From: Wally Plumley [mailto:wplumley@bellsouth.net] Sent: Tuesday, July 23, 2002 4:23 PM To: 928 Subject: [928] Cam Timing There was a question on the RennList Discussion Forum about cam timing. The poster said that one of the cam sprockets on his '90 GT was half a tooth off the alignment mark with the crank at TDC. I thought that the list might be interested in my answer to him. On the 32-valve engines, the cam sprockets can be adjusted on the camshaft - or perhaps we should say that the camshafts can be adjusted on the cam sprockets. This allows you to get the cam timing perfect, even though the sprockets may not be perfectly aligned. There is a four-page procedure in the Workshop Manual. Basically, you set the crank to 20 deg ATDC and see if the #1 intake and the #6 intake valves are lifted a specific amount, which varies by model. If not, you turn the crank until the correct valve is lifted the specified amount, loosen the cam sprocket and lock the sprocket hub and cam in place, then turn the crank/belt/sprocket until the crank is on 20 deg ATC. Tighten the sprocket back to the hub, remove the locking bolts, and do the other side. Bottom line - the sprockets may not be exactly on the timing marks, but if the cams are properly timed, it doesn't matter. You can either hope/assume that the cams are timed correctly, or you can get the manuals and a dial indicator and set them. Wally Plumley 928 Specialists www.928gt.com From: Wally Plumley [wplumley@bellsouth.net] Sent: Monday, January 21, 2002 10:28 AM To: 928 Subject: [928] Cam Timing Info I think that it is good for all of us to publicly demonstrate just how dumb we are sometimes. I posted the following statement a couple of days ago: >One of our customers asked about timing the camshafts on a four-valve engine. I >told him that there was a procedure in the Shop Manual (page 15-133), but that it >was thoroughly screwed. I find it amusing that after puzzling over this procedure many times, I had never noticed that my copy of the Shop Manual was missing pages 15-133 and 15-134! No wonder the procedure didn't make much sense! My only defense, and it is a very weak one, is that my manual went from step 3 on page 15-132 to step 4 on page 15-135. Many thanks to Mike S. for sending me a copy of the appropriate pages. Since several people said that my home-made procedure was easier to follow than the one in the manual, I am repeating it, with a couple of corrections and additions. My apologies for the bandwidth. For an '85 or '86 four-valve engine: 4a. Mount a dial indicator so as to measure the lift of an intake tappet on cylinder #1. The indicator must be mounted directly in line with the valve stem, which will require an off-set attachment for the indicator to get around the cam. Preload the indicator by about 5mm, since valve lift will be down, away from the indicator, then zero the dial. 4b. Rotate the crank about 540 degrees (about one-and-a-half turns), and start watching the indicator. Stop when the indicator reads 1.6 +/- 0.1 mm. Note - NEVER turn the crank backwards! 4c. Check the crank dampener - the 20 degrees ATDC mark must be exactly on the pointer. If it is, move to the other cylinder head. 4d. If the crank is not on the 20 deg mark, remove the distributor rotor, and firmly install three M5 by 15 mm bolts in the rotor mounting holes. Loosen the cam sprocket retaining bolt, counter-holding the washer. Do not let the camshaft move! 4e. Loosen the three 5 mm bolts enough to allow the sprocket free movement on the drive gear and the camshaft (the Woodruff key slot is slightly oversize to allow limited movement). While holding the camshaft in place, with 1.6 mm lift showing on the dial indicator, move the crank until the 20 deg mark is perfectly aligned with the pointer. Note - NEVER move the crankshaft backwards! Louis Ott suggests that it is easier to put the crank on 20 deg ATDC and to move the cams to get the correct lift, using the cast-in flats. Tighten the three 5 mm bolts. 4f. While carefully counter-holding the washer, and ensuring that the camshaft does not move, tighten the cam bolt to 65 Nm (47 lb/ft). 4g. Rotate the crank almost two turns, then carefully watch the dial indicator and stop at 1.6 mm lift. Check to ensure that the crank is exactly at 20 deg ATDC. 4h. Remove the 5 mm bolts and reinstall the distributor rotor. 4i. Repeat steps 4a thru 4h on an intake valve for cylinder #6, but use 2.0 +/- mm lift rather than 1.6 mm. Also note - these values are ONLY for the '85 and '86 engines. For '87 - '91 engines (except as listed below), use 1.8 +/- 0.1 mm for #1; 2.0 +/- 0.1 mm for #6. For the Club Sport (M28.41) and the '89GT (M28.47), use 2.8 +/- 0.1 mm for #1; 3.1 +/- 0.1 mm for #6. For '92 - up (M28.49/50), use 1.83 +/- 0.1 mm for #1; 2.08 +/- 0.1 mm for #6. I find it interesting that following this procedure will put the camshafts and crankshaft in the correct relationship, giving correct cam timing, but may cause the notches in the cam sprockets to be off slightly. Louis also suggests that you can adjust (retard) the cam timing to give a slight power increase on the top end, with a smaller decrease on the bottom. The opposite is also true - you can get a little more bottom-end grunt with a slight loss on the top end by advancing the cams slightly. The easiest way is probably to estimate 23 deg ATDC or 17 deg ATDC, rather than the standard 20 deg., and using the specified valve lift at these points. Thanks to all who responded. Wally Plumley 928 Specialists From: mlschmidt@sprintmail.com Sent: Sunday, January 20, 2002 1:28 AM To: 928 Subject: [928] Re: Cam Timing Info That's definitely a much better description than what's given in the shop manuals. There's just one slight correction though. The three M5 by 15 mm bolts are tightened and the cam sprocket retaining bolt is then loosened as stated in step 4d above, but the three M5 by 15 mm bolts must then be loosened for the sprocket to be able to move in relation to the camshaft when doing the adjustment. The crankshaft is then moved until the 20 degree mark lines up with the pointer, the three M5 by 15 mm bolts are tightened, and the cam sprocket retaining bolt is then tightened. The three M5 by 15 mm bolts are used to make sure that the cam and cam sprocket do not move in relation to each other while the cam sprocket retaining bolt is being loosened or tightened. Mike Schmidt '88 928S4 Auto Black/Black "PORSCHE" cloth 928 Owners Club Charter Member PCA Chicago Region From: mlschmidt@sprintmail.com [mailto:mlschmidt@sprintmail.com] Sent: Tuesday, August 27, 2002 12:59 AM To: 928 Subject: [928] RE: Thrust Bearing/Flex Plate >an initial belt stretch?> > >From the "Guarantee and Maintenance" booklet for my Euro 928 GTS: > >Maintenance 2,000 to 2,500 miles: >Toothed belt for camshafts: Check tension. So that would mean new, regular off the shelf, timing belts were used by Porsche when the engines were initially assembled. If they didn't in some way compensate for the initial belt stretch when the cam timing was initially set, the cam timing would be off from specifications when the belt did stretch. Since the engine rotates in a clockwise direction when looking at it from the front, the passenger side cam timing should be retarded in relation to the driver side cam timing when the belt stretched. That's exactly what the situation was with my car when I checked the cam timing. Mike Schmidt '88 928S4 Auto Black/Black "PORSCHE" cloth 928 Owners Club Charter Member PCA Chicago Region