Engine Build Proposal

[Nicks911]

Background:

Car is an 81 911SC currently stock. Car is my daily driver with 150K miles on it. I put on about 10K a year and drive the car in summer and winter.

Budget ~15K

Goal reliable Daily driver with a bit more HP some where in the low to mid 200s

Thinking about doing the following:

If cylinders are good reuse with 10.5 pistons if not bump in displacement using 98mm or 100mm p/c.
Twin plug spark (haven't decide on a system, but much prefer the look of distibutor over crank fire)
964 Cams
Port head to early SC spec
SSIs
Keep CIS (good for cold winter start up and has been built proof reliable for me over the last 10 years or so)

Questions:

Can carbs be made to work well enough that I Wouldn't mind DD in the winter sub zero temps for starting.

Is 10.5 too high with twin plug for street gas

Why not go with machine in 102mm P/C if I need new p/c?

Any other comments?

Thanks in advance for the help.

Nick

[lindy 911]

The CIS is the limiting factor in your build list. I'd use a hotter cam (GE60) and go with carbs if you don't have to worry about emissions. Look to Arron Burnham @ Rennsport for a distributor and get two MSD boxes to fire it. 46 PMOs are a good choice and 10.5 : 1 with twin plug is fine on 93 octane pump gas (total timing about 28 degrees). I would look around for a rebuild-able set of '78 or '79 heads and leave them stock (other than springs, retainers, guides and a good valve job).

Winter starting is really no big deal. Carbs have accelerator pumps; a few pumps of the go pedal, turn the key and fire it off. It may take a little while to get used to the protocol. This set-up is good for 250+ hp and could be tuned for close to 300. You will like this motor.

[Nicks911]

I guess the question is the balance between drivability efficiency and HP. I would guess that going to carbs would drop mileage from 20 mpg to 10. This is not a small issue on a daily driver. The non fiddle factor. The CIS has been bullet proof reliable for the last 10 years I have had it. I only replaced the WUR in the time I have owned the car. I can't believe that in a 10 year span I will only have to touch the carbs once. Given a 15 K budget am I really going to make that much more power going to carbs. PMOs would eat 5K of the budget alone by the time I'm done. For my purposes I think I might be better off doing mild CIS complaint upgrades saving the money by retain CIS and keeping the convenience of CIS. This car gets driven in the snow and sub zero temps.

[PFM]

Nicks,

If EFI is in the works I would reach for the 3.6 manifold and a solid EFI from Haltech or some other name brand, get the dual head dizzy if you like that look and save another $500 or more. It will not have the WOW factor of Lindy's beautiful example but it will be reliable and start in winter and can be tuned for MPG when not hard on the gas and still scream when needed.

I will pass on the cam guess but CamGrinder is around here somewhere.

Just my two cents worth.

[flat6pilot]

As far as carbs are concerned, why does everyone think they need to be constantly tuned? Is it just me? I used to run a pair of side-draft Webers on a previous car before my 911. Once the carbs were sync'd and jetted correctly (not hard to do) I never touched them in the 5-6 years I had the car.

Don't be afraid of carbs.

[lindy 911]

My 46 Webers have been jetted and adjusted once. They need nothing other than idle set form really cold to really hot weather. My car is rich and still gets around 17-19 mpg on the highway. I could lean it out carefully and add another 2MPG but that's not important to me. Once carbs are set up properly (not hard) they are good to go. The entire 40 mm PMO kit is $3600, and a good set of useable Webers can be had for under $2000. The difference between CIS and carbs is hard to believe; they will completely change the way your car runs and feels (for the better). Again, cold mornings are no problem as long as you have a fully charged battery (same for CIS)

Understand that I'm not trying to persuade you either way, just letting you know the facts.

[Nicks911]

Thanks Lindy that is very helpful! I have been been reading around and have seen people saying that they get ~13 mpg. Maybe the carb personality has a heavy RT foot. I'm getting ~20mpg around town and near 30 mpg on highway runs. I could live with 15 mpg around town and low 20s on the highway.

Another concern is for a daily driver that does see significant traffic will I even want a cam that is aggressive enough to take advantage of the flexibility of carbs.

I must admit an engine bay with a 12 plug distributor cap and a pair of webers is great looking and spacious compared with mine filled with CIS stuff

[HawgRyder]

You just have to remember about the accel pumps.
Every time you push on the gas pedal...the pumps throw raw fuel into the chamber.
So....steady foot = better gas mileage.
If you are a "pumper"...the mileage will go through the floor.
My wife can't keep her foot still...and can't understand why, on a trip, I get almost twice the mileage she does.
Bob

[lindy 911]

Below 4000 rpm, you can't tell my motor isn't stock. It's very drivable on the street. Let that 3.0 spin though and it's Jeckle and Hyde...

[Walt Fricke]

CIS is really a very good street car induction. And, as you have realized, what you have is actually plenty of HP for sane driving on the street.

But remember, if you get the '77-8 or Euro 3.0 heads with the bigger intake ports (or have your head ports bored out), you will need the larger runners which go with the ports if you keep the CIS. And (though not 100% certain on this detail) you will need the early/Euro air box, because I believe the larger runners need a larger attachment at the air box end of things.

While there is some pizzaz to a 12 wire distributor, the crank fire systems are by far the better mousetrap. You could probably pick up a pair of early Electromotives used for less than a twin plug distributor, and have money left over. And there can be bargains on some of the older, less fancy, EFI systems. I am running Electromotive TEC3R on my race car, and it is about two generations old. But worth the savings, and it has all kinds of tuning features I don't bother with (part throttle? a pox on it. Smooth idle, naw).

Or you could try the DIY special - Megasquirt. A friend just put one in his 3.0, and loves it. If he had put in a hotter cam and twin plugged his heads and used higher compression, I am sure he would love it even more. There is a whole cult/support group out there running/trying to run/working on the MS for 911s.

One issue with higher compression (and a slightly hotter cam) tends to be the CIS. Yours was designed to work with the 3.0. Yes, more air through it (as in faster accelleration) ought to cause a proportionately greater flow of fuel. But there isn't a whole lot of tuning you can do with CIS. Richen it overall with the adjuster. Boost the system pressure with shims in the pressure control. Fiddle with the control pressure regulator to reduce control pressures and richen things more. I suppose these fiddles may work better than trying to make a 2.4 MFI work with a 3.0 motor or the like. But something to think about.