Mrmerlin

Greg or Mark any ideas on the cost for a full set of GT cams?>

jcorenman

Just to be clear, Greg Brown built the motor and made all the important choices, we just asked silly questions and wrote checks. And yes, everything you say is correct-- bores need special treatment, using factory pistons is important, as is Not using factory rod bearings. All other choices flow from there. There is a thread here from some years back which will answer some questions in post #1, the rest is just discussing the fine points of various dynos.

GregBBRD

There's a sneaky way.
Talk on phone...
After the 15th!

gb

Mitsu

Hi all, I found an info. on the cams from Dbilas Dynamics Germany that I would like to share with you.

They offer regrinds for 928 32valves as in the following;①　The camshafts from the S4 and the GTS are the same (inlet 254° with 9,5mm lift and exhaust 240° with 8,5mm lift)
②　At 1.0ｍｍ, oemS4 have inlet 209° and exhaust 194° and Dbilas Dynamics GT spec. > inlet 222° exhaust 212°

③ They can also grind another profile:

@1,00mm
inlet 268° with 10,3mm lift 234°
exhaust 260° with 9,4mm lift 220°

I need to confirm again but they are welded cams and induction hardened ?

As soon as i confirm the details, I will ask them to regrind my cams(a set of S4 cams 271-274 00 bought from ebay) Thanks to you all, now I know how to read cam specs !
Also, I am convinced that GTS cams are the same with S4 cams. That is a piece of information I wanted to know.

Strosek Ultra

The idea with welded camshafts is that the cam grinder has the opportunity to move the center of the cam lobes on the shaft as desired in order to change the cam timing and overlap.
Åke

ROG100

​​​​​​​I do not believe that to be correct but I will wait for the cam experts to confirm.

Mitsu

Hi all, Quite a lot of car freaks are interested in camshaft specs and then a notation of LSA appears often. I think it is an important concept for the valve train operated with a single camshaft where LSA is fixed such as OHV, SOHC. However, with DOHC system such as in Porsche 928 32valves, this concept is not really important but the valve timing is. Assuming that I will use the stock intake manifolds and stock ports, that is to say, keeping the intake charge velocity reasonably high, intake close timing should be slightly delayed to make use of the inertia effect of intake charge. For intake, the most general valve timing to achieve high volumetric efficiency with good idle is 10/40. Intake opens 10°BTDC/closes 40°(44° for proposed profile)ABDC. This timing works great except for the exh. emission regulations. It may surprise you that this valve timing works for the motorcycle engines revving up to 20,000rpm. The exhaust valve timing can also 10/40. Exhaust valve opens 40°BBDC/closes 10°ATDC. The available cam has 220° for the exh., so I will set it at 5/35. 5 for a slight over-lap to make use of negative pressure wave from the equal length tuned header and 35 for slightly larger dynamic expansion ratio for fuel efficiency. I could make use of intake cam profile to get 10/44 for the exh. but I suspect that cam ramp for the intake camshaft is smaller/shorter. Maybe the same due to Hydraulic lifters ? If so, I will go for 10/44. 10/44. Anyone knows if the ramp specs are the same for intake and exhaust ?

Strosek Ultra

From top to bottom. S4 intake cam profile, S3 ditto, GTS ditto.
Åke

GregBBRD

In my humble opinion, you might want to reconsider your proposed cam profile.
Although I am not certain of what initial lift you are using to propose your opening and closing events (above), but I'm guessing you are using the factory 1mm (or the US standard of .040".)
And I'm also not aware of what injection/fuel system you are using.
However, if you are using 1mm (or .040") initial lift for your cam specifications and plan on using a MAF, you will quickly find out that 10 degrees before TDC, for the intake and 10 degrees after, for the exhaust, are going to build a very poor running engine throughout the entire lower and mid range rpms.
And the 105 degree lobe centers are going to add dramatically to the complete lack of bottom end.
Yes, the engine might "wake up" at 4,000 rpms or so, but the entire world is going to have already driven by your door.

When thinking about 928 engines, It's important to remember that you are swinging a 60 pound crankshaft around....and the entire engine was designed with torque being the most important factor.
......."Light years" away from a motorcycle engine with a huge power to weight ratio, short stroke, and super light internals, turning 20,000 rpms. (Which never drop below 10,000 rpms, except in 1st gear.)

If you look closely at all of the factory camshafts, none of them have "events" (other than super low flow on the ramps of the cams) on "other side" of TDC.
There's a darn good reason!

Mitsu

Thank you Strosek Ultra for sharing the data. I was told that S4 cams and GTS are the same but It appears to be different. In terms of duration or valve timing, they are nearly the same, but the lift is definitely different. Probably due to increased displacement(5.0→5.4), they increased the lift for GTS. Thanks again for the data. Now this issue is very clear. Do you happen to have the data for exhaust as well? can you share the data with us ?

Mitsu

Thanks for your opinion, GregBBRD ! Your opinion is highly appreciated. Yes, I do agree that 10/10 overlap for the street motor is a bit too large, although in my opinion, not really too excessive. but large overlap reduces the inlet vacuum level which does have some negative effects in drivability. Indeed, I was really so much used to ITB set up, which allows to have large overlap. considering the original intake manifolds and MAF, it might be better off with less overlap. Do you think 5/5 overlap will work with the original intake system? I appreciate your highly experienced opinion.

Mitsu

Sorry GregBBRD, I forgot to mention that The valve timing spec. is all referred @ 1.0mm lift. I think the main reason for not having the overlap is for the emission. especially with the single throttle body set up, the vacuum level in the intake is high and HC will be bypassed to the exhaust and internal EGR will occur more than the ITB set up. Realizing all this, your recommendation not to have overlap definitely makes sense. but I still hope to have a little overlap to make use of exhaust negative pressure wave. this is definitely a trick for NA tuning.

GregBBRD

Yes. Important to realize that everything I offer is an opinion, as it seems like I used to know a lot more when I was younger, for some reason. (The older I get, the less I know., for sure.)
I just get concerned about any intake "back charge" through the MAF confusing the heck out of it.
In the "old days" I confused the heck out of a LJet 911 (3.2 liter Carrera) with some custom cams Jerry Woods ground for me.. Really tough to get it to idle and was a bit "soft" on the bottom end. Sure ran great at higher rpms, on the track, with some "short" gearing.
Of course, "short gearing" in a 928 isn't very easy to come by...

I really don't know how steep the ramps are on these 928 cam profiles...I've frankly never measured it. (Although Ake has been kind enough to hand us that information.)
Perhaps the difference between 5 degrees opening and 0 degrees opening is insignificant, in terms of lift and airflow?
I'll need to get out my good glasses and look.

I do think that 5/40 is going in the correct direction, for lobe centers. 107.5 is getting close to the "magic" naturally aspirated number of 110.
It would be fun to see if Jim Corenman has the data for his engine analysis program (for his engine) and run both a 5/40 (107.5 lobe center) and a 0/40 (110 degree lobe center) combination and see what it says.

Are you listening Mr. Corenman?

Mitsu

Thanks again for your valuable opinion. My background is from motorcycle engine development and I know less on Porsche 928 specifics and non ITB intake set up. With ITB set up and throttle position/Engine speed mapping with compensations, valve timing of 10/40, 10/40@1.0mm with 0.25mm/25°(cam angle) ramp is quite streetable. Idle speed of 850rpm is feasible. but with a single TB, internal EGR which might affect the S/N ratio on MAF can be an issue indeed. I think increasing of plenum chamber volume can slightly reduce the bad effect. not only reducing the negative effect, it also improve the charge distribution to each cylinder.(more volumetric efficiency at the end) There seems to be something in the market to increase the chamber volume. Maybe I will make the whole cover to increase the volume further, so that a little overlap of 5/5 can be tolerated ? It is always interesting to think and exchange the ideas on IC engines. The whole world is directing for EVs and to me it is really ridiculous knowing as long as we stop the use of fossil fuels, such as carbon neutral alcohols, IC engines are far superior to Electric in terms of Energy density, safety and charge time.

Mitsu

Once again, thank you for sharing the data with us. With some careful observation, it seems it is not very clear to read the cam ramp spec. The first graph showing S4 profile, down ramp seems to be readable as 0.15/11° but up ramp appears to be too short. It says the measurement is with the hydraulic lifter, it may have some leak down loss. Looking at S3 cam profile, the situation is reversed. Up ramp seem to be 0.15/10°, but down ramp is short. By observing the profile data, it appears that 928 cams have up and down ramp nearly the same at 0.1-0.15/10°-12°. I am very curious to see the cam ramp spec. on exhaust cam profiles. It may be the same with intake and if so intake and exhaust cams can be interchangeable in terms of valve train dynamics.