A case for carbs. Ditch CIS for carburetors?

[QueWhy]

Car in question is a 1982 SC with the stock 3.0, Carrera tensioners, turbo valve covers and little stuff like that has already been done. The CIS is working, I have some minor idle issues and don’t get very good milage(all signs point to running quite rich combined with a leadfoot although I haven’t done any formal testing) but there is no immediate work needed.

After installing SSIs and a heat backdate this summer I am wondering what is next. I love the way the car feels and sounds after the exhaust swap but alas I want more. Going to carburetors seem like the next step down the slippery slope. (A quick note about EFI, I have looked a little into going this route and it offers some distinct advantages to both carbs an CIS but I have ruled it out for this car at this time for various reasons.)

I have read through a bunch of threads and it seems there are very divided opinions on carbs, especially going from a more "modern" system like CIS back to older tech. I just don't know how they would work for my specific situation.

The carbs bring a lot to the table that I value.

• I like how the heat backdate helped to clean up the engine bay and lust after the pictures of peoples engines running carbs.
  
• Carbs save a little weight, I am always looking to shave a few lbs.
  
• Simplicity, like weight savings if there is an opportunity to make things simpler/easier to work on I'm going to take it.
  
• Sound! The SSI and M&K install transformed the car and I think most of that is due to the sound. Carbs would help tune/turn it up another notch.
  
• Performance. Combined with a distributor recurve and the backdated exhaust they should squeeze a little more out of the 3.0. I’ve seen all kind of claims but I’m not really concerned with numbers, feeling and hearing an improvement is good enough for me.
  
• Performance potential. At some point in the future I am going to need an engine rebuild, either from necessity or simply wanting more. The carbs offer a lot more options for pistons and cams. Going to carbs now would also help free up some $$$ when it comes time to rebuild.

Carbs come with some drawbacks. I just don't know how much they will apply to my situation. This is where the questions come in.

• Cost. I've done quite a bit of research but this is still an unknown. In addition to the carbs themselves(a big delta between a new set of PMO and a used Webers off the clasifieds) I am not quite sure what ancelarry items are needed. A new fuel pump regulator, new wireing harnes, what else? How much money for everyting else needed?
  
• Tuneing. This somewhat also fits into the cost catagory. Am I going to spend a fortune on jets and venturies trying to get it right? Is this going to take a huge amount of time?
  
• Altitude. I live at almost 4,000ft and would like to be able to drive up to 6,000 and go skiing regularly and make occasional trips to the coast. Do carbs offer enough flexiabilty that I can do all these things year round?
  
• Cold start. I drive my car year round and we have plenty of sub freezing tempratures. If I go skiiing for 6-8hours and then try to start the car am I going to get home?
  
• Millage. Though my CIS seems to be working well and always starts I don't get very good gas milage unless its all highway(someting I don't do often) and is most likely a function of how I drive(lots of WOT ). From reading threads this there seems to be widely varried oppinins and experiances of what happens to gas mileage switching from CIS to carbs.
  
• Gas smell. Like milage there seems to be a lot of different experiances and opinions. Some say it stinks up the whole garage and will make you smell like a gas station attendant after even a quick drive and others don't have issues at all. I don't really mind the garage smelling like gas but the better half probably wouldn't like it. I like to drive the 911 to work here and there but if I'm going to smell like gas I can't do that anymore.

Hopefully some of you can chime in and share your advice and experience in these specific areas. Provided they work in the temperature and altitude range I need without stinking the garage up too bad and can be installed and tuned in a reasonable amount of time, it will come down to cost vs. benefit.

[nathanbs]

In my experience you will not be happy with carbs on a 3.0CIS without changing the cams in the motor. It will be relatively flat above 4500-5000 rpms

[Bill Douglas]

Save money and frustration and pay for a Porsche mechanic to set it up right then leave it alone.

Or go the whole hog and go EFI.

Depends what you want. Something to tinker with, or something to drive,

[juanbenae]

ive run carbs on 3 different 3.0l motors, the 1st two were the same motor, but post rebuild I went from weber 40's to PMO 46's. in both instances it was equipped with the stock CIS cams due to race class rules. the spec911 class requires stock pistons, although you are allowed to use the euro higher compression units and stock cams. those are the rules, were they followed by every competitor? not likely IMO.


fuel economy will plummet with carbs. my current 3.ol has hotter cams, 9.5/1 JE pistons and run a set of weber 40's and it's a great set up as far as im concerned. im seeing around 12-13 miles a gallon even at freeway running. less when the drive is more spirited. you are on the right track with the recurve of the dizzy. I plopped a set of 40 webers on a 90k mile early 3.ol and went racing and the motor did 225 at the wheels on a dyno at one point. plenty of power.


im not sure of the cold weather applications, but that could be the fly in the ointment. remember that the early carbed cars in Europe were likely run year around and it's cold and or snowy there, so there's that. when you ask about the jetting and expenses to get them tuned properly and buying a used set of webers you might be better off looking for a set of new PMOs (or lightly used). you can never tell just how good of shape a used set of what could be 50 y/o carbs so there is value in getting new equipment and new PMOs are somewhat tuned to your needs from the outset per your application needs when assembled by PMO. I reacquired the weber 40's I have now that were originally on my spec car prior to the PMO so they were a know entity. you could pay $2500 or more for a set of webers that could be tired, throttle shafts, needing rebuild and be in what a new set of PMO's would have costed when all said and done.


check out the performance oriented website. member 1quicks in the weber guru around here and his site is must read if you are considering carbs.


did I mention you fuel mileage will suffer, significantly?

[juanbenae]

couple more things:::

you can thin out the CIS harness for carb applications. you will need a hotter ignition and coil set up and MSD or the like is the preferred set up.


haters gunna hate on carbs. im not aware of anybody here that has ever removed carbs to go back to CIS. they transform the car even with the CIS cams and once driven you will wonder why you did not do it sooner.

I will be moving to a higher altitude in the spring and will likely have to do some additional tuning to get it right up there, but it can be done. im pretty sure 911's have been driven in higher altitudes all over the world. aren't the alps near germany?

[nathanbs]

Originally Posted by juanbenae

couple more things:::

you can thin out the CIS harness for carb applications. you will need a hotter ignition and coil set up and MSD or the like is the preferred set up.


haters gunna hate on carbs. im not aware of anybody here that has ever removed carbs to go back to CIS. they transform the car even with the CIS cams and once driven you will wonder why you did not do it sooner.

I will be moving to a higher altitude in the spring and will likely have to do some additional tuning to get it right up there, but it can be done. im pretty sure 911's have been driven in higher altitudes all over the world. aren't the alps near germany?

Lol I did but it is on a carrera 3.0 that I want to return to original condition. It ran terribly flat above 4500-5000 rpm. Every mechanic I talked to said it was because I had CIS cams so that’s solely what I’m basing my statement above on. I did just put 46 Weber’s on a 3.0CIS motor but I did cams and pistons among other things. It’s getting buttoned up here shortly. I’m sure it will be a gas guzzler

[zedsn]

If you decide to go with carbs I would recommend that you go with a good used pair of PMO carbs or if you wing it a new pair. They worked well for me over webers. I too ran a msd box and blaster coil but I had a rs spec 2.7 and not a cis 3.0. Car got 12 miles to the gallon with my lead foot but the first guy that drove the car bought it from me.

[juanbenae]

Originally Posted by nathanbs

Lol I did but it is on a carrera 3.0 that I want to return to original condition. It ran terribly flat above 4500-5000 rpm. Every mechanic I talked to said it was because I had CIS cams so that’s solely what I’m basing my statement above on. I did just put 46 Weber’s on a 3.0CIS motor but I did cams and pistons among other things. It’s getting buttoned up here shortly. I’m sure it will be a gas guzzler

well I should have been more specific... for the originality of the 3.oL Carrera that would the be exception (originality = $$$$$$) to my observation. as I recall you have or have had a number of the Carrera 3.0L's over the years? did you buy the one with carbs in place? you'd know better than to mess with it im assuming.

do you know if the distributor was recurved for the carbs on the 3.oL you note going flat at 4-5K? ignition? if the dizzy did not have the greater advance with a recurve and a hotter ignition that could explain it. I realize the OE CIS cams are not ideal, but if you were to ask anyone who went to carbs with the cams in place I bet few would express disappointment in doing so from a performance perspective. the CIS 3.oL motors I went to carbs on were all big port 78-79 (lower compression) motors though. the smaller ports on the later 3,oL motors may contribute to dumbing the carbs down some? somebody smarter than me would need to confirm that.

carbs may not be for everyone, my car is not a DD so they work for me. id love an ITB injection system but due to cost and the desire to continue learning about carbs im sitting pat for now. I think other than the weather concerns and cold weather at altitude the OP suggest are very valid concerns. if you are not going to tear into a motor moving from CIS to the carbs is a stepping stone to the final destination that is worth exploring IMO. carbs are not for the "set it and forget it" type of user, but more for the tinker type. as frustrated as ive been in set up efforts the victories in getting over a specific issue are just that much sweeter. im sick that way though


plus, they look & sound cool as balls!

[nathanbs]

Originally Posted by juanbenae

well I should have been more specific... for the originality of the 3.oL Carrera that would the be exception (originality = $$$$$$) to my observation. as I recall you have or have had a number of the Carrera 3.0L's over the years? did you buy the one with carbs in place? you'd know better than to mess with it im assuming.

do you know if the distributor was recurved for the carbs on the 3.oL you note going flat at 4-5K? ignition? if the dizzy did not have the greater advance with a recurve and a hotter ignition that could explain it. I realize the OE CIS cams are not ideal, but if you were to ask anyone who went to carbs with the cams in place I bet few would express disappointment in doing so from a performance perspective. the CIS 3.oL motors I went to carbs on were all big port 78-79 (lower compression) motors though. the smaller ports on the later 3,oL motors may contribute to dumbing the carbs down some? somebody smarter than me would need to confirm that.

carbs may not be for everyone, my car is not a DD so they work for me. id love an ITB injection system but due to cost and the desire to continue learning about carbs im sitting pat for now. I think other than the weather concerns and cold weather at altitude the OP suggest are very valid concerns. if you are not going to tear into a motor moving from CIS to the carbs is a stepping stone to the final destination that is worth exploring IMO. carbs are not for the "set it and forget it" type of user, but more for the tinker type. as frustrated as ive been in set up efforts the victories in getting over a specific issue are just that much sweeter. im sick that way though


plus, they look & sound cool as balls!

Yeah I have 6 of them lol. I’m severely sick in the head. It came with the carbs. You are probably right, they probably didn’t do anything to the distributor or perhaps even have the appropriate venturis and jets. Yeah I love carbs as well. Can’t wait for this motor with 46’s to be done

[MichaelSJackson]

QueWhy,

You don't say what your long term goals are? Hence all the back and forth.

First off, my CIS worked reliably, but I still ditched it. CIS is a Rube Goldberg, mechanical injection developed as part of a first try at curbing emissions. Look at the path of the air flow. Comes in on the left, goes down thru the filter and then up past a big metering plate, over thru a bridge to the throttle body, down into an air box and then into long individual runners to each head intake. By itself that should convince you to remove it. That doesn't even mention that the mixture is determined by the shape of the cone the metering plate sits in. There is neither map nor jets to reprogram/replace to adjust the mixture. It's a dead-end. Not to mention that the metering plate is sensitive to any blow-back thru the induction so the cams have to be kept mild. There's more, but that's plenty. If you want more performance, the CIS has to go. (You could get a little more from slightly better cams, but the CIS is a limiting factor.)

You don't say why EFI is ruled out, but you say it is. So you only have the remaining choice of carbs. (I'm thinking this is a no-brainier at this point?) A set of correctly sized PMOs are cheaper and more reliable than rebuilding Webers. Six individual carbs at WOT is a bit of mechanical music. I think they look beautiful. The're a lots of Porsche mechanics that can set these up for the diverse driving environments you describe as long as you don't also need it to be a "race" engine. I believe the gas smell if from setting them up to run rich. (A requirement for a race engine that's always running at the limit.)

I went with EFI, but for my needs/desires carbs would have been as good and easier.

You've replaced the exhaust. Replace the induction and the basic engine can breathe as it should. After that you'll have to start changing the internals to get more. But that's a discussion for another day?

[clutch-monkey]

Originally Posted by Bill Douglas

Save money and frustration and pay for a Porsche mechanic to set it up right then leave it alone.

Or go the whole hog and go EFI.

Depends what you want. Something to tinker with, or something to drive,

I’ll second this- carbs to me are a halfway measure, I’ve never been 100% happy with any carb fed car I’ve owned except for a rotary. Ymmv

[Gordo2]

Originally Posted by QueWhy

...Provided they work in the temperature and altitude range I need without stinking the garage up too bad and can be installed and tuned in a reasonable amount of time...

Love my carbs - but if this is part of your criteria to be happy with carbs, I would recommend sticking to CIS...

Another consideration is emissions inspections.

Your vehicle / county may not require active emissions testing (tailpipe sniff), but most if not all have some sort of law requiring a visual inspection to confirm original emissions equipment (including induction and associated components) are still in place.

Gordo

[rbogh901]

you won't smell like gas. the carbs do but it should be mild. if not you need to look at your fuel lines and connections just like any other car.

[Jack Stands]

I absolutely love the looks of an engine with carbs, but certainly believe that EFI is the way to go these days. Both of my cars with Weber’s sound great, but I can’t argue with the drivability of the Motronic. Have you considered the Bitz conversion for your CIS system?

[Tom '74 911]

I find myself in a similar situation with a 2.7L engine I have been rebuilding. The car had a small engine fire (previous owner) and the stock CIS system is non-op as a result. I have considered rebuilding the CIS system, but new parts are not available and based on past experience, used parts are likely to be just as worn out as the ones they are replacing. There are very few places that provide rebuild services for CIS components. The primary one I have found, while highly recommended, has not returned multiple phone and email inquires and I'm not too excited to do business with a non-communicative entity.

I've had some very good, long conversations with knowledgable porsche people and the general consensus is that carbs are still a viable option, especially a new set of PMOs. Buying a used set of Webers could be a crapshoot, not unlike buying used CIS parts. My goal is to try to make it as easy as possible for myself (while still being mostly DIY) and I imagine futzing with a worn out set of carbs could be a frustrating experience. I figure that the stress and aggravation saved are worth the extra $$ for new PMOs - for me at least.

I have/had all the same questions and concerns that you detail in your post...

Cost - In my research, carbs (min. $5k) seem to be about 1/2 the cost of EFI (min. $10k). My numbers may seem high, but I bet they are not far off by the time the car is back to 100%. Can you do both systems cheaper - you bet, depending on tolerance for DIY and hassle...

Tuning - If you start with new PMOs, they should come set up pretty close to where you will need to be. I think extra jets, venturies and other parts won't move the budget needle too much, but the time to swap/tune might, depending if you are DIY or paying someone to do it for you. I think an air/fuel monitor/logger would greatly ease the tuning process & it's on my list of tools to add to my toolbox for that purpose.

Altitude - I live at 5,000' and will drive to 8,800' and 1,500'... carbs will suffer over EFI, but it's unlikely that your car will simply NOT run. I don't think swapping jets is THAT big of a deal for extended stays, but again, it's still time futzing w/your car...

Cold Start - I think w/a properly set up system and proper starting procedure, this is a non-issue.

Milage - MPG will suffer with carbs. Trade-off for sure.

Gas Smell - may be some with carbs. Again, with proper tune, I think less likely to be a big issue.

The EFI systems offered by Al at X-Faktory are really interesting and I haven't 100% ruled out going that route. I'm just not sure I have the brain space and time to learn how to tune an EFI engine myself, or the budget to send the engine to him to do it for me. If cost was off the table, EFI is the way to go for sure. But carbs get you 90% of the way there, for 1/2 the cost and are more DIY friendly I think. I'm sure lots of people will say otherwise...! : ) : )

Originally Posted by QueWhy

Carbs come with some drawbacks. I just don't know how much they will apply to my situation. This is where the questions come in.

• Cost. I've done quite a bit of research but this is still an unknown. In addition to the carbs themselves(a big delta between a new set of PMO and a used Webers off the clasifieds) I am not quite sure what ancelarry items are needed. A new fuel pump regulator, new wireing harnes, what else? How much money for everyting else needed?
  
• Tuneing. This somewhat also fits into the cost catagory. Am I going to spend a fortune on jets and venturies trying to get it right? Is this going to take a huge amount of time?
  
• Altitude. I live at almost 4,000ft and would like to be able to drive up to 6,000 and go skiing regularly and make occasional trips to the coast. Do carbs offer enough flexiabilty that I can do all these things year round?
  
• Cold start. I drive my car year round and we have plenty of sub freezing tempratures. If I go skiiing for 6-8hours and then try to start the car am I going to get home?
  
• Millage. Though my CIS seems to be working well and always starts I don't get very good gas milage unless its all highway(someting I don't do often) and is most likely a function of how I drive(lots of WOT ). From reading threads this there seems to be widely varried oppinins and experiances of what happens to gas mileage switching from CIS to carbs.
  
• Gas smell. Like milage there seems to be a lot of different experiances and opinions. Some say it stinks up the whole garage and will make you smell like a gas station attendant after even a quick drive and others don't have issues at all. I don't really mind the garage smelling like gas but the better half probably wouldn't like it. I like to drive the 911 to work here and there but if I'm going to smell like gas I can't do that anymore.

[QueWhy]

Thanks for all the responses so far!

A couple of the threads I've been using for research
Here
carburetors vs CIS
and
Here
Cis to Carbs...documented power increase

Originally Posted by nathanbs

In my experience you will not be happy with carbs on a 3.0CIS without changing the cams in the motor. It will be relatively flat above 4500-5000 rpms

Worse than the CIS? I haven’t seen anyone claim worse performance when switching. New cams would be part of a longer term upgrade plan, no sense in changing cams and keeping the CIS pistons only to want to change pistons later and then not have the most optimal cam again.

Originally Posted by juanbenae

ive run carbs on 3 different 3.0l motors...

Your posts in some of the threads I was researching is partially responsible for wanting to make the switch to carbs. Some of the same threads also have posts by guys like RSbob who said if given the chance they would keep CIS.

I see what you are saying about the early cars being run year round in Europe and my first car was a 72 super beetle with carbs and I don’t ever remember issue on cold mornings beyond a few pumps of the gas peddle and a few extra cranks of the ignition. Same with the altitude, these cars were driven all over the world. I guess I’m just trying to separate some internet wisdom from actually been there done that experience where someone can say “yes it works” or “no, I had nothing but problems”. Your posts go a long way to helping me understand what to expect from a conversion, a large sample size like spec 911 and some first hand dyno results are the kinds of things I’m looking for.

Originally Posted by MichaelSJackson

QueWhy,

You don't say what your long term goals are? Hence all the back and forth.
I don’t really know what they are either, the journey is the destination kind of thing. With two little kids I don’t get to drive the car as much as I’d like but I still enjoy it immensely and relish a couple of hours here or there to tinker with it too. Anything I can do to get a little more performance, lose a little weight and add some simplicity is the direction I like to see this car go at the moment. I know that at some point the engine will need to be rebuilt and carbs seem to offer a lot more options than the CIS.

You don't say why EFI is ruled out, but you say it is. I have to use a laptop everyday for work, I’d rather not have to use one to work on my 911 too. That and it adds complexity and cost I’d rather not deal with, plus I like the more “period correct” feel of the carbs So you only have the remaining choice of carbs. (I'm thinking this is a no-brainier at this point?) No-brainer if they will start year round and run reliably from sea level to 6,000+ft A set of correctly sized PMOs are cheaper and more reliable than rebuilding Webers. Six individual carbs at WOT is a bit of mechanical music. I think they look beautiful. The're a lots of Porsche mechanics that can set these up for the diverse driving environments you describe as long as you don't also need it to be a "race" engine. I believe the gas smell if from setting them up to run rich. (A requirement for a race engine that's always running at the limit.)Great points. I think setting them up to run rich in an effort to add some safety margin by home mechanics may contribute to a lot of the reports of gas smell

I went with EFI, but for my needs/desires carbs would have been as good and easier.

You've replaced the exhaust. Replace the induction and the basic engine can breathe as it should. After that you'll have to start changing the internals to get more. But that's a discussion for another day? Exactly, those things are further down the slippery slope

Originally Posted by Gordo2

Love my carbs - but if this is part of your criteria to be happy with carbs, I would recommend sticking to CIS...

Another consideration is emissions inspections.

Your vehicle / county may not require active emissions testing (tailpipe sniff), but most if not all have some sort of law requiring a visual inspection to confirm original emissions equipment (including induction and associated components) are still in place.

Gordo

Thankfully I have no emissions testing of any kind to worry about…for now.

Originally Posted by Jack Stands

I absolutely love the looks of an engine with carbs, but certainly believe that EFI is the way to go these days. Both of my cars with Weber’s sound great, but I can’t argue with the drivability of the Motronic. Have you considered the Bitz conversion for your CIS system?

I have briefly looked at the Bitz kit, as stated previously I don’t really want to introduce more complexity and technology to the car and the CIS conversion isn’t going to look or sound as good as the carbs which are also things I value.

Originally Posted by Tom '74 911

Great info Tom! Exactly the kind of stuff I was looking for!

[juanbenae]

the early large port 3.oL motor I built had the 9.8-1 euro CIS pistons and topped with the 46mm PMO's was dyno'd at jerry woods enterprises and did 254 at the crank during break in. wish I had that motor in my current 75S because the seat of the pants dyno on my current motor seems to be at least equal to the CIS cam/piston set up in that motor even with the hotter cams and JE pistons.

the spec car (#2200) did weigh a couple hundred pounds less than my 75 currently sits at. the 75 is a factory sunroof delete, crank window, RS interior, rear fiberglass bumper (front bumper forthcoming) that's about as light as you can get with the impact bumper era cars. the truck scales at the dumps said the 75 with me in + 1/2 tank of fuel it came in at around #2600, 220 of that being my fat ass.

[James Brown]

For me, the carb option was a no brainer from CIS to Zeniths! (3.0 SC) I had new ventures made and this mod along with SSI's woke the car up. CIS cams and all. But when i recurved the dissy and removed the vacuum system, that did it. Butt fell/sound was all i needed.
Today i would go full in, new cams/pistons, PMO's, good headers/muffler. Thats the sweet spot. Not in this for the fuel millage but the driving experience counts.

[Reiver]

Originally Posted by juanbenae

the early large port 3.oL motor I built had the 9.8-1 euro CIS pistons and topped with the 46mm PMO's was dyno'd at jerry woods enterprises and did 254 at the crank during break in. wish I had that motor in my current 75S because the seat of the pants dyno on my current motor seems to be at least equal to the CIS cam/piston set up in that motor even with the hotter cams and JE pistons.

the spec car (#2200) did weigh a couple hundred pounds less than my 75 currently sits at. the 75 is a factory sunroof delete, crank window, RS interior, rear fiberglass bumper (front bumper forthcoming) that's about as light as you can get with the impact bumper era cars. the truck scales at the dumps said the 75 with me in + 1/2 tank of fuel it came in at around #2600, 220 of that being my fat ass.

I have a 930-10 (204 hp/9.8-1 euro) in my US 83SC with the US 915 (taller gearing..nice) SSI's/two inout and a lightened car....too much to list taken off.

I've thought about this but my WUR operated CIS works too well presently and am getting 225 HP now with decent mileage. Getting the WUR just right makes the dif on these models (CIS Flowtech built to stricter parameters than stock)...my on board AFR has me cruising at 13.7-14 and when I stomp it it immediately goes to the mid 11's. I keep it a tad fat as I live at 2600 feet and go to 5-7 grand often and down to 1k. Happy medium.

I suspect I'd get better response (and a nicer looking engine bay) with carbs but I'd want the mileage and modernity of EFI too....I'll get there I guess and reading these threads motivates....

[juanbenae]

Originally Posted by James Brown

For me, the carb option was a no brainer from CIS to Zeniths! (3.0 SC) I had new ventures made and this mod along with SSI's woke the car up. CIS cams and all. But when i recurved the dissy and removed the vacuum system, that did it. Butt fell/sound was all i needed.
Today i would go full in, new cams/pistons, PMO's, good headers/muffler. Thats the sweet spot. Not in this for the fuel millage but the driving experience counts.

that's why james is a "senior advisor"...