down draft vs side draft

[tuckfoo]

I have acquired a rusty 510 with a million parts for stupid cheep its got a L20b and a U67 head CURRENTLY this will prob change but it has a offy intake and a 32/36 webber on it but i found a bunch of SU carbs and a manifold to go with them but i hear tuning them is hard and the linkage is a pain i also cant find the linkage what should I do ? :confused:

[datzenmike]

Run wut ya brung. Why complicate things? Weber downdrafts are simple and it's already there and most likely runs.

 

 

All things equal, the most direct entry of air without bends in the intake or head posts will give the best results. Down drafts have to make an extra 90 degree bend and change direction. Side drafts like the SU are much more direct.

 

SUs would give good power (but not stupid extra power on a stock L20B) and they have fewer parts and are much simpler... but they do need to be tuned and as there are two they must be synchronized too. Then there is the linkage and choke cables. 

 

You could put the SUs and the intake up for sale and pass it on to someone willing to go to the trouble with them. Are they Hitachi SUs? Do you have a Nissan intake for them?

[Ratwagon1600]

If you run the SU's you want the manifold that has "japan 220" on it.

 

This one generally came off the 610 SSS and is the non pollution manifold.

 

Unless you've got experience with SU's its generally better to stick with the downdraught Weber.

 

SU's are a bitch to balance and as mike points out, need the right linkages, choke cables etc. Be careful if you pull them apart, the piston and needle (unless someone has fucked them up) are matched to the bell chambers.

 

SU's are prone to wear on the butterfly shaft where it goes through the body of the carburettor. This can cause air leakage and make tuning a problem. I generally strip mine right down and rebuild them. Silver solder on the shafts can build up wear points and is also self lubricating.

 

The piston chamber dash pots also need the right grade of oil. RW uses sewing machine oil (not too much though).

 

If you do decide to use the SU's there are some great resources on the web about setting them up. The pommies used them on a lot of their cars(minis, jaguars, Rovers, triumphs) so they have some good stuff on setting them up. There is a book called tuning the A series engine (written by a guy called David Vizard). Its about minis but has some really good stuff in it about SU's. If you go the SU route, pm me an email address and ill scan and send you the relevant sections.

 

Hope this helps?, oh, and stick to the Weber.

[Draker]

There is probably a reason they are sitting in a pile of parts and not on the vehicle.

[wayno]

I would run dual SUs over a Weber or any other down draft every time, I think they are easier to tune than a Weber, I have been running dual SUs on the work truck for over 20 years now.

But you have to have serviceable SUs to start with, if what you have are wore out, you will never be able to tune them properly.

Linkage kits are on ebay for dual SUs every once in a while, I have bought one, don't see one right now.

[Stoffregen Motorsports]

One huge benefit to the DGV - accelerator pump. The SU doesn't have them. They help with cold start and improve throttle response.

 

All the other reasons mentioned (to keep the Weber) are great reasons too.

[russaroll]

i love my 32/36.  it never fails.  i've tried su, dual weber, and single weber and i find that my 32/36 is the easiest to tune and it never lets me down.  i've up jetted it so it's got a little more umpf.

[distributorguy]

I'm with Wayno on this one.  If the SUs are any good, performance and drivability will ALWAYS beat  the Weber 32/36.  You don't need an accelerator pump when the SU dashpots work as they should.  Linkages for any HS4s will work if you cut a new piece of steel rod to fit properly.  But if they're shot they're shot.  SImplest carb on the planet so they aren't difficult to rebuild.  

[Stoffregen Motorsports]

Dyno tests show that the single 32/36 makes more power than SU's.

[datzenmike]

I find it hard to believe that two 38mm side draft carbs, properly jetted, can't easily beat a properly jetted 32/36 downdraft. There must have been different engines or different heads or cam.

[distributorguy]

This comparison gets made on 1.8L MGBs all the time.  SUs always win.  Always.  Both in drivability and power.  

[Stoffregen Motorsports]

I feel like we've had this conversation before.

 

If you guys aren't sold, I'm not going to be the one to sell you.

[datzenmike]

nope.

[willz]

I'd like to see the dyno sheets comparing the two. I could see the 32/36 having good low end power and the SU's having good top end. Maybe because of the different style of manifolds, the Weber being longer than the SU.

[datzenmike]

The engine would have to be the same for both tests with only the carbs and intake different and both carbs tuned to their max. The SU has a more direct air flow where the weber has a 90 degree bend in the flow. Then the obvious 38mm + 38mm vs. 32mm + 36mm barrels.  

[Stoffregen Motorsports]

The SU pulls from one barrel for two cylinders.

 

The DGV pulls from 2 barrels for four cylinders, which means that at any given time, each cylinder gets to pull from the whole carburetor (32+36).

 

On an engine that makes power with a DGV (stock L16/L18/L20B) the 90 degree makes almost no difference at all. The plenum makes up for it in its ability to atomize fuel better.

[willz]

 

 

The SU pulls from one barrel for two cylinders.

 

The DGV pulls from 2 barrels for four cylinders

 

 

 

 

Isn't that the same thing? Or am I missing something

[KoHeartsGPA]

The SU pulls from one barrel for two cylinders.

 

The DGV pulls from 2 barrels for four cylinders, which means that at any given time, each cylinder gets to pull from the whole carburetor (32+36).

 

On an engine that makes power with a DGV (stock L16/L18/L20B) the 90 degree makes almost no difference at all. The plenum makes up for it in its ability to atomize fuel better.

With that in mind, how do you believe a 38/38 will do on a KA24?, I'm looking to do this for my 620 instead of EFI, haven't really done research yet, what's your take on this?

[willz]

wish I had access to the shop I used to work at, in ground mustang dyno. I'd have a dyne day just so we can see what's up

[Stoffregen Motorsports]

The SU pulls from one barrel for two cylinders.

 

The DGV pulls from 2 barrels for four cylinders

 

 

 

 

Isn't that the same thing? Or am I missing something

The downdraft manifold with the plenum makes it so that each cylinder has access to the whole carburetor. One 32mm bore and one 36mm bore.

 

The SU manifold is split into two halves where each cylinder only has access to one 38mm carburetor bore.

[Stoffregen Motorsports]

With that in mind, how do you believe a 38/38 will do on a KA24?, I'm looking to do this for my 620 instead of EFI, haven't really done research yet, what's your take on this?

I used to build engines for a guy who ran his KA24E in a circle track car and he had a custom intake and a Holley 2 barrel. I can't remember if it was a 350cfm or 500cfm carb though.  A 38/38 is about 350 CFM.

 

The car ran great, but was rather peaky. But I think that was the way it needed to be for a roundy round car.

[willz]

But wouldn't the twin 38's have ultimately more flow volume than the 32/36? If we could flow bench both manifolds side by side, wouldn't we see more potential flow from the 38's than the 32/36? Ultimately that would be the bottle neck for power. I would also assume that fuel on both would be adjustable and would not be the issue.

[datzenmike]

The downdraft manifold with the plenum makes it so that each cylinder has access to the whole carburetor. One 32mm bore and one 36mm bore.

 

The SU manifold is split into two halves where each cylinder only has access to one 38mm carburetor bore.

 

I've heard this argument before.  The SU intake is connected between carbs so each cylinder does have access to both carbs but mostly the close one. A 90 degree bend in the high speed air flow has got to be more restrictive than no bend at all.

[KoHeartsGPA]

I used to build engines for a guy who ran his KA24E in a circle track car and he had a custom intake and a Holley 2 barrel. I can't remember if it was a 350cfm or 500cfm carb though.  A 38/38 is about 350 CFM.

 

The car ran great, but was rather peaky. But I think that was the way it needed to be for a roundy round car.

Ahhh I see, I'll take a hard look before I decide what size carb I use for it, thanks for the input ;)

[Stoffregen Motorsports]

I've heard this argument before.  The SU intake is connected between carbs so each cylinder does have access to both carbs but mostly the close one. A 90 degree bend in the high speed air flow has got to be more restrictive than no bend at all.

The SU manifold is connected for vacuum only. Any air flow that gets exchanged between the two halves is heavily corrupted and can't be considered as being beneficial to anything other than vacuum balance.