Race Engine Forum UK

Guy

After reading your guidelines please find my post here. I have a Lada 1600 car with 79x80 bore x stroke engine, a real standard one (beside other ones). I could read a lot of about carbs and size of carbs. Even in your post I have seen a sentence saying some bad experience about 48mm Webers for 2 valve 2 litre Fiat DOHC engines. Surely we can see a lot of modifications to twin 40 or 45 DCOE carbs for LADA or even a single 45 Weber conversion for LADA. I am looking for your advice in the following: would it be beneficial to put 2 standard carbs with proper manifold to a standard engine?
The basic data are the following

[mm] 1 2 A1 A2 A1+A2 corr. A
4 cyl carb throttle 28 36 615,8 1017,9 1633,6
4 cyl chokes/venturi 22 25 380,1 490,9 871,0
per cyl manifold dia 29,5 683,5 683,5
per cyl port dia 29,5 683,5 683,5 633,2
per cyl in valve dia 37 1075,2 1075,2
per cyl stem dia 8 50,3 50,3
per cyl lift 9,728 1130,8

((for better visibility I attached a carbs.jpg file))

So in the basic setup one register carb is feeding the 4 cylinders. The first barrel is mechanically operated and after opening cca. 45% of the first the second barrel is opening by vacuum ie. by air speed in the first barrel. So the air speed is quite nice for nearly all range. The data above shows that the narrowest point in the carb has 871 cm2 for 4 cylinders and the port diameter is cca. 683 cm2, same as the manifold for one cylinder. 633 cm2 is for the port minus valve stem area. The data 1130 cm2 is the area of the superficial (skirt?) when the valve is fully open.
Power and torque for the standard engine: 80 HP @ 5400 132 Nm@ 3000
I have started to make the manifold but several "masters" told that I will not get anything from it.
Furthermore I had the chance to measure the exhaust "channels" one by one in original condition. It showed 1% CO for cy 1 and 4 and 10% for cyl 2 and 3. At factory setup the total CO was 4,5% , so it tell that even the flow can not be the same for the central and for the outer cylinders.
Furthermore we use also Horns on it.

Engine cross and long sections:
http://www.zsiguli.hu/index.php?action=cikk&id=125

So what is your experience on it?

Thanks

Regards

B’‚©la
2103i

Hi B’‚©la:

I also own a Lada (a '91 Niva in my case).

I recall seeing both setups (twin 40s and a single 45 on custom inlet manifolds) on the Hungarian Lada Klub a while ago, but since I can't speak a word of your language I can't really tell which setup worked best for the Lada lump.

Whatever you do, I'd suggest NOT using the stock DAAZ carbs. As a matter of fact, almost anything is better than the stock units used by Lada. I put in a single Weber 34DCHE straight from a Fiat 125 1600 DOHC engine; by no means is the carb I wanted (I was looking for a Regata one instead) but my butt dyno (with apologies to the forum admin/members for using such gross expression) felt there was a major improvement in both power and pickup, not to mention fuel economy was vastly improved.

BTW, you did a great job documenting all those Lada engine specs. I'll also be waiting for Guy's input on this.

Regards;

Alejandro

As it looks like the twin 45 Webers are the best for 1600 cc LADA but of course are very costy and requires a lot of changes. Like batttery replacement, etc. None of this issues are difficult at all.

DAAZ carbs are proven here very good. Both emission and also economy. Also the driveability is OK. The carb is very complex but works fine. I consider this as a master piece for fast road usage. However it is not designed for racing surely.
If we race witha Gr. A. LADA where the carb throttle must be the original diameter it seems a very good soultion compared to the other possible solutions. Defenitely the Venturi (the narrowest point) is much bigger than the original. Sometimes we change to not vacuum activated 2nd throttle but mechanical both paralell or register.

Regards

B’‚©la

Putting two standard sequential choke carbs on the engine instead of one?

OK, if I got that right you need a common plenum so that 2 adjacent cylinders can share one carburettor. So you reduce the conflicting pressure wave effects that occur when you run all four ports into one plenum (by plenum I mean the chamber below the carb) but you don't get rid of them altogether. So a slightly better solution perhaps than on carb only.

You must map out and examine the relative valve events vs firing order. Something that does take time and I really must build a model one day. The primary problem is that as the valve opens a negative wave comes out of one cylinder (because it's on the intake stroke) and that wave ideally should see atmospheric at the rampipe and be reflected as a positive wave that will aid cylinder filling. When two cylinders are joined to a common plenum, say 1&2, when one's inlet valve event is taking place no 2 cylinder's valve is shut so that negative wave is just as likely to travel down no 2 port and bounce off the valve coming back as a negative wave. When the valve closes at the end of an intake stroke a positive wave is reflected off the back of the valve as the mixture comes to a sudden stop. etc etc. You do get real interference between these inlet tract waves, splitting the runners is exactly what makes one-choke-per-cylinder so effective. When in the cycle and to what extent the waves will conflict - no idea. Impossible to work out.

Balance that scenario against the cost/benefit of actually buying those 'costly' Weber 40 DCOEs and achieving total independence between the pressure waves. You will probably get a torque benefit from your idea, but if it's real top-end power you want, sorry, there's no contest. Pressure wave effects like those described inhibit the engine's ability to 'breathe' very significantly at high rpm.

All that said I've never done what you propose. You'll probably gather, if I have understood you correctly (and in the nicest possible way) I would not want to either. Low cost solutions rarely bring optimum results in my line of work.

GC

Guy

Thanks for your honest answer. Probably I wouldn't mention the costs, but pleas ebeleive me, the development of a manifold is also a cost. So at the end the DCOE vs. 2 original carbs is not a huge difference on cost side.
My thinking was: all the 2 cylinder single car engines are working on the same principle (as the Fiat 500, 126 etc...) And also some Skoda 4 cyl engine has a factory fitted "sport" conversion with 2 standard carbs.

Again tanks for your answer and please do not misunderstand me with teh cost.

I intended to use this project as a trial to use OE equipment, with good throttle response and better balance (CO 1 vs 10%) between inner and outer cylinders.

Furthermore: based on the inlet cross section data, could you please give us an idea what should be an ideal ratio between the inlet channel for 1 cyl. and the common part for the 4 cyl. So diameter cyl head port / carb diameter narrowest point. If there is such a parameter at all!

Thanks

From actual experience the seperate venturi per cylinder wins hands down over several carbs feeding into a common plenum. Greater flexibilty and tuneabiliy with 40's plus economy ( for those that care about such things ).

I've used 3 twin chokes on a V8 and had to use progressive linkage to make it driveable. A single 4 barrel was ,in that case, more flexible and easier to set up but didn't have as much 'wow' factor, important in hot rodding circles but less important than performance.

No technical reasons I am afraid just results from actual usage


Just occured to me that I used to run a Sunbeam Alpine with original twin Zenith VN2's into a common plenum. Once again a change to a single 28/36 DCD Weber created a smoother more flexible car.

To keep membership.

A tagság megtartásához.

Thanks

OK!

G