its generally accepted to reduce compression to safely run more boost...

ive heard on other threads you need about 2psi of additional boost to make up for a compression loss of "1" (ex. 10:1) ... 1ST OF ALL, IS THIS CORRECT?

my question is which is "better" (on a turbo application)

11:1 compression and 7 psi of boost or
10:1 compression and 9 psi of boost or
9:1 compression and 11 psi of boost

assuming same turbo and similar tune/timing/octane/engine internals...

which is safer for the motor? which produces more hp? bigger powerband?

 

11:1 with 7-9psi should be good. When I had 11:1 with 8psi, I made almost the same power as 8.8:1 with 15psi on pump gas and similar tuning and 11:1 feels a lot better in lower rpms. I miss having power before boost. I plan to run between 9:1 and 10:1 with 14-17psi (race gas with higher boost) until I finally fully build my motor early next year. I am not to happy with 8.8:1 with stock sleeve since I can't get away with more than 16-17psi.

 

I would love to hear this as well, great post!

 

can others chime in on this as well?

 

Just want to add that efficiency (power) change due to CR change is NOT linear.

You lose a lot more going from 10:1 -> 9:1 than going from 11:1 -> 10:1

If you want driveability stick with the highest CR possible.

 

humm...interesting... so you need MORE than 2psi of boost to make up for 1 unit of compression at the lower compression levels?

still my main question is which is safer for the motor? is there more stress/danger involved with more compression or more boost to acquire the same peak horsepower?

ex. if im running 15lbs of boost on 9:1 compression... how much boost would i need to run at 11:1 compression to equal the same HP? and which route is safer?

again assume an ideal timing and A/F for both... everything else constant...

 

The pressure in the cylinder is the driving force in moving the piston downwards. Theoretically, it should require X amount of pressure inside the cylinder to create Y amount of power at Z rpm. Regardless of boost, compression ratios, fuel, whatever.

This is something that I have been struggling to understand for the past few days. Not sure why but its been on my mind.

------------Compression---Boost-----Power
Motor A--------11:1---------8psi------300hp
Motor B---------9:1---------12psi-----300hp

Both engines have the same max power output. This requires that both engines have the same brake mean effective pressure BMEP. (I know there are some automotive engineers in here ). Assuming everything else is the same with each motor, they are both experiencing the exact same amount of stress to achieve 300hp. This also assumes that both engines are operating at the same rpm. This only applies for this one condition of max power. Under minimal boost, motor B will have be experiencing less BMEP, less power, and therefore less stress.

Why, then, is it "safer" to run higher boost levels on lower compression? In the above case, motor B certainly has a higher boost level, but it is producing the same amount of power. Any more boost will make it more stressful on the engine for both motors. But as long as the BMEP remains constant, it wont matter what the boost or compression are for any motor because they will be producing the same amount of power/stress levels.

How about this:

------------Compression---Boost-----Power
Motor C--------15:1---------N/A------300hp

In this case, the motor is naturally aspirated. Still making 300 hp at the same rpm as motors A and B. Still producing the same BMEP and stress levels. Now it gets more complicated. No boost at all, same stress....

You know you want to respond to this

 

It's not about operating stress, it's about detonation. The higher the compression and boost the more likely a fuel air mix is to self ignite or burn too quickly. This can cause excessive engine temp (overheating) or in extreme cases, piston and ring damage.

In the good old world of carburettors we didn't have the sort of tuning control we have these days. So there were supposed set limits to compression ratio and boost. And they used rules of thumb like 2lbs per CR point because they were only working in a very limited CR range.

All of this is pretty much BS these days. There are no set limits (the idea of 11.1:1 and FI just horrifies my father) and certainly no easy rules of thumb. You build to a certain compression ratio, add boost, and tune with fuel and timing. You can work with octane levels but for street driving you know the realistic limits.

My point it there is no point asking for the simple answers; there are none.

 

Detonation can be caused by excessive pressure, excessive temperature, or a combination of the two. To make the same amount of power, the cylinder pressure is the same. Assuming the temperatures are the same as well, the risk of detonation should be the same for both motors A and B.

I have heard what you have stated above many times. It the same answer I have always recieved when asking this question. I am wondering what makes motor A more prone to detonation than motor B. In addition to a lot of other stuff that I'm sure will come up. My IC Engines book isn't proving to be much of a help at the moment.

 

You're correct. Detonation is a result of temperature and pressure. However there are three other factors: cylinder design (irrelevant as we're talking about a specific engine), contamination such as carbon build up (irrelevant except maybe in extremely rich tune) and timing. If timing is advanced and pressure increases the burn rate increases. If the cylinder reaches max combustion pressure before top dead center this is detonation.

I guess what it boils down to is it doesn't matter what causes the pressure (high CR or boost) if it's too high you will get detonation. Neither set up is ridiculously out of range so with adequate tuning you could run either. Of course if you want a definitive answer I suggest you build both and see which one you can run the most advanced timing with before detonation. Expensive exercise however.

If you've heard this answer several times doesn't that suggest it might be true?