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beepbeep beepbeep is offline
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Join Date: Jun 2001
Location: Sweden
Posts: 5,464
My company car has C/R of 9.35:1 and runs up to 1.3 bar of boost, straight out of factory It's a SAAB.

You can run heaps of boost even on hi-comp engine as long as you retard the ignition enough. You won't be making any power though but it's seems that most important thing nowadays is to boost as much as possible, not to make power

Imagine car A and car B, same C/R, same turbocharger (let's say it uses K27), but different boost.

Car A boosts something like 0.7 bar and has ignition mapped for optimal BMEP troughout range (BMEP is all that matters). It will have good torque, good off-boost response, it will be frugal with fuel and intercooler/turbo won't need to work as hard as turbo efficiency will be on-spot and density-ratio would be low.

Car B runs 1.3 of boost. In order to allow that kind of boost ignition has to be dialed back. Turbo will be out of it's efficiencyy range, pumping hot air around. Intercooler will need to work harder and you will be feeding hot air into engine, further aleviating need for less ignition advance. You will need to pump more fuel into it to compensate all that air and it won't neary make as much power as 1.3 bar of boost indicate, use lot's of fuel and will fail more often.

It's always a trade-off. If you want lot's of power trough lot's of boost you need to build it with low C/R and turbo that is made to work efficiently with high pressure ratios. Such engine will be laggy.
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Thank you for your time,
Old 11-02-2006, 12:04 AM
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