CIS to Carbs?
 

1977 911S has been sitting for 2 years. Compression good. New fuel tank, new fuel pump, lines cleaned etc... Will have to replace injectors, distribution block on and on and on. Why not just go with carbs and get rid of all the extra parts needed to keep the CIS in top working condition? Good choice? What about the HP increase?

Thanks, Tom

Carbs are an excellent choice, especially with your larger port (36mm) heads. I can't give you a HP number but it will be a big improvement. You can also go more aggressive on the cams. A pair of SC cams should do nicely.

-Andy

Thanks Andy. Any ideas on brand of carbs that you are familiar with and have had good service with? Where would be a good start for finding the carbs? I know questions questions questions. New porsche owner and have really enjoyed the camaraderie among owners.

Tom

On the 2.7 your choices are either
- Rebuilt Webers
- Rebuilt Zeniths
- Rebuilt Solexes... I think these may be too small though

In the new category, you are looking at PMOs. The price is steep, but everything out there says you get what you pay for. These are basically newly manufactured Webers, with improvements, and built to match your engine.

Also, realize that you will need to recurve your dizzy and replace your exhaust with a '74 style. The most common exhaust mod is to go to SSIs and a 2in muffler.

Additionally, because you have a '77, if its a US spec model you absolutely need to do replace the 5-blade fan with an 11 blade version and remove the thermal destroyers... I mean reactors. Also you should add the Carrerra tensioners.

Its a lot of work, but if you are going to invest in carbs, you ought to do the mods to ensure your engine lasts.

You can also go with an aftermarket EFI setup, which will give nice power bump too, but better fuel efficiency and tunability over carbs. For same or even less money.

Weber 40mm carbs complete with intake manifolds and air cleaners are being sold on this site for $1500 complete, some less. Even if you had them rebuilt by one of the carb guys, you're still under $2,000 for basically brand new carbs. I have 46 Webers ($2,500 complete, like new) on a hot-rod 3.0 SC with headers and twin plug and it's not even from the same planet as the stock SC motor. I am sure the EFI stuff is great but I haven't seen it for anywhere near $2,000 complete.

Lindy

The Bitz CIS to EFI kit is $1500, to which you need to add 6 injectors and a fuel pressure regulator. The grand total should be around $2000.
index

Edit: Fuel pressure regulator is $90, injectors are $227, so grand total is $1,817.
http://www.summitracing.com/search/?keyword=MSD-2222&dds=1
http://www.summitracing.com/search/?keyword=ACC-150630&dds=1

Are you sure the injectors and other parts are bad? If not, it might be a lot easier to get the CIS running. A lot of stuff that is just gummed up and not severely corroded (as I suspect yours is at 2 yrs) can be brought back with good cleaning.

So by looking at this kit, the original intake system including the overly restrictive barn door are still utilized. That's most of the problem. Do what you will. I say Webers.

Lindy

The stock dizzy should be fine on a 2.7 engine with carbs. You just need to check to make sure you are getting 30-35 degrees of advance at 6000 RPM. 40mm Weber carbs would be my choice.

-Andy

Gentlemen,
Thank you so much for your input on carbration. Found out injectors had already been purchased new and just need a fuel distributor and warmup regulator, so it looks like on with the CIS as by now it will have been replaced to almost new. My mechanic tells me the CIS will run great and give many miles of good service. Need a pop off valve, right?
Guess I will have to wait until a rebuild for the carbs.

I have just bought me a pair of Webers that I might put on a 2.7 that I will rebuild, not sure yet.

Can I keep the CIS fuel pump? Do I need a preassure regulator? Are the Webers fuzzy about fuel preassure?

Can I use standard E-cams with the CIS pistons or do I need to machine valve pockets, can I even do that on CIS pistons?

3.5-4 psi max for Webers, yes on the fuel pressure regulator if you run the CIS pump. A Pierburg pump from our host would be a better set-up, no regulator needed. Not sure about your pistons and cams.

Lindy

Pistons will hit valves with E cams. I doubt you could machine enough off to make them work. New and used 90mm RS pistons/cylinders are available at very good prices on this used parts forum.


-Andy

Does the Bitz EFI system give the same throttle response as carbs?

No

Nope, there's (almost) no way an intake manifold setup with a single throttle can compete with individual butterflies.

Now to get that throttle response you have to balance both sets of intakes, then balance all 6 carbs, so there is of course a price.

Note: the almost is because some modern throttle by wire systems can pull some crazy response, but that's using combined throttle, fuel injection, ignition timing, possibly even adjusting the intake manifold geometry and cam timing... None of which can be easily integrated into a 2.7 long block.

OK then.... how about if one went with the new PMO style ITBs setup for EFI?

I would imagine they have fairly crisp respoinse, just not sure how they compare to carbs.

ITB EFI is a whole different story... but then you aren't using the Bitz kit (but can still use something with a megasquirt ECU).

Oh, and the costs go way up - PMO ITB kits cost $2600 (you want the accessories) and you still need the entire EFI system (say throttle position and manifold vacuum sensors).

PMO Carbs for a stock-ish 2.7 are $3800 for 40mm, $3900 for 46mm. I'm fairly certain that 46s are too big for a 2.7 with E-cam, but I could be wrong. Additionally, as has already been noted, going to an E-cam will require new pistons. If you go that far you should run a forum search for all the 2.7 'while you are rebuilding it' lists.

A few other items to note (understanding the OP's engine is currently INOP at the moment) All are valid for CIS as well as carbs or EFI (at least EFI without integrated spark).
- If going to carbs (and even if not), you want to do the exhaust first. I recently did this to my 3.0L, and it wakes up the car nicely (good for you joetiii, if you haven't already). PMO recommends / 'requires' this (I'm not sure why its a 'requirement')
- Re-curving the distributor. Porsche apparently used less aggressive curves on US cars. This is good for CIS, and a must if you switch to carbs as they need a different advance curve
- An MSD box is a good replacement/improvement over the stock CDI.
- Alternatively you can have the distributor locked out at max advance and use an MSD box(s) with programmable advance curve. This has the advantage of re-curving to match needs / future engine modification.

An ITB EFI engine is sweet. All the benefits of a carb but better. But it involves a lot of endless tuning :) Expensive if you can't pull of a DIY approach.


I searched a bit on machining CIS pistons and Henry wrote that you could machine them to clear s-cams. So I guess you could do that with e-cams as well.