Hello, I'm doing this mod to a 2.4t cis. I will be installing a set of 86mm je 9.7:1 pistons to make int a 2.5 liters engine, with the 964 cams, the cuestion is the port size, the stock size is I 30, E 33 wich seems to small for this upgrade, I ask the advice of the engine and cis experts to advice me on this matter

Thanks

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This post was auto-generated based upon a question asked on our tech article page here:

I am not familiar with this upgrade. Or at least not much of an expert. I will defer to the community.

I opened a post in our forums. A Pelican community member may be able to answer your question.


- Nick

The 30mm port size is a little small.

But what you need to do is match your intake. If you are using the stock CIS intake from your '73.5, then the steel intake runners are tiny, and increasing intake port size won't make a difference.

You should find a CIS intake from an '80-'83 911 SC and then make your intake ports match those. I believe they are 34mm intake ports, which would be just about right with the cam upgrade and displacement increase.

Why run 964 cams? I thought guys ran these because they were one of only a few options with stock CIS pistons because of interference.

Based on preamble on the 930 board, I think you could run a much better cam with CIS and the pistons you have selected provided you run anti reversion features (tapered spacers or smaller spacers than your cylinder ports.

Read Wayne's Engine Mod book about changing compression and cams with CIS....U may want to switch to carbs at a certain point....

Ports............
 

I agree with this suggestion. When I built the 3.3SS for my SC, I changed to a set of Carrera heads that had much larger ports than the SC heads. Since I wanted to keep CIS for the time being, I installed an early SC air box with the larger 39mm runners matched to the heads. I also used the 964 cams. Compression is 10.1:1 and ignition is by an Electromotive XDi twin plug system. The engine runs quite well with lots of midrange torque and good top end power. At some time in the future, I may switch to a set of PMO ITBs which will all be external work.

The reason 964 cams are a good choice for the CIS engine is not for clearance but because they are a little more knarly, a touch more lift and duration, but have the same lobe offset configuration (intake vs exhaust) that CIS cams have which minimize air pulsations which occur with "standard" high performance cams.

The air (vacuum) pulsations with CIS flutter the smooth lifting of the air sensor and deliver fuel based on the up and down fluttering. Not good.

That said, with higher compression pistons, OP needs to perform work to ascertain if pockets need to be milled.

Stupid question time (I have an endless supply). Can 964 cams be 'threaded' in without removing the head - just the cam covers? - No doubt an engine drop is in order. And what does a set go for approximately? Thx.

I have read that only the 2.7 S cams work if keeping the stock CIS. Anything else would need a different intake.

I converted my old 2.4CIS T to 2.7 liters with a set of factory Mahle 911S 2.7 (CIS) pistons and cylinders. I had to have the crankcase altered to take the larger cylinders, and I used Dilavar head studs. I left everything else - cams, heads, fuel injection and ignition - stock. It was a great autocross engine - lots of low range torque. It was not good at the top end, really running out of oomph before it would get to 6000 rpm. It was also a perfectly smooth running engine and a blast to drive on the street.

Just a data point for you to consider.