jcorenman

Following from the peanut gallery...
The problem with the engine modeling software is that it doesn't understand intake reversion as it affects the MAF, and the intake modeling itself is pretty generic. ITBs eliminate the reversion issue but introduce a bunch of new issues. I'll have a look in the morning and see what light can be shed.

Cheers, Jim

Rob Edwards

Here's some raw data comparing S4 and GTS cam lobe lift data, measuring every 2 (cam) degrees of rotation, starting immediately before the ramp leaves the base circle . The degree measurements are arranged so 'TDC' is referring to max lift of the intake lobe. If you want crank degrees just multiply by 2.

I have a set of GT cams here but haven't measured them, will try to do that some time soon.

Rob Edwards

Forgot that I also have the data for S3 cams, which as far as I'm aware have the same lift and duration specs as GT cams (but differ RE: LSA, 114 for S3 vs. 110 for GT). Here's S4 vs. GTS vs S3, all together:

GregBBRD

Cool data!
Ramps on intake and exhaust are very similar.
10 degrees of cam rotation results in under .8mm of lift. The next 4 degrees takes off like a rocket...doubling that lift.



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928cs

Here are the GT cams measures, from a spare set that I have on a shelf.
Let me know if you need better pictures.

Mitsu

Thanks to you all for the hard to get data. I am just so impressed that so much information is available from people with kind hearts and so much continuing attractiveness that 928 model provides to us to keep the car still under great attention ! I will save them and study them to make a summary on Porsche 928 camshaft specs in return soon !

Mitsu

I am testing to see if PDF file can be uploaded. Sorry for testing.

Dave928S

Here's some more comparison info to add to what's here which I've collected over years, and keep it all together, with some interesting intake and exhaust LSA combinations .

Mitsu

→Dave928S, thank you for your contribution ! the spread sheet is quite complete. Quite a lot of 928 lovers will appreciate it. I was thinking to make this kind of list. Maybe I don't need to.

Mitsu

→928CS and Rob, the numerical data you have shown us are for the up lift curve. May I assume that the cam profiles have the completely symmetrical profile except for the ramp ? On some cams, acceleration rate may differ for up and down.

Mitsu

928 GT cam valve event. FYI.

Strosek Ultra

From Rennlist: https://rennlist.com/forums/928-foru...is-help-5.html
This has been up for discussion before, it seems that the GTS intake specifications in the Workshop Manual (page 15-124a) are incorrect. Intake duration at 1mm is specified to 228 degrees like in Tuomo´s chart above. I did measure a GTS intake cam once and came up with a duration of 212,5 degrees. Think Rob Edwards also did measure the GTS intake cam and found the same.
Åke

Quote:
Originally Posted by ptuomov View Post
I, of course, agree.

Many people are pointing out things about the intake manifold. I agree that if we'd run this engine with smaller diameter straight 12" or so straight trumpets off the head, it would probably make more power. But let's think about what we need to do relative to that ideal engine if we replace the ideal intake with the S4 stock style intake.

The first thing we need to do is increase the compression ratio. If the curvy intake manifold doesn't fill as well at the peak torque rpm as the ideal intake manifold, then we are filling the cylinders less well. This allows us to increase the compression ratio.

The second thing we need to do is reduce the exhaust duration. There's less burn exhaust gas in the cylinder and it needs less time and/or area to get out from the cylinder at optimal velocity. For the same reason we could benefit from smaller header primaries which would maintain the exhaust port diameter for a fairly long period before the first step.

We should furthermore keep the EVC event and the valve overlap about the same. All of the exhaust duration (and lift) reduction should come from making the EVO later. The overlap is there to start the pull from the intake immediately at IVO, which makes it arguably more necessary with a restrictive intake manifold.

If you look at the SAE paper #962514 on restricted intakes, the paper prescribes a slightly smaller intake cam, a lot smaller exhaust cam, and no change in the valve overlap, relative to unrestricted-intake optimal cam.

Now, let's talk about magnitudes.

According to Colin, his cams have the following specs:
Intake: Lift .437" duration @.050" 230
Exhaust: lift .437" duration @.050" 228
LSA 114.
(source: https://rennlist.com/forums/928-foru...l#post13089746)

Per Colin, GT from the factory has the following specs:
Intake: lift .393" duration @.050" 211
Exhaust: lift .353" duration @.050" 200
LSA 110

That's 28 degrees more exhaust duration at 0.050", and likely more at the seat duration, compared to GT. In my opinion, that's a lot. Personally, I'd go a lot smaller with the exhaust cam for this engine. Keep the EVC as close to the current location and thus keep the overlap, but regrind the exhaust cam to something like 218 at 0.050" by moving the EVO later. Then advance both cams to deal with the very large intake cam also.

The GT exhaust duration is equal to about 95% of the intake duration. If you go to 218 degrees at 0.050" by regrinding Colin's exhaust cams, you'll get about the same ratio that GT cams have with 230/218.

As some background for comparison, here are Rob Edwards's measurements of 928 cams, including his race cams on the bottom for a custom-intake high-rpm stroker motor:



I'm not critical of Colin's or Colt's cams here. I'm critical of the idea that there's one-size-fits-all camshaft solution across rpms, displacements, and intake/exhaust configurations.
This has been up for discussion before, it seems that the GTS intake specifications in the Workshop Manual (page 15-124a) are incorrect. Intake duration at 1mm is specified to 228 degrees like in Tuomo´s chart above. I did measure a GTS intake cam once and came up with a duration of 212,5 degrees. Think Rob Edwards also did measure the GTS intake cam and found the same.
Åke

Strosek Ultra is online now Report Post

Mitsu

Thank you everyone ! With my knowledge combined with yours, I decided to regrind my cams to the following spec. For Inlet: 222deg@1.0mm 10.0mm lift valve timing will be set at Open 2°BTDC　： Close 40°ABDC (GT spec profile)

For Exh: 220deg@1.0mm 9.4mm lift valve timing will be set at Open 38°BBDC : Close 2°ATDC (DBilas optional cam profile)

It will take a bit of time to measure the power to report to you the result because first, cams have to be reground and installed.

BTW, larger size primary will make the exhaust valve temperature lower, preventing detonation. this allows us to raise the C/R. The definition of C/R is a bit misleading because the expansion ratio is the important parameter but not the C/R. It so happens with crank-slider mechanism, that static C/R is equal to E/R. In terms of dynamic E/R, late EVO is better for the efficiency at part throttle operation but not always for the power because the pump loss increases. This leads me to a question, how much can you mill the cylinder heads to increase the Comp. Ratio safely ?

Mitsu

I thought I learned pretty much everything on 928 cams but I found a cam stamped;

9281052926R-292 09 What is it ?

Strosek Ultra

Mitsu, you mention DBilas. Are you located in Germany? Are you on 928 PFF?
292.09 is 928S4 Weltweit (rest of world) intake camshaft cylinder bank 5-8.
Åke