CIS Vs MI vs CARBS~??~!!

goldgunner · 05-25-2005, 01:27 PM

Soon to begin total rebuild of recently purchased 1980 911SC, and I'm pondering the fuel supply options - and I need some helpful info - as to the comparing and the pros and cons of the current CSI system versus mechanical injection verus carbs.

My first inclination is to ditch the CSI and all the associated smog stuff and go with PMO carbs and figure out what the other necessary changes would be - like different distributor? and?

But having read these pelicanized boards for a while, there seems to be some (?) preference for the CSI system, and if I've read it correctly, some preference for 'back dating' to a 'mechanical injection' system.

I'm not entirely sure what a 'mechanical injection' system is - I'm guessing it's pared down fuel injection without the smog apparatus.

I have a lot of decisions yet to make on cam, cylinder and piston set-ups - I know - but the goal here is essentially for an extra fun street car that may eventually see a little casual track time.

So, please - share the fuel system opinions!

Thanks!
Jim

ChrisBennet · ChrisBennet's Garage

CIS - Continuous Injection System. What your car has now. Very reliable though the some of the 25 year old parts may need replacing occasionally. Not a lot of performance potential but reasonable fuel economy, starts on a winter day or a hot summer day, pretty nice to the enviroment, etc. I call it Complicated Icky System but it really is a marvel of engineering for it's time.
MFI - Mechanical Fuel Injection, a high performance fuel injection Porsche used in the late 60's early 70's Very expensive route to go for a 3.0
Do a search for Tbitz and EFI (search string "Tbitz AND EFI") and you'll see an option you might not have considered.
-Chris

goldgunner · 05-25-2005, 03:08 PM

Thanks Chris

Tbitz's CIS to EFI kit and system is quite intriquing.
( http://www.bitzracing.com/index.html )

And certainly worth serious consideration.

So, quickly, it's CIS (as is), EFI and Carbs (PMO) to consider.

As to the Bitz EFI system: it would be helpful if there was even a little more than the very minimal performance measurements available...or even some performance claims!

More! I need more - please - carbs are so...

thanks!
jim

rpiper · 05-25-2005, 08:36 PM

I'd stick with the CIS. I have a 1978 3.0L SC motor (bigger heads than later SCs)with 964 cams, 9.8:1 Ps&Cs and SSIs that makes 196hp at the wheels. The CIS is very tractable and easy to deal with. Just turn the key & go. I used to have an SC with almost identical motor except it had carbs. A beautiful set of PMOs (46s) that really made the engine compartment something to see. Great throttle response and likely about 15 more hp. HOWEVER, they were a big pain for daily use. HORRIBLE mileage (about 10-12 vs 20-22 w/CIS), finicky jets always getting clogged (had to carry around tiny pipe cleaner wires), severe temp and altitude issues. I have no experience with the EFI kit but I think the CIS is fine. If you are planning more than 200 HP you'l have to consider ITBs and programmable EFI (read: $$$).

Jeff Alton · 05-25-2005, 09:20 PM

Last weekend I droved a friends car, a 3.0 with cams/compression/ and carbs. The throttle response was so good from the carbs that I started researching carbs or ITB's for the 3.6 I am building. His car is not any faster than mine but it is way more fun with that imediate throttle response. I had not felt that sinced I had driven a 2.8 with MFI.......

Jeff

goldgunner · 05-26-2005, 05:58 AM

Good points to be reminded about - Thanks!

Finicky carbs but with quick throttle response;
CIS does seem on my limited experience to be 'OK'....

That Bitz EFI system does seem to be totally programmable, and the cost is about half of PMOs.

Hold my hand a little more - what is and what's the story on 'ITBs'?

Thanks!
jim

Mike Bonkalski · 05-26-2005, 06:08 AM

ITB's will give you similar throttle response to Carbs / MFI. They will also allow you to run more cam options as opposed to CIS. Another benefit is EFI and the ability to control fuel and spark throughout the RPM range. Tuning is completed on a laptop and is less of a "black art" like on carbs.

As far as cost goes, if you go with Megasquirt (as with Bitz Racing's Kit), you can get into EFI / ITB's for the same price as new PMO's. This is the setup I am running on my car.

kenikh · 05-26-2005, 07:14 AM

Quote:

Originally posted by goldgunner
Thanks Chris

Tbitz's CIS to EFI kit and system is quite intriquing.
( http://www.bitzracing.com/index.html )

And certainly worth serious consideration.

So, quickly, it's CIS (as is), EFI and Carbs (PMO) to consider.

As to the Bitz EFI system: it would be helpful if there was even a little more than the very minimal performance measurements available...or even some performance claims!

More! I need more - please - carbs are so...

thanks!
jim

Ditto