Fire LS1, what is done is make a port map. You can find this done on this site as its easy to do. Then you convert this map into square mm graph and count. The purpose of the mean line is to determine the amount to count for the time area. Lets say for a wild and easy example we have a square port of 10 x 40 easy right. 10 x 40 = 400 With a mean line of 50% port area this would be 200 still easy right. Now we go and mess with the port timing. The line is moved up and down. Now to 45 % or 55 %. This area above the mean line has a given amount of flow for a given amount of time as time increases the less work the port can do based on area. There is defined time area for given ports. They all have to play well with each other. The other item to consider with this calculation and I do not widely talk about till now is port roof angle to port cord width at a given mean line area. The angle will play a part in the rpm and peak torque. However we are a long way from that. since you have your cylinder on your
Engine just look exhaust port at BDC and go that's a big hole Now move it to a mean line for plot one or 135 degrees ATDC snap a pic of the port. Now go to a base line setting of plot 3 at 136.5 degrees ATDC. Snap a pic and then over lay the two. Depending on the cord width and angle to the top of the port you will loose a lot of area as it is not square. The only fixed number is from the top of the port to the max cord width. The max cord width may need to be changed to the correct port time open. Then and only then can the rest of the port be brought into line base on time area for a given rpm range. So do we go higher which alters the blow down or do we increase the angle with a wider roof width or both. We need to factor in blow down due to there is limits for given rpm. Yes this is where the pipe comes into play and what about that pressure drop of the case according to TO I had mentioned. So the BD has to factored in to any change prior to cutting. Oh ya we have to determine the Intake and scavenging as well before we consider a change to BD.
If we need more Transfer TA ( time area) do we raise of widen or both. again need to factor in cord width of the transfer ports. If we need to taller to get more TA does this not change BD.
Now you see why the basic plot are important for each. If you did your BD for each plot you should have came up with
#1- 29.5, #2- 28.5, #3- 27
#1 plot shows a retard in timing does it not. So why do we need so much BD for a lower RPM. The ramming effect of the transfer at Pressure release will be late. What if the pipe is fast from the heat at an advanced EO. I would say rut ru. Now if was times correctly than we would have a vacuum in place of no or positive pressure at TO. I would say we need a larger head pipe or smaller stinger or longer LT.
Man that a big assumption just from a plot. however accurate.
So where does the 5 degrees of ignition timing lay into this. Simply put later the fire more heat at EO. oops the EO is earlier now and BD is greater. Now look at EC what did we gain in plot #1, 1 degree woopy do compared to the volume at TDC (Top Dead Center) with the piston above deck .055. Your CR was highly increased way above 6.8. remind me what happens when the pressure rises. Would this be higher heat load not only from combustion but also compression heating. As our K- nock brother say run race gas my man. Oops again, dam port open too soon and the pipe just got quicker. Kiss your intake charge good buy and weak. Would that lead to pre ignition and detonation. Did I also mention the ports mean TA being to small and it continues to lean out at the rpm increases. Yes say hello to mister hole in the piston or four corner or all out right failure.
Agin I went BLB BLA BLA, Sorry just trying to simplify thing a touch as we go and there possible effect.
Now for the good news, Kinda our intake duration went up the down side to this is back wash at low rpm( no valve) why?
we need to look at TO or IC to TO degrees for a given case CR.
Now for a what if on plot #1 we find the time area and area angle for the transfer and work from there. The transfers are usually pretty square and flat at port entry. IF, big IF here, we raise the transfer port 2 degrees we soften the blow back to Case CR problem. We also increase TA for the transfers. We now reduce the BD to 27.5. Now with that in check as long as the exhaust ort TA is in check we can factor in CCR ( corrected compression ratio) and wata bing wanta bang we have a new base line for a power pipe. Now what if its a touch pipey. We simple go in and change port roof angles on transfer and exhaust. This will flatten out the power curve and increase rpm.