Ed Blown Vert

iTrader: (2)

Will lowering Compression affect emissions?

Somebody09

Good question. How about raising compression from 10.1:1 to 11.0:1?

charlie c5

iTrader: (6)

I have the same question...

ArcticZ28

iTrader: (14)

I can't explain it better than this, so here you go...

"Most people know that an increase in Compression Ratio will require an
increase in fuel octane for the same engine design. Increasing the
compression ratio increases the theoretical thermodynamic efficiency of an
engine according to the standard equation

Efficiency = 1 - (1/compression ratio)^gamma-1

where gamma = ratio of specific heats at constant pressure and constant
volume of the working fluid ( for most purposes air is the working fluid,
and is treated as an ideal gas ). There are indications that thermal
efficiency reaches a maximum at a compression ratio of about 17:1 for
gasoline fuels in an SI engine [23].

The efficiency gains are best when the engine is at incipient knock, that's
why knock sensors ( actually vibration sensors ) are used. Low compression
ratio engines are less efficient because they can not deliver as much of the
ideal combustion power to the flywheel. For a typical carburetted engine,
without engine management [27,38]:-

Compression Ratio -- Octane Number Requirement -- Brake Thermal Efficiency (WOT)
5:1 -- 72 -- xx
6:1 -- 81 -- 25 %
7:1 -- 87 -- 28 %
8:1 -- 92 -- 30 %
9:1 -- 96 -- 32 %
10:1 -- 100 -- 33 %
11:1 -- 104 -- 34 %
12:1 -- 108 -- 35 %

Modern engines have improved significantly on this, and the changing fuel
specifications and engine design should see more improvements, but
significant gains may have to await improved engine materials and fuels."

Somebody09

lol thanks! Clear as mud. Do you have a "yes" or "no" for us???

ArcticZ28

iTrader: (14)

Yes, it will affect emissions. The lower the CR, the less efficient the burn. Of course this can be mostly fixed with tuning so that the degradation of emissions would only be slightly marginal.

dyls1

iTrader: (11)

So on the other hand, if you slightly raise the cr it would not hurt smogability and could possibly help?

ArcticZ28

iTrader: (14)

Well, like I said, going from 10:1 to 11:1 or vice versa isn't going to make a huge difference on emissions, especially if you tune for it. I wouldn't raise CR just for emissions help, as much more can be done in the tune much easier.

slow trap

iTrader: (22)

something i heard that someone might know also= don't an engine run more efficient at say 210-220* than say 165-180*,smog wise i mean ?

CamKing

When I had trouble passing the smog test, I just blended 20% methenol in with the gas.