about compression ratio

[atsik]

Searched forum but got no good answer....
I have right now 1.8l 66kw 4cyl engine with compression 9.8:1
if I raise it to 10.5:1 then what about fuel consumptiom?
more economic or less?
I drive usually with octane 98 or sometimes 95.
I have no manual, but I guess manufacturer(renault) suggests 95 (or 91 if 95 is not available)
My typical trip: 85% country roads(+highway) and 15% city

[Daox]

If you raise the compression of an engine, you will increase efficiency (all other things being equal). However, altering compression always affects other things, and you should mainly be concerned with ignition advance. If the increased compression causes pinging, the ignition timing will be retarded and you will loose any gain you might have gotten. Running higher octane fuel will help this. Its really a balancing act and until you know your engine and do testing nobody can say how far you can go. It also all depends on the environment you drive in, humidity, altitude, temperature. All those things effect ignition timing. So, what might work well one day won't the next. Thankfully, the OEMs have the resources to test for this and build in some safety factor for us.

[atsik]

So... I know that engine is more efficient, but how about fuel consumption?
66kw car burns 7l/100km(90km/h) or 100kw car burns 7l/100km(90km/h)

- both are wasting same amount fuel....

[EdKiefer]

Originally Posted by atsik

So... I know that engine is more efficient, but how about fuel consumption?
66kw car burns 7l/100km(90km/h) or 100kw car burns 7l/100km(90km/h)

- both are wasting same amount fuel....

fuel consumption should go down as you will only need to use less throttle for same power as older engine . That's as long as fuel octane is good enough for the raised CR as the above poster said .

Now does your engine (any version) come with 10.5 , How are you raising it, by thinner head gasket, different pistons, different head .

You might have to alter the ignition timing curve but from 9.8 to 10.5 is only 0.7 of a CR, and I think you be fine but you need to test for any pinging or pull back in timing if you have knock sensor .

[Phantom]

If you increase the compression by changing the stroke it will use more fuel since it will be a larger engine pulling more air in. Increasing the compression by shaving the heads, using thinner head/ lower end gaskets, or pistons would be the best way to do it.

As said before it should reduce fuel consumption if it does not significantly drop timing. With that said it could allow the use of larger gears or reduce the need of down-shifting when going uphill, and allow it to up-shift earlier with out lugging the engine.

Do your research on increasing the compression with your motor to see what issues you could run into. I assume that the engine has an over head cam so valve train geometry should not be a problem if shaving the heads or using thinner gaskets but could have valve to piston clearance issues.

What engine are you running?

[atsik]

I thought about head shaving, its relatively cheap here-only 40-50$ + new casket.
and because i use RON 98 anyway then timing is not a big prob, i guess.
engine is 1.8l 8v k56 (renault) with 90hp(66kw).