Safe Boost Compression Options

[elitegunslinger]

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.

[DarkPira7e]

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

[bfrank1972]

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.

[Boris_Petrov]

Originally Posted by DarkPira7e

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

What do you mean by dynamic compression?

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[Boris_Petrov]

Does that mean that I can run 20 psi on 12.5 CR safely on 93?

Update: nvm I understood the graph sorry

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[elitegunslinger]

Originally Posted by Boris_Petrov

Does that mean that I can run 20 psi on 12.5 CR safely on 93?

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11 PSI is considered the upper of safe for stock CR.

[Boris_Petrov]

Originally Posted by elitegunslinger

11 PSI is considered the upper of safe for stock CR.

Yeah and that's about 93. What about 100 octane for example? Or water meth?

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[DarkPira7e]

Originally Posted by Boris_Petrov

What do you mean by dynamic compression?

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My use of it was essentially saying raising the compression with forced induction selectively, given the medium, chosen amount of boost, and tuning. So you could be a linear, or throw on a huge turbo, stuff all your torque into the final 2,000 rpm. The tuning of the engine with a higher static compression will limit this ability and essentially will just be raising power all over while you fight knock because of it.

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.

[gtengr]

Originally Posted by DarkPira7e

My use of it was essentially saying raising the compression with forced induction selectively, given the medium, chosen amount of boost, and tuning. So you could be a linear, or throw on a huge turbo, stuff all your torque into the final 2,000 rpm. The tuning of the engine with a higher static compression will limit this ability and essentially will just be raising power all over while you fight knock because of it.

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.

FI raises cylinder pressure rather than the compression ratio. The static and dynamic compression ratios are mechanical characteristics of the engine. Static compression assumes the cylinder is sealed for the entire compression stroke. But in reality, the cylinder isn't sealed until the intake valve is closed, and the dynamic compression ratio takes that intake valve closure timing into account.

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.

[elitegunslinger]

Originally Posted by Boris_Petrov

Yeah and that's about 93. What about 100 octane for example? Or water meth?

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Only example I have found is 14 psi on E85 but there are more factors to take into accont but this is a rough estimate.

[kimcheefists]

Originally Posted by Boris_Petrov

Yeah and that's about 93. What about 100 octane for example? Or water meth?

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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


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[Hags86]

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

[Hags86]

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.