Replace pistons or not

panelbilly · 12-16-2013, 12:14 PM

My 3.0 is at the machine shop now and I'm collecting items for the assembley. I've already purchased new PMO carbs. I'm working on 964 cams. My pistons are in good condition, but should I really be replacing them. The car will be used sparingly for a little cruising and maybe a tour once a year. Can I stay with the stock CIS pistons using this combination?

camgrinder · 12-16-2013, 12:24 PM

What compression ratio? 964 cams and PMO's wont be happy at 8.5-1,

panelbilly · 12-16-2013, 02:20 PM

Whatever the stock pistons for a CIS motor is built with

Plays with cars · Plays with cars's Garage

The CIS piston dome shape was made to work with CIS injection. If you are replacing with carburetors then you can deviate from this piston dome (and would probably get a better result if you did), but you don't have to just for this reason.

The other change you are making to 964 cams must be done with piston to valve clearance in mind and may not be accommodated with the CIS pistons. Others may know if this is an acceptable combination, but if no one chimes you will need to have your machinist check piston to valve clearance and make sure it is okay before you proceed.

fred cook · fred cook's Garage

CR depends on the year model of your car. The later SCs were 9.3:1 while some of the earlier ones could have been down around 8.5:1. A nice set of JE pistons set up to give just under 10:1 would help to optimize the 964 cams and PMO carbs. Any CR higher would put you into the twin plug zone. The 964 cams are just a bit hotter than the SC cams. A little more lift and duration. I have a set in a 3.3SS engine that I built and they run/idle quite nicely. Mid range torque is better than before and they pull well up to 7K which is as high as I have taken my engine. Good luck on your project!

Merry Christmas!

Quote:

Originally Posted by panelbilly

Whatever the stock pistons for a CIS motor is built with

panelbilly · 12-17-2013, 07:09 AM

So I'm a virgin to this and don't want to rely on just the guy who is doing the machine work. What JE pistons would I choose? Would it make sense to look for these used?

carrerarsr65 · carrerarsr65's Garage

i have steve at rennsport in portland measuring my cyls. to get custom je pistons made up, overall better fit , less oil blow by, just a better set up, also more $$$. welcome to porsche land

E Sully · E Sully's Garage

Are the cylinders Nikasil or Alusil? If cylinder walls are slightly magnetic they are Nikasil, which are easier to reuse. Alusil can be a problem. They are difficult to re-ring, and pistons have special coated skirts.

Eagledriver · 12-17-2013, 09:50 AM

To answer your questions. Yes the old Mahle's will work fine in this set-up. (if they are not worn out). I usually use the old rings, they often are less "worn" than brand new Goetz. Unless you want an extra 10 HP the compression ratio will be fine even at 8.5. We have had several racers using those pistons. Save your money and be happy with what you have.

-Andy

camgrinder · 12-17-2013, 10:14 AM

What year or engine number is your 3.0 liter? If you have the 9.3-1 engine, just go with it.

964 cams will clear the 8.5-1 or 9.3-1 pistons if installed correctly.

fred cook · fred cook's Garage

Quote:

Originally Posted by Eagledriver

To answer your questions. Yes the old Mahle's will work fine in this set-up. (if they are not worn out). I usually use the old rings, they often are less "worn" than brand new Goetz. Unless you want an extra 10 HP the compression ratio will be fine even at 8.5. We have had several racers using those pistons. Save your money and be happy with what you have.

-Andy

If you choose to reuse the Mahle pistons, be certain to check for wear in the piston ring lands (grooves). Also check the end gap on the rings to see if they still meet specs. Since you are doing a rebuild and since the (JE) pistons are not terribly expensive, it would seem that a bit more compression and therefor horsepower would be worth the expense. IMHO.

panelbilly · 12-17-2013, 08:46 PM

I checked my records and it a US 3.0 911SC Engine built between Sept. 1978-Aug.1979.

camgrinder · 12-18-2013, 07:08 AM

Quote:

Originally Posted by panelbilly

I checked my records and it a US 3.0 911SC Engine built between Sept. 1978-Aug.1979.

It came with the 8.5-1 pistons. IF it fits the budget a bump in compression will make a big difference. Most people associate a compression increase with HP.
In your case it will give you a nice jump in torque and the engine will come into the power band sooner.

Here is a nicely done build you can look at.
3.0 dyno day - a pleasant surprise

With 964 cams and PMO's I bet you would end up within 10 hp of Don's 3.0L