Ok, so I have seen a lot of threads about decreasing compression to add more boost to get more power. But I want better fuel economy.

First question, Increasing boost, does in effect increase my compression ratio correct?

If my compression ratio is higher, I can extract more power from less fuel?

Therefore I can deliver less fuel with more boost?

I know I am running a slippery slope of blowing head gaskets, etc. but I just want to start with the basic questions.

Cheers,
Devin

 

If you want better fuel economy, one idea I heard is to make a set of higher-pressure injectors. The higher the pressure, the better the atomization and the less smoke(good), but getting a higher pressure during warm-start conditions may be hard. Also, you'll reduce your maximum fuel flow, so less power possible.

As far as compression ratio, basically, the answer is no. The clearances between the valves and the pistons are already almost nothing, so you'd have a tough time of it.

The real question here is: Fuel economy doing what?
If you are talking about highway cruising, the answer is either A, go slower, B, add more gearing(lower revs = better economy, to a point), or C, add a turbo. It seems counter-intuitive, but under heavy loads or above 2K RPM, the turbo will actually make your fuel burn more efficiently due to the extra air available.

 

I have the turbo, and I am considering the gearing. I just want to see now if I can tune the engine for better fuel consumption. I do recall reading about close clearances in regards to heads.

Back to my original question though, aren't we increasing our compression ratio when adding boost?

Devin

 

Compression is a static value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity.

Therefor compression ratio does not change with boost, Cylinder Pressure does!

Diesels like air, lots of it!

-Enjoy
fh : )_~

 

Yes, a turbo increases your dynamic compression ratio. More boost is ALWAYS more effective then more static compression. And alot easier to do.

As far as extracting more power with less fuel, again more boost and less compression is better. If you increase the compression your increasing your pumping losses while under zero boost conditions, which is most of your unloaded driving.

But there is only so much HP in the fuel, and as long as your not getting smoke, your prolly pretty close to as efficient as you can get.

Put in a pyro and learn to drive by it, less egts means less fuel being used.

 

What do you mean by pumping losses?

Devin

 

Your trying to compress air within your cylinders. some of the energy your fuel makes is used to compress the air within the other cylinders, the higher you try to compress it, the more fuel it will take.

 

^^^this^^^

I read a long thread in a Volkswagen diesel forum about decompressing a little in the quest for ultimate mpg. For us that's just silly. It's stepping over a dollar to pick up a dime... or more like a wad of 100$ bills to pick up a penny. The real gains for us is gearing and keeping your speed down. Static CR just isn't going to make any tangible mpg difference for us.

 

The difference between Static and Dynamic compression ratio is the size of the volume used to calculate.
Or simply stated, done with the piston in different places in the bore.

The volume for Static Compression Ratio is calculated with the piston at BDC.

The volume for Dynamic Compression Ratio is calculated with the piston at the point of intake valve closing.
i.e. partially up the compression stroke.

Cam intake timing is what dictates the volume used for dynamic compression ratio. Unless the motor in question has variable intake cam timing it doesn't change!
These IDI's do not have variable intake timing!

-Enjoy
fh : )_~