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Hi all, I know the topic has been discussed a number of times in this forum but can you spare me a bit of time to once again clarify the cam specs on 928GT cams ? I have a 1992 GTS and planning to have a set of S4 cams reground to GT profile. I have looked the previous threads discussing on the topic but not really clear. what I want to know is the duration @1.0mm for GT and GTS cams and preferably the valve timing. I really appreciate if someone can help me on this.
The factory spec for GT cams (from the tech spec book for '90-93 pg. 38) is:
Intake opens: 3 deg CA after TDC
Intake closes 42 deg CA after BDC
Exhaust opens 30 deg CA before BDC
Exhaust closes 5 deg CA before TDC (All at 1mm lift, CA = crank angle)
Intake lift is 10.0 mm, exhaust lift is 9.0mm
That calculates to intake / exhaust duration of 219 / 205 deg @ 1mm, LSA= 110. This matches what Rob and others have measured, the GT specs are correct. (The GTS specs, not so much).
S4 cams are 1mm less lift and smaller duration, and can't be ground into anything useful. Reports on welding and grinding 928 cams are "mixed", i.e. the vendors say it works but others disagree,
Colin's Stage-2 cams (Lizard928 here) are bigger all around (duration is 230 intake, 220 exhaust @ 0.050", 114 LSA, 11mm lift), we had Colin grind a set to bigger-than-GT specs (224 / 213 @ 1mm, LSA 110, lift 10.8 / 10.4 mm). They work well in our 5.9L GT.
Thank you all for the info. Having understood that The GT cams have the duration of intake / exhaust duration of 219 / 205 deg @ 1mm, i would set them ;
Intake opens:0< 4 deg CA before TDC Intake closes 35<39 deg CA after BDC Exhaust opens 30 deg CA before BDC Exhaust closes 5 deg CA before TDC (All at 1mm lift, CA = crank angle)
But definitely, your recommendation sounds more attractive. Are you using MAF and the original intake manifolds with your bigger cams ? 393rwhp(462hp) is quite impressive ! Beside stroking the motor, what other mods you have done to achieve the gain ? The intake system appears to be the bottle neck for improving the power. Any thought on it ?
400 rwhp is all that a stock S4 intake will flow, it becomes the bottleneck (after porting heads and moving to 39 mm intake valves, adding good 1-3/4" primary headers, etc) . Short of a completely different intake, Jim's motor is about as good as normally aspirated 928 motors get. The stock MAF will support 400 hp without issue, and I'm a fan of the sharktuner for tuning purposes. Based on ancient technology but it works.
Last edited by Rob Edwards; 02-01-2022 at 02:45 PM.
Thank you all for the info. Having understood that The GT cams have the duration of intake / exhaust duration of 219 / 205 deg @ 1mm, i would set them ;
Intake opens:0< 4 deg CA before TDC Intake closes 35<39 deg CA after BDC Exhaust opens 30 deg CA before BDC Exhaust closes 5 deg CA before TDC (All at 1mm lift, CA = crank angle)
But definitely, your recommendation sounds more attractive. Are you using MAF and the original intake manifolds with your bigger cams ? 393rwhp(462hp) is quite impressive ! Beside stroking the motor, what other mods you have done to achieve the gain ? The intake system appears to be the bottle neck for improving the power. Any thought on it ?
Yes, stock MAF, still a bit of headroom on that. Stock intake and heads, both carefully ported by Greg Brown. Bored for 968 pistons (104.5mm), GTS stroke (GB crank and Carillo rods), and Sharktuned.
Originally Posted by Rob Edwards
400 rwhp is all that a stock S4 intake will flow, it becomes the bottleneck (after porting heads and moving to 39 mm intake valves, adding good 1-3/4" primary headers, etc) . Short of a completely different intake, Jim's motor is about as good as normally aspirated 928 motors get. The stock MAF will support 400 hp without issue, and I'm a fan of the sharktuner for tuning purposes. Based on ancient technology but it works.
Oh yeah, headers (and 49-state cats). We stuck with 37mm valves, the tradeoff is that higher velocity is good, to a point-- which seems to be above 6L. And more stroke is more torques, no one ever complained about too much torque (except the rear tires).
Even if welded, you can't make GT cams from S4 cams.
Although cam grinders can duplicate the lobes, they can not "move' the basic location of the lobes.
The S4 lobe centers are a far cry from GT cam lobe centers.
And lobe centers are what make GT cams so special.
And why S3 cams, even though they have basically the same lobes as GT cams, will never be the same as GT cams.
Even if welded, you can't make GT cams from S4 cams.
Although cam grinders can duplicate the lobes, they can not "move' the location of the lobes.
The S4 lobe centers are a far cry from GT cam lobe centers.
And lobe centers are what make GT cams so special.
Greg in your opinion besides a GT engine and 5 speed,
What other 928 engine would benefit from adding a set of GT cams to it ?
Like would an auto benefit from them for example?
Or a GTS engine with auto or 5 speed or an S4 auto or 5 speed.
Whats the current cost of a set of 4 GT cams?
I think Bill VanVlack (928 OC) out here (PacNW) has a set in his 89S4 AT. I believe he said there is a noticeable difference, but that it took some work to get them to work correctly. Maybe send him a note or perhaps he'll see this and respond..
Thanks jcorenman ! Considering the total balance of all the components, drive train, chassis rigidity etc, 400rwhp appears to be the target. I have 4 928 motors, 2units, 5.4 GTS and 2units S4. I understand that 928 incorporated Aluminum-silicone bore and iron coated piston. When you bore out the cylinders, don't you have to do any surface treatment on the cylinders ? because I already have the stroker crank, so to get your displacement, i just need to get the big bores. Your pins are eccentrically cut to gain the stroke ? In that case, the big end size for the rods will be different from GTS crank.
Like I mentioned, I have a spare motor that needs overhauling, so i thought I might follow your mods on that one.
Greg in your opinion besides a GT engine and 5 speed,
What other 928 engine would benefit from adding a set of GT cams to it ?
Like would an auto benefit from them for example?
Or a GTS engine with auto or 5 speed or an S4 auto or 5 speed.
Whats the current cost of a set of 4 GT cams?
I had a client who had some GT cams installed in his S4, before I ever saw the car.
It ran very well, once you got it moving and up a bit into the rpm range.
We installed a higher stall torque converter.
And it ran very, very well.
it's important to remember that it is extremely difficult to go too far wrong with 110 degree lobe centers on a naturally aspirated engine. (GT cams have that, exactly.)
Unfortunately, Porsche no longer has a complete set of GT cams....they are missing one exhaust cam.
.....There are ways around this supply problem.
A couple of ways, actually.....
02-01-2022, 01:07 AM
928 GT cams
