Hi All -
Got a line on a rebuilt 2.7 out of a 1977 911S. It is 100% complete including the full CIS system. What kind of power loss would I experience if I stripped off the CIS and went with carbs? The motor is only 165hp to begin with...

Thanks!
Scott S

Scott,

I heard about that motor
seemed like a really good deal !

I don't know that you'd loose power
you probably would loose mileage is all
but I'm no CIS expert either.

brant

I cannot support the following with empirical evidence, BUT;

I would say that you would not lose any peak hp with the switch. You will probably lose a little driveability as the CIS gets you a feedback Lamba circuit (correct me if it doesn't have a O2 sensor) and is more flexible for all kinds of weather.

Also, CIS has a large plate (Read: Restrictor) in the airflow which controls the amount of fuel in the fuel distributor and this has an impact on the pumping losses and efficiency of the engine.

All in all, I'd think you could get about the same peak HP.

That being said, I'd stick with the FI as it will be a better car to drive.

From what I have seen on graphs you get peaks and valleys instead of a smooth power curve.

I would think that you would H.P. & Torque with the switch. 2.7s are fairly low compression motors (remember when they were built - Fuel availability & emissions) and the CIS is made to work with this compression. Without a Cam change to work with the carbs it will suffer. However you can make a 2.7 into a great motor. I expect my 2.7 to put out 225-240HP

T.C.

Why not just keep the CIS?

When I bought my car it had a 2.7L motor built up to Bruce Andersen's 911 Performance Handbook spec's with 40 IDA carbs, headers, the proper cams, shuffle-pinned... everything except the Euro pistons. It had CIS. I wish I could give you numbers but never got that engine dyno'd. That said...

The car ran strong but I had to change jets often. Kind of a pain actually, and I'm sure I would have made more power with the proper pistons and more compression.
How much is impossible to tell... But I wound up with a full set of jets and lots of experience balancing and jetting those carbs. Gave up all that when I traded away the motor for a 3.6L.

Just make sure you have the 11 bladed fan, not the 5-bladed one, unless it's a track only car. I'm not near my books and don't remember what year those came on the 2.7 engine.

That engine sounded great.

If you don't have it, I would get Bruce Anderson's book and follow his advice on the 2.7 motor. I have one with the RS piston/cylinder set, E cams, and the Weber carbs. It runs very nice, and certainly has more power than the CIS system, which used a very mild cam (210 if Anderson is correct in his figures). A wilder cam will give you a higher peak hp, but at the expense of driveability.

I think that if you put on a modern, electronic fuel injection system (ie Bitz Racing with the Megasquirt controller), you could do better than the carbs as they are really not all that happy with today's gas formulations, and the EFI should have better control.

If the engine is complete and working, you can always use it that way initially, and always modify it later if you wish. Even 165 hp in a 914 is a blast to drive.

If the CIS is intact and working stick with it. You won't be disappointed. The only problem will be fitting the engine in the car. I decided to cut a little "hatch" in my trunk to axcess some of the components and make way for other parts. You could probably make it fit without doing this but you'll have to get a new fitting for the fuel distributor and relocate one of the vacuum regulators. Here's what I finally ended up with. I am in the proecess of making the cover. The parts circled are what got in the way.

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You can't have carbs and CIS. The fact that you talked about the jets means you had carbs

That's correct. I should've said "... it had CIS pistons instead..."

Thanks

Wayne at Pelican claims to have installed a CIS motor w/o having to cut the trunk.

I know a few folks that run Webers on an otherwise stock 2.7L.
You won't loose power...more likely you'll gain a few ponies and a hell of a lot of throttle response. Your gas mileage will suffer....if only because you'll play the Weber organ every chance you'll get.

There are very few things in life more convoluted than the CIS intake tract.

I'd probably just use the CIS. But if you buy it and carb it Scotty I need all of the CIS gear.

Totally!

Warning: the Weber Organ is closely followed by an annoying Siren Song if you're not careful when you play.

I have a 2.7 in Huey at the moment, with Webers. It's nice, but not exceptionally fast. Runs good from low to high rpm. But, John Forbes my mechanic said since it was set up for CIS, it's low compression and thus not the best for carbs. Whenever it needs to come out, we'll increase compression, change the carbs, and get more power but not crazy as I want to be able to keep it street.

Just want to throw this out there - since the title says how much HP loss.
It's my belief an engine will make the same power on any non forced-induction system providing it is tuned to take full advantage of the air/fuel mixture provided. I realize the path that the air/fuel take to enter, and the point(s) at which it enters also make a factor in detination within the cylinder.
So what are we really talking about here ?
Camshafts - provide the amount of intake (fuel and air) and how well the cylinder is evacuated (exhaust) - so a CIS engine has lower lift ? shorter duration ? (looking for comparison information).
CIS - mechanical fuel injection that is not direct-port that will take aproximately the same path to the cylinders except for within the assemply itself (right?)
Carb(s) - The main advantage to be had is the more direct path, thus "getting a good run at it" and the ability to easily change the air/fuel mixture ?
Fuel Pressure / Pumps - Carb(s) take less pressure to run, hence need a return mechanism to return the unused fuel. Wouldn't the supply be a bit more dependable on injected vehicles ?
Pistons (compression ratio) should be matched to the camshafts providing they don't hit the valves.
I had a 2.0 carb'd car (delorto) and now run a 3.0 CIS. A daily run will take me over 4000+ foot change in elevation at high altitude - From 5000 to 9000 feet does a carb display that much difference (mine didn't - it ran great)? Between the two I would take CIS because I don't get the fuel smell though it doesn't have the great looks you can get from carbs.
Sorry - I rambled - I swear it was my keyboard
To summarize - are you all saying there is more horsepower from carbs only if the cams are changed or simply because of one of the above other factors ? (I.E. what is the total package?) I don't believe just changing the top end would increase horsepower unless you are saying the CIS is not providing enough fuel/air for the engine to begin with.

You've covered a lot of ground there.

A good illustration of the possibilities of the 2.7L is the difference between the 165 hp @ 5800 rpms CIS engine and the 2.7 MFI RS engine which is conservatively rated at 210 hp @ 6300.

The piston's are different but the compression is the same. The difference is the MFI, the cams and 1mm larger ports on both sides...that's it. The RS pistons have reliefs for the valves at overlap. The RS is freer flowing at intake & exhaust.
The cams allow the cylinders to scavenge....actually suck the intake charge into the cylinders at a higher velocity using the departing exhaust pulse inertia. Scavenging is minimal with CIS cams with their lack of overlap and the CIS won't allow it. The overlap pulse screws up CIS operation.

My hot rod 2.7L makes 198 hp @ 6300 at the wheels. This is with less cam, more compression (than the RS), and Webers. Peak torque is 190 @3800 ....again at the wheels. A very drivable street motor other than the required race gas. I formerly has street gas compression (9.7:1) and it was a hoot.

Humm.... I got off the point also. The big problem HP wise is the CIS itself. There is around 15-18 inches between the butter fly (throttle body) and the valves and the path is convoluted. With Webers that distance is about 6 inches and nearly a straight shot. That is where the extra punch & throttle response comes from even with stock cams. I would hazard a guess that in a 914 with headers & Webers, the HP would be around 180-185 hp at the crank...half of that increase being from the headers. Both let the engine breathe as well as the CIS cams allow.

In addition to the differences you listed, any MFI->anything else comparison must include the observation that while CIS and carbs both have restrictions in the intake tract (CIS has the metering vane, carbs have venturiis) MFI doesn't have anything in the path but the throttle butterfly plate. Slide-valve MFI doesn't even have that.

Even if you figure that the MFI pump must take some HP to run, it's a big (~10) net gain being able to run without power-robbing restrictions.

The price is efficiency - MFI has -no- clue as to engine load - all it can know is throttle position and rpm.

Yeah,yeah, some folks go all gaga over the MFI setup. If someone gave me an RS set up in perfect working condition, I'd put it on my engine, but.....the cost to buy it is 3/4 of the way to an ITB, EFI system which will make more power and run more efficiently. I'll leave the MFI to the Grope R wonks.

This is a good discussion....

I have carbs. I like them, but have thought about something different for Huey and relatively daily street use.
I'd like to stay away from CIS - heard too many horror stories
Not big on MFI just because again, it seems a bit more tempermental
So what's the deal with EFI? what are the choices, costs, peoples experiences, etc?

Not something that's happening anytime soon, but just for info...

I wasn't recommending MFI, mind you - it was just an observation that a big part of its power increase is the lack of intake restriction. It can achieve this because it does not measure airflow. The downside is that it 'knows' how much air is flowing because its cam is matched to the engine displacement and camshaft.

For a period in time where real-time computer control was unknown, it's pretty cool. Nowadays your average iPhone has more than enough CPU power to run a full engine management control system. MAP's and MAF's have essentially zero restriction (taking away one of MFI's last few advantages). Add in closed-loop feedback with an O2 sensor and there's no reason why you wouldn't want an EFI system - if you were starting today.

I'm keeping (and uprating) the MFI in my application because I am building a replica/homage of an earlier car for my own enjoyment - not because I believe it offers any kind of performance advantage.

If I hit the lotto......

Jeff Gamroth has an EFI set-up that produces an astounding amount of HP & torque.

http://rothsport-racing.com/

Our latest Parade AX winner has this set up on his car, a 2.8L, and makes 285 HP and is tuned for autocross. G Fordahl told me that they gained 30ft/lbs of torque across the range but only 10 HP....they bumped it back for more torque.
How about an all stock internal 3.0L @ 250 hp.....the EFI & exhaust are the only changes...the motors are blueprinted, of course.

Around 8k installed & dynoed, I understand.

As long as the carbs are optimized for the engine, you shouldn't give up too much or any HP over the CIS.

The CIS engine you've got is by comparison, low compression, and cams with virtually no overlap. It works well, but it's not the highest performance branch on the tree. CIS is a decent system for a street car, and doesn't have idle jets to clog up.

CIS and MFI have been lumped together, which from a performance perspective isn't correct. CIS and MFI are totally different systems. With MFI you can run a high compression engine, and cams with more overlap. The MFI pump puts out about 240psi at the injector which gives you killer throttle response, but the lack of any vacuum or intake pressure sensing can result in surging throttle response, ie part throttle down hill (low load) compared to part throttle up hill (load).