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Join Date: Aug 2010
Location: Küsten, Germany
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2,7s CIS performance upgrade pistons and cams

Hi there,

i have got here a '77 2,7s Sporto i am planning to redo the engine.
Actually it is a 165HP with a pre '74 type SSI exhaust mounted so probably 175HP.

I am after a slight increase of power while leaving the CIS more or less untouched.

Pistons: I would like to get some 9.5 compression (we have good pump gas over here) and i beleive CIS will want "bowled" piston tops, correct? Actually mounted seem to be K+S P/Cs but i would also have Nicasil sets available. But they allow only 8.5:1 and can probably not be upped to 9.5 by just decking.
Along with that:

Camshafts: What are the best options which do not cause interference with the valve to Piston clearance (using which piston?) and keep stable idle and working conditions for the CIS. DC cams? and which one, if?
In case there is no good P/C combo available machining deeper valve pockets into existing pistons would be no problem for me.

What combo do you recommend and what performance would you expect?

Usually i am working on carbed engines so i am not really used to the restrictions of the CIS when its coming to performance upgrades.
However i want to keep the original setup of the engine, maybe just some minor blueprinting of the heads (and decking) if any.

Your thoughts?

Thanks and greets,

Robert.
Old 05-08-2018, 04:19 AM
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Location: Cologne Germany
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Hi Robert,

I have just finished the rebuild of my 1977 2,7S Engine and I can share some details with you.

From what I have learned about the CIS is that the best method for getting the most out of the assembly is to ensure that the system is calibrated correctly (Fuel system pressure in tolerance etc) and running properly. I am still learning about this system too but it is my understanding that there isn't much that can be done to 'upgrade' the CIS. There are some great people on this forum with lots of experience with the CIS that are always willing to help you out (Boyt911sc). I am sure that they will offer their advice at some point too.

I installed JE pistons which can go up to 10:5 compression ratio but I was aiming for and achieved 9:5:1 CR. These are 'domed' pistons with deep valve clearances. I wanted the option to go higher in case I swapped over to Carbs at some point in the future. Have a look on their website and you'll find some good information.

Camshaft wise I took a profile that is closer to the 3.0ltr SC. Nick Fulljames at Redtek in the UK supplied these for me and I basically built my engine with his advice and recommendations. With these Cams and the Pistons with large valve reliefs there is a huge amount of room to the valves. I wound in my valves approx 7 mm and there was still no touching.

My car is for the street, not track so I too wanted something that had a little more 'pep' but retains it's driveability. Mine was originally a Californian car so I had to remove the Thermal Reactors, EGR valve, Additional air pump etc. With all of this removed, plus the SSI's and the new spec motor I hope to have gained some more BHP. At this point though I just hope to feel some difference compared to how it was before.

I'm having problems uploading photos at the moment so I will try again later.

Cheers and good luck,
Jason.
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Old 05-09-2018, 07:46 AM
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Hi Jason,

thanks for sharing and, interesting.

So you consider non-"bowled" pistons not necessary for CIS use? My understanding was the special piston shape should help tumbling and fuel distribution.
Correct?
I assume the JE ones you used are elevated flat top?

Anyone to chime in?

Did you run your new engine already?

Best,
Robert.
Old 05-09-2018, 11:09 AM
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I'm not aware of any reason why you have to use a particular type of piston shape for the CIS. If you don't have them already I recommend that you get a copy of Wayne Dempsey's and Bruce Anderson's engine rebuild books. They both cover a vast amount of information for upgrading your motor.

Here's a photo of my Pistons....






I did the first start up procedure last week and so far it sounds great. I'm very happy to hear her running again. I had a few weeks of chasing down a 'hard start' issue which meant that I became very familiar with my CIS. With the help of this forum and especially Tony (Boyt911sc) I tested and calibrated the whole system to 'perfection'. Turns out that my problem was the Fuel Pump safety switch cut off not functioning properly. Got there in the end with a much better understanding of the CIS.

First drives will be over the next few days so I hope to get a feeling of the potential of this rebuild.

Keep us posted.
Cheers,
Jason.
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Old 05-10-2018, 12:06 AM
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The shape of the CIS pistons had more to do with single sparkplug and larger bore than CIS. Later 3.2 Motronic pistons look the same as CIS. The 964 piston shape changed as those engines were twin plug from the factory.

john
Old 05-10-2018, 12:05 PM
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Hi Jason,

i know the books as well as 911 engines which i am doing power enhancements and rebuilds on since over 25 years.
I just left CIS aside due to its relatively low potential and its restrictions.
But OK, now i have one.

Regarding the initial question its probably time to dig deeper.

However keep us updated how it works, also in terms of fuel consumption.

Any other thoughts here?

Thanks and greets,

Robert
Old 05-12-2018, 06:47 AM
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Hallo Robert,

Interesting thread and thoughts - I have been thinking about this question (whether CIS can be operated with non-CIS domed pistons) too for quite a while as I have a 2.4 TK which needs a rebuild.

Searching the literature and forum wisdom doesn't really provide a definite answer. Nobody clearly states that it's definitely not going to work - but there also seems to be little practical experience with using domed pistons for higher compression with CIS, probably because for many people it's been easier to just replace the CIS with carbs.

As I see it, the main reason for Porsche to go CIS (with the half year only 73.5 2.4 CIS being something like a test-balloon on the US-market) was to achieve better fuel efficiency and to better meet emission standards - and for that, it seems plausible to me that the main purpose of the new piston design might have been not mainly to create additional intake swirl (which it probably does), but to push the mixture more towards the spark plug, thereby optimising cumbustion - B. Anderson seems to support this view as he states in his book: "The CIS pistons had offset domes to alter the effective combustion chamber and ensure more complete combustion." (p. 146)

So... after a long time of contemplation and also looking further into the development of the factory engines from the 2.4 TK up to the 3.0 SC, I finally decided to start building and just give it a try:

For my 2.4, I have sourced a set of good 2.2E-pistons which will bump the CR up to something about 9.3:1.

Looking at factory CIS cams, you'll find an evolution in 3 steps:

-First there have been the moderate CIS cams used in both the 2.4 TK and the standard 2.7 from 74 on, with an intake duration of about 212 Deg, an exhaust of about 200 Deg, and a lobe spread of about 145 Deg (numbers vary a bit in the literature). Timed to open at TDC, this leads to an intake center of 106 Deg ATDC and an exhaust center of 110 Deg BTDC.

-Then there was the 2.7S cams (the cams you have) which had little more lift but a longer intake duration (about 10 degrees more) while the exhaust duration remained the same, and the lobe seperation angle was 5 deg narrower.
IF this cam was timed like the standard CIS cam (intake opening at TDC) the only difference would be a 10 Deg longer intake with the intake center 5 Deg later and an untouched exhaust behaviour.
BUT what Porsche did for the 2.7S was to spec this cam to be timed 6 Deg retarded (intake opening 6 Deg after TDC). The purpose of this I believe was to shift the power band to higher RPMs to come closer to the characteristics customers at that time expected from an S-engine.

- Finally, there had been the SC cams first introduced in the 3.0 Carrera. These had more lift (I think to meet the bigger displacement, along with bigger valves) and again 10 more degrees of intake duration plus 5 degrees narrower lobe seperation and also a longer exhaust duration (so in comparison to the first generation of CIS cams, we find 20 Deg more intake, 20 Deg more exhaust and a lobe seperation narrowed by 10 Deg).
The SC cam profile remained the standard cam for CIS cars from 77 to 83, the only difference being two versions of timing: In factory engines, we find it timed either with an intake opening at 1 Deg BTDC or advanced by 6 Deg with an intake opening at 7 Deg BTDC.
The first, more retarded timing puts the intake center to 116 degrees, very similar to the 2.7S timing which had an intake center of 118. Timed like this, the SC cam is also very similar to the 964 cam, which also has an intake center of 116 and an exhaust center of 110 with just again a bit more duration. This "retarded SC" timing was used for the 3.0 Carrera, US and Japan SCs 78+79 while the ROW SCs used the "advanced SC" timing of intake opening at 7 Deg BTDC with an earlier intake center of 110, as well as the US cars of 1980 did. From 81-83 Porsche returned to the "retarded SC" timing for all SCs except Japan - together with a raise in CR from 8.6:1 to 9.8:1 ROW and 9.3:1 US.

All this seems to support the "common wisdom" that advancing the cam is good for low end torque while retarding the cam supports top end power (by moving the power band up - it led to 204HP in the 81-83 ROW SC together with the compression bump.) However, I think the changes these two variants of timing will lead to should not be overestimated - interestingly, with the 84-89 3.2 Carrera, Porsche settled for a timing that lies just in the middle of the two SC-settings with otherwise unchanged durations and lobe seperation.

NOW - what can be learned from this for our CIS rebuild projects?

For my 2.4, I came across a pair of good 2.7S cams (the ones you have). These give a little more lift and intake duration over the stock ones. As my car is a Targa, low end torque is more important to me than top end power so I will probably not retard them like in the stock 2.7S config (intake center at 118 Deg) - but rather shoot for an intake center that meets the middle of the two SC settings. Doing so leads to the following timing events: In opens 1 ATDC, In closes 45 ABDC, (Center 113 ATDC), Ex opens 29 BBDC, Ex closes 7 BTDC (Center 108 BTDC). Admittedly, this is a "If you cannot decide, shoot for the middle" approach, but it seems reasonable to me. Also, If I would advance that cam further, I'm afraid Exhaust closing time would be too early.

What about your 2.7S Sportomatic Targa now?
I guess it's also not supposed to be a race car and it looks like your situation is very similar to mine. The variations in CIS cams are not as big as with cams for carbed engines, so in your case I'd probably also go with the next step regarding duration and lobe spread - that is, go for an SC profile on a three-journal cam (or something similar; I'd be interested in what profile the SC-like cams Jason mentioned above would have).

Also, I believe it will still take a while until I have the engine back in the car and running (I'm about to time the cams, long block is finished so far but still need to put the chassis back together) so probably you and Jason will be finished before me.
It would be great if we could keep this thread alive to share experiences with this kind of builds with non-CIS pistons running on CIS!

Regards,
Arndt
Old 06-17-2018, 01:26 PM
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I have a set of 3 bearing cams ground to 964 profile that would make a nice upgrade for a 2.7. PM me if interested.
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Old 06-18-2018, 02:29 AM
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