930 manifold runners

RJM3 · 01-10-2002, 07:31 AM

Hello after reading several threads the question of unequal distribution of gas on cis intake runners...I guess different lengths as I look at mine is a downside of cis. does this effect the power punch? Do some cylinders run lean? any information or comments are greatly appreciated regards Jerry

JohnC · 01-10-2002, 08:20 AM

Over at the Protomotive sight Todd Knighton says this:

"Well, just looking at it makes you think that they did a pretty good job of keeping it low profile and low volume for better boost response.
However, if you look up inside of it, you'll notice that cyl.'s #2 and #5 have about 300deg. of opening around them, vs. #1 and #6 have about 180deg, and #3 and #4 have only about 90deg of flow area. So, you might be scratching your head and saying, "how does this thing flow equal amounts of air to the cylinders"? Well, it doesn't... If you put it on a flow bench, the results will scare you. It's amazing those engines don't blow up on a much more regular basis, #2 and #5 flow anywhere from 25 to 30% more than do the rest. And with the fuel injection giving equal amounts of fuel to each cylinder, you end up with 4 rich and 2 lean, or 4 just right, and 2 very lean, or 4 very rich and 2 ok... No matter what, it's not a nice story to tell. You can yank off the exhaust manifold and check out the port coloration, or maybe even just a spark plug check might do it. You won't find them all the same color, as the air / fuel ratio's are all different. So, what's the fix? Well, the easy one is to trash it and put on a good manifold from a Carrera, C2/C4 or 993 that all flow within about 2% from cylinder to cylinder, and the later the manifold, the better...The hard way is to try to make the 930 flow the same at all cylinders, and it get's much more difficult as the port sizes go up. I know one guy that actually would weld little dams in front of #2 and #5 to slow the air down to those cylinders, trying to equalize the flow... But that's only good at specific flow rates, not everywhere. Hope that tells the story well enough... "

beepbeep · 01-10-2002, 10:07 AM

Ughh...exactly what i tought. Carrera-intake, here i come!

RJM3 · 01-10-2002, 10:16 AM

Now your scaring me....how can this be......doesn't it follow that under boost this inequality is more unequal???? Surely porsche must have thought of offsetting this situation in some way ???remember vw distributors with #3 cylinder with different dwell reading than other three cylinders....I think it retarded the timing on #3 by changing the breaker point timing in the distributor on #3 breaker cam....Any further ideas???regards Jerry

JohnC · 01-10-2002, 12:31 PM

".....doesn't it follow that under boost this inequality is more unequal???"

Yep.Which really makes it scary, considering how many people like to run higher boost levels.Lean burn with the original intake seems inevitable.

Look...the guys at Porsche are engineers......not miracle workers

You've got that huge monstrosity of a fuel distribution/metering device and all that plumbing not to mention the monster air box...oh and I forgot...that bullet proof pop off valve....its cramped in there. A compromise had to be made somewhere!

"Any further ideas???"

The guy from rennspeed does rework the original intake for equalized flow other than that, run a rich mixture

RarlyL8 · RarlyL8's Garage

I must be missing something in this thread, it doesn't make any sense to me.

The 930 intake doesn't have any runners. It is one open plenum to all cylinders.

Also none of the 911/930 intakes flow fuel, only air.

The 930 intake is restrictive in its volume, but not in its ability to feed air to each cylinder. This system is under boost. That means pressure. Each cylinder opens at a different time allowing the pressurized air to enter. I don't see how this is a big problem.
Folks have been running 1.0 bar boost on these things for over 20 years. If done properly (read big intercooler and premium fuel) things are just fine.

RJM3 · 01-11-2002, 04:40 AM

I see what ya mean with the 930 3.3 being just a air distribution with mechanical injector at the end...but what about the comment some cylinders would run lean? I'm not knocking German engineering in any way...It's the best!!! would a flow bench show any differences in air flow to each cylinder?regards Jerry

JohnC · 01-11-2002, 06:17 AM

1. The CIS flows equal amounts of fuel to all cylinders.

2. The air distribution to all cylinders is NOT equal.(I have not flowed the individual "runners" of the intake so I can't speak to the cfm specifically.This is information I have gotten from those who have flowed the intake i.e. "#2 & #5 flow 25% to 30% more than the others" )

3. Pressure here is irrelevent.
Imagine these ports were orficies, for the sake of simplification, all recieving an equal distribution of air. If one orfice is smaller it will flow less air than all the others regardless of pressure, now remember all the cylinders still recieve the same amounts of fuel.That means LEAN burn in the "smaller orfice" cylinder.

4. The total volume is not the issue here.The issue is equal distribution to all cylinders.

I have'nt peeked inside the turbo intake but from accounts and confirmed by the flow bench, there seems to be some shrouding of the port (on the head) by the intake causing impeded flow to those particular cylinders.

RarlyL8 · RarlyL8's Garage

The CIS system on the 930 is set to deliver the same amount of fuel to each injector at a constant continuous rate.

If any one cylinder is restricted in its air flow the ratio of fuel to air becomes RICH, not lean. The cylinders that flow the most do not run lean, but at the correct ratio. This is evident by the poor fuel mileage of these vehicles and the fact that many have gone 100,000 miles before requiring a rebuild.

A lean condition would very quickly destroy any turbocharged vehicle that does not have the engine management capabilities to offset it.

finadict · 01-11-2002, 09:27 AM

The intake valve and port size is the ultimate restricter in this system, The cam on a stock turbo is very mild, considering that a stock SC grind is considered an upgrade. (does mean hotrod 930s qualify for scwdp membership?) The seperate runners in the manifold may flow differently but utimately the air and fuel mixture has to pass the fairly small valve and port area and even under pressure the size of each charge is less than a liter at normal atmospheric pressure. The uneven flow characteristics of is not important in these smaller displacement ,low RPM ,low boast motors. Rarelys right IMHO

JohnC · 01-11-2002, 09:58 AM

I agree....welding a piston to the cylinder wall is not something thats likely to happen with the run of the mill 930.

Rarlys correct( I misled myself and you) that the cylinders with LESS flow give a RICH condtion but does'nt that mean that the other cylinders recieve more than their correct share of air resulting in a LEAN condition?

Again all this does'nt amount to a "hill o beans" unless you're building a high HP engine.

Having unequal flow in the intake manifold puts you precariously closer to the dreaded detonation monster!

perhap someone should ask Steve W. at Rennsport what he does to the stock intake manifold?

Roland Kunz · 08:39 AM

Hello

3. Pressure here is irrelevent.
Imagine these ports were orficies, for the sake of simplification, all recieving an equal distribution of air. If one orfice is smaller it will flow less air than all the others regardless of pressure, now remember all the cylinders still recieve the same amounts of fuel.That means LEAN burn in the "smaller orfice" cylinder.

Well sorry my english isn´t the best but some people don´t can think to the final point. The runners are not from interesst in a normal bostet system as the end from the runner is a equal chamber. You only can fill the combustion chamber and the filled amount is based on the boost level.

The problem with the 930 manifold are when the engine is runing at no or low boost or someone is changing the system and try´s to feed more air then normal.

In fact the 930 manifold was a compromise and the engine is constructet to work in specific parameters designed to do the best from the compromise. Porsche sure could have done better and if you look at Porsche tuned engines or race engines you will see they did better. But everyone will have big horses and don´t pay them and so Porsche had to save costs. If not they would have sold the raceengines to the costumers.

If you read trough this board most have the problem that they think Porsche made lousy engines not performing like they could while others have problems to pay expensive parts and think that technology will work with cheap things. And some think both ways togehter. And rant about Porsche if things are to expensive and don´t perform like a race engine. But not willing to spend money like for a race engine.

YES IN FACT PORSCHES ARE POOR MADE AND POOR ENGENEERED RIP OFF`s.

Get a Viper, Corvette or a Ferrari for the real powershoot, Don´t waste your money or time on a Porsche as those cars are not made to be the fastest they are made to be a solid ride and work longer then a normal car. A porsche is the oppesite from a throw away car.

Grüsse