Change 3.0 from cis to carbs

[jhelgesen]

Hey Guys,

After reading Bruces sugestion in his book on upgrades to a 3.0 cis car, what's involved in the change. I know yanking the cis off. I've got a 79 motor, so no computer. I guess change to an earlier lower pressure fuel pump and loose the acumulator. Anything else?

TIA

[RickM]

A recent thread suggested that while it can be done the pistons are not optimal for this conversion....more research may be needed.

[MotoSook]

Rick,

I followed that thread a bit, but there is no reason one can't put carbs on a CIS engine. It will be a step up from the CIS, but it won't make a "built" race motor. Optimal is a relative word.

A buddy ran 46 MM Webers on a stock 3.0 engine in his race car for years. Granted the car saw most of it's life in the higher rev ranges, but I just don't see any draw back to swapping the CIS for carbs.

So while one can buy new pistons and cams to be used with carburetors for better results, it is not critical to tear the engine apart to install carbs on it.

The caution should be to not over size the carb. As it's commonly known, 46 MM carbs will not give you very good results on a 3.0 for the street.

Some say the intake pulse (reversion) associated with the CIS pistons make the carbs a bad choice, but that's just not the full story.

[emcon5]

Originally posted by Souk
I just don't see any draw back to swapping the CIS for carbs.

Fuel economy and cold starting have been listed as downsides to carbs vs. CIS.

I would think they are harder to get set up properly as well. With CIS (particularly K-basic) you make sure your pressures are in spec, set your mixture with a CO meter and you are done.

Tom

[Formerly Steve Wilkinson]

My SC engine has carbs (PMOs), but I optimized it for them. Pistons, exhaust, twin ignition, compression, displacement (3.4), cams, valves, etc. I don't think you want to just pull off the CIS and slap carburetors in their place.

Last time it was on the dyno, a couple of months ago, it did the equivalent of 275 hp at the flywheel. Saturday I pick it up at Rick Deman's after another dyno session and carb optimization and will find out what he got it up to this time.

Stephan

[Randy Webb]

My '75 street car has the original cams, pistons and 40 mm Webers -- it's great! But it would be better to change everything. Jet the car a bit rich if you leave the CIS cams in.

It isn't the pistons per se ... The cams should be changed to get more overlap and provide better filling with carbs. Ok, but! The pistons prevent using the best cams. So then you have to change the pistons also. JE makes some and so does Mahle. Why not start with carbs, then do the rest later -- after the next technology stock bubble.

[Tim Walsh]

Souk,
It's actually reversed. The metering plate in the CIS system is prone to flucuations due to fuel reversion. That's why you can't run wild cams with the CIS. The 906 had carbs on it and those have lots of fuel reversion at idle. (making for lots of top end power)

another thing to think of is you might have to plug the return line to your fuel tank as carbs don't use it.

[MotoSook]

I guess my mind was on performance Tom, but you are right, fuel economy and potentially cold starts can be considered drawbacks.

There are some established jetting combinations for 911 engines that can be used as a base for tuning, and as many times as I adjusted my CIS for changes in weather..or age, I could have developed the right jetting for carbs by now.

As little as I drive my car in the winter, cold starts are less of a concern to me given the advantages that will come with carbs.

The CIS engine is terribly restrictive in stock form, wouldn't you agree?

If the intake and exhaust can be improved upon, the performance gain should out weigh some of the inconveniences associated with carbs. CIS has its own set of inconveniences too, no?

[MotoSook]

Yes, reversion affects the CIS, I agree Tim. It is also a factor for carbs, as you mentioned in the case of the 906. A tuned intake can "attempt" to target the desired engine RPM and tune out any pulses from the engine, minimizing their effects on the flow through the carbs.

The option I think John is entertaining is to do what Randy did with his 75. As with most guys who have CIS and a need for better performance, carbs on a stock engine is a step in the right direction. Then when the planets are aligned the engine gets further work.

In the meantime they get to enjoy better throttle response, more HP and torque...well, it's all about having fun modifying this and that after all, right? I can spend 5-10 grand working on my engine to make it a monster, or just a fraction of that to bump it up to the next level, without major surgery.

A back pressure regulator can be used with the stock CIS fuel pump and the return fuel line. The regulator will hold back as much pressure as it is set for. The rest gets dumped back into the tank.

It's essentially what the some guys do with the pinching off of the fuel line, but with a product that was designed to do so with much greater accuracy and safety.

[Formerly Steve Wilkinson]

The "some guys" who pinch off the fuel line and use a gauge include me, Steve Weiner and Richard Parr of PMO. Works fine. How much "accuracy" do you need after all. A carburetor is not a chemistry lab.

Stephan

[MotoSook]

True, carb operation is not a chemistry experiment, and the pinching off the line works. It's a little too make shift for my taste, but that's why I get accused of being too anal. Blame my parents..blame my engineering training. If the pinching off method works for you great. It should work for John too.

The big difference between the two method is that the back pressure regulator will adjust for changing flow better. For something that operates at 3-3.5 psig, there is not much room for droop or spikes. Perhaps the real effect of the pinching off method will not reveal itself in the life of your system, or is identified as a problem caused by something else. I just assume not take any risk for the same money or the small difference.

Your money, your car, your decision.

[chuckw951]

I'm running a 82 3.0 SC motor with PMO carbs. P&Cs are stock with 9.3 to 1 CIS pistons. Works fine...actually makes great power according to my seat of the pants dyno. I'm using the PMO pressure regulation device with a MFI fuel pump (using the supply and return line). Fuel pressure is adjusted to 3 psi, works fine...IMHO there's is more of a risk of pulling another #$%@#^$ head stud than the motor grenading because of that gizmo...perhaps in time I'll upgraded the fuel pump with a regulator...YMMV.

I'm running 40mm PMO carbs...I was told that 46mm would be too much for my application. Also note that you will need intake manifolds with the carbs to match your intake ports...PMOs are built to order so that's no problem...buying ebay webers might be another story (and someone else's problem carbs). Since you have '79 motor you'll have the bigger ports and like you said the distributor doesn't need to be recurved.

Removing the CIS can be kind of a PITA. I had to drop the motor a couple of inches. Get a swivel head socket to removed the intake nuts. When you go to PMOs you need to install new intake manifold studs...which can be a pain. Also be sure to cover the intake port with rags (or something). A misc nut or washer WILL drop into an uncovered intake port!

Will the stock SC ignition work fine with carbs? I've seen recommendations that the ignition should be upgraded to MSD, but others might have an opinion about that. I've also read that it is recommended that you get SSIs or early exhaust with the carbs to maximize the performance gain.

The only other thing I that I can think of at this minute is that you will loose your heat unless you backdate using early sheetmetal and ducts around the fan housing.

I suppose the larger question is what are you looking for....a 3.2 conversion might give you more bang for the buck...again YMMV.

Chuck

[RoninLB]

Everybody's offering good info.

and I guess carbs w/CIS cams is definately doable if your comforatable w/carbs.. and obviously it's not the big blast as doing the full monty with carbs.

I tend to think that 22/21 cams with CIS is a better blast than carbs w/stock cams, not sure.

But IMHO carbs w/stock CIS cams is like having an itch that you can't scratch..

Is there a Doctor in the house?

[Randy Webb]

I think you can use 964 cams or something like that w/o chg to the pistons -- but at some pt. it's better to do it right.

Be sure to chg. to SSis.

I also am a fule line pincher -- but I aspire to a more suitable fuel pump... it's not my deisre to (re-)enter chem. lab -- just a functional aestheitc (+ saving a bit of wt. maybe).

[Formerly Steve Wilkinson]

Do be aware that your fuel mileage will plummet with carbs. I'm lucky if I get 12 mpg.

stephan

[RoninLB]

Originally posted by Formerly Steve Wilkinson
I'm lucky if I get 12 mpg.

Hey Steve,
What cams are you running on the street.. I bet your torque curve is up there, huh?

[ckissick]

I better get in on this thread. I have a '70 911E with a very tired '74 2.7L engine with Webers. My newly rebuilt bone stock '78 3.0 is coming next week. (The only mod is the chain tensioners.) Since I have the Webers and SSI's, I figured I'd go the Bruce Anderson route and put those on the 3.0, and re-jet the Webers to the specs that Bruce and others all seem to agree on. (I don't have the numbers with me.) I can't afford the full Monty, so I won't be tearing down the engine any time soon.

I've only been told good things about how the set-up will work. You know, HP, throttle response... It doesn't get cold where I live, and if I wanted good mileage, well, let's say good mileage is not high on my priority list.

So, any advice on the '70 to '74 to '78 transition? From what I understand, most aspects of the switch work out pretty well for me; ie, the '70 has the right electricals, and of course I have the optimum SSI's. I even have the bigger clutch starting in '70. (I'm keeping the 901 tranny.) So are my planets all lined up? What about this back pressure regulator business?

Charlie
Montara, CA

[RoninLB]

Originally posted by ckissick
What about this back pressure regulator business?

historically, routine carb fuel systems have the carb at a "dead end".
If you were to use a high volume/high pressure fuel pump you would have the excess pressure re-circulated to the fuel tank. The control on the returned fuel would be after the carbs are fed. There is 2 types of regulation that is routine today.. One is to restrict the returned fuel with a "hose clamp" tightened down till you get your 3.5psi. Two is to use a low psi fuel regulator instead of the clamp. I'll be installing the Holley low pressure regulator ++. The hose clamp is a simple install and is uncomplicated, somewhat.. The more you look into carb plumbing the more complicated it can become. The hose clamp routine is "good enough" and makes the carb conversion a less complicated affair.

For hard facts and info Tyson Schmidt is one of the experts on track carb plumbing around here. He's been doing the carb fuel loop with a regulator with great results on the track..

I would explore your carb fuel filters also.. the routine 10 micron filter still leaves much to be desired with Webers and PMOs, IMO.

[chuckw951]

Originally posted by RoninLB

and I guess carbs w/CIS cams is definately doable if your comforatable w/carbs.. and obviously it's not the big blast as doing the full monty with carbs.

And I don't disagree with that. When I did my set up I ran into, um, budget considerations. Sometimes these things need to happen one step at a time.

So, to replace cams the motor has come out or can it be done with a partial drop?

chuck

[RoninLB]

Originally posted by chuckw951
.

So, to replace cams the motor has come out or can it be done with a partial drop?

I'm not sure Chuck.. I did the full drop to make things easier and do other stuff.

But IMO [as Randy said] cam overlap is a consideration with carbs, SSI, and Monty.. My Monty was kinda castrated with CIS cams, almost boring. With some high lift/ high overlap cams the Monty will become wonderful tunes.. kinda like what cars sounded like in the 60s.. But my CIS cams were the stock tame flavor..

But the good news is that I just timed my new E-cams last nite..