CIS vs. Webers?

Hi everyone. Can someone tell me any advantages in goin carbs as opposed to retaining my CIS system? Its for a 2.4T and just wondering as to the performance and reliability of going carbs. What about the difference between Webers and Zeniths? I have a pair of rebuilt Zeniths that I could throw in, but am thinking whether I should go with Webers. Any insights on this? Thanks.

Advantages of carbs over CIS:
(insert tongue in cheek)

1. You can get to tinker alot with jets.
2. You can get flat spots in acceleration that are fun to diagnose.
3. You can get horrible gas mileage which keeps you going back to the fun gas station.
4. You get to wash the back of your car alot as the soot builds up fast.
5. You get to experiment with that new fire extinguisher in the garage.
6. You get nice flames out of the tailpipe which are fun to showoff at night as you backfire.

The only truth in the above statement is reduced fuel mileage. All the other things happen only if you don't know what you are doing. If you can tolerate reduced gas mileage then carbs are a HUGE improvement over CIS when it comes time for modifications. If your car is a daily driver or commuter I'd stay with the injection, fun weekend toy - go for the carbs.

Now I am probably wrong in saying this, but I will say it anyway. Isn't CIS constant injection system. In other words, it is pretty much like carbs in the fact that fuel is going through it all the time and a constant spray. CIS isn't that much better economy-wise. I know in one of my previous cars...a Saab 99, it had and the injectors were the exact same ones from a '74 911. When I checked to see if they were working once, after rebuilding the engine, they constantly sprayed a cone of fuel...no pulses. This is going from memory...it has been a while. While the CIS may give you better gas mileage(not much) I would go with the carbs and different cams. Not sure what cams you can get for your car, but the Solex grind comes to mind...at least for the 2.7. Those definitely will change the way the engine functions...ie HP gain.

Anyway, I am sure if Warren sees this, he will chime in.

Cheers,
Paul

You're right the CIS injector sprays fuel constantly and the flow of fuel volume is based on the injector pressure.

BUT...

Even though the Motronic injector pulses it is also a continuous stream. You're probably thinking that the pulses are based on the cylinder firing order but they're not, the pulsing is a very high frequency and the frequency of the pulses is what determines the fuel flow volume. So in effect the Motronic is also a continuous flow injector.

Well, you certainly have more options for tuning with Webers, whereas with CIS you could put in a set of 2.4S pistons & cylinders, port the heads, and put in 964 cams ... and be at a dead end at around 150-160 DIN! My own suggestion would be to put in 2.4S p & c, Solex or 'E' cams, port the heads, and Webers, and I don't really think the resulting 165-175 DIN hp would be under-appreciated in the least! If you really wanted to be brave, you could go with 'S' cams or GE-60 cams ... but I don't think I would recommend it unless you have driven an 'S' and are familiar with its' torque curve and operating characteristics ... but the resulting 180-190 DIN hp (172-181 hp SAE net) would be satisfying to say the least!

If you plan to keep your car for a long time, and do your own work, it really doesn't matter which way you go in terms of reliability, but with Webers you have the flexibility to make changes over time, especially if you don't mind the idea of tearing down top-end components, or should you decide to, even going to 2.7, 2.9, or 3.0 liter sizes by having the block bored out for the larger cylinders! One set of Webers can handle all of those displacement changes, too!

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Warren Hall
1973 911S Targa

Quite true. If there will be engine modifications the CIS system will suck. Other advantages of carbs is they look WAY cooler. WAY, WAY cooler. They sound cooler. And I guess I disagree wiht reliability criticisms. We heard from someone having trouble wiht Zeniths recently, which agrees with my Zenith experiences, but how often do we hear folks whining about carb problems? Almost never. They don't have that many parts.

CIS, on the other hand, was developed much earlier in this century that most people think. It is actually a Rube Goldberg invention. All those crazy parts are just things that can go wrong. O2 sensors, air sensor plates, aux air regulators, warm up regulators, yadda yadda yadda.

Okay, having said that, my car is CIS and there are some advantages. When all those parts work properly (which is 99% of the time) the car fires up instantly, hot or cold, drives the same, hot or cold, gives excellent fuel economy, leaves clean cylinders, and generally runs perfectly.

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'83 SC

Yes, I believe the 57 chevy "fuelie" came with a Rochester morphadited version of this, licensed from Bosch of course. To the original unanswered question of Webers versus Zeniths, the Zenith's are Solex's copy of the Webers, with a few detail differences like accelerator pumps for each throat instead of one supplying 3 throats and that goofy decel enrichment device which was the main cause nobody liked these carbs. The main reason these aren't popular is that you were stuck with 27mm venturis where Webers had a wide variety of venturis available. So the Zenith Solexes weren't popular with the hotrod crowd. I believe Motor Meister has bigger venturis available now so there is that option. I have a set of the Zeniths that are from a Euro car that don't have that lame aux. enrichment device and I'm going to send them to MM for the mods. and use them as a back-up set. Webers are very reliable and once you get them set up properly, you're done. Only weak point is the needle and seats which should be replaced with the Grose jets. The fact that they don't have a choke kinda sucks too in cold weather. Superman's right though, look at all the CIS complaint posts in this BB. And look how few Weber probs. CIS is a great system it's just not a performance system.

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Tyson Schmidt
72 911 cabriolet (964 bodywork)

well, unless you like to constnatly fiddle with a car that is running too lean or too rich; bogs down in wet weather, and changes performance with every elevation change... keep the CIS.

all of Porsche's race cars since the 904 have been fuel injected (whether mechanical or electronic). the stock CIS system is capable of keeping the engine well supplied all the way up to twice the rated output of whichever motor you have. mthere are also ways of modifying the CIS system for so much power that you will end up busting your engine way before you end up starvign it from fuel. the Bosch fuel pumps pressurize the CIS with so much pressure it is absurd (ever tried to bleed the excess pressure out of a CIS system after it has been fully charged?)

i once asked about upgrades to the CIS system. the best authority on it is probably a friend of mine who is an engineer at Bosch. he showed me lots of data that supports that fact that you can get an assload of power out of the system with hardly ANY modification. when you start cranking out monsterous Hp... then all you need to do is upgrade the fuel distributor, lines, and mixture screw. CIS works pretty darn good considering it's simplicity.

carbs are okay if you are into circle track, drag racing, spending a LOT on gas, or if you are afraid of technology that is newer than the 1960s. if you want something that looks COOL, is VERY reliable, and will let you get extreme horsepower out of your car, without the crappy side effects of carbs... then look at this:
http://www.twminduction.com/Home/Home-FR.html

they look WAY cooler than carbs, and there is no hassle. i've seen TWM induction systems on more race cars than i can count. if you are still a die hard fan of carbs... how about putting a set of carbs on, and driving in cold, wet weather across the adirondacks, or the rockies, and try to keep up with someone that has a fuel injected car. keep their cell phone number handy because after you get out of NYC, you will be more than a few miles behind them.

obin

I have been running a 2.3 with 9.8-1 compression ported/polished breathing through 40 IDA webbers. Love It; took a while to get it dialed in, went through 3 main jet changes, but finaly got it right myself and havent had to touch then for the last thousand miles. If you do a lot of city driving I would not recommend them. Also if you have 3000 ft elevation differences where you drive you might have problems. I knew nothing about these carbs when I got the engine; but after reading a webber book and some patients I figured them out. Short of Motec or some other $10K system, mechanical fuel injection by far has the most HP potential; carbs a close second and the restrictive CIS a distant third. CIS was designed for good emissions with funny shaped low compression pistons for use on engines where power output was not the prime goal! It is a wonderfully simple and reliable system if maintained but does not have the HP potential of carbs and a hot to moderate cam(I like my solex cam, especially about 3800 rpm in second it makes my lousy(19mpg) gas mileage worth it, of course with an SUV at about 14MPG 19's not so bad )

My experience with Webers is the same as Mr.Land Locked. High tech fuel injection is easily more complicated and harder to dial in than a simple set of carbs. Why put high tech crap on a simple old car? How much power do you want from 2.4L for the $$ spent? These are the issues. CIS was cobbled together for emmisions use in the 70's. You cannot get more than 195 rear wheel hp out of a 3.0L SC with CIS without spending stupid money. A friend tried and has all the dyno sheets available. My current SC has CIS, I like the fuel mileage and reliability, but the SECOND it gives me any trouble it will be gone in exchange for more power (I think it knows that and behaves itself). Your decision should be based on the true use of your car, where you live, fun money to spend on it, etc. Have fun, these are the enjoyable choices!

Tangential question.

Rarly8
Is the 204 hp for the later stock Euro 3.0L SC not measured at the wheel then?

Would the 20 or 30 hp Bruce Andersen talks about adding to this figure using SSI's etc not equate to more than 195 hp at the wheel?

bill

[This message has been edited by billyb (edited 12-01-2000).]

I posted this earlier...excerpt from Bruce Anderson's book on the 3.0 and HP ratings...

On my 83 euro version it is stamped 930/10. On page 110 of Bruce's book, this engine is a "Rest of the World" (ROW) engine and has a 204 HP rating @ 5900 rpms. This is compared to 180 HP for most of the other 3.0 models....the following HP ratings are quoted from Bruce's chart...HP ratings per engine type stamped.


930/02 (78/79) ROW CARRERA 200HP
930/03 (78/79) ROW 180HP
930/04 (78/79) 49 state
930/05 (78/79) Japan
930/06 (78/79) CA
930/07 80 CA
930/09 80 ROW 188HP
930/10 81-83 ROW 204HP
930/11 NOTHING LISTED
930/12 76-77 SPORTO ROW 200HP
930/13 78-79 SPORTO ROW
930/14 78-79 SPORTO JAPAN
930/15 78-79 SPORTO JAPAN 180HP
930/16 81-83 US
930/17 81-83 JAPAN
930/18 83 SC RS 255HP
930/19 80 ROW SPORTO 188HP

BTW, the engine "type" stamping is on the passenger side of the engine, on the horizontal surface behind the engine "serial" number. The serial number is on the vertical surface of the fan housing support.

Simple: Go with the Webers.

Much aggrevation......

.....followed by ZEN.

Rich
70E

Actually, I think CIS is actually the "anti-Rube Goldberg" injection system.

I find that it is actually quite elegant in its simplicity, especially in its pure hydromechanical pre-O2 sensor form.

It uses a plate in the airflow to directly raise and lower a plunger that meters fuel. All housed in one small, simple unit. Can't get more direct or mechanically simple than that.

It has a very simple warm up regulator that changes pressure on the plunger for cold starts.

It uses the simplest fuel injectors around.

And that's about it.

You can debate mileage, performance, etc. etc. but I think its wrong to call it an overly complex system.

I thought CIS stood for "C"an't "I"ncrease "S"peed.

I will maintain my CIS only untill it acts up, then its webers. I'll just put is a VW motor or something every 2 years when the govt desides to sniff my butt.

Jim T -- You left out the two aux air valves, the thermovalve, the cold start injector, the thermotime switch, and the fuel accumulator, and that's just the K-basic! In it's basic operation it is pretty simple, but once you start adding cold start, altitude and full throttle compensation it starts to lose its elegance.
Still for daily driving and all weather performance, the only thing better is DME with its mass flow sensor.

Anyway to retrofit a Motronic DME system into a 3.0L?



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Bill Krause
'79 911SC Euro

yes, but that's stuff that most injection systems, including Motronic have. Even Motronic has a cold start injector, an aux air valve and temp detectors.

As far as the overall system, I still think that CIS is pretty simple and elegant, esp. given that it is a pre-computer system,l and esp. given the huge range of conditions it has to operate seamlessly in.

After reading Probst's book about 10 times cover to cover over the years, I really have an appreciation for the theory and mechanics of the CIS system (actually, all injection systems are really pretty amazing from a technical standpoint). Sure, the most modern direct injection engines are a world ahead, but they have 30 years of additional technology to work with.

I got real familiar with that book too, while troubleshooting and finally fixing a cold start problem. Nine years and three mechanics later, all it took was a couple of taps on the WUR, and a shot of carb cleaner through the thermovalve switch. It started like a champ at 2 degrees F, last week!!!

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Bill Krause
'79 911SC Euro

I refer to rear wheel horsepower for clarity. The 180HP reference for the common SC engine is not measured at the rear wheels. All the folks I deal with use the chasis dynos to measure real-world power. The SC motor that my fellow PCAer obsessed with put out 191HP at the rears, about 225 estimated at the crank. This was with head work, special cams, modified fuel distributor and CIS, etc, etc. God knows how much he spent on all that. He has sense moved on to turbos where the sickness continues.
I can understand not wanting to use carbs if one is not comfortable working on them. It does seem to me that they are a whole bunch simpler and much more asthetically pleasing than CIS.