Engine/Head efficiency and ignition timing
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- BigBlockMopar
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Engine/Head efficiency and ignition timing
What makes a head (or engine setup) more efficient in regards that it can get away with less ignition timing?
Reason for asking;
I've had a fairly low cr, stock roller cammed, mid/late-'80s 318ci in my daily '73 Dodge Dart with '302' casting heads, known as 'swirl' heads.
CAI, Edelbrock RPM intake, full length headers, full 2.5" exhaust, X-pipe, Borla ProST Mufflers.
With MegaSquirt digital ignition and some chassis dynoruns, I found max 32° advance at 2800rpm would make most torque (fuel is LPG/Propane).
Currently I have in the car, a 11.3:1cr 360ci, KB-pistons, balanced, CompCams XE256H hydraulic cam, same small 318 casting '302' heads, but chambers polished shiney, sharp edges removed, deep portmatching and milled to 60cc chambers. Quench height is near 0.029-0.030". 208-210psi CC.
CAI, Edelbrock AirGap intake, same exhaust system.
Even though the 360ci has almost 3 points higher compression ratio, I can't help noticing (butt-dyno) the 360ci likes more overal timing than the 318ci, as I've been creeping up the advance at various points in the ignition table and found better performance most of the times.
Haven't taken it to the chassisdyno yet but will do so soon.
I would've thought with same/similar heads, timing requirements would be rather equal, and maybe even limited or less because of the higher compression ratio and smoothed chambers.
So, have I 'screwed up' (the 'swirl' in) the heads by polishing the chambers or removing any sharp edges?
Is the high compression (close piston to heads distance) making the heads less efficient perhaps?
Or is the (tame) camshaft making things appear less efficient?
Reason for asking;
I've had a fairly low cr, stock roller cammed, mid/late-'80s 318ci in my daily '73 Dodge Dart with '302' casting heads, known as 'swirl' heads.
CAI, Edelbrock RPM intake, full length headers, full 2.5" exhaust, X-pipe, Borla ProST Mufflers.
With MegaSquirt digital ignition and some chassis dynoruns, I found max 32° advance at 2800rpm would make most torque (fuel is LPG/Propane).
Currently I have in the car, a 11.3:1cr 360ci, KB-pistons, balanced, CompCams XE256H hydraulic cam, same small 318 casting '302' heads, but chambers polished shiney, sharp edges removed, deep portmatching and milled to 60cc chambers. Quench height is near 0.029-0.030". 208-210psi CC.
CAI, Edelbrock AirGap intake, same exhaust system.
Even though the 360ci has almost 3 points higher compression ratio, I can't help noticing (butt-dyno) the 360ci likes more overal timing than the 318ci, as I've been creeping up the advance at various points in the ignition table and found better performance most of the times.
Haven't taken it to the chassisdyno yet but will do so soon.
I would've thought with same/similar heads, timing requirements would be rather equal, and maybe even limited or less because of the higher compression ratio and smoothed chambers.
So, have I 'screwed up' (the 'swirl' in) the heads by polishing the chambers or removing any sharp edges?
Is the high compression (close piston to heads distance) making the heads less efficient perhaps?
Or is the (tame) camshaft making things appear less efficient?
Last edited by BigBlockMopar on Tue Jul 31, 2018 6:55 pm, edited 1 time in total.
Re: Engine/Head efficiency and ignition timing
Do you have pictures of before and after the port and chamber work? Are these the heads in which the valve stem is not visible from the intake manifold flange and have a corkscrew spiral ramp in the valve bowl?
Re: Engine/Head efficiency and ignition timing
Stock valve sizes? I have one set of those and I am not sure why they call them swirl heads they should have called them quench heads since they have an actual quench pad. I do not think you changed the swirl much with polishing. This design head will have a different spark advance curve than a non quench head. I have had the misfortune of tuning till you hear a ping and then back down a bit. I had more than one engine that you coukd not hear a ping but on disassembly there was evidence that the timing was more advanced than needed. Be sure your timing marks are right on the $ too. I can't see the 360 wanting much more advance than the 318.
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Re: Engine/Head efficiency and ignition timing
The 302 castings I have are not screw port. Very small straight port. Tiny valves.
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Re: Engine/Head efficiency and ignition timing
A whole LOT depends upon the fuel being used and the amount of time taken to get to maximum cylinder pressure with that particular cylinder volume at that point.
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Re: Engine/Head efficiency and ignition timing
Stock small valves in both sets of heads.
At the time I wasn't quite sure if I wanted to keep these heads on the engine or upgrade to aluminium heads later on.
Here is a before pic of one of the chambers (I had just started polishing when I thought to take a pic for a before/after comparison;
Here's an after pic of both heads when I was all done 'wasting my time';
At the time I wasn't quite sure if I wanted to keep these heads on the engine or upgrade to aluminium heads later on.
Here is a before pic of one of the chambers (I had just started polishing when I thought to take a pic for a before/after comparison;
Here's an after pic of both heads when I was all done 'wasting my time';
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Re: Engine/Head efficiency and ignition timing
The fuel system (propane with 425 Impco carb) was unchanged on both engines.
A change in cubic inches, a big change in compression ratio and cranking pressure were the major changes.
NGK Iridium sparkplugs were used too.
A change in cubic inches, a big change in compression ratio and cranking pressure were the major changes.
NGK Iridium sparkplugs were used too.
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Re: Engine/Head efficiency and ignition timing
Do both pair of heads have "Singh Grooves" ...?bigblockmopar wrote: ↑Tue Jul 31, 2018 7:16 pm The fuel system (propane with 425 Impco carb) was unchanged on both engines.
A change in cubic inches, a big change in compression ratio and cranking pressure were the major changes.
NGK Iridium sparkplugs were used too.
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Re: Engine/Head efficiency and ignition timing
A bigger plug gap will require less total timing.
Because it'll burn quicker.
Quicker means the time taken from the spark to peak cylinder pressure.
In order to make use of a bigger plug gap, you need an ignition system with the max energy possible.
That usually means getting rid of the distributor.
Afterall....compressing an already burning mixture seems a bit silly?
Because it'll burn quicker.
Quicker means the time taken from the spark to peak cylinder pressure.
In order to make use of a bigger plug gap, you need an ignition system with the max energy possible.
That usually means getting rid of the distributor.
Afterall....compressing an already burning mixture seems a bit silly?
Re: Engine/Head efficiency and ignition timing
Agree with the above post, gaseous fuels, propane and natural gas, require more spark energy than gasoline, particularly natural gas.
Is it the same A/F ratio as before? Flame speed varies with A/F and leaner A/F will require more advance.
Noble metal spark plug electrodes have been proven to encourage preignition. Propane is high octane but is more susceptible to preignition than gasoline, somewhat like methanol. Natural gas is 130 octane but very susceptible to preignition. When preignition starts in a NG engine it can be so violent and the preignition can advance so rapidly it will stall the engine in just a few firing cycles and nearly make the engine turn backwards. Standard nickle alloy electrode plugs are probably a safer choice with propane.
The experience I've had with propane is it needs more initial but less WOT and heavy load advance, and more light load vacuum advance than gasoline. The light load advance is dependent on the A/F, and propane (and particularly NG) can run so much leaner than gasoline it will need more advance if it is run at or near the lean limit. Lean at part load will reduce engine temperature (more or less because the fire is smaller) if proper advance is used.
Is it the same A/F ratio as before? Flame speed varies with A/F and leaner A/F will require more advance.
Noble metal spark plug electrodes have been proven to encourage preignition. Propane is high octane but is more susceptible to preignition than gasoline, somewhat like methanol. Natural gas is 130 octane but very susceptible to preignition. When preignition starts in a NG engine it can be so violent and the preignition can advance so rapidly it will stall the engine in just a few firing cycles and nearly make the engine turn backwards. Standard nickle alloy electrode plugs are probably a safer choice with propane.
The experience I've had with propane is it needs more initial but less WOT and heavy load advance, and more light load vacuum advance than gasoline. The light load advance is dependent on the A/F, and propane (and particularly NG) can run so much leaner than gasoline it will need more advance if it is run at or near the lean limit. Lean at part load will reduce engine temperature (more or less because the fire is smaller) if proper advance is used.
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Re: Engine/Head efficiency and ignition timing
A/F ratios appeared very similar between the engines. But the car's AFR-gauge or sensor usually craps out after some driving time so I can't keep an eye on it anymore when the engine is fully warmed up, unless I change the O2 sensor, again.
The quench pad grooves are only on the heads in the 360ci engine. I haven't opened up the old 318ci yet any further than the intake.
Ignition system is a HEI-7 with a MSD Blaster SS, timing and dwell controlled by MegaSquirt. Stock 'lean burn' Mopar distributor, brought over from the 318ci.
The same Iridium sparkplugs were also used and later transfered from the 318 to the 360.
But there's something I had noticed when I changed from regular Accel 134 or 137 sparkplugs (.035 gap) to the Iridium ones, I was able to give the 318ci more idle timing before idle quality got worse. The 318ci idled best with 24° advance in Drive before it started to get worse from 28 and higher. With the Iridiums 24° was still good but, upto 30° advance, the idle was still good.
I don't think the plug gap was different on the Iridium plugs. Upon installation I just checked them for gap consistency but didn't change them.
Some pics;
This is how the plugs look after about 2 months of daily driving in the 360ci.
This the ignition advance table currently in use on the 360ci;
(The last 4 columns are 3000 - 4000 - 5000 - 6000)
Compared to the ignition table I used before on the 318ci engine:
The quench pad grooves are only on the heads in the 360ci engine. I haven't opened up the old 318ci yet any further than the intake.
Ignition system is a HEI-7 with a MSD Blaster SS, timing and dwell controlled by MegaSquirt. Stock 'lean burn' Mopar distributor, brought over from the 318ci.
The same Iridium sparkplugs were also used and later transfered from the 318 to the 360.
But there's something I had noticed when I changed from regular Accel 134 or 137 sparkplugs (.035 gap) to the Iridium ones, I was able to give the 318ci more idle timing before idle quality got worse. The 318ci idled best with 24° advance in Drive before it started to get worse from 28 and higher. With the Iridiums 24° was still good but, upto 30° advance, the idle was still good.
I don't think the plug gap was different on the Iridium plugs. Upon installation I just checked them for gap consistency but didn't change them.
Some pics;
This is how the plugs look after about 2 months of daily driving in the 360ci.
This the ignition advance table currently in use on the 360ci;
(The last 4 columns are 3000 - 4000 - 5000 - 6000)
Compared to the ignition table I used before on the 318ci engine:
Re: Engine/Head efficiency and ignition timing
Those tables are only appreciably different between 1000 - 2000 rpm.
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Re: Engine/Head efficiency and ignition timing
1000-2000k is about 90% of the operating range of the engine in daily use, so that's where the focus of the timing map lies.
Max. advance is '1 column' earlier in the top map too, so earlier than the old engine.
There's also about 4-6 degrees more advance below 1000rpm in the mid-load ranges where the engine idles.
I'm currently experimenting with more light-load advance for economy reasons, which is also a thing this 360 engine seems to like more than the 318.
I used to limit the advance at 42° with the 318 because no improvements were found, but am using 45° now with the 360ci and might try some more. This seems to be approaching the gasoline 'vacuum-advance' limits at around 50. Propane usually doesn't like or need that much.
Max. advance is '1 column' earlier in the top map too, so earlier than the old engine.
There's also about 4-6 degrees more advance below 1000rpm in the mid-load ranges where the engine idles.
I'm currently experimenting with more light-load advance for economy reasons, which is also a thing this 360 engine seems to like more than the 318.
I used to limit the advance at 42° with the 318 because no improvements were found, but am using 45° now with the 360ci and might try some more. This seems to be approaching the gasoline 'vacuum-advance' limits at around 50. Propane usually doesn't like or need that much.
Re: Engine/Head efficiency and ignition timing
What process do you use to optimise light load ignition advance?
Re: Engine/Head efficiency and ignition timing
With longer duration cam it's perfectly natural that low-rpm ignition timing has to be more than with lesser duration cam. WOT VE will be less, and there's still plenty of exhaust gas left until rpm rises to cam's operating range.
With wild cams there's usually need to much more ignition advance at lower rpm range WOT than higher, this two-way rpm correction was pain to achieve with mechanical distributors - with ecu controlled ignition easily solvable problem.
With wild cams there's usually need to much more ignition advance at lower rpm range WOT than higher, this two-way rpm correction was pain to achieve with mechanical distributors - with ecu controlled ignition easily solvable problem.