Can Someone Help Me Identify 90 Degrees BEFORE TDC

[Tidybuoy]

I'm in the process of installing Clewett Engineering Cam Sync Adapter and I need to set the engine at 90 degrees before top dead center.

in the photo below, I have the engine currently set at TDC and confirmed with the distributor pointing at cylinder #1

Is 90 Degrees Before TDC at the right of TDC or to the left. I understand the 90 degrees, but am having trouble identifying Before vs After tdc. I don't want to start drilling into my cam until I am absolutely certain.

Thanks!

[Discseven]

Vern, I'm thinking "before" TDC would be to the left when looking at crank from front (fan side) of engine given the crank rotating clockwise... BUT don't drill on my comment alone. Wait for others.

[john walker's workshop]

looks like the pulley has 4 holes drilled in it. the one at 3 o'clock would be 90 degrees BTDC.

[boyt911sc]

Originally Posted by Discseven

Vern, I'm thinking "before" TDC would be to the left when looking at crank from front (fan side) of engine given the crank rotating clockwise... BUT don't drill on my comment alone. Wait for others.

Karl,

Look closely at the black timing pulley and at one point I had similar interpretation until I used a strobe timing light. Ignition timing between Z1 and 3 o'clock marking (hole) is advanced ignition (BTDC) and ignition between Z1 and 9 o'clock marking (hole) late (ATDC).

Look it another way, the pulley is rotating clockwise and imagine it is your clock or timer. Z1 is your appointment time and you arrived at 3 o'clock (you're early) and if you arrived by 9 o'clock (you're late).

Tony

[Tidybuoy]

Thanks! I know what to do now.

PMO/Electromotive project should really pick up speed this week. So far, it's been mostly behind the scenes work (i.e, fuel lines, engine cleanup, wire harness, sound pad, rear suspension bushings, exhaust mod, etc...) Now for the fun part!

[911pcars]

I assume the setting should be exactly 90º? If so, the bolt hole at 3 o'clock is approximate.

One way to be accurate is to determine the circumference of the pulley. Use the formula:

C = 2 pi r

where:
C=circumference
pi = 3.1415
r = radius of pulley or 1/2 diameter

E.g. if pulley is 8" diameter, then the radius is 4", thus
C = 2 x 3.1415 x 4

2 x 3.1415 x 4 = 25.132 (C)
Divide 25.132 by 4 = 6.283"

Thus, 90º from the TDC mark on the pulley = a mark 6.283" on either side of the TDC mark.

Sherwood

[TCracingCA]

Come up with a simple question, just so you can show off/show us your cool new parts!

[Discseven]

Originally Posted by boyt911sc

Karl,

Look closely at the black timing pulley and at one point I had similar interpretation until I used a strobe timing light. Ignition timing between Z1 and 3 o'clock marking (hole) is advanced ignition (BTDC) and ignition between Z1 and 9 o'clock marking (hole) late (ATDC).

Look it another way, the pulley is rotating clockwise and imagine it is your clock or timer. Z1 is your appointment time and you arrived at 3 o'clock (you're early) and if you arrived by 9 o'clock (you're late).

Tony

Had to think about your explanation a bit... then the dawn came. Thanks Tony!

[Tidybuoy]

Originally Posted by TCracingCA

Come up with a simple question, just so you can show off/show us your cool new parts!

Do you think I should use stainless polish on these stack thingys?

[Tidybuoy]

Originally Posted by 911pcars

I assume the setting should be exactly 90º? If so, the bolt hole at 3 o'clock is approximate.

One way to be accurate is to determine the circumference of the pulley. Use the formula:

C = 2 pi r

where:
C=circumference
pi = 3.1415
r = radius of pulley or 1/2 diameter

E.g. if pulley is 8" diameter, then the radius is 4", thus
C = 2 x 3.1415 x 4

2 x 3.1415 x 4 = 25.132 (C)
Divide 25.132 by 4 = 6.283"

Thus, 90º from the TDC mark on the pulley = a mark 6.283" on either side of the TDC mark.

Sherwood

Thanks! I will double check using your formula. Good to have a math wiz on the forum

However, it does not need to be exact. This is because the crank trigger is what the computer uses but because the crank turns twice for all 4 strokes of the cycle, the computer does not know if it's at TDC or 180 degrees off. Therefore, the cam sensor just lets the computer where the cam is positioned during the cycle. It can actually be off by many degrees as the computer just uses the cam info for a reference point (according to the manual).

[scarceller]

Since the pulley already has a 0 (Z1) and a 30 deg mark you can simply use a piece of paper to measure the distance between the 0 and 30 mark. Then transpose marks to the paper and then simply rotate the paper and mark off 2 more points using the paper template. You'll end up creating a 60 deg and 90 deg marks.

But I'd also point out that those current marks on the pulley may not be spot on. I use a special 6" threaded pipe that has same exact threads as a spark plug on one end and a simple balloon on the other end and spin the motor by hand and watch the balloon inflate when it reaches the exact TDC the balloon is at it's most inflated point. This will double check the current marks that are on that pulley. It's not that the pulley is wrong but often every motor is slightly different.

[911pcars]

Originally Posted by scarceller

Since the pulley already has a 0 (Z1) and a 30 deg mark you can simply use a piece of paper to measure the distance between the 0 and 30 mark. Then transpose marks to the paper and then simply rotate the paper and mark off 2 more points using the paper template. You'll end up creating a 60 deg and 90 deg marks.

But I'd also point out that those current marks on the pulley may not be spot on. I use a special 6" threaded pipe that has same exact threads as a spark plug on one end and a simple balloon on the other end and spin the motor by hand and watch the balloon inflate when it reaches the exact TDC the balloon is at it's most inflated point. This will double check the current marks that are on that pulley. It's not that the pulley is wrong but often every motor is slightly different.

Pulley marks relative to the TDC mark assume the factory TDC mark is accurate, and if not, all other marks are also inaccurate. I'm pretty sure the factory TDC mark (v-notch) is created as part of the mass-produced crank pulleys from Porsche's suppliers. Thus, it's just a pretty close approximation of true TDC. The only way to know exactly is to determine true TDC on the pulley installed on the attached engine.

Because the piston can move several degrees while at "TDC", non-mechanical methods of identifying TDC (e.g. whistles, observation) are inherently inaccurate. A mechanical stop is an absolute method.



The homemade TDC tool features a threaded and lockable stop to block piston travel close to TDC. Once set up, rotate the crank in one direction so the #1 piston gently but positively stops against the stop. Mark the pulley relative to the split in the crankcase. Next, rotate the crank in the opposite direction so the piston again stops. Place another mark on the pulley. The midpoint between the two marks is true TDC.

The faint white mark in the photo below represents true TDC. It'll be pretty close to the factory marks. In this case, it's maybe a 1º+/– difference. The same degree of accuracy may also apply to aftermarket pulleys, but it's always better to know than to assume. YMMV. Once performed, apply a permanent ID mark to set cam and ignition timing.


Sherwood

PS: While you're at it, place marks @ 120º on either side of TDC. These marks will be handy for those who position each cylinder at TDC when adjusting valve clearance.

[scarceller]

100% with Sherwood, a mechanical stop is the very best method. I've even seen stops that thread into the spark plug hole.

[304065]

Consider the Stomski TDC indicator or a facsimile

[TCracingCA]

Originally Posted by Tidybuoy

Do you think I should use stainless polish on these stack thingys?

Maybe if I see more cool parts, then I could figure out an answer!

[JJ 911SC]

Originally Posted by Tidybuoy

Do you think I should use stainless polish on these stack thingys?

If you use the rain hat or the sport plate, it does not matter as they will be cover by the filters, but it's looking good.