carburetors vs CIS

[mikey911]

I have an 1983 911sc USA with CIS. I am attempting to learn about performance modifications. Through reading Wayne's book it appears that carburetors offer better performance than CIS. Is this true for this motor?

I definitely like the look of carbs compared to CIS. If someone could offer advice reagarding this and is there a resource that outlines how to do this as I cannot find in any of the books I have.

Thanks
Mike 75 911S

[randywebb]

yes

To really make it right is very expensive as you need carbs, cams, and new pistons (!)

You can put carbs on and jet them rich - that works, adds little or no hp, but is a huge improvement in terms of sound and feel.

Use the search engine - best is the button at the bottom of the screen

you will want SSIs in either case

- What are your state's emissions regs?

[ianc]

Do not abandon your fuel injection unless you are totally modifying the car including replacing the entire exhaust and camshafts and will use it for track.

The only performance improvement to be gained with carbs is that they will let you use a more aggressive camshaft, since the CIS will not tolerate this.

In installing carbs, you will:

A) Spend a huge pile of money on the carbs themselves. At least as much again on other mods to even make them worthwhile.
B) Get worse fuel economy.
C) Significantly decrease the resale value and originality of the car.
D) Decrease day to day driveability significantly.
E) End up with something that is much more difficult to troublshoot and maintain.
F) Not be able to pass emissions or smog tests.

If you want a performance increase and to not suffer the above problems, look at Tony Bitz's EFI conversion.

ianc

[Eagledriver]

I disagree with most of the information in this post. Carbs are a huge upgrade for the performance of the SC. I race in the Spec 911 class in the Porsche Racing Club on the west coast and here is what you can expect.

With 40 mm webbers, watershields and sport exhaust you will easily get 220 HP and with careful tuning and racing headers/exhaust you can get up to 240.

We have one engine with 39mm ports, and euro 9.8 to one pistons that dyno's over 250 and pulls to 7000 RPM.

These engines all have stock SC cams and CIS pistons.

Now none of these engines will pass smog and of course they don't run as well when cold as a CIS engine but if power is what you want....

-Andy

[ianc]

40 mm webbers, watershields and sport exhaust

racing headers/exhaust

one engine with 39mm ports, and euro 9.8 to one pistons

Now none of these engines will pass smog and of course they don't run as well when cold as a CIS engine

I don't see that you're disagreeing at all.

ianc

[randywebb]

240 hp? Did you use an engine dyno? I'd like to see those results.

I agree with a thru F above -- but it is worth it. You can do the pistons & cams later on. Again, your state's emissions laws are critical to this upgrade. IF they won't allow it, you can drop in a 3.6L.

[ianc]

you will easily get 220 HP

Oh yeah, and I'm sorry, but there is absolutely no way you are going to net 40 HP by adding carbs and a muffler.

ianc

[randywebb]

Well Ian that's a bit extreme. Magic does happen!

(But I always like to see the schematic for the wand....)

[afterburn 549]

I would go with aup to date injection sytem b4 caveman carbs....they were better in thir day then cis, but injection has come a long long way baby ......with carbs your limited to other mods, with injection ther are no boundries.

[randywebb]

I agree - injection will be better ... but the carbs are "only" $3,000 -- a good EFI setup will double or triple that....

[ianc]

the carbs are "only" $3,000 -- a good EFI setup will double or triple that....

I believe the Bitz kit is $1600. Of course you have to install it, but you'd have to install carbs too...

ianc

[randywebb]

Yes - I'm talking about Motec and the next level down.

[Superman]

I'm going to disagree. With all due respect, and calling nobody a liar, all other things being equal I have never seen credible, or should I say objective and conclusive, evidence that carbs can make any more power than CIS. There is no way that carbs are going to atomize fuel as well as CIS, let alone better. And yes, CIS is less finicky and works more reliably in a wider range of conditions

But there's a caveat. CIS throttle response sucks. Carbs feel like 50 more hp, because of the instant throttle response, as opposed to the "count to three" experience withe CIS. But in my experience, dyno after dyno tells the tale that CIS meters and atomizes fuel just about perfectly. More than a match for carbs, frankly. But again, without the really cool throttle response advantage of carbs. And yeah, carbs look WAY cooler.

Also, CIS systems will not tolerate much cam overlap. So yeah, you can build a 270 hp 3-liter with carbs but not with CIS. But that engine will not be as fun to drive around town as my '83 with 20/21 cams and stock CIS.

[ianc]

And yeah, carbs look WAY cooler

Until it comes time to adjust linkages or synchronize every throat. Or (God forbid) rebush them all...

ianc

[dd74]

Randy - if you search out "Tony" and/or "dyno results" in the engine rebuild section, he (Tony) put PMO carbs on his "unopened" 3.0 and netted about 240 hp, IIRC.

As for Carbs vs. CIS - you may not have a choice any longer. Upon talking with Tyson at TRE, he informed me that CIS parts, particularly the distributor used for the CIS, are getting sparse and very expensive.

Now, if you want to celebrate what, in fact, saved Porsche from extinction, as MFI and/or carbs were becoming inefficient and/or dinosaurs, you will employ CIS. But that's mostly a sentimental reason.

If I had the chance to jump on some Webers for a reasonable price, I'd do so. If my CIS suddenly fails, I will also explore the carb route.

I'm certain you will gain horsepower. And 20 is not completely unrealstic if your engine is in good shape. By the time you do this modification, however, you should consider appropriate exhaust at the same time. SSIs, '74 heat exchangers, etc.

Another benefit of Webers is they are lighter than CIS.

[ianc]

Upon talking with Tyson at TRE, he informed me that CIS parts, particularly the distributor used for the CIS, are getting sparse and very expensive.

Does this mean that Webber parts are now in some kind of parity (or even close to it) with the availability and price of CIS parts?
Didn't think so..

I'm certain you will gain horsepower. And 20 is not completely unrealstic if your engine is in good shape.

Just explain to me how this could possibly be, given stock exhaust and cams. Is there anyone here who disputes the idea that port injection is a good thing?

What this really boils down to is how you're going to use the car. If you will use it on the track only, then yes, you will be able to use a hotter cam, less restrictive exhaust, etc, and make more power.

If you drive the car on the street and want a fun, reliable, driveable, maintainable daily driver, then don't even think about carbs.

ianc

[svandamme]

Originally posted by dd74
as MFI and/or carbs were becoming inefficient and/or dinosaurs

oi , nothing inefficient about MFI , and definately nothing dinosaur about it ... the only reason for ditching MFI was the cost , they still kept using it on factory racers for another 10 years or so because it did things that CIS simply couldn't do... how's that for efficient.

[dd74]

Originally posted by svandamme
how's that for efficient.

Lousy, actually! Because at the time, Porsche needed something a bit more (gas) efficient than the paltry mileage of MFI. How's that for reality?

[dd74]

Originally posted by ianc
Does this mean that Webber parts are now in some kind of parity (or even close to it) with the availability and price of CIS parts?
Didn't think so..Just explain to me how this could possibly be, given stock exhaust and cams. Is there anyone here who disputes the idea that port injection is a good thing?

What this really boils down to is how you're going to use the car. If you will use it on the track only, then yes, you will be able to use a hotter cam, less restrictive exhaust, etc, and make more power.

If you drive the car on the street and want a fun, reliable, driveable, maintainable daily driver, then don't even think about carbs.

ianc

1) I've seen full CIS systems going for $500 or $600 bucks. Do you have any idea how much they'll be in five or six years as parts become rare. Yes? No?

2) To each their own what anyone drives, how they drive it and how much a PITA they want their car to be.

[ianc]

at the time, Porsche needed something a bit more (gas) efficient than the paltry mileage of MFI. How's that for reality?

If Porsche thought they could be faster or more fuel-efficient (both count you know) with carbs, they would have been running them.

They weren't; end of story.

ianc