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Old 03-10-2014, 07:09 AM
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Compression vs. Boost

I am no engineer, but I was thinking about the operation of these 6.4 or really any turbo/supercharged internal combustion engine. My question for any of you is my 6.4L/390 CID has a compression ration of 17.5:1, if my turbo is producing 20 pounds of boost does that raise the compression ration? Or I am thinking about it in the wrong terms?
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Old 03-10-2014, 08:18 AM
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Compression ratio stays the same.

A turbo compresses more air into the chamber. Inject more fuel then you have more power.
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Old 03-10-2014, 08:53 AM
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respectfully, I disagree a titch Capt, by no means completely do I disagree, but just a little...

the rate of compression is 17.5:1.. the 'pressure' of compression varies hugely depending on the density of what you're compressing (in this case fuel and air, one which is compressible one which isn't)..

cylinder pressures and compression ratio's are tricky... there are levels of under pressure air that change cylinder pressures, there are valve events that alter both compression ratio and pressures, and amount of fuel present that alter the volume of the cylinder which has dramatic affect on both compression and pressure.

boost is better described as 'dynamic displacement'....

for every atmosphere crammed into the engine (14.7" of boost) and accompanying fuel to provide catalyst for burn, you've in effect doubled your displacement. if 6.4 is the displacement while naturally gulping air, cramming air into the machine under 15psi (and fueling it properly) will make it produce the power of an engine displaced 12.8 naturally...

hitting 30ish psi of boost is two additional atmospheres, which would equate to a 19.2L (fueled at the same ratio) engine.


but here is something tricky, that gets lost in the sauce too much..

static compression ratio = mechanical calculation of stroke vs. bore.. easy to figure..

dynamic compression ratio = same as above but with valve events factored in.. example: a piston is 20% into it's stroke when the intake valve shuts completely, part of that 20% has to be removed from the 'static compression' to best determine 'dynamic compression'.. It's not out of the question for dynamic compression to be more than a full digit drop from static.. (like, a 17.5:1 static reduced to a 15~16:1 dynamic)...

when you up the naturally aspirated pressure in the engine, you are upping the compression ratio 'dynamically' too.. an engine compressing air naturally filling the void at a rate of seventeen times over before reaching the end of it's stroke is 17:1... an engine compressing the natural air PLUS the air in there under pressure (say, one atmosphere or 14.7psi) will be compressing 34:1 mechanically, and likely 24ish:1 dynamically.. the compression has changed under boost..

too much boost and too much fuel (fuel doesn't compress, by the way) will blow gaskets as they reach their ability to contain it... this is why high boost passes are dangerous- but not only for the pressure added to the cylinder via boost, but for the amount of fuel in there too.. fuel doesn't compress...

If a cylinder held one liter of air, and the compression at TDC was 10:1, and then you added a half a liter of fuel or another liquid, that same cylinder would be 20:1+.. the stroke is the same, but half the volume was replaced with something that doesn't compress.... add an atmosphere to that cylinder in addition to the fuel, now you'd be looking at 30+:1 ratio's...

each atmosphere allows more fuel to be completely burned- so while your adding pressure/compression ratio via air, you're also doing it via filling the void with uncompromisable fluid...
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Old 03-10-2014, 10:01 AM
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You sure are wordy for just a titch lol.

I agree with you. Was just explaining it in the simplest forms, basically mechanical compression ratio.

Even after you calculate the a/r of the turbo, pressure drop in the intercooler, given rpms, weather, etc etc compression ratio is still not a completely accurate if you're speaking dynamically. But close enough to get the desired results.
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Old 03-10-2014, 10:07 AM
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ha!

true, and even funnier? I never said plainly what I intended to say in the first place..

compression ratio and cylinder pressures are related but different..

dang it..

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