Need help figuring out timing related issues
10-04-2014, 11:14 AM
#1
Instructor
Thread Starter
Need help figuring out timing related issues
Cross posted from 944online. Its been quite some time since I last posted to this site.
So, just to clarify, I understand both ignition timing and cam timing. What I am trying to understand however, is adjusting ignition timing on the 944. I am aware of the FQS switch, and I have tampered with it in the past. I had it running @ +3% fuel delivery for a while because my engine was running really lean, due to a multitude of things. I also tried retarding the timing by 3%, but that gave me ignition ping or knock.
Since then, i have completely rebuilt the motor. Rod bearings, piston rings, WebCam camshaft, rebuilt head from R7, to list the majors of the rebuild. Before all of this, an issue I have been chasing down for a couple years now, an intermittent one at that, was that every so often, I would get an intake pop, like a back fire for the intake system, also known as a 'Popback' in other car cultures.
I knew that this had to do with lean running, which is why I adjusted the FQS and tampered with the ignition timing and fuel delivery rate. I was battling leaking valve guide seals, tiring piston rings, just an overall tired motor that needed a rebuild, in short. I figured that with a rebuild, it should take care of that problem. This is what it seemed after everything was said and done.
Well, I have had and experienced Popback 3-4 times now since the rebuild. This only happens when i 'blip' the throttle when rev matching for downshift's. From what I have read, is that it is directly related to timing. Here's where that makes me tilt my head.
In the last 8 months, I have had to establish and recheck my engine timing so many times due to the rebuild, installing the camshaft, doing new lifters, yadayada, I could practically do this in my sleep. Every timing mark used for establishing TDC on these motors is lined up dead nutz. I put my life on that statement. So, knowing that my cam and crank timing is perfectly lined up, and my FQS is at stock settings, how is it that my ignition timing, or even cam timing, could be a culprit to getting Popback? Is it at all possible that my DME/ECU could not be working 100% properly, and once in a while, send the signal for combustion to happen while an exhaust valve is still open due to blipping the throttle? I am just having a hard time wrap my head around this one. Its not like I can just loosen a bolt on the distributor, and give it a little turn to adjust my ignition timing by just an integer, like you can with American cars.
Any ideas are welcomed and appreciated.
So, just to clarify, I understand both ignition timing and cam timing. What I am trying to understand however, is adjusting ignition timing on the 944. I am aware of the FQS switch, and I have tampered with it in the past. I had it running @ +3% fuel delivery for a while because my engine was running really lean, due to a multitude of things. I also tried retarding the timing by 3%, but that gave me ignition ping or knock.
Since then, i have completely rebuilt the motor. Rod bearings, piston rings, WebCam camshaft, rebuilt head from R7, to list the majors of the rebuild. Before all of this, an issue I have been chasing down for a couple years now, an intermittent one at that, was that every so often, I would get an intake pop, like a back fire for the intake system, also known as a 'Popback' in other car cultures.
I knew that this had to do with lean running, which is why I adjusted the FQS and tampered with the ignition timing and fuel delivery rate. I was battling leaking valve guide seals, tiring piston rings, just an overall tired motor that needed a rebuild, in short. I figured that with a rebuild, it should take care of that problem. This is what it seemed after everything was said and done.
Well, I have had and experienced Popback 3-4 times now since the rebuild. This only happens when i 'blip' the throttle when rev matching for downshift's. From what I have read, is that it is directly related to timing. Here's where that makes me tilt my head.
In the last 8 months, I have had to establish and recheck my engine timing so many times due to the rebuild, installing the camshaft, doing new lifters, yadayada, I could practically do this in my sleep. Every timing mark used for establishing TDC on these motors is lined up dead nutz. I put my life on that statement. So, knowing that my cam and crank timing is perfectly lined up, and my FQS is at stock settings, how is it that my ignition timing, or even cam timing, could be a culprit to getting Popback? Is it at all possible that my DME/ECU could not be working 100% properly, and once in a while, send the signal for combustion to happen while an exhaust valve is still open due to blipping the throttle? I am just having a hard time wrap my head around this one. Its not like I can just loosen a bolt on the distributor, and give it a little turn to adjust my ignition timing by just an integer, like you can with American cars.
Any ideas are welcomed and appreciated.
10-04-2014, 12:15 PM
#2
Rennlist Member
I once installed a webcam that had the keyway indexed in the wrong place... I had to mark a new spot on the cam gear for TDC (it was one or two teeth off).
This might be the issue you're having - the "cam timing" is coordinating the valve events with TDC. You might have your valve timing off so the intake valve is closing late, allowing some fuel/air mixture to be pushed back into the intake... which is then ignited from the heat of the exhaust gases from another cylinder's overlap (exhaust and intake open at the same time).
Get a cam index kit and do this:
This might be the issue you're having - the "cam timing" is coordinating the valve events with TDC. You might have your valve timing off so the intake valve is closing late, allowing some fuel/air mixture to be pushed back into the intake... which is then ignited from the heat of the exhaust gases from another cylinder's overlap (exhaust and intake open at the same time).
Get a cam index kit and do this:
