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911st 02-01-2010 02:01 PM

Not the cam, the intake port!
It looks like it would be much more effective to open up the intake port to 38mm than to change out the cams.

I seems that adding an SC or C2 cam dose not even come close to making the breathing improvement that just opening up the intake ports should.

This is dramatized in my rough drawing that started as a graph of an SC cam. I drew in about what a 930 cam’s total lift and timing should be.

The cross hatch (xxxx) area represents flow potential of a given port size.

The bubbles (oooo) area represents what is gained by opening up the intake ports from the stock 34mm to 38mm.

Why 38mm?

That should to put the ports in balance with the 49mm intake and 41.5 exhaust valve diameters. How much a valve can flow is a function of lift and the valves diameter (curtain area). However, this assumes the valve balance is near ideal and it may not be.

For comparison a 993TT runs 43mm/38mm ports. This suggests the intake relative to the exhaust can be even a little larger.

It also suggests that our 3.3 port sizes are on the small side relative to the 993TT’s 3.6. For a 3.3 to be equal we should be at 41mm intake and 36.4mm exhaust.

With even larger ports, it should be that changing cams would then have a more significant impact.

911st 02-01-2010 02:03 PM

Just some of my calculations so they are in one place:

Port cross section:
32mm port is apx 1.23 sq inch area
35mm port is apx 1.50 sq inch area (930 exhaust)
38mm port is apx 1.77 sq inch area
40mm port is apx 1.96 sq inch area

Area under valve seat:

49mm intake valve has about a 6.1" of circumference.
41.5mm exhaust valve has about a 5.15” of circumference.

6.1" x .455" lift for an SC cam = 3.21 sq inch.

6.1" x .378" lift for a 930 cam would = 2.31 sq inch.

Compare port volume to area under valve seat:
2.78-3.21 sq in....area at full lift for 930 to SC cam verses,
1.23-1.95 sq in....area for 32-40mm intake ports.

32mm ports require .2” lift for area under the valve seat to equal the port area.
38mm ports require .29” lift for area under the valve seat to equal the port area.
40mm ports require . 32” lift for area under the valve seat to equal the port area.

35mm ports require .29” lift for area under the valve seat.


118% larger circumference.

If 35mm exhaust port is 1.77 in sq, and the intake is 118% larger, a 38mm intake matches.

993tt ports:
43mm = 2.27 sq in. (28% larger)
38mm = 1.77 sq in

911nut 02-01-2010 02:17 PM

Keith, if possible lay out the 930 cam angle vs. lift over the SC chart you have there. You will see that the SC intake valve is open longer than the 930 intake valve. That's where the increase comes from.
A valve that's lifting higher has to open sooner and close later. Think mass flow vs effective diameters.
Better yet, Google cams. There's lots of stuff to read about.

911st 02-01-2010 04:29 PM


Good suggestion, on the Google. I am very much in the learning stage as to cams and will do so.

There has been much conjecture as to why Porsche put such small intake ports on a 930 compared to most normally aspirated or turbo cars.

I heard it referenced once that someone had talked to a Porsche Engineer asking about this and was told it was to purposefully to limit power. Not sure I buy it.

My guess is it has mostly to do with creating velocity for better air fuel mix to help with emissions at idle. (Note the SC piston crown is also for this.)

For what ever the reason, it would seem that the 930 would respond to any thing that improves cylinder filling with such a restriction.

You can only force so much through a small hole. But why force it.

No argument that a longer duration cam might help some.

However, I suspect until the intake port restriction is addressed the potential that should come with a cam change will not be well be limited from what I think I am seeing.

Forgetting cam timing as it relates to where the TQ and HP peak's are going to be, the existing cam has the piston sweep area covered pretty well.

That is, when the piston is best moving during the intake stroke, the intake valve is mostly fully open and the port is the restriction. Not valve lift or duration.

It looks like increasing the duration is going to have the valve open longer at that point where the piston is stalled at the top or bottom of the cylinder. That can not have much of an impact.

Along with increasing duration comes about a half point loss in effective compression, earlier opening of the exhaust valve such that more energy is lost out the exhaust, and more residual exhaust pressure will remain in the cylinders when the exhaust valve dose close.

If we are going to time our SC cams to make peak HP at 5500 rpm where the stock cam dose, I suspect much of its improvement is going to be lost.

Having said that I recall Brent 930 making over 430whp through stock sized ports. He did however run a lot of boost and had ported his intake manifold substantially and nicely transitioned his injector blocks.

I have lots to learn about cams.

My biggest question is why should an SC cam improve off idle, pre-boost, or even boost onset on a 930 when a 930 cam seems better suited to be a better choice in those areas?

911nut 02-01-2010 05:20 PM


Originally Posted by 911st (Post 5160033)
Good suggestion, on the Google. I am very much in the learning stage as to cams and will do so.
My biggest question is why should an SC cam improve off idle, pre-boost, or even boost onset on a 930 when a 930 cam seems better suited to be a better choice in those areas?

I think that the reason that the intake ports were made small is due to the low static compression. Small ports keep the mass velocity up at low rpm. In the case of the 930 engine it made the engine less sluggish off boost. Once the blower kicked in it didn't matter all that much and the car was fast.
The no overlap of the 930 cam was for the same reason; no reversion of exhaust gases which dilute the air/fuel charge and make for poor low speed response. Remember that a turbo engine has greater backpressure than a normally aspirated engine so reversion is a big problem. Smart tuners will raise static compression when running big ports and aggressive cams to try to get back some low speed efficiency, on both n/a and turbo engines.
Based on this, I don't think that a cam with overlap will improve off boost performance. The SC cam would improve mid range performance but will not be as good as a 930 cam at low engine speeds. By rights the static compression should be raised but now the price goes way up.
This all gets back to what characteristics we want out of our turbos; how we want them to drive. Some want the instant jolt much like a large displacement n/a engine. Others like the "light the JATO" feeling of big boost.
As for me, I've been kicking around what my car would feel like with 964 cams and a lightweight flywheel. Would better acceleration get through the "dead spot" of low rpm, off boost running? Or maybe the answer is stock cams and a Garrett GT35R. Good low speed performance and more compressor efficiency up top (the K27 is right up at the choke point at .9 bar)?
Ain't bench racing great?

WERK I 02-01-2010 05:21 PM

Fuel mixture has mass. A higher velocity gas has more kinetic energy than a gas moving at a slower velocity. This aids cylinder filling. Add what Paul told you about a higher duration cam is going to have a longer duration, it is easy to see why the SC cam is much more effective in cylinder filling than the 930 cam. Even though the piston is traveling towards TDC, the filling is continuing because of inertia.

911st 02-01-2010 08:08 PM


So the small port creates momentum which keeps or pulls the mass with it.

And if we get this right, such effect is stronger than the losses with less effective compression that comes with the SC cam. I understand this with long runners can work well with a current EFI motor, MFI, webers, or even a SC that have a longer intake track (straw).


My gut feel was that with only about a 3" long runner and a restrictive metering system this effect might be minor or reduced.

That there would be more to be had from the mechanical pull of the piston during the intake stroke (930 or RV cam).

That reducing the already significant intake restriction via larger ports and keeping the longer intake cycle of a 930 cam might yield more improvement.

I wonder if there is anything in that Porsche's moving from 39mm intake ports to 34mm's with 1980-83 CIS 3.0 SC?

Again, I was thinking the small ports increased air flow right at the point where the CIS injectors had been spraying fuel in wait for the intake valve to open. This would help atomization and possible low end combustion.

In any case, better breathing results in more power (cam or port change). The more power we can create early, the faster the turbine is going to get us to boost.

Thank's for indulging me.

Lots to learn.

Speedy Squirrel 02-01-2010 10:51 PM

The port area is related to momentum tuning. This isn't a Porsche invention. All engines are designed with this concept in mind.

Momentum is mass x velocity. The air has mass. Smaller cross-section ports produce higher velocity than larger ones, so for the same mass of air they have higher momentum.

When the intake stroke reaches bottom dead center and starts up on the compression stroke, the intake valve is still open, and air continues to flow INTO the cylinder. Why? Because the velocity of the inlet flow comes to a halt in the cylinder. This converts the momentum into pressure, which crams more air into the cylinder. The higher the momentum, the more air that gets crammed in.

The down side is that small cross section ports produce flow losses, so it takes more pressure to get air to flow. For the same boost you will get more power at the top end with larger ports.

Thus, port sizing is a compromise for low speed torque verses top end power. I'm sure when Porsche used EFI, full ignition mapping, with short, pulse conversion exhaust manifolds and twin turbos on the 911tt they found they had improved low speed torque so much that they could do with less momentum charging, so they opened the ports up some to make more power.

I would say an EFI car running with ignition timing control would be a good candidate to open up the ports a bit. I would personally not go over 40mm on the intake side for a single turbo 930.

911st 02-02-2010 08:09 AM

Good info.

Plus the 993 had a decent runner length and much less restrictive metering system.

Long runner low end, short runner high end.

With our stubby runner - small port and restrictive metering assembly, der gut thinks any packing or increasing in VE from idle to boost onset is going to be modest.

With boost onset I am thinking we no longer are as worried about momentum created port velocity.

Also it seems Porsche only sacrificed the balance between the intake and exhaust on there CIS motors.

And, in 1978 when the CIS 3.0 SC got a 39mm intake ports the larger 3.3 turbo got tiny 32mm intake ports. Adjusted for motor size our ports are 40% smaller than the SC that was designed at the same time. (Later SC's were pulled back to 34mm being 30% larger).

I wonder if there is a noticeable off idle difference between the large port 8.5/1 CR SC and the small port 9.3/1 cars? The boost bump alone should be good for about 3-4% more power.

Lots to learn.


copbait73 02-02-2010 01:59 PM

Well as much as 911 purist do not like to admit it the intake system for the 911 moved from the sexy individual port induction design favored by European performance engineers to what they considered a crude common plenum long favored and well know to those in the US. This happened in mid '73, 37 years ago.

Racing was going good, but for the street they had emissions, CIS and this damn common plenum. They had a lot of ground to make up, all during a time when the air-cooled motor was scheduled for obsolescence. It's entirely possible they threw up their arms and said “bumsen Sie es gerade fügen einen Turbolader hinzu” (loosely – “screw it, just add a turbocharger”). I say don't try to make sense of their actions during this timeframe.

Where does that leave the 930 motor? For racing Porsche kept what they knew about porting vs. cams vs. carburetion, even into Turboing. If you want that level of performance follow the concepts in their racing Turbo motors.

911st 02-02-2010 03:14 PM


934 CIS Turbo: 41/41 ports (equal size), major-major overlap cams, TQ peak at 5400rpm, HP peak at 7000rpm. 530hp @ 1.35 bar w 6.5/1 compression.

If 41mm ports support a 7000rpm peak , a 5500rpm peak thus 'might' require 36/36mm ports.

Not a perfectly valid comparison but fun.

Bruce Anderson's book notes on tuning a 930 recommends opening up the intake ports from 32mm to 36mm for what it is worth. That would give us 36 intake/34mm exhaust.

He also basically notes that if the exhaust remains restrictive with a change to SC cams it will lose low end.

So far:

I am thinking the stock cam for pre boost response and a 5500peak, SC for mid range depending on timing and 5500-6000peak, and C2 for upper mid range depending on timing for +/- 6100 rpm peak.

36mm intake also fits between the 34 & 39mm 3.0 CIS normally aspirated ports.

The question remains for most non stock builds is if a 32mm intake port is small enough to inhibit in any way the cylinder from seeing full boost and filling at any point.

Again, still learning and having fun. ;)

David 02-02-2010 04:40 PM

The stock ports are 36mm, they just taper down to 32mm at the intake manifold. I discovered this when I opened mine up to 40mm for the Carrera intake.

Also keep in mind that small ports don't limit flow until sonic velocity is reached. They do have a little more pressure drop (head loss) though.

911st 02-02-2010 05:24 PM

Sorry, I do not understand. Is the manifold 36mm, the head 32mm and the injectors tapered?

David 02-02-2010 06:05 PM

At the port opening where it mates to the intake manifold, it's 32mm. Before it makes the bend to the valve, it's 36mm.

When I opened the ports for the Carrera manifold, I set the heads up in a mill and cut a 40.5 to 36mm taper to match the new manifold to the lower port diameter.

911st 02-02-2010 06:41 PM

Interesting. Make's it easy to go to 36mm intake.

911st 02-02-2010 08:01 PM

It is my understanding that one point compression increase from 7/1 adds about 5% more power.

In another thread we were talking about the effect of cam valve timing on effective compression ratio. Someone much better at math than I cam up with the effect of different cams on our effective compression. It was as follows.


7 /1 measured compression ratio assumed.
6.7 for the 930 (96% of stroke)
6.1 for the SC (88% of stroke)
5.8 for the C2 cams. (83% of stroke)
There are other factors but this makes the stock cam look like it should be the best off idle until boost starts.

Just as the stock cam makes for a higher effective compression, it also captures more of the power stroke before the exhaust valve starts to open.

911st 02-04-2010 01:44 PM

Was reading about CIS and ignition on another thread.

CIS seems to have some challenges with efficient air fuel mixing. If there is not a good mix it is possible that the molecules or mix at the spark plug may be rich or lean and more difficult to ignite.

It makes some sense as we know, CIS is delivering fuel 100% of the time to the injector and it collects at the intake valve in wait. I am guessing that a CIS injector also dose not atomize the fuel as well as an EFI injector.

Additionally, there is a significant amount of intake volume between the metering assembly and the throttle plate. This makes for a lag in fuel delivery and a lean spot with acceleration. Many other CIS WUR's lower control pressure with loss of intake vacuum upon acceleration to help add fuel. The 3.0 Turbos also had this. 3.3 CIS turbo's only add fuel with significant boost.

Add to that low compression and even lower effective compression with an SC or C2 cam, and getting ignition 100% of the time with CIS is a known challenge.

I am accepting that Porsche went with such a small intake ports in an effort to mitigate a CIS ignition issue. Increasing volocity of the air entering the cylinder would help promote better mixing.

As such, if we balance the intake port size by making it larger, we need to be aware of this.

Concentrating on good AFR's pre-boost or improving ignition with MSD or Twin Plugs could be part of this.

David 02-04-2010 05:48 PM

Good point on the port size and CIS fuel delivery. Kind of like the weird piston tops on SC's.

Speedy Squirrel 02-04-2010 07:00 PM

The atomization of CIS is very poor by today's standards, but that was not the purpose of the 32mm port.

Carburetor 911's (2.7's) have 36mm ports, and they have even worse atomization. The CIS 3.0L has 39mm ports.

The "other CIS WUR's lower control pressure with loss of intake vacuum upon acceleration" statement is misleading. The warm-up regulator enriches the full load mixture on an N/A engine due to loss of vacuum by lowering control pressure, but the acceleration enrichment is from the metering plate overswing (see "Automotive Electric/Electronic Systems" by Robert Bosch GmbH, any edition).

Paul Frere states in "The Porsche 911 Story", the turbo port size is for improved load speed torque, which as I mentioned before, is a very common engine design tactic.

The evaporation of fuel is dominated by droplet size, fuel temperature and air temperature. Port velocity is effective primarily on gaseous fuel/air mixing. The intake valve produces flow velocities many times higher than the highest port velocity, and that is where the evaporation action is at; when the intake first opens. Here is a link to a video showing the actual effect: YouTube - In Cylinder Video ( How a 4 stroke engine works ).

I would not be afraid of ignition problems due to increasing the port size.

The S/C piston top is another long story ...

911st 02-04-2010 08:02 PM

Speedy Squirrel.

I like what our are saying and love the video.

Now I am just spit balling here but here is what I am thinking.

With a carb it only pulls fuel when the intake pulls air. Little of the fuel is going to sit in the intake tract.

The vid is of an electronic injection of unknown timing and duty cycle.

CIS sprays fuel full time. I do not know that it puddles but some of it probably coats the intake port like when we use Windex on our windshield.

Thus, no mater how well the injector atomized the fuel it may not matter as the spray settles.

I like your point about the beginning and ending lift making for the highest volocity. This is true.

However, on a 930 when the valve is only about 30% open the port determines the velocity.

Also, when the valve opens and closes it is not when the piston is moving very fast do to the poor angle of the rod and crank shaft. The bulk of the air filling the cylinder is going to be when the sweep of the crank is at its highest, the valve is mostly fully open, and the port is determining the air speed.

In the vid after the initial rush you can see to fuel sheathing down the ports.

I could be all wrong but this is what I am thinking.

I just do not see the runner being long enough to make for much of an increase in cylinder filling no mater what the port size is.

A 3.3's 32mm intake ports have about a 40% higher velocity than those on a 993TT.

They are probably that much more restrictive.

As to the WUR's. Many makers CIS WUR's have as you noted a provision to change the AFR under load. When vaccum in the intake goes away, control pressure is lowered and the AFR's get richer to support higher output. I do not know the goals, but say 14.7 is the goal at idle and cruse, 13/1 might be the goal with acceleration.

The 3.3 Turbo dose not have this function. It keeps its idle and cruse AFR untill boost reaches about 5 psi, then control pressure is lowered, even more so than on N/A CIS cars.

The 3.0T however did, it enriched with loss of vac, not with achieving a boost hurdle.

A 930 has a lot of volume between the metering plate and the throttle body.

When the TB is opened, the metering plate is isolated by the turbo compressor, the metering assembly has sizable volume, the intercooler, and all the many feet of plumbing that connects them. Thus, there is a lean surge or condition with acceleration off idle and for a brief time off curse. Instantly lowering control pressure with acceleration helps and I found it quite noticeable on my turbo.

I believe this is part of the reason many 930 tuners recommend setting idle CO to 3 to 3.5%. This is closer to a 13/1 AFR that better supports making power.

I could be off on this but this is where my studies and gut are taking me so far.

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