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03-13-2019, 02:32 PM | #1 |
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Safe Boost Compression Options
This chart I made utilizing information I've found from previous safe boost levels on the stock motor of 12.5:1 it seems like these engines are good for a final compression of about 22 or less on 91/93 octane (green) and FCR of 35 or less for E85 (orange). I haven't been looking into E85 as it's not readily available in my area.
Compression Vertical and PSI Horizontal Are there additional factors or limitations of the FA20 platform I should be taking into account before proceeding with building a boosted motor? Any reason there aren't more 13.5 CR boosted FA20 in the wild? Note: These are extremely loose numbers and proceed with caution and the guidance of your engine builder / tuner. Last edited by elitegunslinger; 03-14-2019 at 02:42 PM. |
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03-13-2019, 02:50 PM | #2 |
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I think the reasoning is that people would rather have the flexibility to run a dynamic CR using boost, rather than fighting knock on a higher static CR engine before boost is added. That, and not many suppliers waste money building higher CR pistons when they know the aftermarket caters towards lowering it.
Very cool chart though |
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03-13-2019, 03:52 PM | #3 |
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It's a good rule of thumb reference, but I'd imagine a lot of things come into play, like rpm, cam timing, ignition advance, fuel, cooling etc.
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03-13-2019, 04:53 PM | #4 | |
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03-13-2019, 04:54 PM | #5 |
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Does that mean that I can run 20 psi on 12.5 CR safely on 93?
Update: nvm I understood the graph sorry Sent from my SM-A520F using Tapatalk |
03-13-2019, 04:57 PM | #6 |
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03-13-2019, 04:59 PM | #7 |
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03-13-2019, 05:18 PM | #8 | |
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Not that it can't be worked around, but it's easier to just go 10.0:1 CR with boost. If I wasn't using ' dynamic compression" accurately, I'm sorry. |
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03-14-2019, 11:39 AM | #9 | |
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Before variable cam timing became commonplace, static and dynamic compression had a direct correlation because cam timing was a fixed variable. But with variable intake cam timing, the dynamic ratio can be varied while keeping static compression constant. This is advantageous for engine building and tuning since you can use a higher static compression than you normally would, and then use cam timing to reduce the resulting dynamic compression in parts of the rev range where that high static compression would normally be problematic. |
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03-22-2019, 04:52 PM | #10 |
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03-22-2019, 05:43 PM | #11 | |
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I was running 14PSI on stock injectors with 100 octane. The motor is being built now and when me and the mechanic looked at the cylinders/head, there was no evidence of knock Sent from my iPhone using Tapatalk |
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03-23-2019, 03:19 PM | #12 |
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Old school recommendations was to lower your static CR for boosting so you didn't get detonation (knock). 12.5:1 + boost was unheard of years ago. However the DI on these cars and a good cylinder head design does a wonderful job of preventing this. E85 does even more to help with its 107 octane, 40% more liquid in the cylinder and the cooling effect of alcohol evaporating. If you were running e85 I'd say go for it.
The more CR the lower the boost. Lower boost means a smaller turbo which is a good thing
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03-23-2019, 03:24 PM | #13 |
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I don't think you can have a discussion about "safe" boost levels without including fuel octane rating and ignition timing.
This assumes the bottom end is strong enough, which it isn't. I bent 4 rods on e85 with 7psi boost.
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