924Board.org

[fergie]

A couple of days ago I installed a '79 euro spec cylinder head on my '77.5 usdm block. Today I tried to set the timing, and it doesn't seem right. With my original head, the timing marks would line up perfectly and the intake valve on cylinder 1 would begin to open right at tdc mark on the crank pulley. With the new head, when the marks were lined up the teeth on the belt wouldnt go into the grooves on the cam gear, but i figured it didnt matter and rotated it a little so they would go in. So the cam mark was about 1 tooth advanced. But when i checked to see if the valves were opening at the right time according to the tdc mark on the crank pulley, it turns out that the intake valve on cylinder 1 doesnt begin opening until well after the tdc mark goes by the pointer. Its probably between 1/8 and 1/4 turn on the crank before it starts opening. So i'm wondering, is there a difference in timing marks between euro spec and usdm heads, or maybe between 77.5 and 79 heads? Does anyone have the measurement of how far btdc the intake valve is supposed to open? Any help here is appreciated.
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1996 Honda del Sol Si Turbo
1977.5 Porsche 924 (for sale)

[Paul]

The intake valve on any cylinder needs to be closed as the piston starts its compression stroke which ends at TDC. It must also remain closed during the power stroke that starts as the piston passes TDC.

The exhaust valve must also be firmly closed.
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White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

[fergie]

I mean the other tdc, between the exhaust and intake strokes. One way ive learned to check cam timing is that the intake valve opens right around the tdc mark after the exhaust stroke.
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1996 Honda del Sol Si Turbo
1977.5 Porsche 924 (for sale)

[Paul]

Oh now I see, I thought you still had the cam pointer lined up too!

Remember that the cam only turns once for every 2 turns of the crankshaft so the one tooth may be enough to explain what you are seeing.

I'll go look up the valve spec for you, more later.
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

[Paul]

For 76 to 77.5 with 1mm clearance:

Intake opens 5 degrees BTDC

For 77.5 to 79

Intake opens 6 degrees BTDC

1980 on 6 degrees

Sounds as if your running retarded not advanced.
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

[D Hook]

[fergie]

Well i figured it was supposed to open a few degrees btdc, but the problem is, that with everything lined up where it should be, its not opening until probably 30 degrees after tdc. So yea it's really retarded. But the timing mark must be off. With my old head the timing was perfect when the marks were lined up. I could set the timing just by watching the cam and tdc mark and estimating where it should be, but I dont know how to set it to exactly 6 degrees btdc without a proper alignment mark.

and where the cam timing mark is on my cam gear, it is sort of between the teeth, so its only 1/2 tooth off, and i put it on the advanced side figuring that advancing the cam would be better than retarding it. Its not one tooth off on the crank, but on the cam gear, so it wouldnt be as much of a difference.
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1996 Honda del Sol Si Turbo
1977.5 Porsche 924 (for sale)

[fergie]

I think i might've found the problem. Are there supposed to be 2 marks on the cam gear? I just discovered that both of mine have one on each side of the gear. In my haynes manual it shows lining up the one on the rear of the cam gear with the pointer on the valve cover. But when i did that along with lining up the timing mark on the crank pulley, it was off by quite a bit. Then I found the mark on front of the cam gear, lined that one up and everything seems to be timed good. Is the mark on the back of the gear supposed to be lined up at tdc instead? Ive been using a pink dot that is on my crank pulley, which i thought was a timing mark, to set my timing. My old cam gear also had a pink dot where the timing mark on the front was.

I am pretty sure my timing is good now, because the intake valve started opening just before tdc, but im wondering if its not a coincidence that this random pink dot on my crank pulley works to set my timing, or if it is a legitimate timing mark. Or am i supposed to be lining up the dot on the rear of my cam gear with the pointer when at tdc?
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1996 Honda del Sol Si Turbo
1977.5 Porsche 924 (for sale)

[leadfoot]

Do you by chance have your cam gear on backwards?? Is the wear on the gear different from when you had it installed on the other head??

This would explain your situation, the cam gear dot should line up with the pointer on the cam cover when looking at the front of the car from the firewall side of the engine bay. It should correspond to the flywheel centre timing mark, there will be one either side corresponding to +/- 10 degrees.

To find the true TDC of the crank you either need to dead stop the engine, which you should do at construction so as to not damage your bearings or use a dowel rod and place it down the no 1 spark plug hole, turn the crank over a revolution and find where the piston is at its highest point. this should be close to the factory location if not spot on it.
Leadfoot
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1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...

[fergie]

Each of my heads have cam gears that were already installed when I purchased them, i have not changed the orientation of either one. And with each gear, i have been able to set the timing using the marks stamped into the side of the gear facing the front of the car. Are you saying that possibly the mark on the firewall side of the gear corresponds to the flywheel timing mark, and the front side mark aligns with my timing mark on the crank pulley? On my crank pulley there is one mark that I am almost positive is for cam timing, and there is a notch on the edge of a pulley that should be a tdc mark for cylinder 1. Im not sure about where to find the flywheel timing marks. I was able to get the car running using the crank pulley mark before... how far off would it have to be for it not to run?
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1996 Honda del Sol Si Turbo
1977.5 Porsche 924 (for sale)

[John Brown]

Find TDC through the spark plug hole.

Check that the flywheel shows TDC

Check that the timing mark on the accessroy drive pulley is aligned with the point on the front of the water pump. (Believe it or not, the flywheel can be installed wrong, but the pulley only goes on one way!)

Check the cam orientation correct.

The cam pulley can be installed backwards. There are two marks, one on each side. One is aligned with the valley between teet, the other is aligned with the teeth. Installed correctly the 'valley' mark will be on the engine side of the pulley and will align with the pointer on the cover. (Somebody double check me on this - its the valley mark correct?)
_________________
John
80 931 - #931 44Cup
99 Escalade - tows track cars
gone but not forgotten: original 924.org car - 82

[leadfoot]

If you stand on the brake booster side of the engine bay and look down at the bellhousing there is a trapeziod shaped cover over the inspection hole held in by a small screw. Remove the screw and you will see the teeth of the flywheel. Turn the crank using a 24mm socket on the center bolt with a ratchet and you will see the timing marks appear (there are three) when you are at TDC. There is a line on the bell housing that should come in line with the timing mark.
Cam gear should have "valley dot" for TDC on the firewall side of the engine.
Confirm that the centre flywheel mark and cam gear dot both line up when the dot is in line with the pointer on the cam cover.
If your gear is on backwards this may be more difficult to line up so I would suggest turning it around the right way.
Then when these markers align you remove the no 1 spark plug and stick a rod down it to verify that the piston is at the top of its bore. turn the crank on one direction to see if the rod moves up or down, then reverse the direction and see if it moves the other way. TDC is between the two points at which the piston moves down.
Note that this is a rough indication, but also a cheap and effective and less time consuming way than a dead stop technique.
If it is then you also need to confirm that the valves are both closed by removing the inspection cover on the side of the head or by removing the camshaft cover and checking to see that the tappets and valves are at full height. Look to see that the lobes on the camshaft are pointing relatively upwards.
If all this checks out then TDC has been confirmed and your gear is aligned. If you can't get the dot and flywheel markers aligned then the belt may have slipped a tooth and the belt will need to be taken off and replaced in the correct position.
Remeber that the cam gear turns twice for every one rotation of the crank so find the flywheel markers first.
Then set timing using a timing light on the flywheel markers. Before you start get some paint and mark where the factory setting for your car is, just make a small line in the groove.
With the clamp down nut on the ignition distributor loosened start the car point the timing light at the new marker and turn the ignition distributor body until the marker lines up with the reference marker on the outside of the bellhousing again.
Leadfoot
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1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...

[fergie]

Ok... I think i understand what i need to do. Thanks for all the help.



But the crank turns twice for every one rotation of the cam gear, not the other way around.
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1996 Honda del Sol Si Turbo
1977.5 Porsche 924 (for sale)

[CBass]

Boy was my face red the other day, I put the timing belt back on my 924, lined up the crank pulley mark perfectly with the mark on the front of the cam pulley. Cranked it over and chugga chugga chugga CLANK.

I realized what I had done and set it up properly, turned over and purred, although I do have a little tick now... I'm about due for a new head build anyways.
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'81 931 in various states of assembly

[leadfoot]