Sonett Carburetion

SONETT CARBURETION

by Bill Lee

When I originally started out to do this column, I was going to write about the different carburetors that can be used with the Sonett to increase performance. About halfway through the first draft of the column, however, I began to feel that I had jumped into the Boston Marathon at the halfway point.

To different people, "better performance', may mean more power, better gas mileage, smoother engine performance, or a combination thereof. These goals are no incompatible, and a new carburetor can, and frequently does, improve all three. But there is another, easier and cheaper way to improve "performance', however one defines it. That way is to see that the carburetor now on the engine is working at its best. This is the "first half of the marathon" for the following reasons. A carburetor does essentially two things: it controls the amount of air passing through he engine, and it dispenses the proper amount of fuel into that air before it goes into the engine. Putting a bigger carburetor on a car allows more air to go through the engine at full throttle and increases maximum power. However, if the ratio of fuel to air provided by your carburetor is not perfect, and the odds are that it is not, you can improve performance (power, mileage, and smoothness) by properly adjusting and tuning the carburetor.

Carburetors are no different than the other key parts of the engine and require tuning up and periodic maintenance for top performance. Some carburetion problems have sources other than the carburetor itself, e.g., an intake manifold air leak. Even if you have decided to put a new carburetor on your Sonett, you should still tune up the engine so that it is running as well as possible. The alternate carburetors discussed below are more difficult to adjust than the stock carb, and properly setting up the new carburetor will be much easier if the engine is properly tuned first. If, after tuning up the engine and adjusting the carburetor, the engine still does not perform as it should, a new carburetor is not the solution to your problems-not yet , anyway. Therefore, this column will put the cart before the horse and will detail several different carburetors that can be used on the Sonett. Next newsletter you get the horse, and the Technical Corner will be about how to rebuild and/or tune the stock Sonett carburetor to get better performance.

You could probably use almost any carburetor with the V4 engine, with enough effort. Whoever, for people who want a carburetor that will fit under the stock Sonett hood and which can be installed without making major modifications to the carburetor or the intake manifold, there are only two or three carbs to choose from: for Sonett IIIs, the Solex carburetor used on the SAAB V4 engine (including the Sonett V4) from 1967 through 1969 (all models); the Weber 28-36 DCD; and a single-barrel carburetor from Motor Sport Services (MSS) which has been specially modified for the SAAB V4. Each of these carburetors is discussed in detail below.

Sonett V4s (as well as 1967-1969 SAAB 95s and 96s) used Solex carburetors, rather than the Autolite (Ford Motor Company) carburetor used on the Sonett from 1970 on. Two different Solex carbs were used: the 28-32 PDSIT-7 in 1967-1968, and the PDSIT-4 in 1969. These two carburetors are very similar, the principal difference being the manner in which the float chamber is vented. Both are single-barrel carburetors with a throat diameter of 32mm, slightly larger than the Autolite carburetor.

Installing one of these Solexes on a Sonett III is relatively simple. The Solexes fit onto the stock Sonett III manifolds. The only change, which may be necessary, is installing longer studs on the manifold for mounting the Solex. If you scavenge a Solex from a Sonett V4, get the air cleaner and the throttle linkage, it you can. The Solex will fit under a Sonett III hood with the Sonett V4 stock air cleaner. A Sonett III air cleaner will not fit under the hood when using the Solex carburetor. The throttle linkage for the Solex is a little different from the stock Sonett III linkage. If you get a Sonett V4 linkage, use this is in place of the Sonett III linkage. Otherwise, you will have to fiddle a bit with the linkage. I do not know whether the linkage from a 95 or a 96 will fit a Sonnet III.

If anyone tries it, let me know. If you get a used Solex remember the cautions above about old carburetors, and unless the carburetor is in mint condition, it is well worth your while to rebuild it.

I can't tell you how much you may have to pay for a used Solex, of course the real financial advantage lies in the fact that the Solex fits onto the stock Sonett manifold. The Weber and MSS carburetors discussed below, both require buying a new or specially modified manifold, as well as the carburetor, and the manifold costs about as much as the carburetor.

What kind of performance can be expected? Since the Solex is slightly larger than the Autolite carburetor, it should provide a small increasing power. Also, many people contend that the Solex is a better carburetor than the Autolite. The one person in the club who has a Solex on his Sonett III feels that he gets a little more power, better mileage by 2-4 mpg, and smoother running. People who have added Solexes to post-1969 95s and 96s report similar small improvements in overall performance.

For a much larger increase in power, a Weber 28/36 DCD can be put on the Sonett engine, both in the V4 and the III. The 28/36 DCD is a two-barrel carburetor with a progressive linkage. The main throat diameter is 28mm and the secondary is 36mm. The Weber will not fit on the stock manifold and requires a new, two-barrel manifold, discussed below. The linkage again will require some work to make it fit. If you buy a Weber new, you can save yourself some trouble by ordering the 28/36 DCD 23, which is intended to go on a Ford Cortina GT and has a throttle shaft link that is compatible with the SAAB linkage. The 28/36 DCD, like all Webers, has an impressive number of different flow control devices that can be changed to achieve the optimum mixture over the entire operating range. There are ten different adjustments that are available, not counting idle speed and mixture adjustments.

There are two two-barrel manifolds that can be used with the 28/36 DCD. SAAB originally offered a "high-rise" manifold that was primarily intended for use on the 95 and 96 (SAAB part # 00-14000.) Even without an air cleaner, the 28/36 DCD will not fit under the hood of a Sonett V4 and III. SAAB later developed a "low-rise" manifold (SAAB part #00-14001). With this manifold, the 28/36 DCD fits under the Sonett V4 and III hoods, although the stock air cleaners cannot be used. The manifold is not currently listed as being available from SAAB. It is, however, available from Ford Motor Company under part # 448,614. (The V4 engine was originally developed by Taunus-Ford of Germany. It is still sold by Ford in the U.S. as an industrial power plant 91H.)

The 28/36 DCD is a progressive carburetor. When the accelerator is depressed, only the main throttle plate (or butterfly) in the main throat opens for the first 80% or so of the throttle travel, while the secondary throttle valve remains closed. The throttle plate in the larger secondary throat begins to open only after the throttle travel exceeds the 80% point and is completely open when the accelerator is fully depressed. Thus, the 28/36 DCD will cruise along on only the primary barrel most of the time, the secondary coming into operation only when the throttle is at or near maximum power, i.e. when you floor it.

A progressive carburetor properly set up will give better gas mileage at low power settings than a similarly-sized carburetor that is not progressive. Carburetors tend to be more efficient-i.e. get better gas mileage as the velocity of the airflow through the carburetor increases. This is for two reasons. First, the main jets are more efficient at higher velocities. (This is why venturis are located at the main fuel supply orifice in the carburetor throat. A properly designed venturi can increase the air velocity past the fuel orifice while offering less overall restriction to the air flow than simply decreasing the carburetor throat diameter. This is why an independent idle fuel supply system is necessary. The air velocity at idle through the carburetor is so slow that the main jets supply little or no fuel. Secondly, fuel from the carburetor is atomized more completely and more evenly dispersed as the air velocity increases.

Thus, as the carburetor throat diameter is increased, the fuel efficiency of the carburetor goes down for low and medium power throttle settings, which is the case for most driving. A progressive carburetor provides the best of both worlds. When the throttle is at or near full power, both barrels are open for maximum power. At lower throttle settings, below about 80% or so, the secondary throttle valve is completely closed, and the carburetor operates more efficiently on the smaller primary throat.

How much does the 28/36 DCD cost? (The following prices are the result of a few phone calls and may not be the lowest prices available. If you know of some place with lower prices, let me know, and I'll put the information into the next column ) The 448,614 "low rise" manifold lists for $ 156 from Ford. It is available from Northeast Ford Engines, Inc. (Magnolia, MA ) for $125. It is also available from MSS (part # 474,010) for $128. The 28/36 DCD lists for $182 and is available from Far Horizons Automotive Accessories, 1905 East Deere Ave., Santa Ana, CA 92505, for $160 in single quantities. There is a very substantial quantity discount, however. In quantities of seven, the price goes down to $110 ! There are already seven people in the club who are interested in getting a 28/36 DCD at this price. If you might be interested, get in touch with me (Bill Lee) or Scott Prentice.

The only 28/36 DCD installations I know of are not on Sonetts, one is on a 95, and one is on a 96. Both cars are owned by the same person. He reports that he gets better mileage by a few mpg and " noticeably more" power, compared with a stock Solex carburetors. One of these Webers has over 100,000 miles on it, and it is still running well. For more specific performance figures, see the section on performance following the description of the MSS carburetor below.

The third carburetor is from Motor Sports Services, 1400 East Second St., Jamestown, NY, 14701, run by Jack Lawrence. Jack specializes in SAABs, and currently races two Sonett IIIs. Last year, Jack took both 1st and 2nd place in SCCA (Sports Car Club of America), F production, so he is definitely knowledgeable about sonnets.

The MSS carburetor is a 43mm, single-barrel carburetor which MSS buys from Holley and modifies for the SAAB V4 engine. The MSS carburetor does not fit on the stock Sonett manifold. MSS furnishes a manifold for the carburetor, which is the stock manifold machined to take the MSS carburetor. You can buy the manifold, or save money and exchange your manifold for the MSS version.

The MSS carburetor will fit under the Sonett hoods with the stock Sonett air cleaners. The linkage requires minor modifications, and a few other modifications may be necessary, depending on the year of the car. On late 1973 and 1974 Sonetts, the distributor advance mechanism also must be modified.

There is no fitting for the PCV (positive crankcase ventilation) valve on the MSS carburetor, and MSS recommends eliminating the PCV valve. I personally oppose this. A closed PCV system does decrease pollution, and any decrease in power attributable to the PCV system is completely insignificant, especially for a street car. Also, removing the PCV valve may result in an oily odor near the engine compartment when it is hot and the car is not moving, according to Jack Lawrence. The PCV valve can be retained by using an intake manifold from a 95 or 96 that has a power brake fitting, or MSS will add the fitting to your manifold for about $10.

Jack went to his system with the MSS carburetor and modified manifold because, he claims, both of the SAAB/Ford two-barrel manifolds discussed above are "lousy." Jack believes that the air passages in both the high-rise and low-rise manifold are poorly designed, resulting in poor mixing of the fuel to the cylinders. I talked with Jack at some length about this, and he feels that his carburetors with the modified stock manifold are definitely superior to the Weber 28/36 DCD with the manifold described above. The difference is not large until the engine reaches high rpm's-over 6000 rpm or so. Jack claims that the MSS carb is better than the Weber by about 2-4 hp from 3000 to about 6000 rpm. This increases to an improvement of about 13 hp over the Weber set up at 7000 rpm. Jack was very friendly and helpful to me and he indicated that he would be happy to discuss his carburetor with anyone who was interested.

How much does the MSS cost? The price for the MSS carburetor (MSS part no. 379,011) is $98. The manifold with and without the power brake/PCV valve fitting (par nos. 373,011 and 375,011) are respectively $53.50 and $43.50 and there is an additional refundable $80 "core" charge if you do not send your manifold with the order. The refund is, of course, subject to the returned manifold being in reasonable condition. (Note, MSS has just revised its price list and the prices quoted above are probably low.)

What sort of performance improvements can one expect with the Weber or MSS carburetors? Jack gave me a whole bunch of performance specification for his carburetor. While he may be a bit biased, his figures are based on extensive dynamometer testing and should be more accurate than most information of this type, which is of the "...it felt like about 10% more power ..." type of guessing. Although these figures are for an MSS system, they are probably close to being correct for a Weber system, with the adjustments given above in comparing the Weber and MSS performance. I want to stress that I am not in any way pushing MSS equipment by giving the following figures, nor am I knocking it. I vie you these figures as probably the best guide for predicting what sort of performance improvements can be expect from the V4 engine.

According to Jack, a stock V4 gets a maximum of 60-61 hp (SAE). This is true for both the 1700cc engine and the earlier, higher compression 1500cc engine. Adding an MSS carburetor alone to a stock engine will increase power by about 7%, or by 4 hp. A low restriction exhaust system added to a stock engine with the stock carburetor produces a similar increase in power. The MSS exhaust system, for example, has a 1.75" inside diameter, and provide a 70% increase in cross-sectional are over the 1.34" inside diameter stock system. This exhaust alone, added to the stock engine also results in an increase in power of 7% or 4 hp. The MSS carburetor and exhaust together, however, increase the stock engine's power by 20% or 12-13 hp.

Now this may sound like saying two plus two equals six, but it makes sense when you think it through. Engine power increases as more air flows through it. Both the exhaust system and the carburetor, to some extent, restrict the air flow through the engine. By substituting a carburetor with a larger throat, the carburetor provides less restriction to the air flow and more air flows through the entire system. If the old exhaust system still restricts the air flow, however, the maximum performance of the carburetor may not be fully realized. This sort of synergistic effect may explain why one person reports a large increase in power, while another person adding the same carburetor to a similar care reports a much smaller increase.

So with an MSS carburetor and low restriction exhaust the engine power has increased to 73 hp. What happens to gas mileage? The answer is that the care can get both better or worse mileage, depending on how it is driven. The MSS carburetor is more efficient than the stock carburetor and require less fuel to keep the car going at a steady speed. In other words, the MSS carburetor require less fuel to put out the same constant power and can get better mileage for constant speed highway driving. The car will get much worse mileage when you floor it, though. A larger carburetor allows more air to flow through the engine (when the throttle is wide open) which require more gas. The loss in mileage when you floor it can be as much as 10% to 20% and is much greater than the small increase in efficiency for constant speed driving. So if you use the extra power at all, and why get it if you don't, your gas mileage will probably go down.

The following figure don't really have anything to do with carburetion but are interesting. The source is again Jack Lawrence. For the 73 hp engine described above, using the stock compression ratios (8.0 or 9.3:1), adding a relatively mild "street" camshaft, stronger valve springs, and lighter lifters, grinding and matching the ports (but not polishing the heads), the engine should get 90-95 hp at 6800 rpm. With a full race cam, 13:1 compression ratio and large dual 40mm throat carburetor, the engine gets over 120 hp at 7000 rpm. Jack claims he get 143 hp out of his F production race cars at 8500 rpm.

There are two other carburetors which can be used on the Sonett but which are supposedly not suitable for anything but racing. The Weber 40 IDF, and the Solex 40 PII (again as modified by MSS). Both are tow-barrel, 40mm, no progressive carburetors. The Weber requires either of the Ford two-barrel manifolds listed above but wont fit under a stock hood. The Solex requires a modified intake manifold from MSS.

Which carburetor is for you, if any? The choice is yours, but I'll mention a few factors to be taken into consideration. Putting a Solex on a Sonett III is fairly simple, and should result in modest increases in power, mileage, and perhaps smoothness. To get a large increase in power, you must go to the Weber 28/36 DCD or MSS carburetors. Remember, however, that you can get the same 7% increase in power much more simply and cheaply by just putting on a larger exhaust system. Putting on a new carburetor alone does not make economic sense. (So when did that ever stop anyone....?)

Between the 28/36 DCD and the MSS the MSS is a bit cheaper, and more powerful. It is also probably easier to set up and tune properly that the Weber, plus Jack Lawrence can probably help in solving any problems that arise. The Weber does not come correctly jetted for the Sonett. I only know of two 28/36 DCD installations. Setting up the carburetor on the first car, a 95, was so difficult that it was eventually done on a chassis dynamometer. With the data from the first installation, however, setting up the second car, a 96, was relatively straightforward. If anyone is interested in the Weber settings get in touch with me. On the other hand, if you like fiddling around with your carburetor, and I'm told there really are people who are that masochistic, the Weber is for you. Everything in the Weber cab be changed, venturis, main jets, air corrector jets, emulsion tubes, etc.

So there you have it.