Effect of compression on Air/Fuel ratios

[AE86fan]

I have been internalizing a really complex situation in my head........

I have changed my set up from 4age turbo (compression approx 9:1) to 4age 16 valve NA with quad throttles (compression approx 12.5:1).

I havent changed the injectors or fuel pressure. I started the car up while its still running the turbo tuning (link V5) and its running really rich. I havent got an o2 hooked up yet but the idle is really fat and the exhaust is obviously rich.

As I decrease the amount of fuel in the map the idle increases and it starts to run sweet, but it is requiring about 30% less fuel than the old setup.

So the question is.....is the increased compression the reason why the engine needs less fuel (at idle), i.e more compression = more effeciency....or would it be the effect of the engine having to drag the air through the turbo setup compared to the now very short and direct throttles.

I have been reading up on engine management tuning but just cant get the answer and explanation Im looking for.

Cheers

[iOnic]

Youve decreased the size of the combustion chamber and bumped compression up ~33% so you're injecting the same amount of fuel into a smaller combustion chamber which = too much fuel.

I'm drunk as $&#$% so if that makes no sense, that's why.

[Grrrrrrr!]

You are using a link, which use MAP sensor (vacuum/boost) as a load factor in fuelling calcs. You've changed the intake and exhaust dynamics significantly, so 10psi vacuum no longer means the same thing in term of airflow, hence old map = useless. Even more so if you've changed the cams too.

[AE86fan]

iOnic, I understand what you are saying but the cc hasnt changed so whats happening inside the combustion chamber, regardless of change to the squish area shouldn't change.....I think??

Grrrrrr!, what your saying kinda makes sense to me......so the MAP pressure isnt relevant because the air volume is different due to changes in the exhaust and inlet tract....... which going by that would indicate that there is less airflow for a given kpa ie the engine is running rich. Which is the opposite to what I would have thought given I should have a better flowing inlet and exhaust than turbo setup??? WTF!!

[AE86fan]

Sorry I should clarify, Im only looking at while the engine is idling. I understand the engine needs completley retuning Im just trying to get my head around theory involved in map, air/fuel ratios and how airflow works. Im thinking about having a go at doing some basic road tuning and theres a few things I need to get my head around.

One other question is how does vacuum vary with throttle and load?

[allencr]

Nothing to do with the CR at all, only the air to fuel ratio.
It's how much air/oxygen fills the cylinder when there is a pressure differential - piston sucks it in - NA, compared to how much enters when it is being forced in by a turbo/super/whatever charger.
30% less volume for something that was already at about 1/13 by its weight

[jondee86]

By increasing the compression ratio you have increased the
charge density. This means more power and efficiency out of the
same amount of fuel and air at the same engine conditions.

So you can cut back the over-rich mixture and bring the AFR back
closer to stoich. Your timing will need a bit of tweaking also.

Cheers... jondee86

[RomanV]

GGrrrrrrrrrrrrrrrrrrrr is on the money.

If you tuned your car with an air flow meter, your (fuel) tune would still be pretty well near correct, (or closer at least) as it samples the air directly.

However a MAP sensor works on assumptions/known variables in your intake, to make calculations about how much air there is.
You've changed these variables away from what it is expecting.

[Grrrrrrr!]

Grrrrrr!, what your saying kinda makes sense to me......so the MAP pressure isnt relevant because the air volume is different due to changes in the exhaust and inlet tract....... which going by that would indicate that there is less airflow for a given kpa ie the engine is running rich. Which is the opposite to what I would have thought given I should have a better flowing inlet and exhaust than turbo setup??? WTF!!

Biggest factor would probably be the change to quads, the vacuum signal from quads can be a bit of a bitch to work with from what i've heard.

The change in compression ratio won't make much difference at idle. bugger all air (closed throttle) compressed 9 times isn't really much different from bugger all air compressed 12.5 times.

[QikStarlie]

short story. its the itb's

you need to retune in tps mode

dont bother with map at all