engineermike
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I wonder if Mazda is using some level of stratified charge to control detonation as well. Ford definitely isn’t in the coyote.
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WildHorse
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Yeah not so much that Ford is stupid as opposed to the sheep being stupid.
As for Mazda running a big ole 13:1. Good on them. You can run 15:1 safely on 87 too. Mazda did it it the name of efficiency, not power.
Still it always cracks me up when its only the ford people that buy a 11 or 12 to 1 car and insist on using 87.
On a side note. I get better economy on 94. Way better than the Ford published 87 octane numbers. Thank you timing advance.
TX5OH
Well-Known Member
Don’t worry about static compression. It’s all about dynamic compression. Ideally you want to be in the 8-maybe 8.5 range max dynamic compression ratio. The comment regarding the cam allowing more compression is spot on. The intake valve closing is what drives the bus. The longer this timing point is stretched out the more compression that can be run statically and ultimately bled off by the cam. However, the static compression numbers you are throwing out there will most likely require some very long duration cams. The GDI system might help matters, and that may be how Ford gets away with 12:1 on pump gas considering the parked location of the factory cams isn’t conducive for bleeding off the compression.
Regardless, I suggest you look at the combination you want to run as a system such as what cam, rod length, type of fuel etc. and not focus on a static compression number. Rather look at hitting a target dcr to ensure you can make the most power without being stuck with needing race fuel.
Regardless, I suggest you look at the combination you want to run as a system such as what cam, rod length, type of fuel etc. and not focus on a static compression number. Rather look at hitting a target dcr to ensure you can make the most power without being stuck with needing race fuel.
Gogoggansgo
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mazda is running a very special piston and combustion chamber design it’s very knock retardant. But for a performance engine it might not be the best setup, it’s all about efficiency with mazda
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TX5OH
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Actually with the stock cams/stock rod length you can get away with 13:1 and possibly 13.5:1. The factory dcr on the Coyote is in the 7’s. That’s why they can get away with 12:1 static.
engineermike
Well-Known Member
When you guys are quoting DCR what valve timing are you assuming? The stock 2018 cal moves the intake cam 44 deg at wot and another 16 at part throttle for a total swing of 60 deg IIRC. That’s a pretty drastic difference in DCR. It actually lowers DCR as engine speed rises at wot, but this is for increasing cylinder pressure not reducing it. DCR may be going down but inertial effect increases and you have to delay IVC to capitalize on it.
Plus, delaying IVC to reduce DCR defeats the purpose of increasing compression ratio to begin with. What I’m saying is that stock cal BK spark timing is for 87 and should be pretty close as a starting point for 93 if compression ratio is increased 1 number.
However, keep in mind that the 26 or so BK timing maps will all need to be recalibrated for the new compression ratio if it is to be optimized. That’s only about 2500 cells... Technically the 26 MBT tables will be off as well.
Plus, delaying IVC to reduce DCR defeats the purpose of increasing compression ratio to begin with. What I’m saying is that stock cal BK spark timing is for 87 and should be pretty close as a starting point for 93 if compression ratio is increased 1 number.
However, keep in mind that the 26 or so BK timing maps will all need to be recalibrated for the new compression ratio if it is to be optimized. That’s only about 2500 cells... Technically the 26 MBT tables will be off as well.
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And fuel trims.
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engineermike
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Dynamic compression ratio isn’t measured; it’s calculated. But you can’t calculate it unless you know when the intake valve closes (IVC), but this varies 60 degrees using the tivct system. People were quoting what DCR the coyote uses so I’m just wondering what cam timing they are assuming to calculate the DCR since it is not constant.
The intake valve doesn’t close until the piston is rising on the compression stroke. The theory behind DCR is that air flows out of the cylinder back into the manifold while the intake valve is still open, thus reducing the air trapped in the cylinder, thus reducing cylinder pressure and detonation. I used to subscribe to this theory and calculated DCR many times. Now, I have 2 problems with this theory:
1. The intake valve closing late during the compression stroke is actually to increase cylinder filling not reduce it. If your cam and intake work together, which they do in the coyote, then the inertia of the air in the runner continues to fill the cylinder after BDC. Delaying closure of the intake valve increases this filling, particularly at high rpm. The stock Ford coyote cam timing starts the intake cam advanced then retards it 44 degrees from the starting point as rpm rises. This is to increase cylinder filling at high rpm, not to reduce cylinder pressure as the lower DCR implies. It’s ironic that the “dynamic” compression ratio is anything but dynamic since it doesn’t account for dynamic effects of the air charge.
2. Even if you figured out and applied a cam timing scheme that overly retarded the intake cam early in the rpm range to reduce cylinder pressure and detonation, what would be the point of raising compression to begin with? You would be undoing the benefits of the compression ratio increase. The only time I would consider this is to improve power in one context (high altitude, high octane, etc), while sacrificing in other conditions (lower octane, etc). This, however, can get quite complicated. If this was the goal, it would be much simpler to reduce spark, or even simpler to clip torque demand.
Grimreaper
Well-Known Member
Wouldn't it be best to use the entire cam curve to map out each set points impact on dcr? It's only 7 or 8 points.
Mike are you taking the angles direct from the tune as your ivc at 0.050 or 0.006 lift?
Mike are you taking the angles direct from the tune as your ivc at 0.050 or 0.006 lift?
engineermike
Well-Known Member
I’m looking at wot intake cam timing ranging from 20 advance to 24 retard, and 8 steps along the way with infinite variation between them. You could calculate DCR at each of these 8 steps but there would be no point since we already know the cylinder air load and pressure is not proportional to just IVC and compression ratio.
I am not because I believe DCR to be pointless so I don’t bother calculating it at all. Air load would be a better measure of actual trapped cylinder pressure. You can see in the BK tables that knock is more likely with higher air loads.
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