Ongoing Detonation Issue Help!!!!!

[Ishiftem]

If you haven't already, download the MSD software and read the instructions in the Help file a few dozen times and play with the dots on the Run Curve Graph.

You will need to drill a hole in the distributor cap to check the rotor alignment and you will probably need to use an adjustable rotor.

If you haven't already checked the rotor blade position it may be part of the problem you have now, as even with the retard curves zeroed out the computer control of timing discharges the spark some amount (a few or several degrees?) after the trigger point.

In production distributors the rotor blade is aligned with the trigger point, but with computer controlled timing the firing point is however many degrees after the trigger point, so the rotor blade is some amount past the cap terminal. To accommodate a large range of timing change the rotor position must be adjusted to compensate for the trigger point offset.

Your modified unit with the Chrysler pickup and reluctor should be arranged to trigger with the rotor in line with the terminals when using analog ignitions with no computer control introducing timing retard, but is it for sure? You need to check this with a timing light and hole in cap, etc.

This situation of the cam walk is throwing a curve ball. You say timing 34 at 1000, but then it retards 3 or 4 and you can see the timing cover bulge, so the result of this is the timing is actually at 30 deg. in the race RPM range.

Are you sure of the compression ratio? Did you CC it yourself?

Besides the cam walk which you can see, analog electronic ignition systems, whether induction types or CD boxes, retard with RPM increase because of electronic slew rate. Slew rate is the time required for electronic components to change state, switch on or off. The system slew rate is constant but the engine speed is not, so if the lag is 1 degree at 1000 RPM the same time lag becomes 6 degrees at 6000 RPM.

Computer controlled timing in ECUs and systems like MSD digital ignitions use a microprocessor which corrects for slow electronics by being triggered in advance of the desired firing point and calculating time vs. RPM for when to discharge. Eazy Peazy This why the MSD Graph View timing curves are called "Retard Curve".

I have not checked rotor position with it running but I will now! Yes 13.3 is accurate. I have checked it myself a couple times. I was extra sure to capture all the crevice volume above the top ring. Since the head gasket isn’t round, I squeezed it between two plates to cc that. I am putting an electric water pump on and since I need to make the ears for the 409, I’m going to space it out enough to put a flat thick bar across the front of the cover. Hopefully that keeps the cam where it belongs. Great info. Thank you for typing that. It helps my understanding a bunch.

[Ishiftem]

Are the chamber and dome of the Z11 engine much different (more development)?

W/r/t "Dynamic compression is 11.3"
Don't know if this addresses your problem?
That's calculated from the .050" IVC point, and it's not correct. It will show higher DCR than actual, and huge CCP.
The IVC position at nominal or lashed point shortens the compression stroke. Try it again with the number on the cam card, or estimate at least 15° later (=+30° on the nominal duration) than you used.


z-11 is the same chamber with the valves rotated slightly. Piston doesn’t have as much squish area as the Z-11 was 12.5:1 and doesn’t have the exhaust valve not in the block like my truck block. Stone cold after sitting a couple days, it cranks at 225. I can’t say what rpm the starter spins at.
Thanks!


I did the rod geometry based on 4.00", 6.385", & 55° and it does return 11.335:1 DCR. Using 1.25 as the ratio of variable heats, CCP: 291 psi.
Run with IVC closing at 70°, it drops to 10.123:1. CCP: 251 psi. At 75° it's 9.662:1. CCP: 236 psi.

I'll do this with your actual IVC if you like. Do you have actual CCP numbers I can us to correct the psi numbers?

[Tuner]

Are the chamber and dome of the Z11 engine much different (more development)?

W/r/t "Dynamic compression is 11.3"
Don't know if this addresses your problem?
That's calculated from the .050" IVC point, and it's not correct. It will show higher DCR than actual, and huge CCP.
The IVC position at nominal or lashed point shortens the compression stroke. Try it again with the number on the cam card, or estimate at least 15° later (=+30° on the nominal duration) than you used.

I did the rod geometry based on 4.00", 6.385", & 55° and it does return 11.335:1 DCR. Using 1.25 as the ratio of variable heats, CCP: 291 psi.
Run with IVC closing at 70°, it drops to 10.123:1. CCP: 251 psi. At 75° it's 9.662:1. CCP: 236 psi.

I'll do this with your actual IVC if you like. Do you have actual CCP numbers I can us to correct the psi numbers?

z-11 is the same chamber with the valves rotated slightly. Piston doesn’t have as much squish area as the Z-11 was 12.5:1 and doesn’t have the exhaust valve not in the block like my truck block. Stone cold after sitting a couple days, it cranks at 225. I can’t say what rpm the starter spins at.
Thanks!

That was the cranking compression pressure of a fresh 1967 427 435HP BBC I measured once upon a time 1973 or so. It had detonation problems because the previous 'tuner' put "all in at 2500" light springs in the distributor and the 100+ octane gasoline was long gone in 1971. I slowed the advance curve so it had 18 initial and reached 36 "total" at 6000.

Because it was a MagnaPulse ignition magnetic trigger distributor I had to use a complete MagnaPulse ignition system, including the wiring harness with ballast resistors, amplifier and coil, in my distributor machine, which was an old Allen that was the simple kind that fires a neon tube with a regular ignition coil, so it could work with the MagnaPulse system.

That particular Corvette was my first step to gaining a practical understanding of slew rate in ignition systems. The first obvious clue was points don't retard after the advance hits the stop and the MagnaPulse (with original slow-switching germanium transistors) retards close to 2 degrees per 1000 crankshaft RPM. ...... WTH?

I think because your 409 has such a high percentage of squish area it has a faster burning combustion chamber than most similar engines of the time period, so it does not need or want as much timing as the BBC.

Another thought, the detonation specks are as likely as not from the locked 34 at 1000 and lugging it on the return road. With open headers you are not going to hear knock if you jab it even a little bit at 2000 RPM.

Do you have a longer duration cam laying around, 270 @ .050" or so?

The Polyspherical Mopar heads, 318, 326, etc, have a fast burn chamber that is allergic to more than about 22 degrees. Some Flathead Fords (depending on the heads) and the Hudson Hornet don't tolerate more than 17.

[Ishiftem]

I don't have another cam. I would like to get a custom cam made but that is going to have to wait. Not enough money and 409 cam cores are about non existent right now. Maybe next winter. The timing makes sense. I mean, if I put fresh plugs in and cut it at the stripe and pull the plugs they are dry. Put them back in and drive back to the pits and they are wet with oil. If say the engine makes peak hp at x amount of timing at 6700 rpm because peak cylinder pressure is occurring at 13-15 degrees after tdc, it would stand to reason that timing would have to be less at 5000 rpm or less for peak cylinder pressure to occur at the same 13-15 degrees after tdc. Am I going in the right direction with my logic?

[Tom68]

Am I going in the right direction with my logic?

You can run fixed timing 34 degrees (or whatever) all the way when you correct that retarding issue.

I'm getting a little concerned you're chasing degrees up and down the rev range but haven't checked rotor alignment, I may have missed it, but are you running a big diameter cap ? if not rotor alignment is critical and if yes, it's still bloody important.

[allencr267]

I have not checked rotor position...

Just look at it.
Rotor & cap terminals will show where it's arcing.
On the very edge of each, NFG.

[In-Tech]

Hiya,
To me it looks like you could "soften" the edges of the pistons and eliminate this problem. A couple tenths of compression is nothing compared to controlled combustion

[Alkyfool]

How about some gas ports or Singh grooves towards the thick side of the piston dome.

[Ishiftem]

Am I going in the right direction with my logic?

You can run fixed timing 34 degrees (or whatever) all the way when you correct that retarding issue.

I'm getting a little concerned you're chasing degrees up and down the rev range but haven't checked rotor alignment, I may have missed it, but are you running a big diameter cap ? if not rotor alignment is critical and if yes, it's still bloody important.

Small GM points style cap. I will have to confirm rotor alignment.

[Ishiftem]

I have not checked rotor position...

Just look at it.
Rotor & cap terminals will show where it's arcing.
On the very edge of each, NFG.

Definitely not off the edge.

[Tuner]

I have not checked rotor position...

Just look at it.
Rotor & cap terminals will show where it's arcing.
On the very edge of each, NFG.

Definitely not off the edge.

When you start using the retard curve feature in the 6530 you need to check it and may need an adjustable rotor.

Do the rules allow computer control of the timing?

[Ishiftem]

How about some gas ports or Singh grooves towards the thick side of the piston dome.

Singh grooves are an interesting thought.

[Ishiftem]

Just look at it.
Rotor & cap terminals will show where it's arcing.
On the very edge of each, NFG.

Definitely not off the edge.

When you start using the retard curve feature in the 6530 you need to check it and may need an adjustable rotor.

Do the rules allow computer control of the timing?

Yes I can pull timing till the cows come home.

[Tuner]

Definitely not off the edge.

When you start using the retard curve feature in the 6530 you need to check it and may need an adjustable rotor.

Do the rules allow computer control of the timing?

Yes I can pull timing till the cows come home.

Have you downloaded the MSD software and played with the GraphView screen to see what you can do with it?

[Ishiftem]

Yes and found a video online explaining setting up the timing curve or I guess more accurately, retard curve. So I can set the distributor to the max advance I want. Then by moving dots on a graph, I can retard from there in tenth of a degree increments. I knew I could take timing out based on time but didn’t know about rpm. The instructions msd sends make no mention of it.