I wonder how a 3.0 porsche engine would run with 3-duces on it, or one big quad. Think it would work?

Steve

Well, years ago ... 28-30 or so, there was a four barrel manifold for Corvair engines! Of course those heads were 3-in-1 on each side and had that log-type manifold for mounting two 1-barrel Rochester carbs on each side ... and the four-barrel manifold just plopped down in place of the Rochesters.

I wonder how a 650 cfm Holley with vacuum-operated secondaries would work on a split-plenum bolted to CIS runners ... somebody want to send me a tired CIS engine to play with on my transaxle test-bed???

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Warren Hall
1973 911S Targa

i think that the best thing to honestly do is to think of what you are going to be doing with the car. if you are running your 911 in a little asphalt circle for 45 minutes.. go with carbs. that way, Billy Joe Bob and his 15 cousins all will know how to tune them up for you in a jiffy.

i'm not sure about the part of the country you are in, but in Central New York we have some pretty unpredictable weather. though it's not impossible to live with carbs, it is pretty damn difficult. i can leave my house in the morning and it will be drizzle and cold, then by mid afternoon it will be hot and dry. carbs were phased out partially due to the fact that they can not take changes in temperature, altitude, and humidity like a fuel injection system can. if you read books printed back when gas was cheap and muscle cars ruled the pavement, you will see that it was Mercedes and BMW owners that bragged about how their cars instantly started up on even the coldest of mornings when Mercurys, Lincolns, and Plymouths were stumbling and sputtering to start (provided they weren't frozen).

in their defense, carbs are wonderfully simple. in many applications they do work effectively.

on the other hand... if you like to turn the key and have your car start, then get to the destination without 3 dozen annoying little things popping up... just modify your CIS system. if you have honestly have the cash and the patience to convert to carbs, just do one of those TWM induction systems instead (for the same price if not less). you can have the reliability of fuel injection with the style and power of a high performance racing induction.

it was a simple mechanical injection system that fed all 1100Hp in the 917-30. another marvelously simple mechanical injection system fed the 935s. the last thing Porsche wants during a pit stop is to worry about vapor lock, flooding the engine, carb butterflies sticking, or crap clogging the venturis.

in the end, it is your money, but just make sure that if you gut the CIS system... that you really know what you are doing when it comes to carbs.

obin

offhand, if you have ever heard a car with that TWM induction system, you will know just how musical and spine chilling it is. your car will sound like an 800 Horsepower track-bred BRUTE!

Obin, I agree that CIS is great. On a turbo charged car. The problem with CIS in a normally aspirated performance application is not its ability to deliver fuel, it's the ability to deliver AIR. With CIS you're stuck with one big throttle plate delivering fuel to 6 intake runners and a big ol' sensor plate right in the intake path. Not as good as having 6 throttle plates positioned as close to the intake valves as possible and fed by tuned equal length intake stacks. Plus CIS requires that the camshafts have virtually no valve overlap. Not ideal for breathing at higher revs. and really limits your camshaft selection. CIS is better for the day-in-day-out tractability, but I believe we're talking about the hotrod crowd here so take that into consideration. Porsche mostly used different versions of MFI on their race cars, not CIS unless fuel consumption was a major issue in the race. I prefer MFI to Webers and Webers to CIS for a hotrod. For a daily driver I'd choose CIS. Webers allow the most flexibility for mods as you only need to change venturis, jets and emulsion tubes to match. MFI requires a space cam switch and a pump recalibration. With CIS you can't run the fun cam profiles. And you don't get that cool intake resonance that makes the hair on the back of your neck stand on end. Ever heard a 67 911R at about 9 grand? Or a 73 911RS at 7500? GLORIOUS!! I believe your Bosch engineer friend was refering to turbocharged applications where adequate fuel supply is the main concern, not breathing. Oh, and after about 30 seconds of nursing, my Webers have been quite nice to me in all conditions and altitudes. I do miss my MFI though. Can't wait til I have my 2.7RS MFI motor someday. Cheers.

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Tyson Schmidt
72 911 cabriolet (964 bodywork)

I think you might be able to take out the complete injection system, fabricate a intake manifild and plop the holly right on top. The manifold would have to be desingned for balanced fuel distribution, but it seems like it may be a possilbility.

Steve

Brainiac,

ahh, i see your angle and i agree. there are a few upgrades to the CIS, but if you're going to do it right, just gut it and go from the bottom up with a whole new system.

as for sound? the thing that makes my hairs stand on end is the induction on a Spice Chevy GTP. that and a racing Chrysler Viper (both use TWM induction http://www.pelicanparts.com/ultimate/wink.gif ).

i have some of the sounds on my website. absolutely chilling!

obin

I am not so sure about the TWM being as cheap as Webers - I (sort of) priced it up and it is $$$. You need the actual throttle bodies, fuel delivery bits and a decent computer. I seem to remember hitting 4-5k pretty quick, when carbs can be had for $2k tops.

Sure the TWM setup would be way cooler though!!! http://www.pelicanparts.com/ultimate/smile.gif

Cam

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Cameron Baudinet
1975 911S

well, in basic form...

you'd need the throttle bodies, velocity stacks, intake manifolds, and if you were ahndy with tools you could just mount the existing injectors into the new system.

computer? get something basic. though the Motec ones are quite nice http://www.pelicanparts.com/ultimate/wink.gif

obin

you know, Brainiac had me thinking...

wht about doing some throttle plate modifications, and upgrading the fuel distributor to a larger unit. that would be the cheapest of all... basically the cost of a nail file, and a used distributor from either a 928 or a Mercedes V8?

from what i've been told that is good for up to 300Hp... then upgrade the injectors and you can get a nice 500Hp+ out of that setup.

obin

Stormcrow and Warren,

I've attempted the Holley 450 carb conversion. I ran into several problems. One was that the aluminum runners were not of equal length, another was that the stock rubber clamp-on connectors may not weather fuel exposure for the long haul. My CIS box was aluminum as well so this was not a problem. I did not want to run the engine with these potential problems so that was as far as I got. Time deserted me (demanding job , wife, an 8 month old and a 2 year old). The system I used was from my '78 SC before all the mods. Obviously I put it back together, but I'd sure like to give it another go someday! It is enormously frustrating not having the time for the hobbies.

As for the Webers on the 2.4L, I ran mine year round (in the midwest). Temperatures varied from 100F to 0F. The carbs were set on the rich side with a lot of overlap from the idle circuit to the power circuit. There were no problems more anoying than the occasional "pop" when the car was cold. Startup in the winter was so cool - the throtle was blipped as fast as my foot could go until it would hold idle with the hand throttle. Sounded like a sprint car with the supertrapps!!!

Hey RarlyL8, the gas pedal pumping is easier if you have someone scratch your belly at the same time. http://www.pelicanparts.com/ultimate/wink.gif And Obin, that idea would help it breathe and hence increase HP potential but you're still stuck with a long distance between the throttle plate and intake valve which hinders throttle response. And you're still stuck with cams with no valve overlap. Your idea would work great with a large displacement torquey engine, but with the smaller displacement engines, they work best when they are as sharp edged and peppy as possible. With close ratio gears to take advantage of the free revving and throttle response.

hmm,

ok, gotcha! now that i think about it...

hte modified CIS systems i've seen were in heavily modified engines to begin with... crazy cams, upgraded ignition, heavy duty exhaust, ported, polished, and so many exotic materials that it would make a geologist drool.

the throttle plate and fuel distributor tweak is common on VWs and FEWC Porsches. i've seen those heavy duty 928 and Mercedes distributors in several Porsche engines. the plumbing alone looked like it took a weekend (or four).

in either case, i'd go with fuel injection. maybe after putting that many mods into the CIS to extract more power... the time would have been better spent on a more powerful system to begin with.

obin