False. Only the few droplets of fuel nearest the spark are ignited. All it takes is one- and the rest takes care of itself. Lighting more than one changes nothing with respect to flame front propogation.

This is comparable to thinking that a BBQ grill burns hotter depending on how the flame is lit.

 

Fuel does not burn in liquid form. So droplets per se are not ignited.
Ever had misfire due to a weak coil? It takes considerably more energy to initiate a spark at high pressures. A weak spark at idle becomes no spark at higher rpms.

 

Are you saying that the compression ratio increases at higher rpm?

 

'Vapour' then.

A misfire is when the spark is too weak to light the 'vapour' at all, not 'just lights some of it'. Old myth.

 

CR is a mechanical ratio. It can't be changed without rebuilding the engine.
Cylinder pressures most definitely do change with RPM. Cam dynamics mostly dictates the change and to a lesser degree blowby.
Density altitude will also change your cylinder pressures.
Hotter day, higher altitude, more humidty, all contribute to less cylinder pressure and less power output.

 

Correct. Although down here we call it vapor.
How is the fuel vaporized for ignition?
Heat.
Heat from what?
Spark plug is one.
And in the localized area of the plug gap the heat is very intense for an intense spark, less intense for a weak spark.
Obviously the plug only initiates the burn. The remainder of the combustion process once the "kernel" of fuel is lit can be said to be dependant on other variables.
However the initiation is all about the spark and it's quality. Sufficient for the job is just that.
More than is needed is not better.
But less than needed is inadequate and leads to misfire.

So how much do you need becomes the question.

As much as I can get to insure good light off for best possible outcome.

 

If the cylinder psi was 150 at an idle, your saying that the psi would be higher as the rpm goes up?

 

Yes.
Up to approximately your max torque produced by your combo.

After that the cylinder pressure will begin to fall as will torque due to lack of time to completely fill the cylinder with each intake charge.

Hp will continue to rise for a period of time beyond peak torque (typically speaking for a street driven car) due to RPM but eventually enough cylinder pressure and torque is lost that it too will begin to fall.

 

On a much simpler note:

I removed the ancient nasty rusty AC spark plugs from my great idling/running transplanted 1970 350 engine (by P.O.).

The ignition system at the spark plug will toss you across the garage (don't ask)!

Then I threw the old AC plugs in the trash...........

......seemd like a good idea at the time

I set Bosh plug gaps at .045 and installed brand-spanking-new "Copper" spark plugs......

now it idles/runs like C-R-A-P!

Since I am on "meds", I could use some advice.

Back to new AC spark plugs..............OR?

 

We agree on that.

 

The cylinder CR is set as well as the valve train geometry, duration. The only variable is the speed at which the piston comes up TDC. This is what increases the psi up to the max torque?

 

As the RPM's rise there is less time and more inertia on the intake charge entering the cylinder.
As the piston rises in the cylinder some of the intake charge is forced back into the intake tract past the closing valve.
This is dependent on the intake valve closing point. The later the intake valve closes the greater possibility for intake reversion or reversal of flow out of the cylinder. It is an inevitable byproduct of closing the valve after the cylinder has started up the cylinder on the compression stroke. Greater speed, ergo inertia on the intake charge can minimize the effect.
So as the speed increases the less time there is for the flow to reverse and leave the cylinder. Since more of the intake charge is retained then more pressure can be created by the greater volume of intake charge available to burn.
Another factor is the overlap period. Although no compression takes place here some contamination of the intake charge from the remaining exhaust in the cylinder can occur. In addition some of the intake charge on the opening intake valve can be drawn out the closing exhaust valve by the "vacuum" created by the exiting exhaust. Both reduce cylinder pressure.
At some RPM the inertial forces of these two events are sufficient enough to reduce losses due to intake charge loss and exhaust contamination.
Exhaust valve closing and intake valve opening times obviously play a big part in how significant and when this event called "full capture" occurs. Again, usually about max torque.

This is how I understand it. If I got it wrong I'm sure someone will chime in for me.

 

It's just that my understanding considered that when a gas mixture gets hot, especially at WOT, the pressure increases. I really thought it was about the physics of adiabatic compression. I guess a very simple way of looking at that is that hotter gas will have a greater pressure.
Anyway

 

The volume of air that enters a cylinders compared to the static displacement of that cylinder is called "volumetric efficiency", or VE.

When you're tuning a fuel injected engine it's all about VE at different rpms and loads, as the VE determines how much fuel the ECU has to command.

At WOT you can see the VE increase all the way to peak torque rpm, and then start decreasing. Even a reasonably streetable engine can have a VE above 100% at peak torque. At the sweet spot there is exhaust scavenging (the speed of the exiting exhaust leaves a low pressure zone after it that helps the next cylinder). There's also a ram effect on the intake charge (speed of intake air makes air continue to enter cylinder even after the piston has passed bottom center and started the compression stroke).

VE is highly dependent on rpm, and VE correlates to cylinder pressure.

 

Points ignition? I think .035 gap may serve you better.

 

OOOOPS...forgot to mention..,aftermarket HEI/tach-drive.....with lots of zap (my forearm still aches!)

I'll re-gap 'em to .035" because of this 10% E-garbage-gasoline that you practically have to shove a lighted match into to get it to burn............and if necessary buy some good old AC plugs like the originals!


My new motto..................................

"Never replace nasty-looking plugs that work perfectly!"

 

Ethanol vapour is just as easy, or is easier, to ignite than gasoline vapour.

Look elsewhere.

 

Thanks.....will do.

 

The ONLY thing you did was change the plugs? Possibility that you mixed up the firing order when putting the plug wires back on?
My personal preference are NGK plugs. Heat range 5 or 6. .045" gap is what I'm running right now.

 

Wow this thread took off, lots of good info there. Back to my original issue, I put in a new 10 gauge wire from ignition at fuse panel to HEI 'BATT' terminal. Then I came back and read this updated thread, especially door gunners very familiar experience (minus the shock treatment) and I'm going to leave well enough alone for the rest of my short corvette season. I'll try the .045 gapped plugs again next spring. Thanks all