Couple of questions,with this being a quench build, why would the total timing need to be 50 degrees?
This is
NOT Total Timing. Total Timing is PowerTiming and is the sum of the Idle-Timing, plus whatever is in the mechanical at any given rpm, which usually reaches a maximum between 3400 and 3800 rpm
This is
steady-state Cruise Timing. Which is the total of the above plus whatever you can coax out of the Vcan
.
>But before I unpack this; you gotta know the difference between Cranking cylinder pressure, and Running Cylinder Pressure.
>Cranking Cylinder Pressure is what you read on your Compression gauge. It is created as a function of your Static Compression Ratio, your Intake Closing Angle, and the ability of the valves and rings to not leak it away.
>Running Pressure is what you get after start up, and the Hot gasses are expanding inside the chambers driving the pistons down. The amount of Pressure is directly related to the amount and density of the air coming in thru the carb, and the temperature surrounding the chamber.
IDK what your Max Running Pressure might be, I'm not that smart. If I had to guess I'd say 800 to 1100psi, depending on way too many factors for me to understand.
At WOT;
Depending on the load and Rpm, you need to start the fire quite early, to have max Running Pressure occur at the optimum point After TDC, to put the maximum push on the crank at the right point in it's rotation; that is how you achieve max power.
Squish allows you to run a lil more CRANKING Pressure on the same fuel, and that pressure makes heat which makes power. So , as it applies to getting the max out of your fuel, Pressure is the biggest deal,....... right up until you hit detonation.
With iron heads and 91gas, AND tight-Q with a short stroke, 165/170psi Cranking Pressure, might be possible at WOT.
But if your timing is retarded, optimum pressure will never be achieved as the expanding gasses chase after the descending piston.
And if the timing is over advanced, then the pressure will be too high, too early, and the crank will offer too much resistance, and so between the the heat of compression, and the too high chamber pressure, the Running Pressure might rise beyond what the fuel can sustain, and pow!, you get detonation.
So you have to limit your Max Running Pressure to whatever will NOT cause detonation at WOT, with the gas you are using.
Cruising at Part Throttle;
At this time, your Running Cylinder Pressure will be only a fraction of the Max Allowable before detonation. Therefore; you could probably run 200 or even 300psi cranking psi IDK. But you couldn't drive that engine because it will rattle the skirts right off the pistons when you pour the coals to her.
Cruising at 2400; if you are NOT at WOT, then your engine will not achieve max pressure, because it is not inhaling a max load of air, so the RUNNING Pressure is well below the detonation limit, or is supposed to be,lol.
So now, we can talk about Cruise Timing:
Since the Running Pressure will be so low, because;
1) the carb is heavily throttled, and
2) the rpm (2400) is so low, and
3) the Ica is barely trapping what it is getting... and
4) in all likelyhood you are gonna try and run this beast as cool as possible, and
5) the Scr is predicted to be just 9.5
This will require an exceptionally early ignition event, so that meager max pressure can arrive on top of the piston, at the optimum point in it's rotation.... which never changes..and is always in the neighborhood of 25 to 28 degrees ATDC. That is how you achieve max fuel economy.
When you start the fire is of NO CONSEQUENCE, the important thing is to have max pressure occur at the right time. (See note 1)
You could get better fuel economy;
with more Running Cruise Pressure, or
a bigger piston to create more downforce, or
a longer stroke, or
you could just run it hotter, or
install a cam with an earlier closing intake to trap more pressure.
And then, hitching the new engine to a lower cruise rpm.
But the question then becomes; how small an engine can you build, and still have adequate acceleration and reserve power; Power to pass, to climb, to operate at a higher altitude, or to keep up in traffic.
See note 1
When you start the fire is of NO CONSEQUENCE, the important thing is to have max pressure occur at the right time.
How can you know when you are there? That's what I wanted to know. So I got me a stand-alone, dash-mounted, dial-back, timing retard computer with a range of 15 degrees.
So then, I got my car up to my chosen cruise speed. And when it had settled in to it, I read the roadspeed, then retarded the timing 3 degrees, expecting a LOSS of efficiency and so a reduction in roadspeed. And that is what happened. So then I put the 3 degrees back in plus 3 more, expecting an increase in efficiency, and so an increase in roadspeed. And that is what happened.
And so that is how it went all summer. Then in fall, I pulled The D out and modified the Vcan to get me the timing she wanted. Then, next summer back at it I went.
Yes I was serious about fuel economy, because I intended to drive this car until I died. That was 20 years ago..... But I outlived the body, which is now succumbing to the elements. I did manage so far to accumulate about 130,000 miles on it tho, some of which cruised at up to double the mpg, of what most other guys can get out of them.
Note 2
where do you start? I started by reving it up in neutral, to my chosen cruise rpm and then just pulling in the timing, while simultaneously reducing the throttle to maintain that rpm. When the rpm no longer increased with additional timing, then I knew the engine was fighting itself. I backed up 3 degrees and that is where I started.
Trying to do this any other way is an exercise in frustration, and could take years. Especially, if, like me, you tend to run several rear gears or transmissions, even in one summer,lol.
