jasonturbo

I would think that so long as each component fixed to common axis (C/L of crankshaft in this case, the axis of inertia) is balanced individually or as a couple (IE: Opposing cylinders that counterbalance) then no consideration would need to be given to the mass of any one component along the axis relative to another component to which it is not coupled for the purpose of counterbalancing.

Said another way, the mass of the flywheel relative to the crank pulley is likely of no consequence so long as they are both balanced individually.

Tom@TPC Racing

Thanks, Jason. Makes sense.

powdrhound

The #2 and 3 pulleys above are under drive, NOT over drive. They spin the accessories slower, NOT faster. I use the 996Cup/997.1Cup pulley, part number 996.102.017.95. It's also an all aluminum under drive pulley. Been running that for years without issues.

Tom@TPC Racing

You are right! Thanks for the correction! Duh on me. I was thinking smaller diameter pulley drives accessories faster but I was wrong. Thinking of a bicycle, selecting smaller front gear is slower speed and that is correct.


The 996 Cup/997.1 Cup pulley is the same 127mm diameter(measure at the peak of the belt contact area) as the #1 997.1 RS street car pulley
The #2 and #3 pulley are both 123mm.

Tom@TPC Racing

Before getting back on to completing my long block my friend Nick at work rebuilt my cam actuators and machined them from bigger diameter M7 bolts. The original bolts are M6 and the M6 bolts fit quite loose in the bore. Using M7 Grade-12.9 bolts allows for higher tightening torque(increase from 10 ft-lbs to 20 ft-lbs) which results in increased clamp force of the components, and with M7 bolts precisely fitted to the new bore the components of the actuator have a higher chance of staying centered during moments of peak vibration. I feel this is a good proactive measure for a longer stroke engine.

Nick begins the procedure by fitting each cam actuator into a fixture that he machined from a solid block of 6061 aluminum to ensure the components are centric. Then he uses a pilot pin to index the machining operation.



Once the original bore is indexed the new bore is drilled on CNC vertical mill.


Nick prefers to cut the new M7x1.0 threads by hand to feel for the end results.


Before and after. Thanks, Nick! I have one less thing to worry about. Back to working on the engine tonight. Have some exciting stuff coming up. Stay tuned.

AudiOn19s

The 996 under drive pulley ending in .95 the same % reduction as the 3.8 997 pulley? Interweb search shows it's approx the same weight as the standard diameter 996 RS pulley (.64 lbs). I've got the standard diameter 996 RS / Cup pulley on my car but I need to do front main again so I'm thinking I have pulley envy at this point and need to upgrade! I've gone radio delete in my car so I'm not exactly concerned about spinning the accessories slower.

powdrhound

Hey Andy. I'm going to stop by my shop on Friday and can take an accurate measurement of the .95 pulley. My engine is there in pieces right now getting ready to be reassembled so measuring the pulley will be easy. I'm also going to throw it on an accurate scale to get a precise weight. I thought the .95 pulley was slightly smaller than the standard .93 pulley but could be wrong. According to PET, the 996.102.017.95 is the same pulley used on the 996GT3RS and the 997GT3RS pulley also. I don't have A/C or the engine driven power steering pump on my car so under driving the water pump is probably not the best thing for me from a cooling standpoint.

a_Y

Why not consider a dampener?

Tom@TPC Racing

They're good quality.
Its of my personal preference to address the actuators while the engine is apart.

Tom@TPC Racing

Here's my measurements. Let me know if its consistent with yours.
I don't have a caliper with deep jaws so I zero'ed the caliper to stainless steel bushings for the measurement.
Using this method for measuring the numbers are within 0.5mm of the number I posted previously(which I came up with by by measuring the most outter lip of the flange then subtracting the different between the highest point of the belt groove to the highest point of the outter flange lip, and then rounded off the fraction to the nearest whole number. For determining the length of a belt that's >1600mm a fraction of a mm is not crucial.)


View of standing heights. Not that it matters since the belt length is not relevant to the height of the flange lip.




Weight in grams.

Tom@TPC Racing

I have folded to peer pressure and temptation, well, mostly to the latter. After seeing the factory spec 3.8/4.0 pistons(made for Porsche by Mahle) and the Mahle's aftermarket motorsport version side by side I fancy to have the Mahle motorsport pistons. In fact, it was love at first sight but I couldn't have stepped to buying them at the time. Fortunately, I had a local customer needing a set of pistons for a factory spec 3.8 rebuild so he took the new set of factory pistons off my hands, with that money I was able to make the move on the motorsport pistons. I was so happy and excited I felt like going out on a first date with a beautiful woman! I guess its like dating, the right timing and situations need to align for things to happen.

Both the factory and the motosport version are 102.7mm bore, they have the same ring lands. The difference is on the motorsport piston the dome area is greater for higher compression ratio, the underside/wrist area has more structural support, have smaller diameter wrist pin, lighter weight, and the weight tolerance is smaller for the set. Without further ado, here they are, my own set of Mahle motorsport pistons-

The weight tolerance of the motorsport set from heaviest to lightest is 0.6 grams. The factory set was 3.0 grams tolerance.



Here's the underside/pin area of the factory piston(Left of image) and the motorsport piston(Right of image). The factory piston is by no means weak, this is standard equipment for factory 3.8/4.0 street cars, Cup cars, and GT America 4.0 race cars proven in 24-hour pro races. I think of the factory pistons with all of its accolades to be the benchmark, the motorsport pistons being way overkill.


The motorsport wrist pin(Right of image) has Mahle's black DLC(Diamond-Like Carbon) coating.


Blind Date Situation- Nearly all aftermarket pistons have some sort of published compression ratio rating or have published displacement difference relative to factory displacement measured in "cc" (cubic centimeter) to determine engine compression ratio. These motorsport pistons happen to have no published info for advertised compression ratio nor any cc value of the dome size. So, as much as I am still excited and happy the pistons I need to size them up...well, not exactly, I need to measure the cc difference between the factory and the motorsport displacement to calculate compression ratio.

To do this, I made a spacer to set each piston to a set height in an actual 102.7mm cylinder sleeve, then I inject fluid to full to the top and count up the number of cc's it takes for each piston and then subtract the difference.




I also cc'ed the cylinder head chamber so that I can calculate "actual" compression ratio as well as doing comparative to factory Porsche advertised compression ratio. This was a very time consuming task having to do each one a number of times to ensure consistency.



The results are in...Drum roll please...
I measured the motorsport piston volume to displace 10cc more than the factory pistons. The factory piston is not a "flat top" piston, by my measurement it is -4cc from a true flat top. That -4cc value is know as a "dish" rather than a "dome" for + value in the hot rodding world. That means the motorsport piston is +6cc from a true flat top.
Comparative CR(Compression Ratio) using the factory advised CR of 12.6:1 for the 4.0 engine, offsetting the piston displacement by +6cc comes to 15.04:1 CR !!!
Actual CR using numbers in an online formula to calculated ratio it come to a 14.49:1 CR (that's almost exactly same as factory GT3R race cars!)
So, emmm, this date turned out to be more than I can handle...on 93 octane pump gas that is. And my next move is...

jasonturbo

This is all strangely arousing to me.

powdrhound

Tom,
What's the all up weight of the Mahle Motorport piston INCLUDING the piston rings, wrist pin, and the wrist pin C-clip? Would be interesting to have those weight for the factory 3.8/4.0 pistons also. I'm using Mahle Motorsport 105.9mm pistons in my turbo Mezger and curious how the weight compares. I'm using these with the latest versions of the 76.4mm Motorsport crank giving a 4.05 displacement. Thx..

Also, do you by any chance have picture of the piston rings used on the Motorsport pistons. When we switched from the 102 Motorsport pistons to the 106, the rings were a different design. Just curious.

Slippery slope, you'll obviously need new rods also...

AudiOn19s

Tom@TPC Racing

The set of factory spec 102.7mm pistons that I had weight 435.3 to 438.3 grams.
Pins weight average of 110.6 grams each.
Factory ring set(made by Goetze) weight 24.1 grams per cyl.
C-clip 1.1 gram per clip
Averaging the factory 102.7mm pistons come to 573.7 grams per slug.

The set of Mahle motorsport 102.7mm pistons I have now weight 408.1 to 408.7 grams.
Pins weight an average of 96.9 grams each.
Uses the same factory ring set as above at 24.1 grams per cyl.
C-clip 1.0 gram per clip(estimated, I didn't weight these clips)
Averaging the Mahle motorsport 102.7mm pistons come to 531.4 grams per slug.


These pistons were made for factory rings, which I think are high quality, same part # as 2011-2016 GT3 Cup, GT America 4.0.


You know it!