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