Tom@TPC Racing

Let's talk engine dynamics- The pistons I used for my 4.0L engine build has a static compression ratio of 13.5:1 (that was after I machined the domes off, before they were 14.5 !!!). While the engine power has been satisfactory, the compression ratio-to-valve overlap is not fully balanced(two much of one and not enough of the latter). Basically what's happening is the higher compression ratio with stock 3.6 camshafts build up too much cylinder pressure too soon. The ignition timing is dialed down to prevent denotation. Running very minimum amount of ignition timing advance the engine is still making a healthy 440+whp. I think the solution to better balance the situation is to upgrade to camshafts to create more valve overlap to bleed out some of the cylinder pressure. For those not familiar with the term, valve overlap is the duration of which both the intake valve(s) and exhaust valve(s) are open at the same time in the same cylinder. Holding the exhaust valve barely open during intake stroke reduces the filling of the cylinder at lower rpm range, while improves scavenging effect at higher rpm range. That's why high-compression pistons and long-duration camshafts go hand in hand in hot rodding. If all goes well the engine might even make a little bit more power at the top end from better scavenging.

So, I need cams(camshafts). For many other brands of cars such as Chey's, Fords, Honda's, Mazda's, Nissan's, VW's.. upgrading cams is a very common thing. Its so common that there are many aftermarket options at very reasonable price. Guess what? GT3 isn't one of those cars. The 997.1 GT3 has Vario-cam on the intake side making the intake cams very versatile for producing a wide powerband. The exhaust cam however is just "okay" for a 3.6L engine, let alone a 4.0L. The popular exhaust cam upgrade, probably made popular by Porsche Motorsport, is the "Cup S" exhaust cams, which later become the standard exhaust cam 997.2 Cup and 991.1 Cup. Back in the mid to late-2000's Motorsport supplied the Cup S exhaust cams to Grand-Am and World Challenge teams to make their cars more competitive. Cup S exhaust cams are race proven and should be a home run, so easy choice there. The not so easy part, for me, is they cost $4K for two cams! For reference a Small Block Chevy race cam cost as low as 100 bucks. Its a metal blank that's been machined to spec...

I don't have 4g's to spend on cams but I can have cams regrind to match the profile of Cup S for a fraction of the cost. The metal blanks for GT3 is pretty much non-existent so I can take my engine apart and send the exhaust cams out and wait for weeks for find cores to send out. I stumbled upon new factory cams that I can use as cores for less 219 bucks each! They are the 997.1 GT3 Bank 1 exhaust cam. For some reason the other three factory cam for this car is like $2K each, same price a Cup S cams. So I took the chance and bought two Ban 1 exhaust cams from my local Porsche dealer.

Here they are, my cores to be send out for regrind.



I am not sure who has a data base of water-cooled Porsche race car cam profiles, but its difficult to get info. Perhaps people who mapped out the profiles aren't willing to share which is understandable if that's their business. Air-cooled Porsche cams on the other hand, aren't cheap, but there's plenty of profile specs to go around. So...I caught a lucky break by having a 997.2 Cup engine in the shop for rebuild, which the customer agreed to lend me the cams to send out to have the profile read. I did a quick measurement here of the peak lift before sending, I zero'ed the dial caliper at the base circle of 32mm and came up with a peak lift of 12mm. For comparison the stock 997.1 GT3 exhaust cam has peak lift of 11.1mm.



Three weeks later I got the cores back. They went through Web Racing Cams hardwelding process to build up the material and then re-grind to the profile of the Cup S sample cams I provided. Oh, I had them tweaked the specs a little bit.



A quick visual of the cam lobes here shows not only is the peak lift increased compared to the stock 3.6L cam(Left side of image) but check out the big increase of the area under curve!

HarmonyJim

Very cool, Tom. Any duration numbers available? I am aware that Cup Cars are non-actuated on both of their cams (as opposed to the 997.1 GT3 which has variable intake timing). However, is the lift also 0.9 mm higher on the Cup Car intake cams than the lift on the OEM GT3 intake cams? Could you duplicate the additional lift while retaining the duration of the street cams (thereby able to keep the actuator functioning on the intake cams)? Are the GT3 engines already sufficiently 'cammed' on the intake side for your 4.0 liter requirements? Will Web Cams or TPC be offering your creations? Such a great thread.

Tom@TPC Racing

Yes! I'll put the numbers in the next post.


The 997.1 GT3 street car intake cams actually has more peak lift than the 997.1 Cup and the 997.2 Cup/991.1 Cup/GT America 4.0 race car! My guess is having less peak lift may be better for durability in 24-hour racing. I'll post the numbers in the next post.


While the direct answer to this question is Yes, the cam grinder can create just about any requested profile, but given that the street car intake cams already have higher lift than the race cars listed above it would be unwise in my opinion.


Yes, for producing peak hp at 8100rpm. We have proofed this on all the 4.0L engines we have build using stock cams. Here's my dyno results from March. The Red lines are 4.0L engine with stock 3.6L cams. The Blue lines are all stock 3.6L engine.




TPC will be offering the cams with core exchange now that I have the stock 3.6L cores from my engine available.

Brainz

Thanks for sharing, Tom. That's really great info on custom cams.

Tom@TPC Racing

Measuring the cam duration and lift has turned out to be a real chore, at least for me it is, because I don't have a tool called The Cam Doctor. The tools I have is the factory Porsche cam timing fixture and the Stomski Racing cam timing fixture(see image below, the factory tool is the less colorful one on the right side of the image).



Both of these are very effective for finding the cam CL (Center Line- the point of peak lift) but far less effective in mapping out the duration due to the size of the contact pads to the cam. As you can see in this image the cam lube will lift the leading/trailing edge of the contact pad causing inaccurate reading for the amount of left to crankshaft position expressed in degrees.



I also tried using a dial indicator with pin tip on a lathe. This method requires converting camshaft degrees to crankshaft degrees. And I draw the degree marks on a piece of tape based on circumference of the lathe head divided 180(I didn't do 360 because I thought it was too tedious to measure and draw 360 lines, instead I drew 180, so one line is 2 degrees @ the cam which equals 4 degrees at the crank).



After hours at this late in to the night I concluded that the results from neither method exactly matches the little bit of info I can found for these cam on the internet. So my conceded on the numbers are only as good as the method. With that said, I measure the degrees again on the engine itself using a degree wheel knowing that the actual numbers may not match what is published but the Delta value from using the same method by the same person is valid.


Here is the baseline with stock 3.6L cams:

Stock 3.6L Intake
Intake Open @ 0.001" 8 BTDC , Intake Close @ 0.001" 116 ABDC , Duration @ 0.001" lift is 304 deg
Intake Open @ 0.005" 1 ATDC , Intake Close @ 0.005" 108 ABDC , Duration @ 0.005" lift is 287 deg
Intake Open @ 0.050" 20 ATDC , Intake Close @ 0.050" 84 ABDC , Duration @ 0.050" lift is 244 deg
Max Lift is 0.499" (12.67mm)
CL @ 141-142 ATDC

Stock 3.6L Exhaust
Exhaust Open @ 0.001" 77 BBDC , Exhaust Close @ 0.001" 37 ATDC , Duration @ 0.001" lift is 294 deg
Exhaust Open @ 0.005" 66 BBDC , Exhaust Close @ 0.005" 28 ATDC , Duration @ 0.005" lift is 274 deg
Exhaust Open @ 0.050" 46 BBDC , Exhaust Close @ 0.050" 4 ATDC , Duration @ 0.050" lift is 230 deg
Max Lift is 0.437" (11.09mm)
CL @ 114 BTDC

OL (Over Lap)
@ 0.001" lift is 45 degrees
@ 0.005" lift is 27 degrees
@ 0.020" lift no OL
@ 0.040" lift no OL
@ 0.050" lift no OL


Notes:
1) The 997.1 GT3 street car the intake VarioCam actuator can adjust up to 52 crankshaft degrees, which is why the static intake CL is an unusual number(normally the intake CL is closer to 110 ATDC)
2) Personally I think the stock 3.6L engine is way under cammed on the exhaust side.
3) This is for entertaining purpose. I am not responsible for any errors.

Still tuned for more next week.

HarmonyJim

Bolt in modification, or will the piston crowns need to be machined on stock, unmodified 3.6 GT3 engines?

Tom@TPC Racing

I am not sure if 997 GT3 3.6L street car engine has the same pistons as 997.1 Cup.
On a 997.1 Cup (MY2007-2009) exhaust cams can be upgraded to Cup S without machining pistons.

I would like to rephrase what I wrote in the previous post about the stock 3.6L exhaust cams being "undercammed" - What I meant was undercammed for an increased displacement engine.
I think the shorter exhaust duration cams were purposely selected to operate in conjunction with the vario-intake cams to produce a wider usable power band for the 3.6L displacement street engine.
Increasing the exhaust cam duration and lift in a 3.6L race car will produce more power from ~6500rpm to redline. I think there will be some loss in torque and driveability on street car of the same displacement.

Tom@TPC Racing

Doing the cam "swap". The word swap makes it sound swift and clean but I assure you its a pain. I am committed so let's do this.







I have been pretty unlucky when it comes to lifter tappets. This car is on its third set!! I replaced all 24 lifter tappets in January 2015, and one of them started tapping in April of this year and that's what prompted me to thinking of swapping cams since I needed to replace one lifter in Bank 1, but didn't get a chance to narrow down to which cylinder before my car got rear-ended and had to take the engine out to fix the body. So, might as well replace all of them, again. The factory lifter tappets have been revised three times, each time with different part numbers, hopefully the third time is the charm and they got the glitches worked out. What upsets me is that my previous Porsche, a 997.1 Carrera S has over almost 100K miles on the original lifter tappets and they were quiet. These latest lifter tappets are factory equipped on 991's.









Timing the intake cams is standard procedure since they are unchanged.. But the different exhaust cams, particularity on Bank 1 won't fit the factory alignment puck.




Fortunately, the shop has a number of alignment pucks, one happens to fits the Bank 2 exhaust cam at near TDC. Then I was able to measure and find the difference in the desired CL(Center Line) and factor in the difference at TDC and then manually degree in the exhaust cams this way. The Bank 1 exhaust was manually degree in as well because the CL using the alignment puck was too advanced, but Bank 1 exhaust cam did fit the factory alignment puck at TDC.




Here's what I'm going to run:

Stock 3.6L Intake (Unchanged from before)
Intake Open @ 0.001" 8 BTDC , Intake Close @ 0.001" 116 ABDC , Duration @ 0.001" lift is 304 deg
Intake Open @ 0.005" 1 ATDC , Intake Close @ 0.005" 108 ABDC , Duration @ 0.005" lift is 287 deg
Intake Open @ 0.050" 20 ATDC , Intake Close @ 0.050" 84 ABDC , Duration @ 0.050" lift is 244 deg
Max Lift is 0.499" (12.67mm)
CL @ 142 ATDC

Cup S / 3.8 Cup Exhaust
Exhaust Open @ 0.001" 89 BBDC , Exhaust Close @ 0.001" 49 ATDC , Duration @ 0.001" lift is 318 deg
Exhaust Open @ 0.005" 79 BBDC , Exhaust Close @ 0.005" 39 ATDC , Duration @ 0.005" lift is 298 deg
Exhaust Open @ 0.050" 55 BBDC , Exhaust Close @ 0.050" 14 ATDC , Duration @ 0.050" lift is 249 deg
Max Lift is 0.472" (12.0mm)
CL @ 110 BTDC

OL (Over Lap)
@ 0.001" lift is 57 degrees (before was 45 degrees, delta is 12 degrees)
@ 0.005" lift is 39 degrees (before was 27 degrees, delta is 12 degrees)
@ 0.020" lift is 12 degrees (before was 0, delta is 12 degrees)
@ 0.040" lift no OL
@ 0.050" lift no OL

Hopefully the extra 12 degrees of Over Lap will do the trick for me. Keep in mind this is a static OL value, the vario-intake cam can be adjusted to increase the OL. Whatever the max dynamic value is, its 12 degrees more than before.

AudiOn19s

Great work as usual Tom, can't wait to see the results.

This takes me back to my HS days studying cam cards for my small block chevy to determine which one was going to be the best as that engine evolved.

don't have much cam experience with multiple cam engines though...hence can't wait to see the results. 12 degree difference in centerline on a conventional small block would have been a MASSIVE change. Are you going to have to run a higher idle similar to the cup cars that I see that idle around 2000rpm?...or will the variocam still allow it to have a much more civil idle?

doritos

be interesting to hear the ilde chop on this thing <3

Tom@TPC Racing

Before buttoning up the engine I checked the "upgraded" M7 Vario-cam actuator bolts. Not one of them budged. A-Okay after 30+ dyno runs to 8500rpm, and 6 hours of track time in 2018.

Tom@TPC Racing

Thank you, Sir.


I loved studying cam cards too! SBC was a great engine to learn the these dynamics and to get hands on.


The idle should be pretty smooth since the bore was increased 2.7mm and the stroke increased by 4.0mm. This is almost like taking a SBC 350 to a 383 stroker. The increased engine displacement soaks up the longer cam duration like me eating ramen! LOL
We shall see. Now I am more eager than ever to get rolling chassis back from the body shop.

LateBraking

I'm so glad I got this upgrade done while I was on the east coast. What fantastic peace of mind.

Have to say I miss being within driving distance of Tom and TPC, was such a fantastic group to rely on for my Porsche needs.

Tom@TPC Racing

We have done the M7 bolt upgrade on a number of cars. Including heavily track cars with increased engine displacement. So far haven't had any bolts come loose.

Thank you for your kind words. We enjoyed working with you and on your GT3!

rbahr

Hi All,

I had measured my Camshaft info when I did my motor. Not wanting to add yet more to these 64 pages :-), I simply added the info to the end of my post:

https://rennlist.com/forums/996-gt2-...l#post15545714

Ray