Conversion-CIS to Webers
 

I have just purchased a 1981 911 SC with a 3.0 with CIS. I have a 1970 911 T with Webers. Can I swap these out without doing any modifications or do I have some work to make em swap? Also will it effect anything with my A/C on the 1981? Also the 1970 has the SS muffler. Should I swap it out also? I would like to get a little more power out of 3.0. The 2.2 is much stronger.

Sound like a fun experiment. You will need to change the jet/chock settings to get the best performance out of the carbs, but you can just slap them on to get a rough idea of the difference. In my opinion, you will see a fair improvement in performance and throttle response, but a fair drop in gas mileage. You will need to change your fuel pump since the CIS pressure is much higher than the 3 psi for the carbs. I would backdate to the early exhaust, if you have it.

The real magic in carbs is the wild cams and pistons you can run with them, but they are still fun on a stock CIS engine.

Good luck,

JP

JP

My problem is, I'm Porsche illiterate...... but I still wanna go fast. Could you swing by west Texas and install em for me. I looked at your project. That is quite impressive. By the way, would the fuel pumps swap out?????????

Come on over to Austin. We can make the exchange. Best bet would be to change out the entire exhaust from the 71 along with the carbs.

You will need to change to a low pressure fuel pump or pinch off the fuel line as instructed by PMO. The jets and chokes will be about $200 by the time you buy sets of six for each part.

another change will be to add a vacuum port to the manifolds for the power brakes on the 81. This has been documented on the board.

JP has it nailed - better throttle response but worse gas mileage.

Do a search on the tech board for 3.0 and carbs or CIS to carb. conversions. There is a good bit of info there.

Bolting on carburetors is probably not worth it - you will still have the mild camshafts that are associated with the 3.0 CIS system. Plus, you'll have to do a lot of work, and swap out your fuel pump. Probably not worth it, unless you swap cams & pistons...

-Wayne

I'm in the planning phase of the same thing. Rebuilding my 3.0 to a 3.2... You can't use CIS pistons. The top profile is incorrect for the carbs. This comes from several people I have discussed this with. Particularly, Steve at Rennsport. Also, you need a different cam, preferably early "s" or grind the CIS cams to "s" specks.

Do all this and add headers etc, gain around 40-50 HP.

Chris,

The drive from Florida is a little long for me, but most of us love to help with projects like this if we can. We all started and illiterates, but that's what this board and the book are for, so don't let that hold you back. I always defer to Wayne's opinion on these matters. I did try a set of webers from a 2.2 T on a 2.7 CIS long block last year, and I thought it was an improvement, and a fun combination, but Wayne is right, the real performance improvement come from piston and cam changes.

Good luck!

JP

You can get some improved "feel" from the carbs - maybe a little more HP, but in my opinion, it's a lot of work for not that great of a gain. I generally recommend keeping the CIS instead of carbs, if you're not touching the longblock. Plus, your gas mileage will suffer, big time, which is a large detriment, in my opinion...

-Wayne

OK, I'll leave it alone. When I have a few (lots) dollars maybe I'll do the whole engine upgrade. Thanks everybody.

I hope people don't mind me jumping in here but I am also doing the same thing and have a few questions.

If webers are fitted to a late SC motor.

1. Can you use the CIS pump if a fuel regulator is fitted?
2. Can I use the pump from an early car that originally had the webers ?
3. If not what fuel pump shall I fit?
4. If this was then going in a an early car can I simply remove the used wires from the engine loom?
5. What dizzy should I use with this motor?

Thanks for any advice. I stripped the CIS from the motor yesterday so I am getting close to actually fitting it soon. Motor will have S grind cams and 98mm B&P.

Alan.UK

1. Not recommened - just get a simple carb pump (inexpensive) and don't mess with the high pressure pump - too dangerous

2. Yes, probably a good solution

3. See #1

4. Which wires?

5. You have to still use the SC distributor as it runs backwards. If you had the option of replacing the distributor drive gear, I would use the 2.7 distributor - good for carb cars.

-Wayne

regarding #5 - you ahve three options. One is to use a 78-79 Sc dist. It has sufficent advance but not optimal curve. two is to have your SC dist recurved. three is to swap gears with an earlier dist.

Wayne and Jamie,

Firstly thanks for your replies.

1. Can you tell me, does the 78, 79 dizzy have a vacuum as I do have a early SC dizzy but I am unsure if it’s a 78 one as it has a vacuum.

2. When you say swap gears, I take it you mean the gears on the crank? If so I was not planning on splitting the case so where would I find a re-curved distributor?

3. Wayne , Point 4. I was referring to the wires on the engine loom that will no longer be used, as the CIS is now removed. Can I just tape these up or do I need to take them completely out of the loom.

4. Finally if anyone knows what changes I need to make in the 14 pin connector that would be very helpful.

Thanks

Alan,

Regarding the case split... you almost need to do the cams in order to get the most out of the Weber's. The CIS cams are too mild to of much benefit with the carbs. EVERYONE I have discussed this with concurs on this point. You can run the CIS cams but it is far from optimal...

The 65,000 dollar question... anyone looking at this thread know if it is possible to smog a 3.0 with cams, carbs etc... in Colorado??? Could it pass with some serious manipulation? I'll most likely wind up in the 9.5:1 compression ratio zone...

I am actually doing two 3.0 conversions into early cars over the next few weeks. One for a friend whom is not to worried about using the CIS cams. But my motor is having 98mm barrels and pistons as well as Andial S grind cams. I believe I may also have some clearance issues ?

Re: CIS pistons and stock cams with webers.

I've found the combination to work well for the time being. At some point I may go back in their and swap the pistons and cams for something more exciting. All a function of cash flow :)

Gas mileage stinks, but throttle response is great.

Re: distributor. Another option to consider might be the HPX ignition system. Can do fine tuning with the HPX.

Re: fuel pump. I used the MFI electric pump with the regulator, but the MFI doesn't generate the fuel pressure that a CIS does! Do a search for carb fuel pump there are many options. The 70 or 71 fuel pump for the 911T looked like it would be an easy solution, but my memory is fading. Also suppose it depends on if you are going to use the return line to the tank.

Yes, the 'S' cams will not work with the stock CIS pistons, unless you cut pockets into them. Most people don't like this approach because the finished product doesn't quite look right. However, it can be made to work...

-Wayne

Thanks,

Chuck, how much is the HPX set up you used ?. As for the fuel set up I will be using a pump from a 69 T and therefore will not use the return line to the tank.

Wayne, I will be using my Max M 98mm with the S grind cams. Do you know if there are still clearance issues with these, and if so what are my best options? Is it best to deepen the pockets or maybe use spacers, I guess it depends on how tight they are and also the desired CR. I will be running standard fuel with single plug.
Any advice is much appreciated.

Thanks

Alan.UK

Not inexpensive, Pelican has them. You'd probably also need a set of custom plug wires. On the other hand, at the time of my project I didn't have the right distributor, CD box and wiring. So while it was a bit more expensive the wiring was easier for me at least. But with the vaule of the dollar these days it might be something for your to consider.

The other thing that I thought about with my upgrades was the "upgrade path." While I couldn't do everything at once, later on I can use better pistons cams and with the electromotive going to twin plug wouldn't be to tough.

http://www.pelicanparts.com/catalog/shopcart/911M/POR_911M_igniti_pg7.htm#item32