Weber 40 or 45 for 102 Spider?

[Touringspider]

Twoliterlover is in the process of making up a manifold and airbox to convert my 1961 2000 Spider to Webers. I asked him for recommendations on jetting and other settings for 40 DCOE's, and he replied that he uses 45's on his car. I sort of took it for granted that 40's were the preferred choice, but now I am wondering if a 42 or 45 is the way to go (I have a set of 40s on the shelf, but I think I can get a hold of the other carbs if need be).

What is the collective wisdom of those who have made this conversion on a 102? Also, what settings did you use? I don't want to over carb the car, but I don't want to do something less than optimal either. The engine is being rebuilt totally to stock specifications, and I want the best combination of power and driveability.

TIA,

Arno Leskinen
AROC-USA National Concours Chair
102
115
116

[2600]

Hi Arno,
What do you want to do with the car?.......As you are involved with the Cocours, obviously you will exclude yourself by converting to a non original induction system.
I retained the original Solex PHH 44's, and had no trouble with either performance or function. My friend and mechanic replaced his Solex' with Weber 45's, and had the same result. I drove both cars and attained identical times, at the track meets, and on the street could not notice any difference other than the louder Weber "throat" noise under acceleration.
As Jay has, so often said, "It's not a race car". Cheers Adrian

[Gordon Raymond]

The 40 and 45"s are, for the most part, identical . The 45 allows the use of larger venturi's. All other parts interchange . Unless you are using very large venturi's, the 40's you have should work nicely. Some feel the 45's are "easier to work with", though I have not found that to be true , and use both. Parts are available either way. I'm not sure why, problbly the Harley guys, but used, 40's and 45's seem to go for the same $ on E-Bay . Many Americans always go with "bigger is better" , but unless you have radical cams and plan on the engine living at high RPM, 40's will work just fine. Ask your friend what size venturi's he runs in his conversion. If they are available for the 40's, you will get the SAME performance. Best, Gordon Raymond

[Touringspider]

Thanks for the replies. I will stick with the 40's. The engine will remain bone stock. My only real motivation for the conversion is simplicity of maintenance and reliability. I will of course keep the stock Solex carbs, intake, and airbox on the shelf so the car can be swapped back easily.

Arno

[Gordon Raymond]

Arno,
If your friends conversion runs right on, just use the same venturi/choke combo and jetting. The 40's will be right on as well AND save buying any major new stuff!
Gordon Raymond

[spiderman2600]

Arno take the 45's and match the progression holes with the solex phh 44
or there wil be a flat spot of idle

The dcoe that was factory mounted was 45 also and the reason for this is that the intake valve is bigger than 40mm.
So if you take the 40mm dcoe you restrict the intake traject .
you won't notice this @ the first rpm's but from 4000 and higher there is a difference , and not only for sound .
the italian engineers of those day's knew what they were doing .

[DPeterson3]

I read a lot of mis-assumptions about Weber carbs. Of course, there is a danger to any brief summation, but based upon what I've read there are a few points that folks might benefit from considering

The main reason for using 40's vs 45's has to do with the RPM range you are using. Yes, the primary venturis can be the same between the two (up to a point, but the diameter of the butterfly and exit bore are different. In a given application with the same primary venturi, you will have a bigger transition of flow rate for the 45 vs the 40. The effect slows down the airflow (bernoulli). This is disadvantageous for low-RPM torque creation, which is benefited by having the maximum manifold velocity that you can maintain. Upper RPM horsepower is benefited by mass of air, and so prefers the larger bore. This is why race engines tend to use the 45 and street engines are happier with 40's.

It is meaningless to compare the 44pHH and the Webers. The 44mm bore of the PHH is essentially a single barrel carb (until the secondary kicks in at high power), and so has about half the flow-area of either the 40 or 45 for most of its operational use. It also maintains the highest velocity as a result. Comparing progression hole placement or anything else is a step in the wrong direction.

I recently noted that the 2300 RIO engine appears to use true 2-barrel Solexes of 40 mm diameter (40DCH), which should hint at the preferred choice for this low-RPM high-torque-producing engine.

Lastly, if you mismatch the bore diameter of the intake manifold and the carburetor you get interesting standing-wave flow-rejection behavior at the junction. This can work to one's advantage in correct exhaust header design, but is normally a mistake on the intake side.

Enjoy,

Don

[Gordon Raymond]

Don has managed to state in his post what I have found true from experience. Most seem to think "Bigger is Better". However even with a 2L street engine, this is not necessarily true. Bernoulli, the word of the day. Air speed. The same thing that keeps an airplane in the air with it's airfoil. With an Alfa race engine, living in the 6500 / 8000 Rpm range, with cams that build combustion pressure at that speed, a larger butterfly and "condotti",
- throat, has advantages. For an engine with non radical cams, expected to pull from low rpm, the smaller 40's will prove more than enough. Thanks Don! Gordon Raymond

[DPeterson3]

Years ago I helped some guys try to make 45's work on a 190SL, replacing the 44 PHH. Bad idea, for the reasons I've tried to describe. The thought at the time (remember, this was the 70's), was that 45 was closer to 44, so why not?

We probably killed the intake velocity by about half, and introduced a 1mm "step" between the carb flange and the intake manifold. No wonder the car would just gasp trying to come off idle, and up through the low to mid-range.

As for diddling with the progression holes... DON'T!. These are drilled with great precision at the factory, and then inspected using an optical comparator that can spot very very very small errors in placement. An engine using 2 Webers will run about 90% of its time on just the progression hole circuitry, and not really start pulling on the primary / secondary venturi feed circuit except under significant acceleration or high speed cruising. Around town, it's all progression hole - or mid-range feed circuit. Small errors in hole placement and/or butterfly choice, will mess up your drivability, gas mileage, or both.

As for comparing intake valve with carb bore size.... False reasoning. Although the diameter of the valve has a bearing on total flow, it never achieves the maximum unobstructed opening that the carb butterfly achieves. There are far more dominant variables, such as intake manifold runner size and configuration, length of runner, etc.

Probably the biggest challenge to making 40's work on a 102 engine is to get the correct intake runners set up. The siamese configuration has to go, and the correct diameter has to be set up all the way down each runner. Jay Nuxoll claims to have sorted this out, but personally I think a custom manifold would be the right solution compared to modifying the original.

Enjoy,

Don

[franco-veloce]

hi ,

If you want more power and torque under 4000 rpm use the 40 with the modified 102 inlet , or the 2300 RIO inlet

if you want more power above 4000 use the 45 , and use "hot" cams

rgds Franco

[dretceterini]

You are never going to turn a 102 into a race car, so I would use the 40s...

[twoliterlover]

Gentlemen: Most interesting comments -- some learned and some totally without bais of personal experience. Frankly, I use 45's because my son is involved with many race cars that have 45's as stock set up. That means I have unlimited parts -- venturies, jets, emulsion tubes, etc -- available. But I can't see why 40's wouldn't work as well.

As for Adrian Ratcliff's comment that it makes no difference between Webers and Phh44's I can only say that I KNOW that neither he nor his late mechanical master friend, Richard Jarvis, ever modified their intake manifolds. I never got any difference in performance until I did either. As for Don Peterson's comments on weber use on Mercedes 190SL I have to say there appears to be an astonishing difference with webers and modified intake manifold, but not without the different manifold.

The final design to which I use came through research, determination, and sad experience. I tried everything -- different venturies, jets, etc -- with Webers and the stock solex caruburetor. The results were a waste and even dangerous. I determined finally that the point at which the runners cross over in an untreated solex manifold the extra cross section allows the air to slow down just enough to precipitate out a lot of raw gasoline from what previously had been aerated particles. That raw gas eventually ran out the outside ends of the carburetors or, Lord Forbid, into the cold air box iwhen I developed "gender menders" to allow the factory box to be used on webers. The raw gas would sit in the little trough in the top of the framework connections on the inside edge of the right front fender. It smelled bad, made me carry a fire extinguisher, and quickly ate off the paint under the carburetors. Ever wonder why Phh44's have the two tubes to drain extra raw gas down to below the pan? Even the stock solex set up has that problem when the secondaries start to kick in.

So I studied what Weber said about how their carburetors worked. Instead of the Solex "increasingly wet plenum" theory, Webers use the "faster puff of air carries more aerated gasoline" theory. They suggest individual runners to each cylinder to move air without slowing down.

In fact, subsequent tries proved faster more important that bigger. Thinking of full throat power usage I had Vintage Racing Motors modify an intake manifold for me by "sleeving" it straight through with 45 mm internal diameter auminum pipe, welding up the jackets so I could still use the water passageways (probably not necessary with webers as much as with solex). Over 4500 rpm the engine ran fantastically strong, but it was hard to start, and would hesitate and often almost stall on start of accelleration. The runner size was too big. Unless the air was flowing at full capacity the engine did not carry enough aerated gasoline into the chambers. And there was almost the same raw gas problem that I had with solex intake manifolds with webers. THIS WAS A BAD IDEA.

So I had another intake manifold welded up and then routered out to have smaller but round and equal size diameter runners. Probem was that 26mm was the largest size ball I could pass through the runners. I thought that would be a detriment. But it did not seem to be all that bad. The engine ran fine, used less gas and had more power until at the top end. It did not seem to be better enough for me at full throttle -- maybe better than the old car "dragging" a log experience, but not worth the extra money.

So, I finally got into Hemmings and made call after call to people who advertised as Weber carburetor specialists. Almost uniformly they referred me to the intake manifold specialists in Gilroy who do manifolds for Nascar racers. They were the ones who showed me and explained to me what they had long before come up with for SL 190 intake manifolds. They pointed out the need for the separation of the runners, but suggested only a flat plate be welded in vertically to separate them. That would permit a larger throat and also provide a slight "roll" to the air as it passed into the chamber. In fact, they also suggested I use copper gaskets on intake manifold cut with turned fingers stuck into the air passageway to get the air to spin (haven't done that yet, but mgiht in the future).

And so I started welding in plates. I am happy to say the modified intake manifold DOES MAKE A GREAT IMPROVEMENT, giving both better lower end and top end without raw gas anywhere. Problem, of course, is that the intake manifold cannot be used again with Solex again once it has been modified. Those carburetors each feed two cylinders, not just one, and if you separate the runners only the cylinders to the front side will get enough gas to run decently. Arno will have to keep a special solex intake manifold is he wants to have it for his lousy solex carbs to be "authentic and beautiful" in a concours. Yuk!

But, 40's or 45's is up to you. I use 36mm venturies in my 45's anyway. Should be able to get those into a 40, I would think. I am an old man, but I confess I run my car a bit hotter than I should. It makes me feel young and still the guy my wife used to welcome back and call the "old two liter lover" when I returned from a crisp hot ride, especially with my favorite German music playing on my radio. I guess the name can have many meanings. Lately, I am sorry to say, the emphasis seems to be mainly on "old". I drove my first two liter, the factory demonstrator, on the autobahn in Germany in December 1959. IT HAD WEBERS. It also had a dry sump. I have always hated Alfa for neutering the car when they started to mass produce it for sale. It took almost 30 years of trial and error and effort to get one to perform the way that first one did. I used to be able to pass cars passing cars back then. However, "The older you get, the better you were."

But don't be deceived. While they never will be race cars, they can drift pretty good and make you hold on pretty hard in the corners once you get the engine running like it should. The engine is basically the same engine the 1900 racers used. But I now believe the factory demonstrator had a sportiva engine. I had no prior experience, so I didn't realize then just how great it was. Amazing what a good carburetor set up will do. But don't ignore the need for a dry sump if your are going to corner a two liter long and hard. I've never run Seattle Internation Raceways (now Pacific International Receways) without blowing an engine because the oil would all get shoved away from the sump intake by centrifical force on the old "Indianapolis" turn (now turn two). And I think Adrian will confirm that. He blew one on his red two liter too (and only did a 2.08). Maybe the track was longer then, because even Berlinas can do it in 1.48 now. But, Happy New Year to you all.

Jay

[dretceterini]

What's interesting about the 1900s and may be connected to the intake manifold problem on 102s, is that most of the successful 1900s that raced used two DOWNDRAFT Solex carbs; the same ones that were used on Porsche 550 spiders.

[1,6 HF]

Don P has it exactly right; unless you have a separate runner to each cylinder, don't bother with DCOEs. And, with due respect to Jay, for street use 40s are a better idea than 45s for most people's driving style.

Yes, you can use a 36mm venturi with both 40s and 45s (it's at the upper limit for 40s). And, yes, theoretically a 36mm venturi should have a 45 barrel, but this rule of thumb (barrel = 1.25 x venturi) is less important than controlling the airflow velocity. With a smaller venturi you'll sacrifice some top end power, but you'll gain a tremendous amount of low end torque by keeping the airflow velocity up at lower rpm; by contrast, a 36 venturi can seem gutless at lower (below +/-3500) rpm, because the air flows too slowly at the lower rpm. What this means is that, when Jay says '40s or 45s is up to you', what he really means is that you need to assess your own driving style and decide whether you want more torque in the low-mid range or more power above 5000 rpm. If it were me, I'd go for the torque (40s with a smaller venturi than 36), but your preference might be different.

The attached PDF is the best basic explanation I've found of how to select and set up DCOEs, and should help you with some jetting & emulsion tubes. Weber DCOE setup.pdf

You'll notice that the 'baseline' settings are based on the venturi size, so those settings are theoretically the same for any given venturi, whether in a 40 barrel or a 45 barrel. In practice, however, this isn't quite true--airflow velocity downstream of the venturi does affect the settings--and you'll need to so some experimentation. Still, the theoretical calculations will give you a starting point that shouldn't be too far off. When you do experiment, though, do yourself a favor and only change one variable (main jet, air corrector, emulsion tube, idle jet) at a time.

So decide which driving style works for you, pick the venturi (and the 40s or 45s) on that basis, and good luck.

[Erik S]

Quote:

Originally Posted by dretceterini What's interesting about the 1900s and may be connected to the intake manifold problem on 102s, is that most of the successful 1900s that raced used two DOWNDRAFT Solex carbs; the same ones that were used on Porsche 550 spiders.