Boost & Comp Ratio, good rule of thumb

Posted by someone other than me, but it makes a great point.

There are certainly reasons to try to raise compression ratio, namely when off-boost performance matters, like on a street car, or when using a very small displacement motor. but when talking purely about on-boost power potential, compression just doesn't make any sense.

People have tested the power effects of raising compression for decades, and the most optimistic results are about 3% more power with an additional point of compression (going from 9:1 to 10:1, for example). All combinations will be limited by detonation at some boost and timing threshold, regardless of the fuel used. The decrease in compression allows you to run more boost, which introduces more oxygen into the cylinder. Raising the boost from 14psi to 15psi (just a 1psi increase) adds an additional 3.4% of oxygen. So right there, you are already past the break even mark of losing a point of compression. And obviously, lowering the compression a full point allows you to run much more than 1 additional psi of boost. In other words, you always pick up more power by adding boost and lowering compression, because power potential is based primarily on your ability to burn fuel, and that is directly proportional to the amount of oxygen that you have in the cylinder. Raising compression doesn't change the amount of oxygen/fuel in the cylinder, it just squeezes it a bit more.

So the big question becomes, how much boost do we gain for X amount of compression? The best method we have found is to calculate the effective compression ratio (ECR) with boost. The problem is that most people use an incorrect formula that says that 14.7psi of boost on a 8.5:1 motor is a 17:1 ECR. So how in the world do people get away with this combination on pump gas? You can't even idle down the street on pump gas on a true 17:1 compression motor. Here's the real formula to use:

sqrt((boost+14.7)/14.7) * CR = ECR

sqrt = square root
boost = psi of boost
CR = static compression ratio of the motor
ECR = effective compression ratio

So our above example gives an ECR of 12.0:1. This makes perfect sense, because 12:1 is considered to be the max safe limit with aluminum heads on pump gas, and 15psi is about as much boost as you can safely run before you at least start losing a significant amount of timing to knock. Of course every motor is different, and no formula is going to be perfect for all combinations, but this one is vastly better than the standard formula (which leaves out the square root).

So now we can target a certain ECR, say 12.0:1. We see that at 8.5:1 CR we can run 14.7psi of boost. But at 7.5:1 we can run 23psi of boost (and still maintain the 12.0:1 ECR). We only gave up 1 point of compression (3% max power) and yet we gained 28% more oxygen (28% more power potential). Suddenly it's quite obvious why top fuel is running 5:1 compression, that's where all the power is!!

8.5:1 turns out to be a real good all around number for on and off boost performance. Many "performance" NA motors are only 9.0:1 so we're not far off of that, and yet we're low enough to run 30+ psi without problems (provided that a proper fuel is used).

Makes sense and its simple enough for the common man to understand. Nice post Tony. :bigthumb:

I thought you might like that little tid bit of info :)

Something it does not mention is the head design, which also is a factor, like what I was talking about on the other thread.

Ya I saw that. Would you care to go into a little detail? Take for example my RPM heads vs a 4v 4.6 head. Why would a 4v head have issues per say with boost? I'm guessing combustion chamber shape, valve angle, ect ect all play an even bigger roll when working with boost. Always been kinda curious why 2 different heads that flow similiar numbers can act totaly different with boost, n20, n/a.

It comes down to the efficiency of the head. Remember that all flow number CFMs are not created equal, but normally a 4v head will still always out perform a 2v head. That is why more modern heads are designed with 4v or 3v in the 2005 Mustang. If you look at the flow numbers of that head, intake is fine, but the exhaust is total crap (I wonder why that is?)

Typically a very good 2v head is in the 65% range, while a 4v head is in the 85% range (off the top of my head), this is where the difference is. Its not that boost increases this percentage, but given if your maxing out the power with a 2v head at a given rpm, timing, volume, flow rate, comp ratio, a/f, displacement, etc. What do think would happen if everything was equal and your head was more efficient? Your ECR will happen at a lower boost level, then you have to add more fuel, or take timing out for that same boost level. Its a dance between a lot of variables that a lot of guys do not understand.



That is what I was trying to explain in the other thread, with a 10.3 and a 4v head, that is not a very good starting point for boost. You would have to drop it to at least a 9.5:1 to add 8lbs of boost. Of course you can boost at 10.3, but your a/f is going to be pig rich, and your total timing is going to be very low, so what happens to your hp when that happens. Remeber you can led a horse to water :)

http://home.comcast.net/~ryan.langford/pics/newECR.jpg

This chart is based on a 2V head, so for argument sake take that 10.3 and run it on the SCR of an 11:1 and see what the 9psi runs on the ECR chart? 14:1, try running that engine with 93oct? Like said before, its going to be det nightmare!

Nice. Bookmarked the link. Thanks Tony.

BTW I just lack fuel lines, fittings, cell, and odds and ends such as valve cover breathers and radiator cap now lol. Gonna start a build thread with pics soon here and at TM.

Nice. Bookmarked the link. Thanks Tony.

BTW I just lack fuel lines, fittings, cell, and odds and ends such as valve cover breathers and radiator cap now lol. Gonna start a build thread with pics soon here and at TM.

David,

You might want to run an evac instead of breathers, if oil fumes bother you of course :)

I can't wait to see the build.