torseilertsen

Hi! all.
Working on a VEMS tune project and trying once and for all to figure out the crank degrees from VR sensor TDC trigger pulse kicks in, to the actual crank TDC (visual mark on fly wheel) (No they are not in sync).
I have engine in, maybe someone could check, possobly by counting teeth on the speed ring.
I found the number 44' degrees, but unable to get it verified. that should be approx 16 teeth.
Anyone here who knows?

NCLA951

torseilertsen

That would be great!

NCLA951

torseilertsen

Yes i know.
It is the time/distance/tooth count between the two i need,
Counting: From the macnetic pulse tooth (on single tooth TDC ring) sits right below the sensor, to the actual crank TDC, measured in teeth count on the speed ring, (that sits outside the tdc pulse ring)
The speed ring has 132 teeth, knowing the count of teeth, i can calculate the degrees, and hence the timing.
.
I assume the TDC mark shows actual crank TDC.
I find it strange this is not avaliable, at least i cant ggogle my way to this info

torseilertsen

WOW! i found this (link) https://rennlist.com/forums/944-turb...side-info.html

it says: "132 teeth on the flywheel, ref sensor detects a stud 21 degrees before TDC"
Anyone that could confirm this?

odonnell

Taken from this website: http://www.vems.hu/wiki/index.php?pa...therToothWheel

However, you want the value between the trigger event and true TDC, not the location of the stud to TDC. Because the sensor is not at TDC. What I would do is try 44deg as your Tooth #1 offset and borrow a timing light. Set your timing to be fixed at 15deg or something, so it cannot change away from that advance. Start the car and set the timing light to pulse 15deg BTDC and point it at your alignment mark on the bellhousing. Adjust the 44deg figure until it's on the money.

torseilertsen

Hmm. It allso says 58.6 degrees.

and this says 58,54 degrees.
https://rennlist.com/forums/944-turb...heel-info.html

Am i getting close?

torseilertsen

Hi Odonnell and thanks.
That strategy makes sense for fine tuning, but still getting different 951 degree numbers...
I need the correct number, to start within a safe range, or not?

torseilertsen

btw, VEMS has a "default" for porsche 944 turbo, it puts 80 degrees in the table...
I'm getting confused..

Tom M'Guinn

I don't know where all the oft-quoted offset figures come from, but have never been able to confirm them on actual parts. Not saying any of them are wrong, and maybe they represent a different measurement than what I've been after. That said, it you wrap a string around a 951 flywheel, it's roughly 34.7 inches around, and about 6.25 inches from the TDC line to the center of the reference sensor pin. By my math (6.25/34.7 * 360) that would mean the pin is 64.8 degrees in front of the TDC line. The ref sensor triggers on the falling edge inside the DME (at the 8051 processor anyway, but I can't say for sure if the S100 chip that processes the raw signal in the DME triggers on the rising or falling edge -- though my hunch is that it triggers on the rising edge, then inverts the signal to the 8051, all of which would add a bit to my measured 64.8). (On the other hand, the sensor face is almost 15mm wide, though I assume the signal starts to fall as the pin passes the mid point.) So, those numbers Odonnel posted seem credible to me for a turbo. I can also tell you that my own ignition advance monitor counts 22 teeth (or 60 degrees) between the Ref sensor signal and the spark plug firing, which would equate to roughly 5 degrees of advance at idle -- consistent with my map. The distance from the TDC line to the center of the ref sensor is about 1.4+ inches (roughly). If you add that to the 6.25, you get about 80 degrees. So, depending on how that software handles offsets, etc., that 80 may be just right.

V2Rocket

Off topic But tom what are your afr values to idle at 5 btdc? My na on vems gets really rough/low rpm if its got less than about 17 degrees aiming for 14:1 or so.

Tom M'Guinn

On gas I rely on the O2 sensor to get me to 14.7. My E85 map does not use closed loop, but that's still my target at idle. My timing is single digit advance -- somewhere in the 4-8 degree range at 840+/-, depending on circumstances, and seems to idle just fine. I'd mark the front pulley and check yours with a timing light to make sure you have all your offsets/programming right. What you think is 17 may actually be less?

Raceboy

VEMS 80 degrees is because it has different trigger tooth and that changes the TDC after the trigger value.

Tom M'Guinn

It doesn't run off the stock flywheel and sensors? I thought that was part of the appeal?