Some general carb (and fuel/air intake) questions. . . .

[Tom '74 911]

So many questions, so little time. . . So I thought I'd start simply with a few questions about carburetors.

Some quick background - My only direct experience w/a carburetor was on my KTM 300EXC (2-stroke dirt bike) - which I incidentally sold to fund my Porsche. That bike was great, but no matter how much I fiddled with the jetting, mixture and exhaust, I could never get it to run perfectly. I live at 6000' in the mountains, and a typical ride might have taken me from 5000' up to 8-9000' - quite an enormous spread. I know it's unfair to ask a motor to run well over that entire spread, but it seemed like I never found the sweet spot and that it always varied some too - whether it was cold or hot, dry or humid etc...

So my first car related carburetor question - assuming the initial installation is well thought out, maybe even dyno'd for optimum set-up etc... how flexible are they to altitude and weather changes? If I go the track (which is 4+ hrs. away and maybe 2000' lower) am I going to have to re-jet them when I get there? If I go on a drive over a nearby pass that's 2000' higher am I going to be sputtering over the top? How sensitive are they in general?

I'm hoping to basically start a conversation that will evolve into further discussion of different air/fuel intake options and the pluses and minuses of each. But lets start simply - I'm sure it won't be difficult to get complicated quickly.

Also, to relate it specifically to Porsches (and me) - I currently have a 2.7L CIS engine that runs pretty darn well as far as I can tell. I do dream of turning into a 2.8L monster. . . but I'm getting ahead of myself.

I appreciate your responses and informed opinion.
Tom

[Steve@Rennsport]

Hi Tom,

In a nutshell, you have discovered the main limitation of carburetors: they simply have limited abilities to correct for air density changes.

It matters not whether a carb is on a MC or a car, their capabilities are limited by their very design. If one needs a seamless transition between low & high altitudes or temperatures to maintain best performance and power, then some kind of air-density metered EFI is required to do that.

There are MANY solutions to this dilemma and all of them depend on your budget so I'd not offer anything specific until I knew what numbers you had in mind. One thing, CIS doesn't work well on a high-compression 2.8 due to its inability to deliver sufficient fuel so keep that in mind.

[Tom '74 911]

Originally Posted by Steve@Rennsport

Hi Tom,

In a nutshell, you have discovered the main limitation of carburetors: they simply have limited abilities to correct for air density changes.

. . .some kind of air-density metered EFI is required to do that.

There are MANY solutions to this dilemma and all of them depend on your budget

Steve -
I apologize for asking a question and then disappearing for a few days. Thanks for your response - you were one of the people whose attention I was hoping to catch.

I pretty much figured that was the answer w/carbs. It seems like so many people are always raving about converting to them, I just wanted to make sure I wasn't missing something obvious. I don't think they would be a good fit for me.

To get a little sidetracked - how does MFI fit into the equation? My assumption is that for the money, going with a more modern system has a better value? Also that parts might be challenging to track down? Less "tunable" too?

So we are left with assembling some kind of EFI system. Forgive me for being very elementary about all this - I know just enough to know that I don't know much! Does that basically mean some sort of throttle body like what PMO does:



combined with some sort of engine management system ie:computer? like Electromotive or Motec? The computer, basically, gets its info about air temp, pressure, exhaust gas etc... from sensors and then optimizes the air/fuel mixture to provide for the best performance?

Can you use a distributer with this system or is it more common to use the electric coil type system (sorry, I don't know the name of that set up)?

As far as dollars go - I'm a value oriented person I think - I'm willing to pay up, if it's worth it. I know we haven't discussed much about the rest of the package. Here's a few things that'll be important to me:

runs on pump gas
twin plug is OK
I need heat
not obnoxiously loud
good street milage is a plus
durability is important - life measured in many, many thousands of miles, not hours

My car is one of those typical stripped down street legal track cars that everyone warns you not to build because it'll be too much of a compromise and not do anything really well. The one thing it does better than anything is put a very large smile on my face - which is it's most important role. So it has to be streetable, as it's a long drive to the track for me and I also drive it work every day during decent weather (2 miles). Really, I'm only looking for more HP for the track. My car is plenty fast on the street (where I try hard not to be an idiot!) Although, it'd be nice to be able to keep up w/the minivans at stoplights too!

Thanks for all the help,
Tom

[Steve@Rennsport]

Hi Tom,

MFI doesn't compensate for air density any better than carbs, and argueably, less so. Its mainly a throttle position & RPM dependent fuel delivery system.

EFI does the best job of adapting to changes in temperature and altitude using MAP sensing (as well as TPS) feedback to the ECU.

There are many options for ITB intake systems such as PMO's, TWM's, and a system that we make here. Most of them have their strengths and weaknesses as well as different price points so the ultimate choice really depends on the engine and how it will be used.

Engine Management also varies and in most cases, you really get what you pay for. We use Motec and DTA in the majority of applications. Ignition can be either a twin-plug distributor with CD boxes & coils or a 6-coil pack setup to fire all 12 plugs. Both are controlled by the ECU.

Call me anytime during business hours if I can be of further assistance. I can also send you a pic of our intake system, too.