1974 2.7 911 - Can it be Carbed?

[Ty Webb]

My dad and I are undergoing our porsche rebuild project for a while now. Buying lots of parts from Pelican.

We have run into the point where a decision needs to be made on fuel delivery. Keep the existing CIS, Upgrade it, or Carb it.

There are a few carb kits that are available, but it has been brought up that the cam and compression of the motor may not support a carb setup.

The car is a mint 1974 2.7 911 that has been sitting for 15 years. Zero rust. We are making it drivable again and want to have a little fun as well in making it unique.

Any insight is greatly appreciated on what our options are for the fuel setup.

[lindy 911]

Carbs will work, your combination is just not optimum. 40 mm Webers can be had here on this site and there are great sources for rebuilds like Performance Oriented (OnequickS). Noticeable difference when compared to CIS. The CIS can be made to work well also. I replaced my CIS with Webers and the sound alone was worth it.

[Walt Fricke]

Tye

What you have been hearing is that just switching to carburetors does not give you as much of a performance boost as switching to carburetors and changing cams at the same time will do.

The carburetors can support cams which are, for lack of a better term, more aggressive. The CIS limits how much overlap you can run, because higher overlap means more backflow through the intake. That messes up the CIS air measuring flap by trying to push it closed some. Carbs don't have this problem (at least are not affected as much), so you can run a bigger cam.

Guys who change cams are tempted to install pistons which do not have the funny dome shape the CIS pistons have, and in so doing are tempted to increase the compression as well.

But as Lindy says, you can get a performance gain from carbs with the stock CIS pistons, CR, and cam.

Once you've built a motor, you realize that changing cams later (when you have saved up more money) is not all that big a deal. Not like a whole rebuild.

Installing J&E higher compression pistons is best done along with a rebuild as you are doing, and that would go along with a change away from the CIS. If you really boost the CR up there, then you are also looking at the expense of twin plugging the heads as Lindy has, and finding a suitable distributor or switching to a crank fire/distributorless system.

However, the stock CIS motor, with the early exhaust you have, is no slouch. If you decide just to rebuild to stock you needn't worry that old VW vans will run away from you up steep hills or from stop lights (unless they have a 911 turbo motor back there). Better fuel mileage, and one less expense.

[Ty Webb]

Thanks!

The motor has actually been rebuilt before it sat for the longest time. I think the overall goal is to simplify the setup as it stands, and make it unique. Not looking for performance gains, just want a fun, reliable driver.

So from what I understand, a nice set of 40mm Webers will allow the car to run just fine with the stock cam and pistons. I think this sounds like a great idea, and the sound would be a great addition to the build.

Thanks again for the insight, and if theres any more info on the 40mm Webers, I will dig around or feel free to link me up to any good resources to pick some up.

[burtm5]

With a 74 you will have the early exhaust which is good. You will also have the 32 mm intake heads, not so good. with the 40 webers you will need to get manifolds. If you keep you 32 mm heads these are the same size as earlier T manifolds. should be fairly easy to find. If you bore out the intakes to 35 or 36 or find some 75 or later heads you will need to get the 35 or 35 mm manifolds. not as common. best Henry

[Walt Fricke]

Well, if it is a '74 non-S motor, it has the small ports and a detuned cam. The S has ports which are plenty large enough for the CIS intake, and for carbs as well.

Ty didn't say which he has, did he?

[larrym]

check also the parallel threads

- "... http://forums.pelicanparts.com/911-engine-rebuilding-forum/662112-dc-30-cams-911rs-2-7-mfi.html .."


Raising compression in 2.7 rs spec



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[JmuRiz]

Isn't another option to keep the CIS and install a SC-grind cam?

I was thinking about doing this, even though I have a set of really nice Webers on the bench I could install. I keep going back/forth on the to keep CIS vs switch to Webers options.

Either way, I may put in an SC-grind cam in.

[HawgRyder]

A good or rebuilt set of webbers is going to cost $$.
There are other options in carbs.
I set up an engine with SU carbs for a friend (4 carbs on a cross ram tubing intake system).
Seemed to work well...and the carbs are simplicity themselves.
They are cheap at auto wreckers and are very easy to tune.
Just another option.
Bob

[larrym]

the cams for fuel injection cars do not have the right profiles for webers

use your nice webers and get an appropriate cam to go with them

i sold a set of webers complete with manifolds & linkage on eBay last summer for about $1500 - and it took several re-lists to find a buyer at that price

there was a time they'd bring more, (2-3 yrs ago)

but "it's the economy" ya know

Originally Posted by JmuRiz

Isn't another option to keep the CIS and install a SC-grind cam?

I was thinking about doing this, even though I have a set of really nice Webers on the bench I could install. I keep going back/forth on the to keep CIS vs switch to Webers options.

Either way, I may put in an SC-grind cam in.

[Eagledriver]

Actually SC cams work fine with carbs. Of course if you are replacing cams and pistons, then by all means put in hotter cams. A euro SC with 46mm carbs and race exhaust can put out 240 HP easily (257 is the record so far, but that's a "blue printed" engine).

-Andy