E46Fanatics - View Single Post - Lower compression
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Old 10-20-2012, 12:25 AM   #30
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Join Date: Nov 2011
Location: Nashville, TN
Posts: 19
My Ride: 04 330i6
Originally Posted by Bdave View Post
The main technical issue with FI and compression is that as you increase boost, fuel/air temperatures increases _during_ the compression stroke. If those temperatures raise too much, the fuel/air mixture will undergo combustion prematurely. Under WOT and full boost, these unwanted occurrences, called detonations, can damage an engine very seriously.

There are several ways to reduce or manage conditions that lead to detonation. Lowering compression is the more drastic of such methods. What is best for you want to do depends on what you want to do and how much money you have to reach those goals.

The most obvious solution is to use a fuel that has a higher octane rating (detonation resistance) such race fuels like 110 octane race/aviation fuels, C16 or E85. Some problems are cost and general availability of the fuel but it's probably the easiest path to running higher boost.

Water or methanol injection can help lower pre-combustion temperatures and/or raise octane levels as well but require significant design consideration of the delivery system. It's best to install these types of options with a kit because the exact application needs to be tuned. With a kit you don't need to start from scratch. I view it as a band-aid approach the problem but it's a nice next step for people who want to fiddle with those types of setups.

E85 is more convenient because it's less expensive than race fuel and more available but it requires that parts of the fuel system be upgraded, rubber components might need attention since some components made of lower quality materials will degrade and fail. E85, as a fuel, has a lower power density than gasoline; so you will need to inject more fuel to burn an equivalent amount of air.

Regardless of the exact fuel, if your motor is consuming more air, it will require more fuel, so you probably will be looking at injector and fuel pump upgrades depending on your goals and the capacities of the components your car has.

The most drastic method is obviously an engine rebuild for lower compression. That's done either by replacing the pistons with a model that has more dish, using a thicker gasket or even removing material from the head.

Overall, boost can be considered to be another form of compression. Between the boost and cylinder compression, and a few other factors relating to intake air temp etc, there exists a compression limit where temperature increases will cause fuel to burn spontaneously. That limit relates to the octane levels of the fuel you are using.

I hope that answers your question.
Did not read the rest of the thread to see but this is information worth quoting

Last edited by wtbass21; 10-20-2012 at 12:27 AM.
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