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Is the 2.5XT engine's compression ratio less than 10:1?

I always thought that turbos were best used with low-compression engines, which would also allow for use of 87 octane.

Any input is appreciated.
 

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Compression ratio on the 2.5XT is 8.2:1 according to the slick brochure that I got from the dealer. Since I know diddley-squat about that kind of thing, I'll use whatever octane the owners' manual recommends.
 

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doc5339 said:
Is the 2.5XT engine's compression ratio less than 10:1?

I always thought that turbos were best used with low-compression engines, which would also allow for use of 87 octane.

Any input is appreciated.
I've not heard of running low octane fuel with a Turbo, whatever the nominal compression ratio is, the turbo will increase that substantially, so the 8.2:1 may end up more like 11:1. Car makers usually lower the compression ratio on Turbo's for this reason, detonation is a big risk to the engine.
 

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Jondalar, you're absolutely correct. i've been having to do this sort of math a lot due to 3 turbo conversions in progress.

say you're running 7.5 psi boost (abt 1.5 atmospheres), which is abt 22.2 psi absolute (7.5+14.7). if you have 10:1 compression, your pre-boost cylinder pressure just before it fires is abt 147 psi. boosted to 7.5 psi, it's about 222 psi, or equiv to abt 15.1:1! max boost on the XT/GT is abt 13.5 psi and on the WRX more than that.

don't even think of running regular gas or anything other than top grade premium with a turbo unless you have less than abt 4-6 psi, factory has equipped it with killer antiknock system, you plan to permanently wedge the wastegate open with a screwdriver, or you want to replace your pistonsand exhaust valves soon anyway.
 
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