2.7 CIS to Carbs - how much HP loss? Options

[Scott S]

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

[Mountain914]

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 (IMG:style_emoticons/default/type.gif)
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.

[J P Stein]

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 (IMG:style_emoticons/default/type.gif)
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.

[ArtechnikA]

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.

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.