximum speed of air flow through the Venturi facilitates fuel pulverization. Creating flow restriction elsewhere will reduce air speed at the Venturi and thus worsen the pulverization.

My theory is that if you really want to avoid reducing air speed at the venturi then your only option is to remove the throttle.

Yes at full throttle any restriction along the intake will not allow the highest venturi air speed. But at part throttle your biggest restriction is the throttle, and I don't see how any restriction along the intake will have any effect at part throttle.

Yes, a dual throat carb can allow the use of a smaller venturi. But I'm not ready to modify the engine to accept a different carb right now.

[freebeard]

When all four cylinders are sucking through the venturi, there will be skin friction all along the path. A restriction at 1/4th" of that path prolly has less effect than the skin friction throughout the full length.

The mass of air will speed up locally, suffer a lot of turbulence and then slow down again.

[BLSTIC]

Originally Posted by Isaac Zackary

My theory is that if you really want to avoid reducing air speed at the venturi then your only option is to remove the throttle.

Yes at full throttle any restriction along the intake will not allow the highest venturi air speed. But at part throttle your biggest restriction is the throttle, and I don't see how any restriction along the intake will have any effect at part throttle.

Yes, a dual throat carb can allow the use of a smaller venturi. But I'm not ready to modify the engine to accept a different carb right now.

Well there goes suggesting you get something other than the most basic carburettor ever to be fitted with a choke. You *could* modify one to have an annular discharge venturi, or a booster, or a central discharge point (all of which will improve atomisation) but that's all more difficult to do than finding another manifold and modifying it to fit a progressive dual-throat carb.

This will help atomisation by nature of the high airspeed through the plate and the turbulence immediately after it. It can't *not* do that, assuming the atomisation of the carb is poor to start with.

You can also make up for *some* of the restriction by detailing the carburettor, ie knife edging the throttle body, making the throttle shaft more aerodynamic, repeating that for the choke, making sure the venturi has no casting marks (but don't make it bigger), and so on. Those won't affect the operation of the carburettor but will increase the flow.

Have you got any kind of manifold heating? It seems like that would help, assuming you can avoid pinging.

[Frank Lee]

Originally Posted by Angel And The Wolf

My ex-brother-in-law went into business with a friend. The friend had built a plate to fit between the carburetor and the manifold. screwed into the plate, at three points from the sides, were three ultrasonic vibrators from a dishwasher.The theory was to pulverize the fuel, and tests on a 390 Galaxie produced 50 mpg. they were blocked at every turn from manufacturing the plate, and ended up selling the rights to Texaco. Someone should give it a try.

[Isaac Zackary]

As far as the Jet-A-Vator is concerned, I am planning on trying the system out both with and without it to see what gets me better fuel mileage. I do think the fine tuning of the carb jetting and the digital ignition map will get me a whole lot better fuel mileage than the Jet-A-Vator will. I just think it's worth the try since I have it.

Originally Posted by BLSTIC

...but that's all more difficult to do than finding another manifold and modifying it to fit a progressive dual-throat carb.

I'd need to find one that has a smaller venture than the one I already have. Weber makes a progressive carb, but it seems to have a 32mm main barrel. I'm not sure what the actual venturi size is but it's likely not much different than the 26mm venturi I already have. Also I'd lose my heated air cleaner and the dual heat risers unless I built my own intake manifold and air cleaner.

Deluxe Center Mount Progressive (DFAV/DFEV) Carburetor Kit, Reline, DP Type 1, K1410 - Aircooled.Net VW Parts

Originally Posted by BLSTIC

Have you got any kind of manifold heating? It seems like that would help, assuming you can avoid pinging.

Yes, and I got about the best you can get on a VW. It a called a dual heat riser manifold, which are rare in the US since they were only put on 1974 Californian VW's.



As opposed to the much more common single heat riser manifold:

[freebeard]

Well there goes suggesting you get something other than the most basic carburettor ever to be fitted with a choke. You *could* modify one to have an annular discharge venturi, or a booster, or a central discharge point (all of which will improve atomisation) but that's all more difficult to do than finding another manifold and modifying it to fit a progressive dual-throat carb.

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

Volkswagen downdraught set

The pictures below show a pair of Reece Fish carburettors with manifold tubing suitable for an air-cooled Volkswagen engine. Note that Reece Fish manufactured the tubing and manifolds as part of an after-market kit for this vehicle.

[Isaac Zackary]

Originally Posted by freebeard

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

That would be interesting to try some day!

[Isaac Zackary]

Originally Posted by BLSTIC

Well there goes suggesting you get something other than the most basic carburettor ever to be fitted with a choke.

I did base my carb selection on the reputation it has in the VW community, although there are quite a few that stick by their favorite carb "just because".

It has taken me five years to get this car running. And right now I need it for commuting. Some day I might try different carburetors and such, but not right at the moment.

[freebeard]

My Superbeetle needed a 34PICT and I couldn't [at the time] find the one I had laying around. My VW shop ordered the 32PICT and adapter instead. They said it was a more tunable/maintainable/known. ¯\_(ツ)_/¯