View single post by 12° North Industries
 Posted: Thu Nov 23rd, 2017 08:21 pm
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12° North Industries
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Just using this as an example for boost all around, doesn't matter if its mechanical boost in the sense of turbo, supercharging, or liquid form.

The Subaru BRZ/Scion FRS for example run their 2.0L engines at 12:5.1 compression (with a 7400 RPM redline). So lets say you have the a 2.0 engine with 12.5:1 compression ratio and you run 5 psi and you achieve 250whp.

Then you take the same engine an lower the compression ratio to lets say 9.5:1 but you now will have to run 10-12 psi to achieve the same 250 whp. For the same amount of stress in both engines the next factor that comes into play is fuel.

High boost + High Compression = Needs high octane gas. (e85, Race gas)
High boost + low compression = Needs not so high octane gas (93)

Then it becomes an issue of detonation. You have to know what kind of build you are going after to determine the compression you need. With that, higher compression simply increases likelihood of detonation much more so than boost. So it IS better to have lower compression so you can run much much higher boost if you are planning on going for big hp numbers... IF not, then you could stay the other route.

A 8:1 motor will swallow 17-20 psi all day long and make more peak hp than the above motor. (notice you are TRIPLING the boost pressure) This is on pump gas. On race gas/e85, the limits of detonation are raised and you can get away with high boost on high compression.

Typically with DI you can stretch 93/94octa a lot more. In the case of most STG 1 turbos for example the vehicles are 8psi tunes and it seems like there is still a bit left on the table. I would start lower and work up till I find the knock sensor, that would then be my limit then dial it back a hair or two. I'm going to go out on a limb and guess 12psi before knock becomes an issue.

But "Tsquare" does bring up a valuable point in the Ford's hypereutectic pistons being cast and even I don't think I would even spend or let someone spend the funds to Cryo those to expound on the information of "would it work"..?



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