Lower Compression question

[Qiko]

Hey Guys,

Out of curiosity. What would happen if I lower the compression ratio on a f22 engine from Honda's 8.8:1 to 8.1:1, and still run the same honda's ECU? Including running the stock head too?

Does anyone knows?

I was looking at the H22 Rods & 4g63 stroker pistons mod thread

[Joey GT-R]

I'm not familiar with the mapping on an F22A1 but for lack of better words your car will be more of a turd. Depending on the condition of the engine overall you could be looking at only making about 100whp. I would estimate that you probably would be running running slightly rich due to the ECU injecting fuel for a higher compression ratio. So you're probably losing gas mileage and power in the same token. If you're trying to build a turbo motor I would just try to source a block on the side.

[Jarrett]

The pistons needed for this setup are aftermarket 4G63 pistons with a 28.7mm compression height. The benefit with them being aftermarket is that there are multiple dish volumes available. The bad thing is that they're all focused toward turbocharged applications. What this means for us is a loss in volumetric efficiency in a "typical" application. Some compression can be regained with milling the head slightly or, the more expensive option, ordering custom pistons. It's not much more expensive, though, and may be worth looking into. If all other specs of the aftermarket, off-the-shelf stroker piston remained the same, you may get a better price. You would just have them cc less material out of the dome. A flat-top piston in that application would net you around 10.7:1 without milling. You can go up or down from there depending on what your application is. You didn't allude to what your goals were so I can't really help you much further.

[wagon-r]

I'm not familiar with the mapping on an F22A1 but for lack of better words your car will be more of a turd. Depending on the condition of the engine overall you could be looking at only making about 100whp. I would estimate that you probably would be running running slightly rich due to the ECU injecting fuel for a higher compression ratio. So you're probably losing gas mileage and power in the same token. If you're trying to build a turbo motor I would just try to source a block on the side.

Sorry, Joey, I have to disagree with most of what you said.

The compression ratio has no impact on the amount of fuel injected. In the absence of boost, this is determined by the cylinder displacement (and engine rpm). Further, decreasing the compression ratio, if anything, is more likely to increase the cylinder displacement. But in any case, the difference would be very small, if not zero, and would be easily corrected by the oxygen sensor.

Higher compression ratio extracts more useful work from the fuel combusted. A point of compression is worth about 2-3% on fuel economy and power (per Professor Haywood at MIT). So, dropping from 8.8 to 8.1 would likely cost you less than 5 hp and 3% fuel economy.

[Joey GT-R]

You sound like a better engineer than I am, so I'll concede to your point. What we can agree on is the loss in power and efficiency, however brash or slight that may be.

[Jarrett]

Wagon-r, are you saying that decreasing the static compression ratio of an engine can decrease it's displacement?

[wagon-r]

Wagon-r, are you saying that decreasing the static compression ratio of an engine can decrease it's displacement?

I was saying the opposite. That decreasing the static compression ratio can increase displacement.

But upon further reflection, I think I was incorrect and it can go either way. For example:
- Decreasing the rod length will lower the compression ratio without affecting displacement
- Increasing the rod length will increase the compression ratio without affecting displacement.
- Decreasing the stroke will decrease both displacement and compression ratio.
- Larger piston bore will increase both displacement and compression ratio.
- Mixing the previous elements can either decrease or increase displacement while lowering compression ratio, depending on what is done.

[Jarrett]

There we go.

[johnl]

The compression ratio has no impact on the amount of fuel injected. In the absence of boost, this is determined by the cylinder displacement (and engine rpm).

More or less. Correct AFR is determined by the quantity of air ingested on the induction stroke, which is a function of the manner in which gasses move in the induction and exhaust systems (cam timing, pipe lengths, diameters etc. etc. etc.). Lowering the CR shouldn't really affect this, but will affect how efficiently the fuel is burnt.

I can't see any reason to want such a low CR as being suggested other than if intending to run very high levels of boost. The engine would be quite inefficient at all times the boost wasn't near maximum (and if boost pressure isn't high then the engine will always be inefficient). I suspect a better result would be to use a somewhat higher CR and to fine tune the tune more carefully.

Trying to supercharge using the stock ECU is unlikely to be advisable, regardless of CR. If you force more air in, you need to inject more fuel, or else the AFR will lean out and burn hot, regardless of the CR. Lean AFR may (depending on how lean) lead to all sorts of potentially serious problems. This pertains to open loop (read WOT), in closed loop the ECU should be able to 'trim' the fuel adequately based on lambda readings (i.e. O2 sensor), to account for changes in ingested air quantity (up to a point).

If the engine is stroked you still have a similar problem, the engine will (in theory) ingest more air, and the stock tune will cause the AFR to lean out (to some degree). You may well get away with it, depending on how much the 'stroking' increases the swept volume, and how rich the stock open loop tune tends to fuel the engine at WOT, but...

Further, decreasing the compression ratio, if anything, is more likely to increase the cylinder displacement. But in any case, the difference would be very small, if not zero, and would be easily corrected by the oxygen sensor.

Swept volume (i.e. cylinder displacement) is completely unaffected by CR. This is a function of stroke length X piston diameter, only. The cylinder volume is larger with a deeper dish in the piston (or metal removed from the cylinder head depression), but this is static volume, it doesn't mean that more air is 'sucked' into the cylinder as the piston descends on the induction stroke.

Higher compression ratio extracts more useful work from the fuel combusted. A point of compression is worth about 2-3% on fuel economy and power (per Professor Haywood at MIT). So, dropping from 8.8 to 8.1 would likely cost you less than 5 hp and 3% fuel economy.

Sounds about right.