Performance gains with PMOs versus CIS?
 

Anyone have any hard numbers for this?

Considering this as my first performance upgrade - after the mechanical assessment of course.

Outside of throttle response I did't think there was a performance gain. That is unless they are accompanied by other performance changes like cams and pistons.

I've heard 5hp but Bobby is correct, has to be accompanied by other changes to get any real benefit.

I'd like to learn more about what happens to drivability when you move from CIS to carbs. They look great, very classic, but I've heard they are a world of trouble to keep tuned. Don't know if this is true, would love to hear from someone who knows first hand about migrating from CIS to PMOs.

I've been running PMO's on my 83SC for about 7 months now and would not go back to CIS and I really haven't spent the time to fine tune them, they are as purchased from PMO.

Simply substituting PMOs for CIS is going to do almost nothing but decrease fuel economy and make a nice noise. Oh, and look cool. At a minimum you need S cams and headers and can go on from there.

almost nothing if the CIS is well tuned. headers and CAMS and a perhaps a visit from Captain Air-flow to the heads is what you need.

The nice thing about carbs vs. cis is that they allow you to run cams w/ over-lap like an 's' or greater.

My CIS had been butchered and never did work well in the time I had the car, so switching to Weber carbs made a huge difference to me, but it's not in hp gain, it's in driveability and reliability. I have put 6500 miles on the car since the Webers were installed and I get 20mpg on the highway and 12-15mpg in the city. The engine sounds great and the engine compartment is nice and clean, but no extra hp, just very nice throttle response. Was it worth the $ to me, yes, indeed!

what's your total budget for motor work?

Save the 3K$ you'd need to install PMO's and just do an engine swap. After you've bought a used 3.0 and sold your existing 2.7, you'll be in the same ball park monetarily, but with better fuel economy, and much more power...

ianc

that's kinda where i was going with my question as well.

If they are tuned correctly, they are not a problem....even with season changes. My 40 IDA Webers on the 3.0L were tuned once....13 years ago and haven't been touched since. I was laughing the other day about this with my mechanic friend who is a recognised engine builder who did that tuning. My carbs run awesome and sound even better! Of course, I'm running SSI's and a Sport muffler with them.....the throttle response is fantastic and instant....they have perfect drivability!

Simple operation....turn the key to on to let the fuel pump prime the system, a single press and release of the accelerator pedal, and then turn the starter and it roars to life.....year round.

Carbs are not going to meter or atomize fuel any better than CIS. Ideally, you'd be able to get AS MUCH horsepower as CIS. But.....

It'll FEEL like 50 more hp because of the much improved throttle response.

Everyone has their own experience with this but I've seen 15HP improvements with properly setup and adjusted carbs on a 2.7 w/CIS "S" cams (different from the '74 150 HP ones); more with the early-style exhaust.


FWIW,......I've had Webers on my own car and haven't needed to touch them since 1980. As long as they are properly configured, installed, and set up, they are VERY reliable. Carburetor problems are usually caused by Human Factors. :)

(edited for clarity)

"w/S cams"

Steve - can you clarify the above? There are "real" S cams - the ones from the pre-CIS cars. And then there are the CIS cams that come in - say - a 1975 911S. The latter might be thought of as S cams by some readers...

Edited for clarity,........

"S" cams cannot be used with ANY CIS piston. Some folks will machine pockets on the intake side of a CIS piston but I surely won't,. .:)

Steve,

Can you run webers or PMO's on an otherwise totally stock SC motor, (with required exhaust backdate of course) and get similar power to a stock SC with CIS?

Same question here! Subscribing...

CIS pistons are domed in a fashion but not relieved for valve clearance...correct?

I just finished this install and have had the carbs fine tuned by a proffessional. Let me say that:

I am very pleased with the change in character of the car. There was a world of difference in me having the car running and an experienced mechanic tunning them. I would not suggest that this is the first place someone should put money in an SC.

I would put effort into the suspension and transmission prior to doing the carb swap.

-Matt

yes to both above questions

You get more HP than CIS,....:)

Thats The "Golden Rule" for any 911,...:)

all imo

CIS is a great reliable system.

Max hp is partially defined by fuel delivery. CIS is a nice compromise system.

CIS is boring to drive and carbs are fun.

Usually if a guy really wants carbs he should not limit the fun awaiting with further induction and exhaust mods.

I wanted either early S-cams or Solex but settled on E [long story].

Even small cam overlap can be music if packaged right.

PMO's can be set up easily to deliver fuel in the consistent EGT range of big bucks FI systems with correct instrument monitoring. [carbs are usually related to wide EGT differentials]

Best place to test carb install and CHT is N Nevada in 5th at 4k, 4.5k, 5k rpm for 20-30mi each full time on a warm day. If you're good there track conditions should be doable with only acceleration adjustments.

It's a carb eat MFI world.

"Usually if a guy really wants carbs he should not limit the fun awaiting with further induction and exhaust mods."

Well sure -- but guys aren't "limiting" this -- their pocket books are. It is expensive b/c you have to not just get carbs, but also cams, and _then_ toss out the old pistons so the cams will clear. Mucho Bucko...

just installing carbs and fuel delivery mods can be a small fortune http://forums.pelicanparts.com/support/smileys/fles.gif

Probably, but why would you want to?

ianc

On CIS?

:D

Because it looks cool, 'cus it sounds cool, gets just a bit over half the mpg and cost about as much as a set of throttle bodies. And i almost forgot, they can be a bit tough to get started in futzy weather when a CIS system will fire right up.

Wasted effort unless you add/or will add a a hot cam and new pistons.

In my particular situation, my CIS was hacked up and the fuel distributor was bad and the cost to repair/replace components and get CIS running correctly was greater than the cost to convert to Webers. I've put 6500 miles on the Webers so far with no adjustments and I drove from Detroit to FL in early March and got 20,pg on the highway. My car always starts right up now and I'm very happy. My CIS was not running right and never had in the 4 + years I had the car and nobody around me really knows how to work on CIS, so the Webers really worked out well. I've been selling the CIS stuff and that helps defray the cost of the carb conversion.

"(with required exhaust backdate of course)"

- remember, you will pick up some hp if you go to the more free-flowing exhaust. See Steve W's comment above.

So you are not just getting throttle responsiveness, induction sound, and a "cool look" -- you _will_ get more power.

Sure, at the expense of decreased fuel economy, driveability and the inability to pass emissions standards...

You'll get more power by throwing a performance exhaust on a CIS car as well for that matter...

ianc

For those asking about their SC motors and carbs the answer is yes you can get more power with the carbs. The SC motor with carbs is the basis for our Toyo spec race motors. A euro spec SC motor with big ports (39mm), 1.5 inch headers, phase 9 mufflers, and 46mm PMO's makes between 240 and 250 HP on the dyno. One of these motors made 260 HP at 6500 RPM and hadn't peaked yet but the owner didn't want to take it higher. I estimate an early US SC motor with SSI and sport muffler will make 220-230 HP with 40mm webers.

-Andy

Hi Eagle Driver,what would your estimate be for a stock 1975 911S 2.7 cis motor with 40mm webers,ssi's and 73 rs exhaust and stock bosch cdi with points.Thanks..Brent

I don't believe that, Andy.

Steve, if you're listening, I'm pretty curious. Guys like you would know from dyno results. So.....let's have it. All other mods the same.....and no hot cams......I don't believe carbs can achieve more hp than CIS. CIS atomizes fuel with 60 psi of pressure. It also can meter the fuel at a perfect stoichiometric ratio. Hard for me to believe carbs can outdo that. In terms of physics, it would seem they can't. But......if you are convinced, then I want to hear it. And....it would be my expectation that the gains would be single-digit, certainly. Speak, oh wise One.

All true, Ian. People will have to balance the benefits with the detriments.

PS Autoart over in England also says that they can get 240 + HP from a Euro spec SC motor - with CIS. They have implied it is very simple to modify the fuel metering unit, but they won't say how. A euro spec motor is going to make more power all over the board no matter how you fuel it.

If I had proper running CIS, I would do all the other mods first. The SSIs, exhaust and maybe even the cams tom 964 spec. Then you will have a real runner, and all those mods will go well with the carbs if you ever decide to get them.

What you really need is access to someone who can set up a 911 engine to give you what you want, CIS or PMO. If that person tells you you CIS is junk or will cost X dollars to get right, then you will feel justified in taking the leap. Of course if you are a leaper, and feel like jumping, hop right to it. So many of us deny ourselves the car we want, waiting for the right economic or emotional security to go ahead . Life is short. if you want it, make it happen.

The reason the carbs make more power is very simple. They flow more air. The CIS system limits the airflow (even the big euro tube CIS) especialy above about 6000 RPM. The CIS seems to have better torque below about 4500 RPM. Probably due to the longer runner length. I think Steves estimate of power improvement for a 2.7s engine is very doable. As far as believing me give Jerry Woods a call and ask him what power you can expect from a carb conversion (or Jeff Gamroth, or Steve Weiner, or Robbie at S-Car-Go, or any other Toyo spec racer). Even Bruce Andersons book quotes a significant power gain.

Don't get me wrong. CIS is a great system. If I had an SC for the street I'd probably keep it. The milage, cold start and driveability are all better than carbs. The question is how much power do you get with carbs and the answer is plenty. CIS doesn't come close.

-Andy

I don't want to start the debate about the "euro dyno"........ :) but over there they seem to make some really ummmmm impressive numbers..................

I have built a couple of CIS 3.0 motors with 964 cams and think they are great motors with a fun power band. But, lets not forget that CIS was not designed as a performance system. Lots of different everyday cars ran CIS.

I would bet that Steve is right, there would be a definete improvement in HP with a conversion to carbs, not just throttle response.

Lets say you have a malfunctioning CIS system and you can't trace the problem down. How many hours do you want to spend $100+ an hour tracking it down plus parts? Who knows what the previous owner did to make the car run okay. So, you look into carbs, maybe you already have a backdated exhaust, does the cost now seem to start making the desicion a litltle easier?

Each case is different and each owner must way the pros and cons. I like CIS 3.0 with 964 cams, provided the CIS is properly functioning. But it doesn't take too many hours of labor before people start hating it........

Cheers

"the carbs make more power [b/c t]hey flow more air."

- I suspect this is correct. I would like to hear from Steve or someeone with a fair amt. of dyno data tho.

- Also, you may not be at an rpm that requires max air flow very often. Of course, when you are -- that may make all the difference...

Hi:

Andy is right on; its all about airflow. Some time ago, we took a 163K bone-stock Carrera motor and replaced the OEM Motronic intake system with an ITB system of our own design and construction. Power went from 217 HP to 249 HP with NO other changes.

Carburetors, being a type of individual throttle butterfly system, make similar improvements even with smog-type cams that have little or no overlap. One advantage of carbs is their ultimate flexibility in configuration. HP & torque can be affected by changing venturi diameters, jets, and manifold heights for effect matching displacement, camshafts (RPM range), compression ratio & exhaust.

As Andy said, CIS is wonderful for emissions, cold-start behavior, drivability and in some cases, fuel economy but it doesn't come close to making the same power as a pair of properly configured carburetors or ITB's with Engine Management.

In the stock classes of PCA club racing, replacing CIS with carbs (no other changes) bumps you up two classes. I don't think they would do that if there were no gains in hp to be had...