4 barrell carburation

[balleta]

Don't know if this has been discussed before but, do any one have installed a 4 barrell carburator in a 911? Maybe a 750 double pumper? On top of a modified CIS manifold? It should be a lot less expensive than Webers or PMOs. Can be a temporary solution for the poor man's Porsche. Any ideas or photos?

[sammyg2]

It will run worse than webers or PMOs.
One of the challenges of a flat 6 (or 4) engine is the intake system.
With a single carburetor you will need to heat the intake runners to keep the fuel vaporized during periods of lower velocity and vacuum. Read major bog when the throttle is nailed.
The fuel will tend to condense on the walls of the runners.
It would work fine under full throttle though.

[Early_S_Man]

It has been discussed before ...

Double-pumper would likely never work ... the vacuum-controlled secondary 3310-style 4150-series would be your only chance at anything close to accurate A/F ratios over the rpm range.

[neilca]

I, like the others, have no data on whether this would work. However, it is on my to try list. I think it would be beneficial to have a carb that can be jetted easily without breaking the bank. As for whether it would work, I think a plenum with relatively short runners would work. It will be a while before I am down to that area of my to try list but I will share the info once complete. If you take on the challenge please let us know.

With a plenum the 750 would be way too large. For a 2.0-2.4 I am thinking of a 450-500 CFM carb.

neilca

[Don Z.]

OK, this is something that the Corvair guys try often. They try a lot more than they succeed. Most success is from those who re-route oil lines to keep the plenum hot.

It can be made to work. Maybe... and then you'll put on PMOs, Webers or FI anyway.

[RarlyL8]

You're talking less than 3.5L and less than 250HP. A 500cfm 2 bbl would be about right. Intake design would be critical. I have a picture of a small 4bbl carb on a Corvair somewhere. I have no idea how it ran but the runners looked way too long.

[Porsche_monkey]

Search. You'll find a picture. Once you see it the idea loses some of it's attraction.

[balleta]

I remember a Corvair 4 barrell manifold sold in the JC Whitney catalog about 25 or 30 years ago. That's where the idea came to my mind but have never saw it working. The air temperature here is always over 80 F; therefore I don't think that warming the mixture is so critical, at least here. Anyone with a photo?

[fred cook]

Holley makes a neat little 390 cfm 4 bbl carb with vacuum secondaries. I had one on a 2.6 Capri V6 ummmmm "several" years ago. It was on a built V6 with Isky cam, ported heads, headers and dizzy with custom advance curve. I had it mated to a 4 speed trany with an open differential. Top end was about 145 and it pulled like a freight train all the way up. With a compression ratio of 10.5:1 it required premium fuel which it drank at the rate of about 13 mpg in town. On the road it would get about 18 mpg. On a 911 engine with a heated intake manifold and insulated intake runners it would probably be just about the right size for 2.4 - 3.0 litre engines.

Zoom, zoom, zoom...........

[balleta]

Here is a picture of how it should look more or less. (This a Corvair engine, not Porsche)

[jackb911]

I put one of the Eelco (as I recall) 4 barrel intakes on my first car, a '61 Corvair Monza, which had a 2.4 liter flat six. It actually worked pretty well, except that in my youthful enthusiasm I used a much too large carb, a Carter AFB made for a 300 HP 327 Chevy V-8. Also, the Corvair was saddled with a Powerglide 2-speed automatic transmission. The setup looked really cool, which was important when I was 16.

Had I used the recommended smaller Rochester 4GC carb, and had the car had a 4-speed, it would have worked great. As it was, it was pretty soft on the low end, but it pulled very hard over 4K.

The 4 barrel conversion kits worked especially well on the 140 HP 2.7 liter Corvairs, replacing the four single barrel Rochesters that came on them. It ended the chore of balancing the carbs with a Uni-Syn (remember those?)

I see no reason why a similar setup would not work on a 2.4 or larger 911 after the proper carb CFM and distributor curve was worked out. I think a 390 Holley might be a bit too small. The Edelbrock 500 (made by Weber, by the way) should be just about right.

[Mr9146]

Why?

[jluetjen]

Originally posted by jackb911

The 4 barrel conversion kits worked especially well on the 140 HP 2.7 liter Corvairs, replacing the four single barrel Rochesters that came on them.

I see no reason why a similar setup would not work on a 2.4 or larger 911 after the proper carb CFM and distributor curve was worked out. I think a 390 Holley might be a bit too small. The Edelbrock 500 (made by Weber, by the way) should be just about right.

Let's see -- The mildest 2.4 911 was already making 130 HP on Zenith carbs -- which you can find for small money, and it certainly wasn't the carbs that was holding it back. Liter for liter the Porsche engine was already at a higher state of tune then the Corvair engine. The slightly more clever people among us have discovered that with a little bit of work, you can replace the Zenith's smaller venturi with the wide selection of Weber 40 venturi. Once you've done that, it's a small matter of jetting it for your configuration of engine. There are a number of engines of various states of tune running well over 200 HP on 40 mm carbs. So what does a 4bbl buy you?

- Is it cheaper then a pair of Zeniths? I doubt by much
- Is it capable of making more HP then a pair of Zeniths? It doesn't appear to
- Does it provide better performance then a pair of Zeniths? While the WOT performance of both configurations may be comparable, under part throttle the IR Zeniths should definitely outperform a 4bbl configuration.

So why bother?
Here's a different question -- why not convert any of the popular (Ford/Chevy/Nissan?) V6 engines to run on Zeniths for racing purposes? It sure sounds like a cheap "6-pack" arrangement to me!

[304065]

Even better, why not convert a Lancia Fulvia V6 to use the 911's IDA Webers. . . wait a minute. . .

[neilca]

The main reason to go to a 4 barrel is to jet the carb you only change two jets. Try that with a Zenith.

[RarlyL8]

I don't know about the price and availability of Zeniths but the P/A of Webers is not good. A rebuilt 4bbl would be 10 times cheaper than a junk set of Webers. Economics come into play when you consider putting a $1500 set of used carbs on a $2500 2.4L engine. It may not be optimum but then again neither are Webers when compared to something even more expensive - EFI.

[quattrorunner]

If your trying to save mony with a porsche, I think your kidding yourself. Give it up and look for something else. Maybe a 69 camero? Wait..... nope, maybe a gremlin? There you go.

[balleta]

It is not a matter of saving money on Porsches, for me it is just plain fun to solve situations the least expensive way with parts easily available. As I said at the begining, it can be a temporary solution. Sometimes we don't find what we want when we want. I'm sure that I'm not alone in this thought.

[ianc]

And what's wrong with the CIS injection that you're considering this?

Why take a step back?

ianc

[RLJ]

Horsepower and carbs. are a myth. Drag racing and carbs are in there own world, period !

Holley came out with the 4118S for the Cobra. 715 cfm.

I worked in for an engine builder that built motors for Winston West champions, APPA (boat racing)record holders, and a bunch of other fast Pacific Northwest go fast guys. I remember working on the dyno one afternoon and we made "more HP' with a "smaller" carb than the one the owner supplied. Slap my head, gee.................. less fuel is better "sometimes".

Four hole carbs are "cool" if the four holes "match" what your runnin'.

In todays world, a bowl of fuel with a "float"l and vacume sucking it out of that container don't get the job done !! My BMW R 1100R bike is an example. Why in todays world would you put carbs on anything, other than a race car. That is where my 46mm PMOs' went, to a good home on a race car!


Best luck,
Randy Jones
1971 911 "Iris"
Now with EFI and headers !