too much carb?

[Moisabamf]

will the weber 32/36 DVG(both barrels open at the same time) be too much for the A15?

should i just get the regular ol' webber?

[ggzilla]

The 32/36 *is* the 'regular 'ol weber'. It is by far the most common and least expensive Weber found in USA.

 

It is not too much for the A15, it will run OK. For performance though you will get a better across-the-RPM perf, i.e. what you want in a street car, with a smaller carburetor. 32/36 is good for 180HP and up, yet a stock A15 is only 85 HP. A typical 'built' A15 if done well may put out 125HP. Still the 32/36 is a bit on the large side for that. However it will run OK at higher RPMs and high throttle openings -- but the 36/36 Weber does even better for this limited use.

 

Still, the 32/36 is better for perf than the stock carburetor which is too small for max. performance even with a stock A15. The stock carburetor is most excellent for fuel economy and is the reason it is undersized.

 

You may like it.

[Moisabamf]

i meant the progressive or the non progressive, DGV, DGEV, too many acronyms for me to keep track of

[ggzilla]

You meant the 36/36 or 38/38 non-progressive carbs. Not that common. The DGV series (ex: 32/36) are progressive: they don't open the second barrel until half-throttle.

 

Yes, I'd say they are too big. They start at 36mm, where DGV ends. So they are even "bigger" than the 32/36. That said, a lot of people are happy with them even though they have no dyno proof, and theory says that a smaller carb would be better for most use. I've only used the DGV and stock Datsun carbs and twin-Mikunis ... and my experience is same as club's: It seems almost nobody can get the DGV tuned just right for an A-series engine, while the Mikuni/Weber/Dellorto types are easily so tuned. And none will exceed fuel ecomony of the stock carb.

 

What is your goal? Drag race-only vehicle? Around-town commuting?

Edited December 31, 2008 by ggzilla

[datzenmike]

I think the 32/36 is rated around 270 cfm.

 

Here's some cfm amounts to satisfy these engines at full throttle:

 

Motor...@ 6K, @ 7K, @ 8K

A15.......159....185....212

L16.......169....197....226

L18.......190....222....254

L20B......212....247....282

Z20

LZ22.....233....272....310

LZ24.....254....296....339

 

Because engines have to work hard to draw air in, carbs are a pinch point like a small straw. Almost any larger carb will out power the smaller stock carb, but not because it flows more air, but because the larger carb is less restrictive. Any power increase is from the motor not having to suck as hard to draw air through the venturi. As you can see it is only too easy to over carb most of these motors. Too big and it runs like shit off idle where there isn't enough air flow to make enough vacuum at the venturi to draw in the gas and mix it.

[Moisabamf]

just daily driver, but im going to put the 32/36 to get more umph out of the engine. im also putting in the rear end from Duncan in order to make the car very freeway friendly

[ggzilla]

That 270 cfm sound right -- it is a 2-bbl cfm rating.

I think your table is using 4-bbl cfm rating.

 

If so then they don't match together. The carb will be too big for optimum usage. They use 32/36 on 4-liter engines.

 

In any case it will run "OK".

[datzenmike]

Absolutely. For economy a 'too small' carb would be better to keep the vacuum and air volume up for it's size. Economy will be mostly part throttle so breathing isn't an issue and good mixture is.

[agentalpha]

There is really no such thing as "too much carb".

 

What carbs need to function is a strong booster signal.

 

Consider:

a 2.8L with triple 40's has(using Holley's method of sizing)@ 1000CFM

(this is rough math in my head, I'm sure I'm off).

 

That's MUCH more than Holley recommends for most street big block V8's...

[ggzilla]

You're off. Triple carbs does not triple the CFM (unless put onto a plenum).

With separated runners and only balance passages between, it's only a little more than 1x cfm.

 

I had a 32/36 on my A-engine, it ran OK, more power than the stock 26/30 carb, but never worked exactly right off-idle.

 

Holley recommends ... yes, they recognize that sometimes a larger carb does not mean more power. Their charts recommend a certain size for max power and a slightly smaller size for street use.

[datsunrides]

I run a single DCOE 40 on my A14 thats choked down to 34mm venturis and while I have not done any tuning yet, it seems a bit over carbed. Has pretty good driveability but does not like to go wide open as it kinda bogs down. Throttle back to about 3/4 and it's happy. I previously had a 32/36 on it and other than the WOT issue, the sidedraft seems to have somewhat better performance. One of these days I will get around to playing with the jetting and maybe going to 32mm chokes.

 

Mark

[ggzilla]

I have twin-40s on my A14 (in the blue pickup) and it seems to run great throughout the rev range. No idea on the venturi sizes. But it's not a stock A14 -- it has a lopey cam, headers, and who knows what else. The exhaust system is definitely too big but let's not start debating pipe sizes...

[agentalpha]

You're off. Triple carbs does not triple the CFM (unless put onto a plenum).

With separated runners and only balance passages between, it's only a little more than 1x cfm.

 

I had a 32/36 on my A-engine, it ran OK, more power than the stock 26/30 carb, but never worked exactly right off-idle.

 

Holley recommends ... yes, they recognize that sometimes a larger carb does not mean more power. Their charts recommend a certain size for max power and a slightly smaller size for street use.

 

I know I'm off on my math, but I'm not sure where you get I'm "tripling the CFM". And engine (theoretically) can only draw so much carb, yes. Holley rates a carb at 1.5 HG drop over the entry (ham handed method, really). And if you "triple" the carbs(whatever that means) in a common plenum, you will see a huge drop in the vacuum over the entry, lowering the rating. I dunno where you are getting these "numbers" and "info", but you are reading(and believing) the wrong things.

 

I said, using Holley's method, the Mikuni or Weber 40's are rated @ 1000 CFM.

Nothing more.

I could do the math accurately, but I don't think this "argument" is worth the effort.

Edited January 1, 2009 by agentalpha

[ggzilla]

I'm sure I'm off

Yes, I agreed with you :-)

 

 

using Holley's method, the Mikuni or Weber 40's are rated @ 1000 CFM

That brings up the question: what is Holley's method? It is the standard 3"Hg method. From the book "Holley 5200 Carburetor Handbook" Holley's method is:

The generally accepted method of rating carburetor size or capacity is by flow rate at a given pressure drop...CFM ... Holley carburetors are rated in CFM...One- and two-barrel carburetors are rated at a pressure drop of 3 inches of mercury ("Hg) or 1.5 psi.

 

 

So what is a Weber DCOE 40 rated at using this 3"Hg method? Depends on which venturies are fitted to the 40. That's beauty of this type of carburetor, it is fully adjustable even down to venturi size. It seems Weber doesn't emphasize CFM, instead they like to talk about venturi size for a specific engine/cylinder size relative to RPM.

 

From the Weber book, a twin-DCOE setup with 37mm chokes (venturis) is good for max power at 8,000 RPM in a 2.0-liter Six, or a 1.3 Four. For 6,000 RPM, a 29mm venturi is called for. Both venturi sizes are within spec for a 40mm DCOE. So a DCOE 40 carb is very flexible for different size engines.

 

Note that this doesn't apply to single side-draft configuration. This is strictly for one bbl-per-cylinder manifolds.

 

Also note that V8s with webers often use larger carbs, like Weber IDF 48mm. Four 48mm carbs with the largest venturis is about right for a 3.5-liter V8 at 8000 rpm. That's as much as a 48mm bbl will do. If however these are put on a plenum-type manifold, the same four carbs will be good for larger V8s.

 

 

Here's a useful chart from Holley regarding their 4-bbl carbs:

 

This chart is for an "all out racing engine" (100% volumetric efficiency):

 

From this chart, I read some examples:

* An all-out racing 240Z (2.4 liter == 146 cubic inches) at 6600 RPM would need less than 300 cfm (4-bbl rating).

* For an all-out racing A15 (1488cc == 91 cubic inches) @ 7000 RPM, a 200 cfm 4-bbl would be called for according to this chart.

 

What about a 2-bbl carburetor e.g. a Weber DCOE?

One- and two-barrel carburetors are rated at a pressure drop of 3 inches of mercury ("Hg) or 1.5 psi. Four barrel carburetors are rated at 1.5"Hg or 0.75 psi... To determine what a carburetor rated at 3"Hg [4-bbl] flows at 1.5"Hg [2-bbl], divide the rating by 1.4... for example, a 600-CFM four-barrel has more flow capacity than a 600-CFM two-barrel -- 40% more

 

OK, so that 200-CFM 4-bbl / 1.4 == 143-CFM 2-bbl. Yet the 32/36 is rated at 270-CFM.

 

 

Conclusion: Weber 32/36 is oversized for an all-out racing A15, let alone a street A15. And yet, it works OK (there's no such thing as "too big"...?). But it will not be optimum for power, economy, or responsiveness. At the same time, the much larger 40mm sidedrafts -- when used in a dual-carb configuration -- are closer to the right size for an A15.

[agentalpha]

Ah yes.

I'm off. I wouldn't disagree.:D

 

An I apologize. I apparently crossed the 1.5 PSI and 3 Hg.

Been a LONG time since I've calculated anything using a Holley,

so it seems I was wrong.

 

As I've stated before, I was using a 350CFM Holley 2Bbl on my 2.4L NAPS Z 720.

 

Your write up is infallible.

 

Great job! ;)

[b210in]

i have the weber progressive in my a13 b210. it runs perfect with no off idle hesitation. in fact it runs a hell of a lot better with this carb. it revs faster to the red line than the stock carb. to improve on it and a little more go, i have a k and n filter, removed the choke, and advanced the timing 5 degrees and only burn super unleaded. it also has no muffler.

[Guest jaimesix]

I had a Weber DGEV 32/36 in my A 15ed B210. It ran beautifull. Certainly better than the stock carby.

In all fairness, the Hitachi was old and worn out, I would have to get a brand new Hitachi to compare it to a new 32/36 progressive.

The 32/36 is a fine carburetor for an A15 engine. Off course, you need to have the jets and venturis adjusted for your A15. Pierce Manifolds has the information on right jets and venturis for a Datsun A series engine. That unless you want to rebuild your Hitachi and then experiment with its jets and venturis ( if there are any Hitachi jets and venturis available for that endeavor.)

The simultaneous 38 DGES would indeed be too much carburetion for a stock A15.

 

Jaime.___________________________________________________________

[rusty12ver]

so just for conversation would a single 40/44mm weber/mikuni/solex etc. do better than a 32/36 weber? or would it be better to run twin carbs?

[denveratsun]

Oh man...one carb is way better as far as reliability is concerned. Economy will usually suffer as well due to two major factors...

1) WAY more carb=WAY more gas...der!:blink:

2) Your foot usually ends up WAY more buried in the floor...cause 2 carbs sound awesome when they open up!:D

 

An A series motor that is already far more then satisfied with a 32/36 Weber will be havin a really hard time with duals of that size...well...any duals really...without some head mods and a bump in compression and/ or displacement.

 

Just my 2cents:cool:

[Guest jaimesix]

so just for conversation would a single 40/44mm weber/mikuni/solex etc. do better than a 32/36 weber? or would it be better to run twin carbs?

 

 

In a stock engine, the Weber 32/36 progressive DGEV downdraught carburetor would be a better choice than the single Weber 40 DCOE sidedraught. ( talking about an A series engine )

 

The single sidedraught has better flow that the single downdraught , but the stock engine does not require the extra mix. If there was something like a single sidedraught 32, that would be perfect.

 

With some modifications ( cam, bigger valves, more compression etc ) then, the single sidedraught Weber would be the right choice against the single downdraught unit.

 

2 carburetors are cool to see, to feel, and to enjoy, but 2 carburetors are meant for high RPM power driving. For a city driven car, not recommended.

 

If you have a built performance engine, which will be driven at high RPM, then a twin set up would be the choice, as racers attest.

 

So it all depends on what condition the engine is at, and the intended driving conditions.

 

Jaime.______________________________________________________

[rusty12ver]

thanks all for the insight. I always ran a 32/36 on my 1200, with a header, but just recently got a cam and a round port head that has the gx valves and ported. I also have a lynx sidedraught manifold and carbs in sizes 40mm and 44mm. So I was thinking of using the lynx and sidey as opposed to the 32/36. Before I even get that far I am still saving for the ajust. cam gear.

[Guest jaimesix]

thanks all for the insight. I always ran a 32/36 on my 1200, with a header, but just recently got a cam and a round port head that has the gx valves and ported. I also have a lynx sidedraught manifold and carbs in sizes 40mm and 44mm. So I was thinking of using the lynx and sidey as opposed to the 32/36. Before I even get that far I am still saving for the ajust. cam gear.

 

 

With a cam, and a ported head with GX valves, then I would use the single 40 sidedraught in town, and the twin sidedraught for highway high RPM driving.

 

Seems like a cool A series set up.

 

Jaime.____________________________________________________________

[Burabuda]

If there was something like a single sidedraught 32, that would be perfect.

 

hmm, how about twin 38mm su's?

[zerow]

hmm, how about twin 38mm su's?

 

With an A-Series manifold from AU?

[rusty12ver]

if i could get a pair of twin 38mm su's I would, just that it is hard to find someone with that set up willing to sell it......