"H", I have not checked this Wet spot as I do not want to open it up at LS. To get an accurate
WOT (Wide Open Throttle) wet line I would need to do a high speed pass under load at high
Engine temp. I could do some here however they wouldn't be very accurate. I may do some testing around here for something to do when time permits.
I also played with the numbers to see where the UCCR would be if I bumped the static to 170. The UCCR would be right over 13.2 to 1 for that Engine. If we look at a stock and slight modified exhaust port the UCCR would be right at and under 11 to 1 for a static of 170. So I do not think 170 would be advisable for pump gas for this Engine configuration. I could discuses that more if you would like or anybody is interested.
I would like to share a little more on the pictures. This time looking at the gasket change and why.
We have talked about raising the CR and I had full attention to do so just not sure yet on how much. Changing the gasket did in fact achieve a change in the CR. It did this by way of the gaskets thickness and ID (inside diameter). The change accounted for 1.94 cc's or -1.94 cc from the equation. So if we look at the UCCR (un-corrected compression ratio) which consist of the bore, stoke and area above the piston at TDC (Top Dead Center) the UCCR has changed or in this case increased. The CCR (corrected compression ratio) has also increased for the same reason as the UCCR. The area above the piston at TDC (Top Dead Center) has been reduced. The bore, stroke and effective stroke (the distance the piston travels after the exhaust port is closed off by the piston) as remained unchanged.
So let’s look at the numbers:
(1) Cometic #1: UCCR = 11.3255, CCR = 6.4268
(2) Athena #2: UCCR = 11.8669, CCR = 6.7113
UCCR change + .5414
CCR change + .2845
The head gasket used which we will call #1 was a Cometic. Not a real big fan of these however there cheep and do seal good. They have their place in the pecking order though. For my application it was not a great choice. For testing though it served its purpose. The #1 gasket has a compressed thickness of 1.1 mm and an average ID of 82.55 mm. You notice I said average ID and that is one item I do not really care for when it comes to the Cometic. If you measure it in an X and Y orientation than you will notice it varies in width or is oval. This gasket out of four I use has the greatest amount of out of round. Not a real big deal however noticeable. The Cometic adhering properties makes it hard to get the head off if you leave the cylinder on the Engine and take the most about of clean up and prep for a new gasket. This could lead to cylinder or head matting surface damage or damage to the head it’s self depending on removal method. Gasket #2 is an Athena and has a compressed thickness of .71 mm and an ID of 81.3 mm. I like this choice due it matches the 80.5mm bore better and is close to a stock head gasket. One can look at this like reducing the head volume by 1.94 cc with the exception that the heads physical shape and configuration has not changes. If this was a stock head than removing that amount would not be a big deal as one could simply mill the head about .5 mm since the stock head has a recessed pocket (see pictures) of about1.9 mm which accounts for about 9.8 ccs. The ID of this pocket is about 81 mm. Since MY head has been milled right at 2 mm’s flush with the squish band already and the squish area ratio and combustion chamber has been modified and is fixed, I do not want to touch it YET so I can use my existing squish data for that head. Looking at the pictures of the static gauge reading you will notice it did not change hardly any if any at all even with an increase in CR.
So why make the change?
It’s a matter of Squish and Squish velocity. It lowers the combustion chambers roof closer and sparks plug electrode closer to the piston. This also reduces the Squish clearance. It takes the Squish from 1.6mm to 1.26 mm. When you reduce this clearance it increases the squish velocity and creates more turbulence in the combustion chamber as well as a higher compressed state of the gas. As the gas compressed state raises the heating effect on the gas increases or for better words Compression Heating. When this takes place you improve thermal efficiency. “Efficiency = 1 - (1/compression ratio)^gamma-1”
This translates into higher heat load on the piston to which needs to be dissipated into the surrounding area and atmosphere via the coolant system. This also heats of the surrounding metal parts which has an effect on the incoming fuel charge. Staying with the IDEA of Fuel Cools the higher we raise this temperature the more fuel we need to resist detonation from higher heat load. Based on this you can see where direct injection has a big advantage with high CR. Unfortunately we do not have direct injection yet. If we look at the Flame Propagation and discuss flame speed the reduction of squish plays a role in this. The mixture strength at temperature also plays a part in the flame speed. This flame speed, degrees of spark ignition is what sets the LPP or Location of peak pressure. Keep in mind the timing is controlled by a
CDI (Capacitor Discharge Ignition) and Fixed Static set point. We do not have a way for the CDI (Capacitor Discharge Ignition) to adjust for differentials in Flame front. So we design the Engine changes around these fixed settings. Every time we re-jet, alter the port timing or space above the piston at TDC (Top Dead Center) or head we alter the Flame speed and Location of Peak Pressure. This LPP changes constantly with rpm. As mentioned above the compression heating effect plays a part in this as it heats things up inside the cylinder and has an effect on the incoming charge. As this takes place the fuel is subject to greater heat for its density and can lead to detonation.
This takes up to the duration of the exhaust port and its effect on the LPP based off of CCR. We go from a static environment to a dynamic one or the engines rotating assembly in motion due to combustion. We all have seen the Engine demo “H” has put on this site if not you should. The exhaust port acts like a super charger and adds pressure to the atmospheric pressure state at port closure. This to say when the exhaust port closes the area inside the cylinder is at an atmospheric condition during cranking compression reading. During combustion the return wave of the pipe pushes the intake charge back into the cylinder prior to closing off the port. This takes that cylinder area from atmospheric to a pressurized state then continues to compress and rise in pressure from there. An important key is to time the exhaust wave with port closure. Wave refraction to soon and pressure will drop off as it bleeds back into the pipe, too late and the wave pressure hits the face of the piston skirt leaving power on the table. The up side to hitting the face is that is does lube the skirt well. The down side is advances the LPP which could be bad. When looking at this one also should take into account just what the port in this case exhaust port can do in way of flow. When we discuss flow of a port we have to do Angle Area calculations to see at what the port is good for. I need to stay on tract so talk about that later and continue to cover, Duration and CR related to LPP.
The pipes wave speed is based off of heat, hotter gases flow quicker. See how a shift to a lower EGT would result in lower speed and affect the dynamic cylinder pressure. This would advance the LPP and build higher cylinder temps and then it could continue to build until either pre ignition set in or detonation would accrue or both if your jetting was to lean for the octane used at a given SI (spark Ignition) timing. You could go from a rich to lean it matter of seconds. A CHT and EGT used together would see this coming. This of course your Engine parameters must be close or at Detonation levels.
Since the effect in UCCR and CR are so minor one might ask, what’s the big deal?
(1) It takes us closer the fuel ability to resist detonation.
(2) This also increases the engines Thermal Efficiency which builds more MBT for the engines timing and fuel choice.
(3) It accounts for a higher ER (Expansion Ratio) which allows more energy to be extracted.
This does not cover the whole picture however gives you an idea on my decision process. This experiment basically provides me with information for future builds and other ideas. These pistons will not reach circulations other than in my personal or loaner cylinder/engines.
If you have any question please ask and I will do the best I can to answer or other on this board can chime in and answer.
Please do not use this to go adjust your Engine parameters based on what you read. Heck I could be all wet and this is all worthless information.
I Hope after reading this you get an Idea of the way my Hacker head works. Please feel free to rate according the new POO scale.
Adnoh