Now, this is like Gossip, I've already told you more than I know myself.

 

Ok, everything you guys said makes perfect sense. Obviously ignition timing would have a major role in this, also the AFR determining how fast it burns.

The only thing that one would think on a theoretical aspect, wouldnt it be ideal that you make the force contact the piston at exactly TDC or a couple * after?

Because when the piston is at its highest point that is when compression is peaked, so? i dont get why you would want the force to start exertion at 14 or 15 vs, exact TDC or a couple degrees after. I am sorry i just happen to be curious and want the full break down Also i dont see how piston speed would be relative to when you would want the exertion on the piston it would still be fundamentaly the same, you would just need to compensate with timing / AFR to get it to hit the piston at that certain timing being X * ATDC.

 

bring up another question, i know different piston speed would requier different ignition timing and AFR to make that ideal force target of hitting the piston at 14 or 15* ATDC, My question is WHAT changes the ideal point of force exertion on the piston from the combustion flame front. like making it go from 14 or 15 ATDC to something like 10 or 20 whatever? You would think it would be a constant in basic engine theory?

 

kind of like the swing aspect, lets say you push somebody on a swing, and to get the most efficient swing out of it, you want to make contact and start pushing when the swing rope is at say a 45* angel,

No matter how long the swing rope or how fast the swing is going, wouldnt it always be a 45*? yes you would have to start to lean in to the push sooner, but your goal would be to make contact with the swinger's body while he is at a 45* angle and about to start swining back the other way. Thats what im fundamentally asking about the ideal peak pressure point of 14 or 15,

Wouldnt the ideal peak pressure goal always be the same regardless of piston speed, stroke whatever. How does that 14 or 15 change? lol damn i really need to learn to ask question in a few sentinces heh.

 

No....think about it. At TDC, your connecting rod is completely vertical. Having a burn at this position ( 0* ) would be putting all of the force right into the pistion and rod, possibly breaking either. The crank doesn't get the power until the crank rotates some so that the rod is pusing the crank in a turning motion. Do you understand what I am saying?


Eric

 

If you think about it in slow motion, the best place would be when the crank/rod were at (3 o'clock) atdc. That is when all movement of the rod would be in a positive direction.
Now back it up some, say 1 o'clock, some of the energy is moving the rod out towards the sides of the cylinder.
At 12 o'clock, the pressure is going straight down the rod into the crank, without imparting any rotational movement. Try pointing your finger straight up and hit it with a hammer as if it were a nail. Did it hurt or did it swing your finger?

The problem with pp at 3 o'clock is the piston has lost half of its potential range of motion. If you push there a lot of energy is wasted out the exhaust.
You need to make contact soon enough that at least some power is pushing as soon as possible, and continues through the down stroke.

 

Where would 15* be at on a clock scale...what about 0100 right? maybe a little more?

 

I'll just add a few more things to the equation. A longer stroke motor has higher piston speeds, since the piston must travel FURTHER during each cycle. With a higher piston speed, you have less margin for error in getting the exact, perfect timing. Remember, the same AFR burns at the same rate, and your goal is to hit peak pressure at 14 degrees after tdc. Now, add the fact that a stroker motor ususally has shorter rods, which causes more side load pressures on the cylinder walls, which increases friction on an already 'speed increased piston'. More side load means more potential to break piston skirts, scrape cylinders, etc. Many things to consider before stroking a motor, or for that matter, tuning it. Also, do some reading on the magical 1.8 numbers and ratios.

 

1.8 or higher...F1 and 4:1 RS ratios...but thats another topic for another time.

 

OHHH this is getting interesting i want to know more I do get it now thanks guys. and ya now i get why the 15* position would be the ideal, btw to compare a clock and * all you do is realize there are 12 hours in a clock rotation and 360* in a full circle so divide and you get 30* per hour, so 15* would be about 12:30 on a clock, that seems to be pretty close to my original though, that seems to only be a little bit after TDC if you think about it on a clock scale. 15*=12:30

 

wow guys - vote for sticky, great information - a lot of things beginning to make sense now...

 

This article covers a lot of the stuff mentioned here

http://www.innovatemotorsports.com/resources/myths.php

 

That is a good artical well worth saving to your favorites. Now if we think about it, why not run a ridiculously lean AFR with a ridiculously retarded timing curve? It would be so awesome if there was an exact formula to figure out when you hit peak pressure with the, a certain timing, AFR, type of gas, combustion design, and specific rpm/load. Hmm tuning would be champion status if we knew what timing/AFR was needed to hit peak pressure.