Pelican Parts
Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help



Go Back   Pelican Parts Forums > Porsche Forums > 911 Engine Rebuilding Forum


Reply
 
LinkBack Thread Tools Rate Thread
Author
Thread Post New Thread    Reply
Registered
 
Join Date: Mar 2011
Posts: 598
Machining piston tops to lower C/R?

Does the 911SC 3.0 pistons have enough 'meat' on the crown to machine down to 8.0:1 C/R to be more 'boost friendly'?

I know this has been done on the 964 pistons many times before and still retains strength, but just wondering if it's possible to do it on the 911SC pistons?

Who offers this service?

Or is it cheaper to fit thicker base cylinder shims to decompress the motor? What thickness shims would one need to obtain 8.0:1 CR from a 8.5:1 SC motor?


Last edited by 911TT33; 11-16-2011 at 08:39 PM..
Old 11-16-2011, 03:37 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #1 (permalink)
Registered
 
Walt Fricke's Avatar
 
Join Date: May 2004
Location: Boulder, Colorado
Posts: 7,275
You can calculate this. You know the displacement of your cylinders (bore and stroke). Knowing 8.5/1 allows you to calculate the volume above the piston needed for that displacement to give that ratio. You can then back calculate to see how much more volume is needed to get the lower CR. Knowing that volume, you can easily calculate what height cylinder equals that extra volume. That's your extra shim thickness.

Porsche uses 0.25mm base shims, with 0.50 reserved for rebuilds. I used 1.00mm shims (EBS sells them, as someone on this list kindly informed me) to get needed head clearance on a race motor. This required that I space out my chain boxes to get the cam seals to line up properly, though it didn't require anything special for the chains. Something to think about.

I've seen a spec for piston top thickness somewhere. Maybe Wayne's book (you do have that, don't you?). But the CIS pistons are a very irregular shape, so machining them seems like it would be tricky, and calculating how much to flatten the top of an irregular dome equally tricky short of a lot of cut and try on a junk piston and measuring with fluid volumes in a cylinder. I'd not want to reduce the depth to the first ring, especially on a blown motor.

How about calculating how much you would have to reduce the stroke to gain a bit more volume? Same calculation. I know that you can have wrist pin bushings offset bored, though there may be a prompt limit to that.
Old 11-16-2011, 09:58 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #2 (permalink)
Registered
 
Join Date: Jul 2001
Location: Portland Oregon
Posts: 7,007
Quote:
Originally Posted by 911TT33 View Post
Does the 911SC 3.0 pistons have enough 'meat' on the crown to machine down to 8.0:1 C/R to be more 'boost friendly'?

I know this has been done on the 964 pistons many times before and still retains strength, but just wondering if it's possible to do it on the 911SC pistons?

Who offers this service?

Or is it cheaper to fit thicker base cylinder shims to decompress the motor? What thickness shims would one need to obtain 8.0:1 CR from a 8.5:1 SC motor?
You'll likely get multiple opinions on this that may conflict, but I'd simply offer mine.

SC pistons are cast and I would not machine them for any boosted applications. I think that's looking for trouble. If you need a particular CR for a turbocharged engine, I would have either JE or CP make a set for you with a ring package that can withstand detonation far better than the OEM Goetze's do.

Hope this helps (and prevent problems).
__________________
Steve Weiner
Rennsport Systems
Portland Oregon
(503) 244-0990
porsche@rennsportsystems.com
www.rennsportsystems.com
Old 11-16-2011, 11:51 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #3 (permalink)
Turbonut
 
Raceboy's Avatar
 
Join Date: Jun 2009
Location: Estonia, Europe
Posts: 1,261
Garage
There is no engine component (piston, rod etc) that would withstand detonation for sustained periods. Detonation happens due to poor tune or bad gas. And forged pistons can't cope with detonation until bad batch of gas is used, they will fail just like cast pistons.
400 hp on 6cylinder engine is nothing with cast pistons (only less than 70 hp per cylinder) but as with ANY part, it requires controlled environment.

I have run my completely stock euro 924 engine (with crappy cast pistons, crappy combustion chamber design etc) with 1 bar of boost, 9.3:1 CR and 250 hp/320 Nm for around 4 years and 30 000 km's without ANY issue and that includes track sessions top speed events etc. This is more than 60 hp per cylinder.
And I remember from somewhere that one cannot boost 924 engine, it will break immediately

My point is that you should leave the pistons as is.
__________________
'83 924 (2.6 16v Turbo, 530hp),'67 911 hot-rod /2.4S, '78 924 Carrera GT project (2.0 turbo 340 hp), '84 928 S 4.7 Euro (VEMS PnP, 332 HP), '90 944 S2 Cabriolet
http://www.facebook.com/vemsporsche
Old 11-17-2011, 12:11 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #4 (permalink)
Registered
 
Join Date: Mar 2011
Posts: 598
Quote:
Originally Posted by Raceboy View Post
My point is that you should leave the pistons as is.
We are considering this option. If we can get away with running 0.8 - 1.0 bar of boost on the 8.5:1 C/R motor with EFI and a full bay intercooler, we'd be more than happy.

8.5:1 would offer awesome pre-boost driveability / response, then when boost builds and kicks in, it should be a very fun street car. It won't be seeing any track events.

If we do end up cracking a piston or ring land, then we'll slip in some lower compression 8.0:1 aftermarket JE's, or source some good used 930 P&C's

BTW, we're considering the BITZ EFI conversion kit for this SC motor.
Old 11-17-2011, 12:59 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #5 (permalink)
Registered
 
sjf911's Avatar
 
Join Date: Dec 2006
Location: Idaho
Posts: 5,727
Garage
Quote:
Originally Posted by 911TT33 View Post
We are considering this option. If we can get away with running 0.8 - 1.0 bar of boost on the 8.5:1 C/R motor with EFI and a full bay inter-cooler, we'd be more than happy.

8.5:1 would offer awesome pre-boost drive-ability / response, then when boost builds and kicks in, it should be a very fun street car. It won't be seeing any track events.

If we do end up cracking a piston or ring land, then we'll slip in some lower compression 8.0:1 aftermarket JE's, or source some good used 930 P&C's

BTW, we're considering the BITZ EFI conversion kit for this SC motor.
I am running 9.5:1 with 0.6 bar and am seeing sub-audible detonation even with near winter temps, a full-bay inter-cooler, and meth injection at 15 degrees of timing. Now, I don't see any evidence of this on my spark-plugs and I only do freeway pulls so I am not doing repeated back to back boost runs or running sustained boost on the track. I suspect if I tried to run my car on the track I would quickly discover the limits of my heat dissipation systems, and pistons. I think 8.5:1 is reasonable in these cars with EFI and complete timing control but I don't know whether the CIS piston adversely affects your detonation risk. The lower you go on compression, the more of a safety margin you have.
The Bitz kit may not be the best starting point for boosting as you don't get timing control with it and you're stuck with smaller fueling tables. You really should think of an MSII or MSIII unit with EDIS wasted spark ignition if you want to go that route.
__________________
Steve
Sapere aude
1983 3.4L 911SC turbo. Sold
Old 11-17-2011, 06:32 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #6 (permalink)
Registered
 
Join Date: May 2003
Posts: 3,346
You might be OK with stock 8.5 pistons. If you actually measure the compression ratio with a deck height of 1mm, you'll probably find that you only have 8.0 CR anyway. It's very common for Mahle pistons to measure the full compression ratio with a deck height of zero.

-Andy
__________________
72 Carrera RS replica, Spec 911 racer
Old 11-17-2011, 02:28 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #7 (permalink)
Registered
 
Join Date: Mar 2011
Posts: 598
Quote:
Originally Posted by sjf911 View Post
I am running 9.5:1 with 0.6 bar and am seeing sub-audible detonation even with near winter temps, a full-bay inter-cooler, and meth injection at 15 degrees of timing. Now, I don't see any evidence of this on my spark-plugs and I only do freeway pulls so I am not doing repeated back to back boost runs or running sustained boost on the track. I suspect if I tried to run my car on the track I would quickly discover the limits of my heat dissipation systems, and pistons. I think 8.5:1 is reasonable in these cars with EFI and complete timing control but I don't know whether the CIS piston adversely affects your detonation risk. The lower you go on compression, the more of a safety margin you have.
The Bitz kit may not be the best starting point for boosting as you don't get timing control with it and you're stuck with smaller fueling tables. You really should think of an MSII or MSIII unit with EDIS wasted spark ignition if you want to go that route.
Good point about ignition control. I was thinking the BITZ EFI conversion in conjunction with the MSD 6ABTM ignition which retards ignition under boost.

Old 11-17-2011, 02:33 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #8 (permalink)
Reply


 


All times are GMT -8. The time now is 03:40 PM.


 
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2025, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2025 Pelican Parts, LLC - Posts may be archived for display on the Pelican Parts Website -    DMCA Registered Agent Contact Page
 

DTO Garage Plus vBulletin Plugins by Drive Thru Online, Inc.