thanks a lot for the info. I'll keep trying. maybe try p&g.

installing that mpg gauge looks a little complicated for me, i dunno I'm sure I could figure it out eventually if I could find one.

Quick explination on that... At 15-20 degrees after TDC is when the connecting rod is at a 90* angle to the "lever arm" created by the offset of rod journal to crank journal. At that point the rod has the longest "lever" to push on to turn the crank, and thus makes the most torque for a given pressure.

For what it's worth, you will likely never be able to get the same fuel economy after installing larger injectors for one simple reason: Reduced metering resolution. Quite simply, at idle the larger injectors will have to deliver more fuel. Unless you can dial back the fuel pressure at idle, you likely can't reduce the pulse width enough to fire the injectors reliably without running on the rich side.

o is that where it's at 90 degrees. give or take for different motors though right? I though is farther down. that makes more sense.

I have a set of 450cc injectors in my car, but I'll easily able to lower the pulse width enough to idle at 14.7 at 40psi of fuel pressure. I can even run way leaner at idle like 20.1 at 35-40psi.

Have you or others verified that via wideband?

The problem I ran into with 550cc injectors on a 1.6L was that the injectors would fail to reliably open for the pulsewidth desired. In other words, I could program it into the map, but there was not enough current dwell time to make it actually happen.

yep, I have a PLX M300.

hmm, 450's support more then effort HP that I want right now so, I'm good, but that's interesting. if I ever up grade. how low did you go on fuel pressure?

I think mine only went down to 45 psi. Now that I think about it, the brand of injector may make a huge difference on this issue however. Not all injectors are built the same, so perhaps the picture I have painted is more bleak than warranted. Certainly I was dealing with 22% more flow at the same pressure over 450s.

another thing that I don't understand is, the pressure inside the cylinder before the plug is ignited is relatively low. when it does ignite and starts to burn, the pressure starts to climb right immediately. so wouldn't that pressure start to push to piston down, even before TDC?? or is force, forcing the piston up is so great, even the peak pressure wouldn't stop it right then?

so, the peak happening at that 90 degree angle, if it happens any where else the total force applied to the crank during the combustion stroke would be less?

I saw a chart either on here or another site on how a gas engine works. It takes time to build power and the best use of the engine is either 2000 rpm or lower and 3000 rpm or higher. Most cars are tuned to run in the 2000 to 3000 rpm range as this works best for city or highway driving. Its not good for mpg however. So the longer i stay under 2k or above 3k the better it is.
I tried early on to shift at 3k or higher with ok results. Better was of course under 2k. The reason is that at 3/4 wot under 2k is the best way to convert gas into forward motion. If you look at my garage chart you can see how i have improved. Launching a front wheel drive car is best at about 1200 rpm, i use this to start when the engine is warm and just "puff shift" through the gears staying under 2k until 5th. I have found when starting the first time when the engine is cold that i can get into 5th gear and cruise at 23mph with my foot off the gas, then into eoc. By the time the engine is warm i have traveled about 2 miles. My mazda at 65 is tacking 3100 to 3200rpm. I usually go 55 to 57mph at 2700 rpm or less depending when i draft and draft with eoc. Hope this helps.

The pressure does start to climb immediately after spark, however the upward force exerted by the other cylinders is far more than enough to keep it from trying to stop. And while the 10-15 degrees of burn between spark and TDC are technically wasted, that is also at the low end of the pressure cycle of burn, so you waste the early, light pressure so that you can better capture the mid burn, high pressure stuff.
If the spark happens at TDC, while you're capturing the early burn pressure that would have been wasted, the real strong stuff about halfway through the burn would be right near the end of the power stroke, and pretty much wasted.

The second question... The total force applied to the crank would be the same, however, with the rod being at 90* to the crank/rod journal lever, that's where most of the force is converted to rotational torque and spinning the crank, rather than the pressure passing through the crank to the main journals and being turned into vibration.
Like if you take a wrench, and turn it with some set force, you spin the nut. However, if you were to put the wrench on the nut and instead of applying the force perpendicular to the wrench, you apply the force in line with the wrench, you'd just transmit the force directly to the nut in a futile pushing manner. Same basic thing.

Technically with the adjusted ignition timing, the total force wouldn't be identical, what with some burn pressure being wasted near the top of the compression stroke, or if it's too late of an ignition, some burn pressure still occurring in the exhaust stroke... But that doesn't matter with the question we're dealing with.

It all boils down that the forces will be the same, however with properly tuned ignition timing, more of that force will be converted into work (rotational force), rather than waste vibration.


Edit: As I forgot to answer one of your earlier questions...
No, if the peak pressure ends up occurring during the compression stroke, that won't stop the pistons... It'll break them. That's called Detonation, and it usually occurs because the mix is too lean, so that little amount of gas just burns right up, very fast. It usually ends up doing nasty things like blowing head gaskets, breaking holes in pistons, and otherwise doing very bad things to the engine.