SU or Weber Carburetors? Either neither, and it depends.

[ECS620]

I found this short article very informative for a relative novice trying to understand the carburetor options for our L16, L18, L20 engines. This article is based on MGB and Triumph engines, but the L series are also a reverse flow cylinder head of about the same displacement so the theory and dyno numbers probably translate. Check out that sick single DCOE intake mani:

 

https://classicmotorsports.com/articles/su-or-weber-carburetors/

 

Edited February 10, 2022 by ECS620

[ECS620]

 

This was also a good technical article on airflow in our reverse-flow heads.

 

https://en.wikipedia.org/wiki/Reverse-flow_cylinder_head

[datzenmike]

4-5 HP difference with almost all the rest basically the same doesn't point to a clear winner. With this being the case, price would be more important followed by looks? Ease of jet changes doesn't enter into the discussion because you won't be changing jets again after the initial adjustment. The 32/36 has the advantage of only needing an adapter plate. The other three need expensive intakes. I would call it the winner though I don't care for them myself. .  

[Crashtd420]

9 hours ago, ECS620 said:

I found this short article very informative for a relative novice trying to understand the carburetor options for our L16, L18, L20 engines. This article is based on MGB and Triumph engines, but the L series are also a reverse flow cylinder head of about the same displacement so the theory and dyno numbers probably translate. Check out that sick single DCOE intake mani:

 

https://classicmotorsports.com/articles/su-or-weber-carburetors/

 

The only thing is with the single dcoe they can be hard to tune on that manifold...

It pairs 1/2 then 3/4 .... so it takes 2 intake pulses per side because of the firing order 1 3 4 2.....

But it is a straight shot to the motor .. the lynx intake is a good option for a manifold but some dont like the bends in it.... the benefit is it pairs  1/4 then 2/3... so it takes an intake pulse one at a time back and forth from each side.....

They are out there but sometimes hard to find.... I did read somewhere they might be doing a new run on them but not sure....

[datzenmike]

 

Hitachi SUs pair 1/2 and 3/4 but straighter more direct shot into the ports. If Weber $ide draft get dual rather than single with individual runner/carburetor per port. These can be even better tuned with trumpet length.

[ECS620]

Great input, thank you both. And @datzenmike I agree on the Weber 32/36. Unless I can find a decent local deal on a pair of side-draft DCOEs with an intake, I'll be keeping my DGV. I can keep the vacuum advance, and I'm already familiar with the carburetor.

 

@Crashtd420, I think that Lynx mani looks awesome!

[Stoffregen Motorsports]

Bolting a pair of dual Webers or Mikunis onto a stock Datsun engine is a waste of time. The engine cannot take advantage of that much carburetion without some basic mods.

 

Therefore, either the SUs or the DGV is the better choice. GIven those two choices, I would opt for the DGV because of the ease of tuning. One often overlooked aspect of the DGV is that while it seems the SUs are "larger", it's not that straight forward. An SU feeds two cylinders, while the DGV feeds all four cylinders, so each cylinder has access to the entire CFM area of the DGV at any given time. In reality, the DGV is a bigger carb because of this and ultimately has more power potential than the SUs.

 

We played around with the SUs on the dyno years ago, but the results just didn't stack up against all the DGV's attributes.

[ECS620]

@Stoffregen Motorsports This is exactly the type of conversation I was hoping this thread would inspire i.e. fact and experience based convo about not only the options, but benefits and detriments of carburetor options for those interested. Of course dual DCOEs should always be accompanied by some a modest cam upgrade maybe some exhaust attention. On an L20 with these mods, should some intake exhaust porting be considered as well?

[datzenmike]

The L head is a good design and breaths very well as is.... but I would gasket match it. Put the manifold gasket on and scratch the outline of all the ports with a nail into the soft aluminum.

 

 

Grind away anything inside the lines. Smooth in at least an inch and a half. Remove any casting flash. Does not have to be polished like a mirror. Rough is fine.

 

Now to complete this, do the same to the intake and exhaust manifolds. The exhaust is steel and will need a bit more effort.

 

 

 

Now when they are bolted together there is no lip or edge or chance of any miss match.

[Stoffregen Motorsports]

Yes, gasket match but make sure you don't create any steep transitions in the intake ports while doing grinding. It should be a smooth, tapered bore all the way through. If you don't have experience porting, I'd go easy and stay close to the gasket surface. You can screw things up if you're overzealous.

 

One of the largest areas of improvement is the combustion chambers, and the valve pockets just outside the chambers. Remove all rough edges and radius the hard corners in the chambers up to and just around the valve seats. This allows free-er flow, but also eliminates hot spots that can cause detonation. Inside the valve pockets, just smooth the transition from the port to the valve seats, and also around the valve guide. Just removing the casting junk around the valve seats (and smoothing out that transition) aids more than you'd think.

 

A cam will go a long way, and there are many choices. Most of the cams from the usual suspects are very old designs and good enough, but if you want a proven cam with more potential, give Rebello a call. They have been developing the L motor for years, and still are, and they have their own cam grinds.

 

M favorite cam on a stockish L motor with a DGV is a 27C. It requires stiffer srpings, but is smooth and torquey with a noticeable increase right out of the box.

[ECS620]

On 2/11/2022 at 8:26 PM, datzenmike said:

The L head is a good design and breaths very well as is.... but I would gasket match it. Put the manifold gasket on and scratch the outline of all the ports with a nail into the soft aluminum.

 

 

Grind away anything inside the lines. Smooth in at least an inch and a half. Remove any casting flash. Does not have to be polished like a mirror. Rough is fine.

 

Now to complete this, do the same to the intake and exhaust manifolds. The exhaust is steel and will need a bit more effort.

 

 

 

Now when they are bolted together there is no lip or edge or chance of any miss match.

 

Ah, Mike with the high-res pics! I will definitely be doing this.

 

@Stoffregen Motorsports Excellent advice. Thank you for the cam suggestion as well, it can be confusing trying to interpret all the cam options without experience with any of them.

 

 

[Stoffregen Motorsports]

Sure. Good luck.

[datzenmike]

It's not worth that much on a stock application but lets say one HP. If you pay attention to detail and make 10 small modifications like this, like indexing your plugs, suddenly you have 10 HP and as the L20B is just over 90- hp this is a good 10% increase. Gasket matching and a little porting is there forever, never needs replacing or adjusting and never goes out of tune. Little things add up. On an engine that has been modified that little change is returns even higher dividends.

 

I don't know about everyone else but I love removing small imperfections in the ports, in the bowl area below the seats and unshrouding around the valves in the combustion chamber. Not worth paying someone to do this but fun all the same when it's your own time. 

[Stoffregen Motorsports]

Yeah it's a kind of sickness. I too feel like removing any imperfection is the road to enlightenment. You should see my restoration work. Strive for perfection and all that.

 

One of the reasons that the chamber work is important is because if you remove the hot spots and eliminate detonation potential, you can now run more ignition timing safely and increase power and throttle response that much again. So yeah, it all adds up.