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  #21  
Old 09-29-2014, 03:14 AM
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Is there a reason not to go with different pistons? Custom rods or custom pistons will both cost a fair bit (though I suspect that the rods will be a bit cheaper).

Generally, for power and longevity, it is better to have a longer rod than a shorter one. That said, some updated rods would probably not go amiss ... Though the same could be said of the pistons.

This is how my projects get out of hand.
If we play our cards right we can probably scope creep Justin into a twin charged LS engine.
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  #22  
Old 09-29-2014, 03:29 AM
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If we play our cards right we can probably scope creep Justin into a twin charged LS engine.
Where is the damn "Like" button?!??!
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  #23  
Old 09-29-2014, 04:04 AM
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I thought about pistons...

Currently the S2 engine has a 10.9:1 compression ratio which is a bit too much for a boosted antique 944. Pistons need to be alusil compatible and strong enough for some boost. I have a set of 968 pistons which are a bit stonger, but also higher compression. The S2 pistons are supposed to be forged and have been shown to be reasonably good with boost. The stock rods are cast and IMHO not strong enough for the amount of force that will go through them no matter what, so rods are a sunk cost. The choice is then new stock length rods with low compression custom pistons, or reuse the pistons that came out of the engine with forged 2mm shorter rods.
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  #24  
Old 09-29-2014, 04:44 AM
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Did some more investigating into pistons. It turns out that the S2 pistons are not forged, but the 968 pistons are and have the same dish as the S2 pistons. Problem is that the 968 pistons I have seem to be for a slight overbore. They say 104 on them, but seem tighter than they should at the top of the bore. I'll have to measure them more carefully.
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E46 M3 Daily Driver
944 Turbo S Weekend/Project car
944 S2 Race car
944 team car (25% owner)
VW Touareg Tow rig

We choose to race 944s. We choose to race 944s, not because it is easy, but because it is hard.

Last edited by JustinL; 09-29-2014 at 04:58 AM.
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  #25  
Old 09-29-2014, 01:45 PM
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Did some more investigating into pistons. It turns out that the S2 pistons are not forged, but the 968 pistons are and have the same dish as the S2 pistons. Problem is that the 968 pistons I have seem to be for a slight overbore. They say 104 on them, but seem tighter than they should at the top of the bore. I'll have to measure them more carefully.
Pistons are not round. They are 'cam ground' as, due to there being more aluminum across the pin bosses the expand differently. We're it not so early, I would remember where you measure them, but I don't want to steer you wrong first thing on a Monday ... I believe it is across the pin bosses.

Anyway, there is more wear half way down the bore as there is more side loading because that is where the rod is at the greatest angle (a longer rod means less angle, and less friction, but more mass ... That is the trade off). Really, 2mm is not going to affect it much ... Much. Truly, the bigger issue I could see is if Porsche designed in some 'quench' ... An Internet search will explain that in more detail ... Essentially, it is turbulence created within the combustion chamber to quell detonation before it can happen. It relies on areas of close interaction between the piston crown and the cylinder head. If this is lost, detonation can still be avoided by lowering the compression ratio further, but at the cost of response and efficiency.

Another generality is that engines with dished pistons tend to make more power than flat tops or domed (all else, including compression ratio,being equal). it has to do with flame propagation.

I believe you are right in choosing a compression ratio in the 9 to 9.5:1 range with a reasonable boost level. Less compression would theoretically allow more boost but off boost performance suffers terribly. I remember a story about the Skylines running in Austrailia.... The sanctioning body, in a effort to slow the cars down, reduced the allowed boost level. Nissan found that they were able to raise the compression to 9.5:1; thereby regaining much of the lost HP and, ultimately, making the cars faster.
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  #26  
Old 07-23-2015, 04:07 PM
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The project is still moving along, though in a slightly different, more expensive direction. The forged pistons I planned to use are oversize and don't fit the bores. The stock S2 pistons are cast and after doing some more reading I'm thinking I really should go with forged pistons. That set me shopping for a stock size set of 968 pistons.

I decided to have a peak at the short rods I ordered and found that they are actually stock length. The plan now is to go with custom 9:1 CR pistons too.

Expensive used headers and intake manifold are also on their way.

Damn scope creep.
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  #27  
Old 07-23-2015, 05:17 PM
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This will be amazing when done. That is all I have to add.
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  #28  
Old 07-23-2015, 05:30 PM
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This will be amazing when done. That is all I have to add.
I hope so. My target is to have a very driveable engine that gives my car a better power to weight ratio than a 997 Gt3RS. It will need about 450 at the crank to achieve that. Let's hope it comes together for next spring.
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E46 M3 Daily Driver
944 Turbo S Weekend/Project car
944 S2 Race car
944 team car (25% owner)
VW Touareg Tow rig

We choose to race 944s. We choose to race 944s, not because it is easy, but because it is hard.
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  #29  
Old 07-23-2015, 08:32 PM
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My target is to have a very driveable engine that gives my car a better power to weight ratio than a 997 Gt3RS.
YESSSSSS!!!!!!!!!!!!!!!!!!
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  #30  
Old 07-23-2015, 09:26 PM
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It will need about 450 at the crank to achieve that. Let's hope it comes together for next spring.
Boosted 3L at 450hp? So you're planning on running on 3 cylinders?

602 RWHP or nothing!!!!
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1987 951 - M193 Version for Japan

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2015 Jeep Wrangler
2009 Subaru Impreza
2005 Chevy 2500 HD

Last edited by chrenan; 07-23-2015 at 09:32 PM.
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