Prechamber Mod "Flame" Front

OM616

10mm MW

572

02-13-2018, 12:09 PM #382

(02-13-2018, 10:15 AM)whipplem104 My point is that if you are having to run increased timing that you are slowing down the flame front. The fact that you are increasing timing means that you are taking longer to reach peak cylinder pressure. So I do not see how the mod is doing what you want, assuming you want more power. If your flame front was faster from the larger holes then you should decrease timing.
I had assumed that the larger holes were allowing a larger amount of fuel to burn at a quicker rate. Because the holes were to small and causing a restriction for the volume.

Increasing the burn hole size is not done to "burn at at quicker rate", it is done to increase the fresh air charge in the PC, which allows more fuel to be burned.

One side affect of reducing / eliminating the restriction between the cylinder and the PC is that the PC and cylinder pressures will rise closer to the same rate, and the PC charge will reach ignition condition sooner than the stock restricted PC will, which allows the injection to start sooner without nailing.

Reducing / eliminating the restriction between the PC and the cylinder has a dramatic effect on the pressures in each at every point during each cycle. The restrictions between the PC and the cylinder causes the cylinder pressure to be higher than the PC pressure prior to injection. Once injection starts, the PC pressure is higher than the cylinder pressure that is already pre-loaded (for lack of a better term, (think of a preloaded spring)) and the added volume from combustion goes to work. For small fuel volume injection, low power, this is efficient. But it limits how much fuel can be injected and requires high boost pressure to force more air into the PC to keep the EGTs in check, which generates crazy high pre injection cylinder pressures (pumping losses), crazy high post injection PC pressures, Smoke, and high EGTs.

Reducing / eliminating the restriction between the PC and the cylinder means that the effective pressure volume (the volume that has to be pressurized to a point where work will start, (how far does the spring have to be compressed before it is solid)) has increased, and the static pressure in the cylinder is lowered, so when ignition starts, the initial fuel is used to bring the cylinder pressure up to working levels (compresses the spring), where as before with the small holes, the cylinder pressure was at that point of work because of the restriction between the PC and cylinder.

What I have done is almost totally eliminate the aspects of the Prechamber design, and made it into more of a high flow Swirl Chamber with the pressure characteristics closer to a DI engine, so to expect that this set up would want the same timing as the stock set up is not reasonable. It is a totally different engine now, and it wants what it wants.

I have also gone a step further and changed the cam timing to further enhance the pumping efficiency which changes the cylinder / pc pressure rise timing. With the current settings, (cam + 10*, and static timing at 28*) the engine idles quieter than it did before the mod, EGTs are way lower, smoke is non existent, and the engine is significantly more responsive and runs stronger. I can tell by how it comes to life when the advance starts to come in that it wants more timing, so once it warms up I will set it to 30*. (I also want to replace my DVs as I have a bad one so I am also waiting until I get them)

I get the impression that you are thinking that I had to advance it 10* or something to make it run lol... I gave it 2*, from 24* to 26*.. who knows, it may have been happy at 25*.. Even an injector change can require that if one really wants to tune the engine. I laugh every time I hear someone quotes a timing speck from a manual as if that is the end all be all, when in reality from a tuning perspective, they are good starting points; each engine will want what it wants.

To your point, have I changed something? Yes.. Did it have a negative effect with a stock tuning? Yes... Did it allow me to run more aggressive fueling and timing, while lowering EGTs and Smoke, making more power, and increasing mileage when not driving like a maniac? Yes... Is this something that everyone should do? No Wink