9.8:1 C/R Pistons in late CIS

[arbita1]

I'm searching around, but just wanted to make sure.

It seems from my research that it doesn't make sense to go to the 9.8:1 c/r euro SC pistons (I currently have 9.3:1 c/r) if you are keeping the US late (smaller runners) CIS system stock.

Can anyone confirm this?

[Grady Clay]

Matt,

The issue is somewhat complicated.
The CIS requires no overlap with the cams.
This makes for high cylinder pressures with low rpm (great torque for street driving).
However, this makes the engine ‘detonation sensitive’ at low performance levels – hence the 8.5:1 CR.

I suspect you may already have undetected detonation issues with your raised (9.3:1 CR) compression pistons.
Hopefully you have twin-ignition and are using very high octane fuel.

With CIS cams (and even twin-ignition), 9.8:1 CR should require 114 octane fuel or risk detonation damage.

I recommend that you start over with a chipped 3.2 DME engine and add twin ignition and 98 mm P&Cs with high compression to suit your fuel octane access.

Best,
Grady

[Bill Verburg]

Originally Posted by arbita1

I'm searching around, but just wanted to make sure.

It seems from my research that it doesn't make sense to go to the 9.8:1 c/r euro SC pistons (I currently have 9.3:1 c/r) if you are keeping the US late (smaller runners) CIS system stock.

Can anyone confirm this?

Seems like a lot of expense for a half point of compression.

Theoretically higher compression increases the efficiency of an internal combustion engine.

The small runners are not really an impediment, their design goal was to raise the speed of intake air at lower rpm, if there is going to be any flow volume restriction it will be at higher rpm. A happy cam might cause you to want to go to higher rpm.

[arbita1]

Originally Posted by Grady Clay

Matt,

The issue is somewhat complicated.
The CIS requires no overlap with the cams.
This makes for high cylinder pressures with low rpm (great torque for street driving).
However, this makes the engine ‘detonation sensitive’ at low performance levels – hence the 8.5:1 CR.

I suspect you may already have undetected detonation issues with your raised (9.3:1 CR) compression pistons.
Hopefully you have twin-ignition and are using very high octane fuel.

With CIS cams (and even twin-ignition), 9.8:1 CR should require 114 octane fuel or risk detonation damage.

I recommend that you start over with a chipped 3.2 DME engine and add twin ignition and 98 mm P&Cs with high compression to suit your fuel octane access.

Best,
Grady

Grady...this is a stock 83 SC engine which already has 9.3:1 c/r. I have not done anything to it yet. I'm just looking to do a rebuild over the winter and weighing my options and budget. Swapping out a 3.2 is definitely out for me.

Originally Posted by Bill Verburg

Seems like a lot of expense for a half point of compression.

Theoretically higher compression increases the efficiency of an internal combustion engine.

The small runners are not really an impediment, their design goal was to raise the speed of intake air at lower rpm, if there is going to be any flow volume restriction it will be at higher rpm. A happy cam might cause you to want to go to higher rpm.

Bill...I agree and if I had Mahle p&c's this wouldn't even be a question. However, last fall when I had my engine out I noticed the dreaded KS on the cylinders. So I'm probably just going to buy a used set of Mahle 9.3:1 c/r's to replace my existing, however if I can get a set of the euro 9.8:1 c/r's for a bit more, maybe it would be worth it. But I don't want to do it if I'm chancing detonation since I'm using a stock CIS from my 83 USA car.

[Scott R]

I run this setup on CIS with 964 cams and single plug, it's very nice. Well let me be clear I'm using larger P&C's as well, but CIS works very, very well for this. In fact it works a heck of a lot better than the webers I had before it.

I do not have any detonation, just nice clean even torque through 7500 RPM. You will need an adjustable WUR since you need to dial the warm control pressure in a bit lower than stock, and you will need to adjust the sensor plate since your vacuum profile will change. (if you go with different cams)

[Halm]

Originally Posted by Grady Clay

However, this makes the engine ‘detonation sensitive’ at low performance levels – hence the 8.5:1 CR.

I suspect you may already have undetected detonation issues with your raised (9.3:1 CR) compression pistons. Hopefully you have twin-ignition and are using very high octane fuel.

With CIS cams (and even twin-ignition), 9.8:1 CR should require 114 octane fuel or risk detonation damage. . .

Best,
Grady

Grady, your advice is always the best and I tend to listen when you talk.

There is a long standing tech article here on Pelican that indicates the higher compression is fine. Does the combination of the higher compression with headers and 964 cams somehow negate the need for twin plugs?

[Bill Verburg]

930/10 used 9.8 Mahle and works fine on US 93

[Grady Clay]

Matt,

You are correct.
Above is my dumb posting relying on my lame (geriatric) memory, not looking things up as I should.

The earlier (’78-’82) 911SC USA 930/03/13/04/05/15/07/08/16/17 were 8.5:1 CR.
The 930/09/19 were 8.6:1.
The 930/10 was 9.8:1.
The ‘later’ (’82-’83) 911SC 930/16/17 were 9.3:1 with the 930/10 still at 9.8:1 CR.

At 9.8:1, USA pump fuel will be an issue.
Twin plugs reduces the octane requirements a bit.

All of the 911SC pistons seem to be fairly robust tolerating some detonation. However, when that ‘limit’ is reached, the piston ‘collapses’ capturing the top compression ring. In turn, the ring breaks apart and things progressively fail. – Not something I want happening in MY engines.

I suppose I am ‘overly cautious’ in this area.
I am one to carefully approach a ‘limit’, not tolerating breaking parts.
The consequences of a broken 911 engine are not for the lightweight pocketbook.

That said, using the Factory 9.8:1 CR should work fine IF you use the correct octane fuel.

Best,
Grady

[arbita1]

Ok. So lets say for the time being that if I use 93 octane I will be fine with the euro pistons. However Scott R indicated he had to use an adjustable wur. So does this mean there is still an issue with using the stock CIS and it needs to be modified?

[rgoodrich]

I don't consider myself as knowledgeable as most in these matters. On the other hand, when we did a top end on my 81SC(broken head studs) we put in Mahle 9.8:1 P&C's, 964 cams, and a set of Fabspeed headers and 2 in 2 out. It had no air, and only a trombone cooler. In the driving part of the summer it's 110 degrees. I ran my SC hard year round on 91 octane, with 1/4 tank of av gas in the summer. and put about 15,000 miles on it. I guess there could have been some silent detonation, but it always ran fine.

[jamesjedi]

Originally Posted by Bill Verburg

Seems like a lot of expense for a half point of compression.

Theoretically higher compression increases the efficiency of an internal combustion engine.

The small runners are not really an impediment, their design goal was to raise the speed of intake air at lower rpm, if there is going to be any flow volume restriction it will be at higher rpm. A happy cam might cause you to want to go to higher rpm.

What Bill V says took me about 10 hours of reading to understand (somewhat). It is a very interseting topic and I am slow to learn.

Porsche did not make a mistake when they designed the intake the way they did at that time. A "happy cam" may want you to use a different intake. At that point, for me, the expense should be for a reason other than a .4 increase in compression.

Excellent short article from Wayne R. Dempsey on your engine (Pelican);

Pelican Technical Article: Extracting Power from CIS 911s...


Perhaps a good question is "what gain for what cost?"

[j911brick]

If you really feel detonation is an issue all you have to do is dial back the timing a couple degrees.

[Tom F2]

I have been driving a 930/10 RoW Targa for years. It works great with the 9.8 CR, and is faster than any US model SC (or 3.2 Carrera) that I have driven. In fairness, I should add that my car is a non=A/C Targa, so it probably has a weight advantage, and I have SSIs, which are a definite advantage on the big port 3.0 heads.

BUT, BUT, BUT, the stock ignition timing, which I follow of course, only gives 25 degrees max advance. The distributor for the 930/10 motor is special, surely for all of the reasons that Grady mentions.

[Eagledriver]

What all of you are missing is that these aren't 9.8 compression ratio pistons. The Euro pistons at a deck height of 1mm are about 9.3 to 1. Mahle over-rates the compression figures. They are only 9.8 at zero deck height. Euro pistons are fine for these engines with 91 octane and 29 degrees of advance. We even race with them "blue printed" to 9.8 using 91 octane and 29 degrees.

-Andy

[Bob Kontak]

Originally Posted by arbita1

I'm just looking to do a rebuild over the winter and weighing my options and budget.

I have an 81 US and had my heads done is '97. They (Engine Builders Supply) shaved six thousandths off the seal mating surface of the heads (and stamped the heads accordingly)

That effectively brought the head closer to the center of the crankshaft by that amount and I calculated the compression increase at about two tenths of a point. I posted my calcs and it was confirmed but I cannot find the post. If you were to do the same you could have 9.5:1 without springing for pistons.

[arbita1]

Originally Posted by Bob Kontak

If you were to do the same you could have 9.5:1 without springing for pistons.

Good idea, but I have alusil pistons/cylinders. So they have to be changed which is what prompted me to change compression in the first place.

[j911brick]

Originally Posted by arbita1

Good idea, but I have alusil pistons/cylinders. So they have to be changed which is what prompted me to change compression in the first place.

And what's wrong with Alusil?

[Bob Kontak]

Originally Posted by arbita1

Good idea, but I have alusil pistons/cylinders. So they have to be changed which is what prompted me to change compression in the first place.

I got you. I have Alusil & KS. Don't remember reading any threads where they let anybody down, although agreed, they are the step child setup - at least in perception.

Bummer the change out is so expensive if you want the whole nine yards new - the full Mahle P&C setup.

[arbita1]

I'm not even going to get into the Alusil re-ring debate. I would love nothing more than to reuse my existing pistons/cylinders. I'm just afraid to go against the advice of the majority.

That being said. I was on the Ollie's website looking at their price list for machine work. They have a price for honing and reconditioning Alusil cylinders. So I am inquiring for more info.

[Scott R]

There is a Pelican member, maybe someone can help with the name "Aurel?" Who did a alusil re-ring and hone and he had a good amount of miles on the motor with no issues at all.