Open Discussion-Cam Advance/Retard

[jsawduste]

Let`s have an open talk about the merits of cam advance/retard and how it affects our strokers.

A few givens to start with:
1. Advancing the cam shifts the torque curve/peak down in the RPM range.
2. Retarding does the opposite.
3. Injector timing is controlled by the CMP in the distributor and is adjustable.
4. Ignition timing (pulse) is recognized by the tone ring/CKP and the event/curve is set by the ECM.

If we advance the cam by 4* we can adjust the injector timing by turning the distributor.

Since the cam runs at 1/2 speed of the crank. The intake/exhaust cycle now moves by 8* of crank rotation.

Since we cannot adjust ignition timing the timing event stays the same. In the old days we could adjust the distributor to whatever timing we wanted. Regardless of cam advance/retard.

IMHO opinion I think we are not seeing the full benefit of advance/retard because of the inability of adjusting the ignition timing. While the cylinder fill/emptying cycle changes in relation to the piston position. The fact that we cannot adjust for optimum ignition timing may nullify the full advantage of changing the cam timing.

Comments, opinions, facts ?

[Mgardiner1]

....we can adjust the injector timing by turning the distributor.

False!!

The injector timing is just as fixed as the ignition timing. Both events are triggered by position of the flywheel. the combination of the tone ring inside the dizzy along with the pulse from the crank sensor, determines WHICH cylinder is to be injected/fired next.

[jsawduste]

....we can adjust the injector timing by turning the distributor.

False!!

The injector timing is just as fixed as the ignition timing. Both events are triggered by position of the flywheel. the combination of the tone ring inside the dizzy along with the pulse from the crank sensor, determines WHICH cylinder is to be injected/fired next.

While I agree with what you say that the sequence is controlled by the TR/ECM.

The actual injector timing in relation to the piston location is variable. Is that not what the CMP does ? And turning the dizzy CW/CCW affects how the engine runs due to the fuel being delivered early or late in the intake cycle.

Which leads us back to the advance or retard of the cam.

Please correct me if I am misguided.

[gradon]

What you say is true J. So to sync up the fuel on a cam advanced 4*, you'd set the crank to 8*BTC and follow the dizzy drop-in procedure. To sync up the spark you could use a FTC1 which will let you retard the spark 352*(which is equal to 8* advance). IIRC slotting the CrankPS and moving it in the direction of the passenger will only give 2* advance(supposedly the high alt adj renix cps can give 8*). I want to advance mine 4* to bring the power down a bit, but the DCR would be increased to 8.7 or so and mine is already on the edge. I never thought past that to realize I'd also have to advance the fuel and spark signals.

[jsawduste]

Thanks for the descussion. I`d like to edit your quote so that I understand better.

What you say is true J. So to sync up the fuel on a cam advanced 4*, you'd set the crank to 8*BTC and follow the dizzy drop-in procedure. To sync up the spark you could use a FTC1 Sorry but what is an FTC1 ?which will let you retard the spark 352*(which is equal to 8* advance). IIRC slotting the CrankPS and moving it in the direction of the passenger will only give 2* advance(supposedly the high alt adj renix cps can give 8*) A high Altitude Renix CKP ?. I want to advance mine 4* to bring the power down a bit, but the DCR would be increased to 8.7 or so and mine is already on the edge. I never thought past that to realize I'd also have to advance the fuel and spark signals.

There is a way to mix a renix CKP with a typical OBD 1 CKP ? Confused here ??

Like you I`d like to try and advance the timing. I believe that my DCR with the ovelap in the cam I am running would allow.

Thanks

[gradon]

The FTC1 is a Fuel and Timing Calibrator made by Split Second. There are other piggy-backs and standalones that can adjust timing--it's just one I'm familiar with since I have the PSC1 that uses the same R4 software. AFAIK, you can't use a renix cps in a obd1 or 2--I was just putting it out there as a possibility of advancing the timing on a renix.

[amcinstaller]

if i recall correctly, the renix flywheel/flexplate has the one set of four notches offset a few degrees to signal to the computer the number one cylinder being at TDC. that would make sense as to why you couldnt use a renix crankPS with other computers. i could have them backwards of the styles of flywheel/flexplate though, this is just off the top of my head.

[jsawduste]

This is a challenge that I would like to attempt. Going to look at the CKP and the bell and see what might be done to move the timing around. I suspect there is a bit of HP that can be had by advancing the timing even as little as 2 degrees. Even more so on the bottom end of the RPM range.

My hunch is that having the cam advanced 4* is making the rather inefficient 4.x intake manifold, intake port even less efficient. As there simply is not enough air velocity for decent cylinder filling. Even more so at low RPM.

On the intake event the cam some 8 degrees (of crank rotation) further into the intake cycle in relation to the opening of the intake valve. Granted the exhaust event is happening earlier. Think most would agree that the exhaust side of the 4.0 is the lessor of the two evils. This 8 degrees is at the lowest point of air velocity build up and may be further hampered by reversion.

Need to think about this cam advance/intake event more but am pretty confident that a few degrees of spark advance would help to offset the low RPM laziness the engine has. Granted the cam moves the torque curve down but the torque starts when the cylinder filling begins to become efficient. Due to the long/short runner intake manifold and the shape of the intake ports. The beginning of optimum cylinder filling doesn't start till air velocity reaches a point where intake charge dampens reversion.

Recall now that this is about a cam that would be much more aggresive then a stock 4.0.

Doing a quick look around I am starting to like the Lunati VooDoo cams. Though on a stock head the extra lift doesnt help all that much. Better ports and perhaps bigger valves might make better use of these high lift camshafts. Dropping air velocity by virtue of an "bad" intake manifold and a high lift cam is not the answer to make power. If you notice, Lunati is keeping the duration fairly short compared to lift.

Side note-Does the Hesco crank balancer pully timing set up allow for any changes in intail timing set ?

[eliv1]

using the hesco dampner and pulley, would allow easier modifications to CKP timing mod's.

simply by building an adjustable CKP mount.

[amcinstaller]

Side note-Does the Hesco crank balancer pully timing set up allow for any changes in intail timing set ?

i would think its adjustable a few degrees either way

[Mgardiner1]

While I agree with what you say that the sequence is controlled by the TR/ECM.

The actual injector timing in relation to the piston location is variable. Is that not what the CMP does ? And turning the dizzy CW/CCW affects how the engine runs due to the fuel being delivered early or late in the intake cycle.

Which leads us back to the advance or retard of the cam.

Please correct me if I am misguided.

Maybe i'm just dumb.... but the cam position sensor is HIGH for one full crankshaft revolution (3 fuel injections) and LOW for one full crankshaft revolution (3 more fuel injections). How could the LEADING edge of the cam sensor, which is seen 1 time by the ECU during 720 crankshaft degrees, establish INJECTION timing for 6 cylinders? I need a little help here explaining how that works.

This is copy and pasted from a 1992 jeep FSM on Electical Theory of Operation:

"Camshaft Position Sensor
The camshaft position sensor is located in distributor. This
Hall Effect type sensor works in conjunction with engine speed signal
of crankshaft position sensor providing PCM with inputs necessary to
establish and maintain proper fuel injector firing order.
When leading edge of pulse ring enters the sync signal
generator on the camshaft position sensor, the resulting change in the
magnetic field causes a 5-volt reference signal to be induced. On 4-
cylinder engines, this indicates to the PCM that piston No. 4 will be
the next piston at Top Dead Center (TDC). On 6-cylinder engine, it
indicates that piston No. 3 will be at TDC.
When trailing edge of pulse ring leaves the sync signal
generator on the camshaft position sensor, the resulting collapse of
the magnetic field causes reference signal to drop to zero volts. On
4-cylinder engines, this indicates that piston No. 1 will be the next
piston at TDC. On 6-cylinder engine, it indicates that piston No. 4
will be at TDC"


Here is a picture i'm sure most people are familiar with

[jsawduste]

the cam position sensor is HIGH for one full crankshaft revolution (3 fuel injections) and LOW for one full crankshaft revolution (3 more fuel injections). How could the LEADING edge of the cam sensor, which is seen 1 time by the ECU during 720 crankshaft degrees, establish INJECTION timing for 6 cylinders? I need a little help here explaining how that works.

This is copy and pasted from a 1992 jeep FSM on Electrical Theory of Operation:

"Camshaft Position Sensor
The camshaft position sensor is located in distributor. This
Hall Effect type sensor works in conjunction with engine speed signal
of crankshaft position sensor providing PCM with inputs necessary to
establish and maintain proper fuel injector firing order.
When leading edge of pulse ring enters the sync signal
generator on the camshaft position sensor, the resulting change in the
magnetic field causes a 5-volt reference signal to be induced. On 4-
cylinder engines, this indicates to the PCM that piston No. 4 will be
the next piston at Top Dead Center (TDC). On 6-cylinder engine, it
indicates that piston No. 3 will be at TDC.
When trailing edge of pulse ring leaves the sync signal
generator on the camshaft position sensor, the resulting collapse of
the magnetic field causes reference signal to drop to zero volts. On
4-cylinder engines, this indicates that piston No. 1 will be the next
piston at TDC. On 6-cylinder engine, it indicates that piston No. 4
will be at TDC"


Here is a picture I'm sure most people are familiar with

[/quote]

Wanted to think about this a bit before a reply.

While I understand what you are saying "The high/low of the CMP determines the injector timing per 720* of crank rotation"

"When leading edge of pulse ring enters the sync signal
generator on the camshaft position sensor, the resulting change in the
magnetic field causes a 5-volt reference signal to be induced"-Which would be the high side with #3 at TDC.

Then

"When trailing edge of pulse ring leaves the sync signal
generator on the camshaft position sensor, the resulting collapse of
the magnetic field causes reference signal to drop to zero volts"-Which would be the low side with #1 coming to TDC

I think the key here is the pulse ring entering/leaving the sync signal. The ECM picks up that #3 is coming up to TDC. If we move the dizzy then the ECM will not see that signal till the crank has moved X degrees. Once the ECM see`s the signal it lines up the next 3 injectors to fire. NOT the cam sensor sending a signal to the ECM. So the cam sensor sends a signal every 360 degrees and the ECM is smart enough to fire off each injector in the same sequence of the ignition firing order. As the CMP/CKP/VSS etc all share the same 8 volt reference.

So if you take a running engine and turn the dizzy you can make the engine backfire through the exhaust on one extreme and through the intake at the other. Just like advancing or retarding the ignition timing. Which remains constant because it picks up it`s signal from the tone ring on the crank.

So while I will conclude that the injector timing can be adjusted and doing so adjusts both banks at the same time.

Please correct me if my thinking is out of sync.

Now back to the cam advance/retard

I still maintain that the injector squirts a load of fuel into the cylinder at a point that could be as much as 8* of crank rotation off. Because of the 4* cam advance/retard.

To be continued/gotta run.......

[Mgardiner1]

Honestly i think the injection timing is far less critical then we are making it out to be. Fuel injection an a gasoline engine happens during the intake stroke of the cylinder. There should be no way to advance or retard INJECTION timing enough to get it to backfire through either the intake or exhaust, BECAUSE combustion does not happen until SPARK occurs. I am still sticking to my statement of the computer using the cam position sensor ONLY to determine which set of cylinders the ECU injects next.....

Now if this was a DIESEL engine, fuel injection would be EXTREMELY critical.........

[jsawduste]

Honestly i think the injection timing is far less critical then we are making it out to be. Fuel injection an a gasoline engine happens during the intake stroke of the cylinder. There should be no way to advance or retard INJECTION timing enough to get it to backfire through either the intake or exhaust, BECAUSE combustion does not happen until SPARK occurs. I am still sticking to my statement of the computer using the cam position sensor ONLY to determine which set of cylinders the ECU injects next.....

Now if this was a DIESEL engine, fuel injection would be EXTREMELY critical.........

OK fair enough. That is why I wanted this to be an open discussion.

Talked to Benny today (after my post) and he too could not totally give an honest answer. We did talk about modifying the CKP to perhaps add a little more initial timing. Which with the cam I am running and the 4* advance, the engine might like.

Another point to consider is the octane requirement. A lower octane will have a faster flame front. Which I have seen make more power. For no real reason I have always run 93 octane. Going to try a tank of 91 and see if it makes a difference. Without inducing detonation of course.

[amcinstaller]

if some ecus run bank fire (fire an entire injector bank at once) then injector timing cant be all that critical. i tihnk actually the magic number is in the injector pulse width rather than the timing.