Z

Apparently sometime in '87 Porsche started to use a belt with some degree of pre-stretching, which would reduce the effect of the cam timing change. Rumor has it that the Porsche timing belts have less of an initial stretch than other brands. I have absolutely no idea if that's actually true or not. Since Porsche is not in the belt production business, and I doubt in the belt pre-stretching business either, I have to wonder if there's really any difference between the belts that the manufacturers make for Porsche, and those that they put their own name on. Everyone did make sure to put their new belt on with the lettering right side up when looking at the engine from the front, right?

Z

But that doesn't necesarily mean that your custom cams have the same lobe separation angle though.

If you know how many degrees of crankshaft rotation is between the exhaust valve closing and intake valve opening events, divide that in half to get that center point I mentioned. Using the GT specs as an example again, the center point would be 4 degrees after the exhaust valve closes, and four degrees before the intake valve opens. That's fixed and has nothing to do with the cam timing in relation to the crankshaft. Follow the adjustment procedure in the shop manuals to get set up, and set the cam sprockets so that the adjustment is roughly in the center of the slots in them. Put a dial indicator on a #1 cylinder intake lifter and slowly turn the engine in the normal direction of rotation while watching the dial indicator. When that intake valve lifter just starts to open the valve, stop and look at the timing pointer on the harmonic balancer. You may have to measure between existing marks on the balancer to determine exactly how many degrees you're at. Let's say the pointer indicates 6 degrees after top dead center on the balancer. You know that the center point that you determined previously is 4 degrees before the intake valve opens. Now you also know that at the current cam timing setting the intake valve starts to open 6 degrees after top dead center. That means with the current cam timing setting the center point is now 2 degrees after top dead center. (4 degrees back from the observed 6 degree after top dead center intake valve opening event) That puts the center point 3 crankshaft degrees advanced in comparison to the GT cam timing specs, and 4.5 crankshaft degrees retarded in comparison to the S4 cam timing specs. From there you can adjust it to where you want.

On the other cylinder bank, put the dial indicator on the #6 cylinder intake valve lifter and use the same procedure. With the firing order of the 928 engine, the indication on the balancer would be the same since both the number #1 and #6 cylinder would be at the top at the same time. The difference would be that one cylinder would be about to start the intake cycle and the other would be about to start the power cycle.

The specs for cam lift and timing are often given at a lift of 1mm. If that's the case, the procedure to determine the center point between the exhaust and intake valve lobes would be the same. It would result in the same physical center point location between the cam lobes, but the degrees of crankshaft rotation would be a smaller number. Then instead of watching the dial indicator to see at what point the intake valve lifter just starts to open, you'd watch the indicator until the lifter opened the intake valve 1mm.

If you do try this, make sure you securely hold the cam when you loosen the sprocket on the front of it. With the engine in that particular position, some cam lobes for some cylinders may be in a position that places them holding some valves partially open against the valve springs. That means that there's the possibility of the spring pressure suddenly rotatingthe cams if you just loosen the bolt(s) that hold it to the cam sprocket. With a pistons close to the top of their travel, any sudden and unexpected cam and therefore valve movements could possibly be not good. It's probably safer to check as indicated above to know where you're at initially, and then turn the crankshaft to close to where the manuals say to adjust the cam timing at. For example, if you want to retard the cam timing 2 more degrees from where it's at, turn the crank to the 20 degree after top dead center position that the manuals state and note the dial indicator reading of the lifter. Then loosen the cam sprocket, and turn the crankshaft to the 22 degree after top dead center position while making sure that the dial indicator reading stays the same, and then tightening the cam sprocket.

drnick

hi z, im still waiting to confirm that the lobes are in the stock axis but i did speak to my mechanic and he went through this with me too. seems the specs for my cams are really not that wild and close enough to the GT and 86.5 on the intake side. his only concern was with the extended duration of the exhaust side, however if we advance the timing as per the GT this should reduce any possible risk of valve to piston contact.

in terms of determining the crank angle between exhaust closure and intake opening, it seems to me that this will only be possible once the cams are installed, with the use of dial gauges. one thing that has me confused is the disparity between the printed specs for stock cam duration as opposed to the timing of valve opening and closure, whats that about?

drnick

here is the spreadsheet data i recieved today from piper cams. the overlap is indicated at 33 deg with full lift timing at 110 deg ATDC for intake and BTDC for exhaust. not sure what the stock full lift timing is though, given that the intake and exhaust are linked by chain.

Z

If the lobes are symetrical, you can calculate it if you know the lobe separation angle and the intake and exhaust duration. With the cam information that you got on that page from Piper cams you don't need to do that though.

I'm not exactly sure what you mean, but the duration is only the period in crankshaft degrees that the valve is open. This is fixed by the characteristics of the cam grind. The timing of the valve opening and closing is what you can set, and is the relationship of the opening and closing events to the crankshaft. Duration is how long they're open. Timing is when they open and close.

You have 33 degrees of overlap according to that information you got. I have to ask if you're planning to try to pass any kind of emissions test with these cams. I'm kind of hoping your answer is that you're not.

Garth S

I've installed the 'dimple' tooth HTD TB made by Optibelt on both the 32v and 16v cars: Through two seasons on the S4 and one on the OB, no loss of tension was noted on recheck intervals. That would be no stretch .... no change ....

drnick

z, no emissions testing needed. the info from piper assumes full lift at 110 deg either ATDC for the intake or ABDC for the exhaust, the difference between them is 180 degrees. however porsche have larger gaps and as there has not been any change to my lobe seperation angle, i calculate the difference will be greater when installed in the standard manner as per porsche giving a difference of aprox 237 degrees. that is max exhaust lift at about 70 deg ABDC and max intake lift at about 127 deg ATDC. maths isnt my strong point but i make my timing figures to then be

intake: opening 0 deg ATDC
closing 74 deg ABDC

exhaust: opening 56 deg BBDC
closing 16 deg ATDC giving 16 deg of overlap!

the discrepancy i mention between published specs for duration as compared with the published valve opening and closing data. take the GT intake as an example; opens 3 deg ATDC closes 42 deg ABDC giving a duration of 219 deg as opposed to the measured 258 deg?

Z

One of us is apparently looking at the wrong numbers somewhere. On the sheet with the cam specs that you posted above, it says that the full lift is 110 degrees ATDC for the intake, and 110 degrees BTDC for the exhaust, not ABDC for the exhaust as you wrote above. With full lift at 110 degress ATDC for the intake, and full lift for the exhaust at 110 degrees BTDC, you'd have a difference of 220 degrees, not the 180 that you mention above.

The exhaust opening and closing events that you gave above are correct. The intake opening and closing events are not correct, because you apparently arrived at them based on the mixup of ATDC for BTDC.

The correct timing figures should be:
intake:
opening 17 deg BTDC
closing 57 deg ABDC

exhaust:
opening 56 deg BBDC
closing 16 deg ATDC

That gives you 33 degrees of overlap. That cam timing also places that lobe center point I mentioned earlier in this thread right at top dead center, which is 1 degree more advanced as compared to the centerline of the stock GT cam timing specs.

The reason for the discrepancy is because of how the Porsche specs are given. The published specs are given at a 1mm valve lift. That means that the measuring of the valve opening period starts when the valve has already opened by 1mm, and that the measuring of the valve closing period ends when the valve is really still 1mm open. At both the 3 degree ATDC opening and 42 degree ABDC closing positions given by the specs, the valve is actually open by 1mm, and not closed completely. The extra degrees of crank rotation required to get the valve open that first 1mm, and to get it closed that last 1mm, is why there's the difference you mentioned. If you don't take this difference into account, and just look at the actual numbers printed, the stock cams will appear to have less duration than they really do when compared to cam specs that don't use those open by 1mm starting and ending points. The duration specs for your cams measure from the actual opening point to the actual closing point of the valves, and not just the duration between those two 1mm open points like the Porsche published specs do. There is less difference in duration between the stock GT cams and the cams you have than just the bare duration numbers would imply.

drnick

quite right z, regarding my incorrect calculations. looking at the timing specs for the stock GTS the manual lists the following:

intake opens 13 deg ATDC
closes 61 deg ABDC

exhaust opens 28 deg BBDC
closes 12 deg BTDC

which i see now are measured at 1mm of lift! from this ive derived full lift times of 127 deg ATDC for the intake and 110 deg BTDC for the exhaust giving a difference of 237 deg. this gives me the same intake times i posted earlier zero deg ATDC and 74 deg ABDC and an overlap of only 16 deg. granted my math were in part faulty before, but is this not what i could expect if set to the stock GTS timings (for full lift)?

if so, should i try any more advance?

slate blue

Hi DrNick, did the cams look real nice after they came back from Piper? Did they assess the hydraulic lifter at all for you? I'm just wondering, if you went to solids, how much extra duration you would get. There is a place in the U.K that sells DLC coated solid lifters for the 928. These will give you 2% extra power due to the lower friction according to the manufactorer. I don't doubt it I have studied a little on the friction front in the valvle train, there is a lot. Plus you may pickup some exta duration.

Cheers Greg

drnick

greg, if you went to solid lifters do you also need shims? how often would you have to check valve clearances? i like the idea of self ajustment with the hydraulics.

Z

I would set them with the full exhaust lift at 110 BTDC and full intake lift at 110 ATDC to start with, like what the cam grinder has on that sheet you have from them. That's probably going to be at least close to optimum, and should also put you pretty close to the point of having the most clearance between the pistons and valves during operation. I would defiitely check the piston to valve clearance to make sure what it is. There's been more than one time where a cam/head/piston combination that somebody thought would work didn't.