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Before we get on to it I have two decisions to make on the clutch/flywheel. The clutch pressure plate is a no-brainer, the 4.0RS version featuring the locking nuts(instead of rivets) is the way to go on a 8K+rpm larger displacement engine. The decision are 1) go with 3.6RS or 4.0RS flywheel? 2) which clutch disk?
For reference, this is the original dual-mass flywheel.
Here's the 3.6RS. Been using one of these old and faithful with zero issue. It is an all time great flywheel. Notice the contact surface width of 40mm.
Here's the 4.0RS. Notice the contact surface width of 32mm.
With the 4.0RS flywheel there's a little bit of compromise on the contact width in order to lose that 2.6 lbs compare to the 3.6RS version. Also the back side of the 4.0RS flywheel has material machined off between the bolt holes. So there's considerable amount of machining involved to shed that 2.6 lbs. Let me say that the reduced contact surface is not a concern unless the engine is turbo making 700+hp.
Both the 4.0RS and 3.6RS disk has a contact width of near 40mm, so that means there's an overhang of ~8mm when using the 4.0RS flywheel. I don't foresee the overhang to be an issue, since its a factory approved design and lot of GT3 owners have this setup over the years. It is just an interesting observation, that's all. I do wonder though, with enough usage/wear if the overhang will cause a little bit of dragging on the release if the contact surface is worn down enough creating a step...only time will tell.
Another observation, when I unboxed the 4.0RS sprung hub disk, it felt noticeably heavier than the 3.6RS sprung hub disk that I had. This is due to the extra thick spline hub and likely heavier spring designed to handle higher engine torque. At 5.15 lbs, the weight reduction of the 4.0RS flywheel is negated by the heavier-duty disk. So, I am going to try something unconventional to the general public and pair the 4.0RS flywheel with a rigid hub GT3/GT2 disk. Even though the rigid hub is not as thick, its factory designed to handle the torque of a GT2 so it should be plenty good for me.
Okay, that's enough clutch talk.
Engine in.
The Moment-
I would not run a solid unsprung disc with a lwfw if you want to retain a measure of civility.. I am running the 4.0 sprung disc with a 3.6RS LWFW (due to the larger friction area) and a Sachs Motorsport 487 pressure plate. The 487 is the bolted 4.0 pressure plate but beefed up by Sachs Motorsport for more torque. May be overkill for your application but something to consider.
Last edited by powdrhound; 03-23-2018 at 01:00 PM.
I did 4.0 Flywheel with 3.6 sprung disk to have the lowest rotational mass possible without giving up the benefits of the sprung center.
No issues for me in the 4+ years it's been in my car however my motor is 100% stock power levels (other than exhaust) so it might not be a good comparison to the increased power you'll see with your build.
P.S. Congrats on it being alive....sooo close now!!!
I drove the car easy for 100 miles. It was a mix of stop and go, medium speed secondary roads, and some highways. I varied the rpm range during the drive. Engine is running smoothly on the factory 3.6 ECU tune for those easy break-in miles. The rigid hub clutch disk isn't as smooth as the sprung hub(which was completely chatter-free). With this particular clutch combination the amount of chatter is noticeable but not intolerable to me. I did stalled the engine three times getting used to the clutch release, after that I adapted to feathering the clutch pedal. As others have pointed out some combinations are uncivilized and even unsafe for busy street commutes, this is particularly true as the pressure plate clamping force is increased for higher engine torque rating, and the coefficient of friction is increased on the friction material for increased torque/race applications. On a scale of 1 to 10 for ease of stop and go, with 1 being a completely stock street car and 10 being a Cup car, I rate this clutch a 3. It feels "Sporty". ;-)
After the first 100 easy miles on the street, I did a few half-throttle acceleration runs, which felt fine then graduated to three-quarter throttle and then to full-throttle. All is well. After that I changed the motor oil and filter. The motor oil was a little bit discolored and looked normal. I inspected the oil filter element and found no metal, which is a good thing. Frankly I expected the full throttle runs to be faster but I was also paying close attention to find any abnormal sound or feel from the engine so I was quite distracted.
With the break-in street run and first oil change completed we are now ready for dyno!
This the baseline comparison run: BLUE is the best run of my 3.6 engine, stock 3.6 ECU tune, .2 Cup exhaust manifolds, 991 GT3 street car muffler. RED is 4.0 engine, stock 3.6 ECU tune, same exhaust as above. Dyno Notes: This dyno run confirms to my butt dyno that under 7500rpm its no faster than the 3.6 even though it ran smoothly. Scooby Doo we got some work to do...on the ECU tune. All dyno comparisons in this post is done on the same DynoJet model 248C dynamometer at TPC Racing. CF(Correction Factor) setting is at SAE(Society of Automotive Engineers). I can make the horsepower results a higher numeric value by switching the CF setting to DIN(German standard) or other standards on the selection but that doesn't matter to me, what matters to me is the delta on the same machine set to same CF. My "first run" with the 4.0 engine(RED lines) is labelled as run #5 is because the first few runs had tire spin prior to getting the tires up to temp. Run #5 was the first valid run.
Using the COBB Accessport to datalog and tune on the TPC dyno. With tuning expertise of Mike Levitas we made run after run.
It took no less than 27 runs and a lot of data crunching in between the runs to come to this. Thank you, Mike Levitas!!! Notes: 3.6L to 4.0L is 11% increase in engine displacement. We got a 20.5% increase in peak HP and 8.4% increase in peak torque. And a table top flat torque curve to power out of corners. I am happy! BLUE is the best run of my 3.6 engine, stock 3.6 ECU tune, .2 Cup exhaust manifolds, 991 GT3 street car muffler. RED is 4.0 engine, TPC Racing custom 4.0 tune via COBB Accessport, same exhaust as above.
Same runs with cursor at 8K rpm. Notes: At 8000rpm, 80.2 HP @ the wheel increase! That's 22.3% HP increase from 11% increase in engine displacement. I am Very Happy! BLUE is the best run of my 3.6 engine, stock 3.6 ECU tune, .2 Cup exhaust manifolds, 991 GT3 street car muffler. RED is 4.0 engine, TPC Racing custom 4.0 tune via COBB Accessport, same exhaust as above.
Same runs with cursor at 4K rpm. Notes: At 4000rpm, 42.5 ft.lb Torque increase @ the wheels. 17.2% increase in Torque. I am Ecstatic! BLUE is the best run of my 3.6 engine, stock 3.6 ECU tune, .2 Cup exhaust manifolds, 991 GT3 street car muffler. RED is 4.0 engine, TPC Racing custom 4.0 tune via COBB Accessport, same exhaust as above.
Just for fun/reference, dyno comparison of a stock 997.2 RS 3.8 to my 4.0 build. Notes: Based on the factory rating of the 997.2 RS 3.8 to be 450 HP @ the crank, my 4.0 build should be 491 HP @ the crank. I am using stock 3.6 camshaft, they are max'ing out, need different camshafts to exceed 500 HP...maybe next Winter. For me, I am satisfy with my 4.0 build. BLUE is the best run of a stock 997.2 GT3 RS 3.8. RED is 4.0 engine, TPC Racing custom 4.0 tune via COBB Accessport, .2 Cup exhaust manifolds, 991 GT3 street car muffler.
Another fun reference, stock 997.1 Turbo to my 4.0 build. Notes: 997 Turbo makes massive torque, my 4.0 can't touch that. Turbo has taller gear ratio to take advantage of its powerband. Speaking of gear ratio, the 997.2 RS 3.8 has shorter gearing than my 997.1 3.6 so that will come in to play during real world acceleration. The shorter gearing on the RS 3.8 is among one of the features that make the RS driving experience special. Put gearing change on my "to do" list. BLUE is the stock 997.1 Turbo. RED is 4.0 engine, TPC Racing custom 4.0 tune via COBB Accessport, .2 Cup exhaust manifolds, 991 GT3 street car muffler.
Yet another fun reference, 987.2 Cayman S with TPC Turbo Kit to my 4.0 build. Notes: Again turbo makes massive torque. These 987's with their short gearing really packs a punch with TPC turbo added. Its another example of power alone doesn't always make for a great driving experience. Power + Gearing = Awesome'ness. I just think its fun making comparisons of very different cars. BLUE is the 987.2 Cayman S with TPC Turbo Kit @ 4psi boost. RED is 4.0 engine, TPC Racing custom 4.0 tune via COBB Accessport, .2 Cup exhaust manifolds, 991 GT3 street car muffler.
PS- I had a doubt on whether or not I installed a piston clip correctly, well, this has been troubling me for weeks. So after 27 full throttle dyno runs I think I'm good!
Last edited by Tom@TPC Racing; 03-27-2018 at 01:52 PM.
Reason: added PS
I'm sorry if I missed the answer or if this is a silly question. Why did you choose the 991 GT3 muffler? I have both the 997 and 991 GT3 centers and I'm curious if there is a benefit to running the 991? Thanks again! Best Josh
I'm sorry if I missed the answer or if this is a silly question. Why did you choose the 991 GT3 muffler? I have both the 997 and 991 GT3 centers and I'm curious if there is a benefit to running the 991? Thanks again! Best Josh
997.2 GT3, 991.1 GT3, and 991.2 GT3 use the same part # center muffler. My thinking is since this muffler is factory approved for 500hp car then it should be sufficient for my build. I bought my muffler used on rennlist classifieds and I happen to like the sound.
Thanks Tom. I like the sound too. I just wanted to know if the internal baffles were the same. Thanks again and congrats!! I hope my Carrara White GT3.1 will make 500 at the crank someday Best JBO
03-23-2018, 09:46 AM
Moment of Truth
