PROs vs CONs of webers over cis?
 

I am in the fence, leaning more towards carbs, and I just wanted to know what you guys that have done the conversion think. thanks

Geoff

Cons on carbs for me were:

Cold starts, can't just start and drive it

Pros on carbs:

Nice power
User friendly, easy to adjust when needed
Parts easily available from PMO

Cons on CIS:

Parts are becoming increasingly difficult to get
Troubleshooting not always straight forward

Pros on CIS:

Can pass my local emissions
Cold starts are automatic


I;m actually back to CIS on my car, due to an emissions issue that I posted about a few months ago. Still nice to drive on CIS, just doesn't sound or look as cool.

Carbs will never run as good as a properly functioning CIS system. That being said, what is your mission? How will the car be driven? Daily driver, only on the track? or? You can definetly make a carbed setup really nice, but there isnt much middle ground. You can either make them run great for the track, OR for the street. For everyday use CIS is simply better.

Lots of posts that reference this topic. Use the search / advanced search function then isolate your search on the 911 technical forum. Many of the threads I see talk about the actual conversion and set up vs. the pros and cons.

I have not done a conversion on my 81

Upside is there will be quicker throttle response a few more ponies and a cooler looking engine compartment.

Downside is the fuel economy drops and you have the investment and install to deal with. Suspect you will have more time fiddling but there is a fiddle factor with older CIS systems.

Folks say to add a non CIS cam and better exhaust for a bigger punch but you probably know that.

Example thread
http://forums.pelicanparts.com/porsche-911-technical-forum/591296-changing-over-cis-carburetors.html

Are you changing the cams too? There's very little point to using carbs with the CIS cams. That said, this decision is more about feel than anything else. I finally got my original CIS setup running well but could never stand the clumsy throttle response so, for me, carbs were a no brainer.
Note that a proper Weber rebuild plus a donor set in decent shape will cost nearly as much as brand new PMO's which have many useful refinements if you choose to go in this direction.

eh?



another issue is the vulnerability of CIS to EtOH blends - I'd expect a lot of that in Ohio.

meaning in all aspects of driveability, like cold starts, hot starts, flat spots, hesitations, etc.. Yes you can tune most of that stuff out of carbs, but on an overall spectrum, CIS simply runs better. And by that, I dont mean, faster, effiecient, more HP, etc...

good running CIS is user friendly

you gotta really want carbs or don't do it

Carbs just pizzoff CIS cams. Ya get a wow factor and crappy mileage. Stay with the CIS unless you plan on doing a lot of other upgrades.

If your CIS is running well, don't change it. If you move to carbs your looking at cams, re-curving your distributor and pistons to really make them work right. Its a slippery slope I am learning.

Do you feel like a bit of a fiddle? Cheaper than PMOs :D

index

http://forums.pelicanparts.com/uploa...1300785122.jpg

I have carbs (46 Webers) in my 76 911 and I do love them. The throttle response is great, the sound even better, but there are certainly some downsides as mentioned above. My car is a pure DE track car, so I can live with the poor fuel millage, cold start issues, flat spots, etc.

for the cost of converting to Carbs you could probably get a pro to redo the CIS and then you get another 30yrs before you have to fiddle with it.

If you get a chance, check the drivers running Euro SC's in the "Stock E"at a club race... you might be surprised at just how well a CIS car can run when setup correctly.

I agree but CIS isn't considered "sexy" like carbs or after market EFI. ;)

A lot of people complain that it's a compromise compared to earlier mechanical FI or carbs but my car still runs like a champ with minimal fuss and is pretty good on fuel.
I have been considering ITBs and a Motec EFI but damn it all starts to add up.

Well, Carbs are not too expensive compared to Pistons! But the Pistons or chaning gear ratios will provide definite power increases and you can keep the solidity of the CIS.

another option to consider is a standalone ECU like Megasquirt. a short time ago someone was selling a complete 911 set up for a very reasonable amount, much less than carb conversion parts. www.bitzracing.com

I use my '78 SC strictly as a weekend toy as it replaces the Ducati my wife won't let me ride anymore. In that situation and the motor I built the 46 Webers I have are irreplaceable. I have a 3.0 motor with 10.5:1 pistons, GE60 cams, twin plug, headers and a car that's stripped of most anything that isn't welded on; 400 plus pound reduction. It's as fun to drive as any motorcycle I've ever owned and I've had a bunch.

It really boils down to what you plan to do with your car. If it's a daily fix the CIS but if you want to have some real fun look at carbs, cams and pistons; you'll never regret it.http://forums.pelicanparts.com/uploa...1300819370.jpg

Lindy

ok, Peter what's up with that fuel rail and injector kit?

That is the bitz efi conversion kit, around $1500 for everything there. Made for the CIS system but can be used on other intakes.

Don't think it would be an upgrade for a 3.2 or 3.6 Motronic.