daveo90s4

Hello all,

I am trying to find precise information on inlet and exhaust cam lift (inches or mm) by degree of camshaft rotation for the various 928 cams available.

The reason I'm after this is to complete a draft spreadsheet model that will precisely determine the piston : valve clearance thoughout the full 4 strokes for any combination of 928 cam and for any 928 crank stroke and for any 928connecting rod length. Why? Well,
(1) Someone posted concerns about inlet valves touching pistons as the inlet valve 'chases' the poston down on the inlet stroke (especially for planed heads) and that got me intrigued
(2) an interesting mathematical and mental challenge and
(3) maybe the results might be of use to someone sometime.

(Maybe I've got too uch spare time on my hands??)

So if anyone can point me in the right direction where I could get information about the cam lift by camshaft degrees of rotation, that's be much appreciated.

Thanks all,

john gill

Hello Dave

All those questions are answered in the manual (book versions, consult them regularly ) not sure about PET

drnick

dave that sounds complex and yes i think you have too much spare time, go chase some women or something... oh thats right your in the desert surrounded by men who lock up there daughters and cut off apendages from those who do wrong! hang on, no youve left australia..

marton

but not the sheep.


There is a good list here (of std cam specs)
http://members.rennlist.com/pirtle/zeng_cam_spec.txt

If you want to include some non-standard cams then look here
http://www.webcamshafts.com/pages/porsche-auto.html

Marton

daveo90s4

Oh we're a funny lot, aren't we now.

John, while the manual may well provide some key data points (maximum lift, degrees at which lift starts, degrees as which lift ends, dwell, etc) it doesn't from my memory show cam lift by degree of rotation. Or is my memory mistaken?

Nick, yes it's a lot of fun calculating crank 'fall' per degree of rotation, and hence big end fall, and with cosines and 'things' rod angle, big end 'offset' and hence piston fall per degree of crank rotation. But maybe I should take up your wise alternative suggestions - fear of failure may well be putting me off that one!!! (The best way to learn a foreign language is of course to have a foreign speaking partner - if I were to change religion then in Dubai I believe I could learn up to 4 languages at a time!!)

Marton, how do you make a you (ewe) turn? Ask it nicely. Not as amusing as the other one. What is thye most common lie told in NZ? "I was just helping the sheep over the fence, officer"

I'll look at the urrls you've posted Marton.

Thanks guys - still smiling :-)

Vilhuer

Your memory works fine. You'd need to calculate lift by degree numbers from what little info is known or take actual measurements from cams. If nothing else works I can do S3, S4, GT and GTS cam measurements while all are still out from engines.

daveo90s4

Hi Errka,

That'd be great. The other option I was toying with, and that me be more simple, would be a hi resolution end on shot of each representative cam lobe. The photo could then be superimposed on a PC over a 360 degree 'wheel' and the relative lift per degree measured off that image. The absolute lift per degree could then be determined by factoring the maximum lift reading from that measurement method by the known actual maximum lift reading.

So if you wanted to make things simple for yourself, some square-on cam lobe photos would be great!

The model now works properly in principle. The piston deck height per degree of rotation is now all computing accurately. Even the valve height and distance from piston mathematics is computing properly, albeit based on a dummy cam lobe lift profile. All I need now is some photos I think.

Cheers

Dave

drnick

For your reference, here are the factory timing specifications for several engines:

Camshaft Int Duration Int Lift Exh Duration Exh Lift
Stock 32 Valve 253* .393" 236* .353"
Stock S-4 236* .354" 223* .314"
Stock GT 258* .393" 236* .353"
Stock GTS * .374" * .334"

info lifted from 928 developments web site. the pics of cams end on is a snazzy idea.

mark kibort

that doesnt look right. the GT lift should be the same as the stock 32valve, which is greater than the stock S4.

mk

drnick

mark the colums under the headings are skewed, intake lift is column number two and exhaust lift the end numbers.

LT Texan

DaveO,

This is a fun spreadsheet to put together. I did one to teach myself trig and to calc piston speed at crankshaft degrees with various rod lengths. It involved drawing lots of right triangles.

Not only will you need cam lift at crankstaft degrees (which you will find nowhere), but you will also need measurements from the cyl head (valve angle, valve seated depth).

I think this is a great exercise. Good luck.

FlyingDog

Check with Louie Ott. He or Greg Brown are probably the most likely people to have the data that would be willing to share.

daveo90s4

Dan,
Yes, good points, and lots of conceptual triangles. Cosines of radians and all that. I'm treating cam lobe lift and vertical valve movement (where in this case vertical is defined as being in line with piston travel) as two separate related elements. First, determine the cam lobe lift by cam degrees, then second from that derive the vertical valve movement by taking into account the valve stem inclination. For the dummy model I've assumed valve stems are set at 30 degrees from vertical. I will need to get that dummy variable right in due course. But thought I'd bite away at the problem. That finessing can come later - or sooner if anyone can give me the right answer. I'll check the WSM I've got on my laptop. I don't think valve set depth is too much of a mathematical concern. I will simply provide for a user defined stationary TDC piston to valve clearance (and a certain cam to lifter clearance too) to represent the static clearances. I think those semi-default values will be available from various experienced engine builders, and the user would modify these values to reflect, say, any cylinder head planing that had been done.

Nick,
Thanks for that. Using that info I've plotted a few approximate curves, smoothing the acceleration and deceleration ramps so I've effectively got a forward facing S curve and rearward facing S curve adjoining at the top - ie at maximum cam lobe lift. Its a start - yip, the end on photo approach may well be a way of 'back-casting' the curve. We'll see.

Matt,
Good idea. But those guys time is money for them, and this is just a bit of fun for me, rtaher than just sitting at my desk waiting for the clock to strike or the next panic to ensue. Hopefully though, if those esteemed engine builders have a chance to read this thread, then they may have some useful info. Hope so.


Has just occured to me, if this model works (and why shouldn't it) then it would clearly demonstrate which engines are interference and which are not - simply set the virtual valve lift to maximum and spin the virtual crank, and see if / where valve to piston clearance is less than zero.

Thanks all, any more info from any much appreciated.

Dave

Vilhuer

I did measure this while back for machine shop which did 39mm valve cuts to 968 piston exhaust side so that they will work in 928 engine. Do not have numbers on hand anymore but htey were much less than 30. IIRR intake was something like 13-14 degrees and exhaust 9 but these can be wrong few degrees.

Picture idea is very good one. It should work on S4 and later cams as rearmost lobes do not have bearing surfaces after them. I'll see what I can do.

marton

about
tourist meeting australian sheep farmer with a sheep

tourist "are you shearing?"
Farmer "no, go find your own"

Good idea about end-on fotos; somebody must have these?

Marton