9.8:1 C/R Pistons in late CIS

[j911brick]

I have re-ringed many alusil motors (some for racing).

[Bob Kontak]

I did not have to re-ring mine as the end gaps were well within tolerance. Although, top end rebuild was premature because of head stud failure.

[Vin-barrett]

I have an '83 small port CIS motor and went from the 9.3 USA pistons to the euro 9.8's... also with 964 cams.
Dont be tempted to run too much timing. I have detonated pistons getting too froggy with advance, the car just feels great until smoke bellows out the tailpipe.
I ran the same CIS without any changes for years, however I have Webers now.

[arbita1]

With all the potential problems with the increased 9.8:1 c/r, for the minimal gain it seems to give, it no longer seems to be worth it to me.

So, I think I've been convinced not to change the pistons/cylinders at all and reuse my existing 9.3:1 alusil ones. It seems alot of people have gone this route with good success, including some reputible shops.

In fact below is Ollie's response to me in this regard:

Hi Matt,

We've been using Sunnen equipment for many years now that has a process to handle Alusil honing. Please give us a call and we can discuss the process with you - ask for George.

Ike


Plus this seems to make my engine rebuild budget much more managable. Thank you all for the info, especially the ones who provided the additional info on PM's.

[Vin-barrett]

good info from Ollie's, I'd love to know what they charge to process Alusils

Looking forward to hearing how your rebuild turns out

[arbita1]

Originally Posted by Vin-barrett

good info from Ollie's, I'd love to know what they charge to process Alusils

Looking forward to hearing how your rebuild turns out

The price list on their website shows $ 45/cylinder.

[j911brick]

Originally Posted by Bob Kontak

I did not have to re-ring mine as the end gaps were well within tolerance. Although, top end rebuild was premature because of head stud failure.

End gaps are not the only reason to re-ring. Cylinders don't wear evenly and a ring will "learn" its place and wear unevenly with the cylinder. Once you take the piston out of the cylinder its impossible to reinstall it back in the same place and this could lead to sealing issues. Its usually not that big a deal on a street motor but if your looking for every last HP you'd be leaving a few on the table.

[j911brick]

Originally Posted by arbita1

With all the potential problems with the increased 9.8:1 c/r, for the minimal gain it seems to give, it no longer seems to be worth it to me.

So, I think I've been convinced not to change the pistons/cylinders at all and reuse my existing 9.3:1 alusil ones. It seems alot of people have gone this route with good success, including some reputible shops.

If you don't need the extra power then what you have will work fine. You can also have your cylinders nikasil'd and that will buy you some reduced wall clearance. My experience has been that Porsche over estimates the CR. Your real CR is probably more like 8.7. There things you can do to get a real 9.3 CR. You can also do a big port conversion. The hot ticket in PCA racing is the 930/10 (9.8 plus big ports) motor and its allot faster than any US spec SC. But it all costs more money.

[Walt Fricke]

Unanswered in all this is the question of how much benefit the Euro 9.8s will give over the US 9.3s if you don't run the big port heads and runners.

It seems pretty obvious that you will pick up a bit of power, but not as much as the Euro motors, right? When the US motors switched to the smaller intake ports and runners in '80, the Euros kept their setup as it was. And when they went to the 9.8/1 with the big ports, they got the advantages which made them able to run with the 3.2s. On the other hand, bumping the CR from 8.5 to 9.3, but making the ports smaller on the US motors ended up with the HP basically the same.

Me, I would not go out of my way financially to switch to the 9.8/1 if I wasn't also going to the big ports and runners.

All moot for me, though, as I run my '82 SC in a stock engine class. All I did when I rebuilt it was buy a new set of Nikasil Mahles to replace the Alusils.

[Bob Kontak]

Originally Posted by j911brick

End gaps are not the only reason to re-ring. Cylinders don't wear evenly and a ring will "learn" its place and wear unevenly with the cylinder. Once you take the piston out of the cylinder its impossible to reinstall it back in the same place and this could lead to sealing issues. Its usually not that big a deal on a street motor but if your looking for every last HP you'd be leaving a few on the table.

That makes sense.