Everything You Wanted to Know About Slant 6 Carburetors but were Afraid to Ask
By Joshua Skinner, revised 2/2/06
Many people have asked me what kind of aftermarket carburetor to use on their Slant 6. There is, of course, no one correct answer. Most people want to improve the performance of their Slant 6. Some people want to improve mileage, and others want to improve power. The really demanding types want both. I will endeavor to help as many of you as possible. But beware! There are no Holy Grails of fuel mixing, and you will find many dire warnings throughout this article. Do not despair! We will come out on the other side! As long as everyone understands that there are compromises that have to be made we will be fine. Having said that, let’s see what we can do.
1. Ultimate power and money, power peak over 5- 6k RPM, good drivability.
Much tuning work to see all the benefits:
If you want power, power, power and love to spend money, Weber DCOE, Mikuni PHH, or Delorto side draft carburetors are for you. But if you don't have $1000+ to spend, you would do well to look elsewhere. Individual throttles per cylinder have the potential to boost torque and make good horsepower. The biggest problem here is the expense. They are expensive to buy and expensive to tune since you have to calibrate three carburetors. The Mikuni PHH is no longer made and I believe the Delorto has met the same fate.
This is the famous side draft Weber that you may have seen on modified six cylinder Jaguars, Datsun Z cars, and many others. These are a great racing carburetor that can work quite well on the street. They look awesome, and terrific amounts of power can be made. They do require quite a lot of work to use them. They use a special intake manifold that mounts three of these two-venturi units, giving you one venturi for each cylinder. The throttle linkage and fuel lines may need to be fabricated. Then, once you mount them, they must be tuned. I won't even try to discuss tuning of these carburetors. It would and has filled a book. If you really want to use these carburetors or want to know more about them, buy a copy of the Haynes Weber Carburetor workshop manual. It discusses the DCOE and other Weber carburetors in detail.
2. Very good power, reasonable cost, power peak over 4-5k RPM, poor to reasonable economy. Not generally recommended for normal street use:
This is your standard 4 bbl manifold and carburetor. You can use a Hyper-Pak manifold and its Carter carburetor. You can use a Clifford or Offenhauser manifold along with a Carter or Holley carburetor. You might even find an old Edelbrock manifold for the Slant 6. The most common setup I come across is the 390 CFM Holley 4 bbl on a Clifford manifold. I had this on my '66 Dart, and with a 4 speed, had very good luck with it.
If you have an automatic with a stock converter and a mild cam, or any near-stock application, this arrangement is just a little too much. I read the recommendations in the Mopar Performance books and they said the 4 bbl is for race use only. I didn't heed the warning. I went ahead and used the smallest new 4 bbl I could fine (the 390 Holley) and actually had quite good luck. My manifold was an old unheated version. This wasn’t a big deal in Mountain View, California, but colder areas should have manifold heat.
4 bbl. Manifolds:
Hyper-Pak: The Hyper-Pak manifold makes the best power this side of the Weber DCOEs. In fact they might just make a little more top end power, but the mid range, in theory at least, should be a little less than a well-tuned side-draft setup. The Hyper-Pak has very long runners and mounts the carburetor toward the driver’s side of the engine compartment. This makes the throttle linkage more interesting (read: time and money consuming), but not insurmountable by any means. Doug Dutra used to reproduce this manifold, but has since sold the patterns to Clifford Performance where you can buy a new one. The Carter carburetor originally used on this manifold is no longer available, but any standard 4 bbl can be used. Be aware that this manifold is not heated and has poor drivability in cold weather.
Clifford and Offenhauser: These manifolds mount the carburetor in about the same location as the stock manifold. They also use standard 4 bbl carburetors, but installation is a bit easier as both manufacturers offer installation kits. The Offy manifold is easiest to use on a vehicle with stock exhaust as it has a stock-type manifold heat arrangement. This is good for vehicles operated in cold weather. The Clifford has to be used with headers (or your exhaust manifold has to be modified) because it does not cover the large hole at the top of the factory exhaust manifold that is used to heat the bottom of the stock intake. These manifolds don't make quite as much power as the Hyper-Pak because there is less of a "ram" effect. The Clifford can also be used with a 2 bbl as they have interchangeable top plates.
4 bbl carburetors:
A Carter WCFB (about 400 CFM) is a good choice for engines that aren't too radically modified. This carburetor is not available new, but used ones can be had at most swap meets as they came on all sorts of early 4 bbl cars. Unfortunately they are getting rare and parts aren't easy to find. They are also kind of tall and heavy.
The 390 Holley will work. Mine does, but it has the nasty habit of pouring too much gas through the engine at low speeds. I blame the lack of manifold heat. Once the car is fully warmed up and on the highway I get near 20 MPG. You can also try a 500 CFM Carter/Edelbrock carb, but they have a manual choke and could be even worse at low speeds.
3. Good power (comparable to or better than a 4 bbl on mild engines), cheap to reasonable cost, reasonable to good economy depending on carb choice, general street performance on stock to moderately modified engines:
These are the 2 bbl carburetors. They come in two basic varieties. The first is the synchronous opening type in which both throttle butterflies open together. The other is the staged opening variety in which one venturi acts as a primary and the other starts opening after the primary is about 2/3 open, giving increased flow capacity. Both types come in many shapes and sizes.
The synchronous type is the most common carburetor found on most American vehicles. They range from little Carter BBDs to big Holley 500s. The Carter BBD and a 2 bbl intake manifold was used on the “Super-Six” starting in 1977. California models had a Holley 2bbl for their Super Six. This carb and manifold alone gave the "Super Six" ten more horsepower than the same engine with a 1 bbl. This is the largest synchronous opening 2 bbl I recommend for a stock engine. One big advantage the Super Six manifold has over nearly everything else is price. If you’re lucky you can get every piece you need to change to a 2 bbl in a wrecking yard. Even the linkage pieces for the transmission are there. Reports are that the mileage losses with this setup are usually quite small. I once tried a 350 (maybe a 500) Holley 2 bbl. on an otherwise stock 225 and all I got was an engine that was flat on its face until 2000 RPM, and a big loss in gas mileage. That particular experiment included removing the intake manifold heat. I would expect better drivability and economy with a heated intake. Live and learn!
The staged type of 2 bbl gives us the best hope for improving mileage and power on a stock slant 6. These carburetors usually use a nice small primary venturi for low speed use that will give good throttle response and economy while having a larger secondary venturi for increased power when needed. This type of carburetor is common on many 4-cylinder engines such as 2.2 Chrysler products, Pintos, Vegas, and Rabbits.
A Slant 6 Club of America member in Sweden used a Weber 32/36 DGAV carburetor on his Valiant wagon. He went from 19 highway/15 city to 22 highway and a little less than 15 city. This is in a wagon with 3.55 gears so you may do better. He reports "a definite improvement in torque" and "with a heavy foot the acceleration is much better than with the old carb". He blames this extra acceleration power for the slight loss of city mileage. He also used an adapter to bolt this carb to his 1 bbl manifold so power should be increased further if this carburetor was mounted on the 2 bbl Super Six manifold. You can read more about it in issue 54 of the Slant Six News.
Another route to go if you have a modified engine would be the Holley 2305. The 350 cfm version should do quite nicely on a mild engine. The 500 would only be for more heavily modified motors. The 350 may sound like its only 40 cfm smaller than the Holley 390 cfm 4 bbl, but it isn't. Holley tests its 2 bbl and 4 bbl carburetors differently. They use a 1.5" of Hg (mercury) pressure drop for 4 bbl, while 2 bbl carburetor flow numbers are generated at 3" of Hg. So the difference is actually much greater than the 40 cfm that the numbers indicate. There is a formula to convert between the two, but I don't have it handy. One other thing about this carburetor: It uses annular discharge booster venturis that give a better metering signal. This lets you get the jetting closer to optimum without suffering bogs and hesitation. If I can stand to part with the money, I may try this in place of my 390 4 bbl to test the results. If not I'll try a Holley/Weber carburetor off of a Pinto or Vega.
Update! The Holley 2305 is no longer available. They didn't sell well and were discontinued.
I did try a Holley model 5200 that is nearly a direct copy of the Weber 32/36 DGAV. It even says made under license by Weber in the float bowl. The carb I got was off a 1980 2.3l Ford Pinto. It had an electric choke that made installation easier on my modified '66 Dart. This carb was a disappointment on my modified 225. I expected better low-end torque over the 390 Holley, but I lost torque. It also had a nasty bog if I opened the secondary too soon. The accelerator pump that works for a stock 2.3L was inadequate for a warmed up 3.7L This would probably be a good carb on a stocker as was the experience of a Slant 6 Club of America member, but my engine didn't respond well.
You can put almost any carburetor on any engine and make it work. Some may work well. Others will frequently work very poorly. These are my suggestions. Some have been tried on the Slant 6 and some have been used successfully on other in-line 6 cylinders.
Two downdraft 1 bbl carburetors: This is a relatively easy and inexpensive setup. Offenhauser makes the manifold so all you need is that and one more stock carburetor. Expect to loose at least one or two miles per gallon, maybe more. The one report I've read on this setup was on a very mild Slant 6 (only a cam change). He said, "It's quick off the line and really 'comes on' at 75 or 80 MPH."
One side draft 1 bbl: If you have a Cortez motor home with the Slant 6 you already have this. The stock setup uses a Carter YF carburetor just like the ones on the old 6 cylinder Corvette. I'd like to try an SU or its Japanese-made Hitachi counterpart. It probably wouldn't make too much more power, but the throttle response and mileage should go up considerably.
Two or three side draft 1 bbl carbs: This would take a custom manifold, but should work great. Again the SU is the carburetor of choice because it is almost impossible to over-carburate your engine since they only feed as much as the engine demands.
Two or three downdraft 2 bbl carburetors:
www.piercemanifolds.com sells a package with the dual carb Offy manifold and a pair of small synchronous opening Weber downdraft carbs. I know of one person running this setup and the oxygen sensors he has installed indicates it runs pretty rich everywhere, but it does run and drive well. I don’t have a mileage report.
For three carbs a custom manifold would be needed. Dave Mueller of Kelso, WA, has done this on his racecar, Vivie the Valiant. He built a sheet metal intake mounting 3 Holley 350 cfm 2 bbl carburetors. Vivie is a serious racecar so don’t believe that this is a street-friendly combination. It’s really a custom tunnel ram on a slant.
Anything else you can think of: Two 4 bbl carbs, three 1 bbl carbs, whatever you can think of. It's probably been done. Just remember smaller is almost always better on a street car.