Cam advance/retard and fixed ignition timing

[fzsk4p]

Hello again Dart folks.

My plans for the 581 tow rig have been put temporally on hold. Hopefully not for long though.

Like to ask a totally unrelated to Dart engine question. I ask as this forum seems to be filled with darn sharp folks. Pretty sure I can get accurate information here.

The basics,
4.9L Jeep I6 stroker. 4.0L block with 4.2L crank as a starting point
Eagle rods and Diamond pistons (+.080) 9.56 SCR, zero deck and .043 quench
New style intake with a 64mm TB
Edlebrock header
Cleaned up the ports and smoothed out the radius.
Mopar roller rockers
Balanced

The cam I used specs out at
274 Adv. Dur. 224@.050 with a 112 LC and a gross lift of .480
A fairly stout cam for a little 6 cylinder Jeep engine.

Cam was installed 4* advanced in order to bring the torque curve/peak down a bit.

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. ECM limits RPM to 5200 RPM
5. Timing is fixed, both the initial and the curve. OBD 1 system with little aftermarket support.

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

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/retarded cam 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.

Tried to give a good understanding of the basic engine set up.

Thinking about this more I am starting to lean towards the cam advance as not having been a good thing. UNLESS the ignition timing can be advanced. Which with a bit of machine work I may be able to gain 2-4 degrees be relocating the CKP.

What you folks seeing with cam advance/retard and the requirement of changing ignition timing ?

The engine is a little soft on the bottom. Picks up steam at around 2300 and really starts to pull by 3000.

Comments, opinions, facts ?

Thanks MUCH !!

[Awesome Bill]

When you advanced the camshaft, you advanced your ignition timing!. All that port work and large camshaft is the culprit on your bottom end power. Here is the thing you want to try, if the fuel your using is high test, try 87. The compression your running may not warrant this and the fuel burn is a little late if not dead late. If it pings now on 93 pump gas, you may want to try a little water injection because your not able to adjust timing. You may gain a little torque by getting the 2-4° ignition timing but when you advanced the cam you advanced the ignition timing also. The stock map in the ecm is pretty darn good. I have found very little power in race engines once the ignition timing is within 2° if any in some cases. So your not going to find a ton of power here. Your engine was never designed to have all that air, and then camshaft, intake and whatever things you have installed. I doubt you will find much. The burn time of gas is pretty slow so the 87-89-91-93 octane deal is what I would start with.

[fzsk4p]

Thanks for the reply Bill. But I am a little confused.

Let me think about this openly. When the cam went in (Cloyes double roller) the crank went to TDC and I used a degree whee/dial indicator to verify the cam specs. Then cam was rotated 4 " (really 3.7*) using the drilled holes in the cam gear to achieve the advance.

The distributor has the cam sensor which adjusts the injector timing. So the dist. had to be readjusted to fire the injectors at the correct time. A process of moving the dist. back and forth to find the sweet spot on a running engine.

The ignition timing is set off a tone ring on the flywheel. Since the crank did not move but the cam did-with due respect-how could the timing be advanced ?

Yes I have been using 93 and no it does not knock. Good idea of increasing the flame speed by trying a lower octane. Will have to try.

While it is a healthy cam it is hooked to a set of 5.13`s and even with the 37 inch tires it spools up pretty fast. My low range crawl ratio is 96:1 so even with a "soft" bottom end it performs fairly well for what I want.

[Awesome Bill]

If its working for you I would stay with it. The gains would be so small you would not even notice. Your not going to find a ton or lbs for that matter of anything with your combination.

[Moshe1]

sounds good to me

[Awesome Bill]

Thanks for the reply Bill. But I am a little confused.

Let me think about this openly. When the cam went in (Cloyes double roller) the crank went to TDC and I used a degree whee/dial indicator to verify the cam specs. Then cam was rotated 4 " (really 3.7*) using the drilled holes in the cam gear to achieve the advance.

The distributor has the cam sensor which adjusts the injector timing. So the dist. had to be readjusted to fire the injectors at the correct time. A process of moving the dist. back and forth to find the sweet spot on a running engine.

The ignition timing is set off a tone ring on the flywheel. Since the crank did not move but the cam did-with due respect-how could the timing be advanced ?

Yes I have been using 93 and no it does not knock. Good idea of increasing the flame speed by trying a lower octane. Will have to try.

While it is a healthy cam it is hooked to a set of 5.13`s and even with the 37 inch tires it spools up pretty fast. My low range crawl ratio is 96:1 so even with a "soft" bottom end it performs fairly well for what I want.

Is this a batch injection set up? If it is, which I think it is, when you advanced the camshaft, you advance the camshaft signal. I think this is the distributor that has the detent and no adjustment. You can adjust the cam signal buy manipulating the lock. The engine gets is signal from the crankshaft positioning sensor which controls the timing and tells the ecm where the crank is. Being you advanced the camshaft, if the camshaft gear is timed correctly everything should be o.k. This would just fire the injectors in a batch sequence and as long as the fuel is there, when the valve opens, it will work fine. I am not saying you could not improve this thing a little but if your not on a chassis dyno and have a program to change the ecm up, your stuck and could not really know by the seat of the pants feeling unless something gets really bad and then you put it back. A track test just going for 60 times would tell you when the torque was better, or even to the first 300ft.

Sounds like a 1 off thing and your the one to find all this out. When using the stock ecms, I usually keep everything right where the factory has it. If I can purchase a program, load it and play with it, I can do a ton more. But, I have never worked with the old jeep controllers. Let me know how this comes along