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High CR Pistons


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47 minutes ago, joshuaho96 said:

I'm actually just hoping to be able to get an RB to be more modern, running something like high 13 AFR deep into boost and revs instead of 11.5 and maybe push back the knock limit enough to get power equivalent to 98-100 RON fuel with 95 RON. RBs also seem to be thermally challenged so water injection might help in that regard as well if it ever makes it to a track. But if the trade-off is I never actually get to drive the car or I have to pull the engine because water contaminated the oil and trashed the engine it isn't really worth it.

You are a funny guy

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E85 done, less thinking and stuffing about. 

You can put timing way past MBT and start losing power on a RB with E85 and it won't even produce any knock.

Anyways back on topic, 10:1 would be sexual. I would still keep the squish pads in there though. 

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Disclaimer: Not a professional engine builder. You'll need to CC your parts to verify these calcs, this is assuming the stock Neo head has a 51cc chamber and you will also want to verify piston to valve clearance.

Playing around with a compression ratio calculator (I've been using https://goodcalculators.com/compression-ratio-calculator/) I'd say an 8.5:1ish R33 RB25DET piston would be a good starting point. I can see two fairly easy/reasonably cheap options so far:

Option 1 - ACL aka Ross aka Mahle 6MKRY9603 with 11.5cc piston dome. Set to zero deck height with a 1.3mm head gasket on a Neo head this should give a flat 10:1. If you bring the pistons 0.5mm above deck you can achieve around 10.5:1 with the same 1.3mm HG while keeping quench around 0.8mm - I've heard 0.7mm-1.2mm being a rough rule of thumb to aim for. Each 0.1mm gasket change on this combination is roughly 0.15 change in CR so you should be able to fine tune the CR easy enough.

Option 2 - Wiseco K578M865AP with 14cc piston dome. At zero deck this will give 10.4:1 with 1.3mm HG. With +0.5mm deck height this will give 11:1. As before a 0.1mm change in HG thickness will change the CR approx 0.14-0.15.

Hopefully this gives you a starting point.

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2 hours ago, Ben C34 said:

You are a funny guy

I like to spend my hours reading about interesting things, not necessarily productive though.

2 hours ago, Dose Pipe Sutututu said:

E85 done, less thinking and stuffing about. 

You can put timing way past MBT and start losing power on a RB with E85 and it won't even produce any knock.

Anyways back on topic, 10:1 would be sexual. I would still keep the squish pads in there though. 

Feels like this has been the answer a number of times I've looked into this. I'm probably going down that road in the end, just want to figure out a good 340 lph brushless in-tank pump solution.

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8 minutes ago, joshuaho96 said:

I like to spend my hours reading about interesting things, not necessarily productive though.

Feels like this has been the answer a number of times I've looked into this. I'm probably going down that road in the end, just want to figure out a good 340 lph brushless in-tank pump solution.

Walbro F90000267 and call it a day (or F90000274 if you're running stupid high base pressure & big silly boost)

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9 minutes ago, Dose Pipe Sutututu said:

Walbro F90000267 and call it a day (or F90000274 if you're running stupid high base pressure & big silly boost)

That is a properly buff pump but probably too much fuel pump for something making 300 kw. Something like the Fuelab 48402 seems to be what I'm looking for, around 10A draw and capable of 300 lph at 70 psi outlet pressure to be able to turn up the FPR to 3.5 bar differential pressure. I might run some experiments to see if the OEM FPCU/FPCM has the right resistances to be able to actually function as the pump speed controller, have to see what the impedances look like. Probably have to just accept that it's a horrendous idea though and just use a standalone ECU to run the PWM controller.

Anyways, I don't doubt that high CRs improve driveability, I'm just skeptical that you can run high CR with low duration cams and no tricks like advanced VVT/cooled EGR/E85. I'm sure that Mitsubishi and Nissan would've raised the static CR on the 4B11T and VR38DETT to 10:1 if they could've done so while meeting power targets on pump gas. The VR30DDTT has a 10.3 CR but uses GDI and dual VVT to get there. 

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32 minutes ago, joshuaho96 said:

OEM FPCU/FPCM has the right resistances to be able to actually function as the pump speed controller,

Don't waste your time, you'll end up killing the pump with the OEM setup. PWM is the only way, not a drop resistor (like how the OEM garbage works).

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1 hour ago, GTSBoy said:

Emissions.

Doesn't that imply that you would have to run rich for too much of the load/RPM map? Modern gasoline is up to 14:1 CR for NA, or even as high as 16:1 for the Skyactiv-X, as high as 12:1 for turbo. I can't think of why emissions would factor into it. NOx obviously goes up with high CR, but 10:1 PFI-T is nowhere near what diesels have done for a long time now.

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1 hour ago, Dose Pipe Sutututu said:

Don't waste your time, you'll end up killing the pump with the OEM setup. PWM is the only way, not a drop resistor (like how the OEM garbage works).

Yeah, probably going to just disconnect the FPCM and use the PWM controller then.

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10 minutes ago, joshuaho96 said:

Doesn't that imply that you would have to run rich for too much of the load/RPM map? Modern gasoline is up to 14:1 CR for NA, or even as high as 16:1 for the Skyactiv-X, as high as 12:1 for turbo. I can't think of why emissions would factor into it. NOx obviously goes up with high CR, but 10:1 PFI-T is nowhere near what diesels have done for a long time now.

You're knock limited with port injection, thus running it leaner is not possible. With direct injection you get better atomisation, cooling effect of fuel being compressed at high pressures then decompressing in the chamber, thus your knock threshold is much higher.

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16 minutes ago, Dose Pipe Sutututu said:

You're knock limited with port injection, thus running it leaner is not possible. With direct injection you get better atomisation, cooling effect of fuel being compressed at high pressures then decompressing in the chamber, thus your knock threshold is much higher.

Doesn't this imply that you would want less CR anyways? I can't imagine running rich all the time is healthy for an engine.

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29 minutes ago, joshuaho96 said:

Doesn't that imply that you would have to run rich for too much of the load/RPM map? Modern gasoline is up to 14:1 CR for NA, or even as high as 16:1 for the Skyactiv-X, as high as 12:1 for turbo. I can't think of why emissions would factor into it. NOx obviously goes up with high CR, but 10:1 PFI-T is nowhere near what diesels have done for a long time now.

NOx is the big one.

Diesel combustion temperatures are quite different to petrol, both at partial load and at full load. Diesel runs very lean at partial load so has different NOx problems than petrol does. Hence why they're down the Urea injection rabbithole these days.

For OEMs trying to meet Euro and US emissions regs, fuel consumption targets, engine life expectancy & warranty boundaries, the space they have to work in is a very tiny portion of the N-dimensional space. So they can't have a combination of high SC and boost and not give up ground in at least one of the other dimensions of their design space. It's a bigger nastier version of "cheap/fast/light pick any 2".

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