Lower compression effects

[Kodename47]

Originally Posted by Irace86.2.0

IAM?

Ignition Advance Multiplier

[Hags86]

Originally Posted by SirSpectre

Good info. Pulled the trigger on JE 12.5:1 pistons.

You won't regret it.

[wparsons]

Originally Posted by Irace86.2.0

I don’t think this is a factor. It is more of a product of the trade off of lower compression and higher boost motors versus higher compression and lower boost. Higher IATs are bad, but higher compression creates higher temps, so 300hp is 300hp, no matter where the temps originate. There might be an argument for better manifold air velocity with colder IATs, but I would have to see the data. The bigger differences are at the low end—not the high end.

That's just not true. Higher compression does not create higher IAT's at all. IAT also doesn't directly dictate combustion temperature or EGT either. If you're getting at heat generated by compressing the mixture you have to remember that the turbo is compressing air alone, the piston is compressing air and fuel and it won't react the same.

Higher IAT's do make it a lot more prone to detonation though.

[Irace86.2.0]

Originally Posted by wparsons

That's just not true. Higher compression does not create higher IAT's at all. IAT also doesn't directly dictate combustion temperature or EGT either. If you're getting at heat generated by compressing the mixture you have to remember that the turbo is compressing air alone, the piston is compressing air and fuel and it won't react the same.

Higher IAT's do make it a lot more prone to detonation though.

I didn't say higher compression creates higher IATs. Read it again.

You said higher boost pressures create higher IATs...

higher boost pressure = more heat in the intake charge too

You were implying low compression with high boost (versus high compression with lower boost) is a worse case scenario because of higher IATs related to needing higher boost to reach the same power. Where you not implying that?

I was saying that higher compression creates higher combustion chamber temperatures, so whether those temps happen during compression or from hotter air entering the chamber, both are similar enough. Essentially, I was saying that a motor that hits 300hp at redline might see similar combustion temps, or EGTs, as you brought up, in a high compression low boost setup as well as in a lower compression higher boost setup.

The biggest differences are going to be at lower rpms.

[wparsons]

Originally Posted by Irace86.2.0

I didn't say higher compression creates higher IATs. Read it again.

You said they both create equal heat, I was explaining how that's not applicable.

Originally Posted by Irace86.2.0

You said higher boost pressures create higher IATs...

Yes it does, that's a scientific fact. Boyle's law in action.

Originally Posted by Irace86.2.0

You were implying low compression with high boost (versus high compression with lower boost) is a worse case scenario because of higher IATs related to needing higher boost to reach the same power. Where you not implying that?

There's nothing implied, I flat out stated that lower compression needs more boost which raises IAT, for the same power output. If the pistons and rods are strong enough for the cylinder pressures, the higher compression engine will make power more efficiently.

Originally Posted by Irace86.2.0

I was saying that higher compression creates higher combustion chamber temperatures, so whether those temps happen during compression or from hotter air entering the chamber, both are similar enough. Essentially, I was saying that a motor that hits 300hp at redline might see similar combustion temps, or EGTs, as you brought up, in a high compression low boost setup as well as in a lower compression higher boost setup.

And that's what I was clarifying... higher IAT doesn't directly mean higher combustion temperature or EGT. Lower static compression typically lowers the volumetric and thermal efficiency of the engine as well.

If you have high IAT you'll have to run less timing advance, which can increase EGT if it goes too far, but a high IAT doesn't automatically increase the combustion temperature or EGT directly.

The amount that the air is heated by a turbo is different than how much the fuel/air mixture is heated by the piston during the compression stroke, so your assumption that 300hp worth of heat is 300hp worth of heat regardless of how it's made isn't accurate. That was my whole original point.

Originally Posted by Irace86.2.0

The biggest differences are going to be at lower rpms.

IAT and combustion temperature isn't really RPM dependent. It'll be more related to load and spark timing. You'll likely find that peak cylinder pressures are at the torque peak, not hp peak. IAT won't be RPM related at all on an N/A engine, but if it's turbo it'll climb with boost pressure (which is indirectly RPM related).

[Irace86.2.0]

Originally Posted by wparsons

]Yes it does, that's a scientific fact. Boyle's law in action.

Slow down. Read what I wrote. I wasn’t refuting that.

The amount that the air is heated by a turbo is different than how much the fuel/air mixture is heated by the piston during the compression stroke, so your assumption that 300hp worth of heat is 300hp worth of heat regardless of how it's made isn't accurate. That was my whole original point.

You can state it, but you haven’t given reason for it. Higher compression creates more heat because of gas laws, so you haven’t proved that higher IATs are bad. You have only moved the heat from the intake to the combustion chamber and said it is better by restating that higher compression is more efficient, so stating IATs are lower doesn’t state anything new.


IAT and combustion temperature isn't really RPM dependent. It'll be more related to load and spark timing. You'll likely find that peak cylinder pressures are at the torque peak, not hp peak. IAT won't be RPM related at all on an N/A engine, but if it's turbo it'll climb with boost pressure (which is indirectly RPM related).

This is what I believe to be the case: at low rpms there is less flow of fresh air into the combustion chamber. At high static compression there is an increased risk of knock at all ranges, but especially at low rpms where there can be more pockets of heat. Turbos can get around having a high effective compression ratio because they don’t alter the compression ratio at low rpms very much. This is why the OP wanted to drop compression because he can keep his effective compression ratio low at low rpms.

I was saying that there will be a safe limit for the upper end too. Let’s say it is 300hp. At that level I was arguing that it doesn’t matter if the IATs are higher for a low compression/high boost motor because the high compression/low boost motor is at it’s limit too, partially because high compression will also raise temps before combustion, reaching a knock limit.

The clear advantage is that this motor can manage low rpm knock with the D4S system, so dropping compression isn’t necessary and only results in poorer low end performance and boost lag/threshold. I don’t believe peak numbers should change much.

[wparsons]

Originally Posted by Irace86.2.0

This is what I believe to be the case: at low rpms there is less flow of fresh air into the combustion chamber.

Less flow how/why? On an N/A engine, the fuel/air mixture is drawn in by the vacuum created by the piston descending, regardless of RPM. The MAF sees more air at higher RPM's because the cylinders needs to be filled more frequently, but the volume of air drawn in by the cylinder on each stroke isn't going to be heavily RPM dependent.

Don't forget that valve timing (and lift on a VTEC engine) will have way more effect on this.

Originally Posted by Irace86.2.0

This is why the OP wanted to drop compression because he can keep his effective compression ratio low at low rpms.

Where did he say that at all?

Originally Posted by Irace86.2.0

I was saying that there will be a safe limit for the upper end too. Let’s say it is 300hp. At that level I was arguing that it doesn’t matter if the IATs are higher for a low compression/high boost motor because the high compression/low boost motor is at it’s limit too, partially because high compression will also raise temps before combustion, reaching a knock limit.

Again, missing the point. A higher compression ratio will have a better volumetric efficiency regardless of boost pressure. The temperature increase through compression by the piston isn't going to be the same as by the turbo. Both create heat, but they won't be equal or equivalent.

Originally Posted by Irace86.2.0

The clear advantage is that this motor can manage low rpm knock with the D4S system, so dropping compression isn’t necessary and only results in poorer low end performance and boost lag/threshold. I don’t believe peak numbers should change much.

Even pure port injection can run high compression and boost. Direct injection is better at managing knock in any application, especially when running leaner fuel mixtures.

[Irace86.2.0]

Originally Posted by wparsons

Less flow how/why?

As engine rpms rise, residual gases are flushed much faster from the combustion chamber in a more efficient manner. These residual gases at lower rpms can create knock problems, which is discussed here by Mazda in their new engine:

https://www.mazda.com/en/innovation/...iv/skyactiv-g/

The important quote here is that even with the 4-2-1 exhaust, there is still a negative effect of lingering gases at extra-low engine speeds. You can see from the graph that the impact on torque with the higher compression motor is more severe at lower rpms.

Where did he say that at all?

Say it/Imply it. Why else do it? Why do manufactures drop compression on a motor when adding a turbo system over the same NA motors? Do you think he meant something else?

Again, missing the point. A higher compression ratio will have a better volumetric efficiency regardless of boost pressure. The temperature increase through compression by the piston isn't going to be the same as by the turbo. Both create heat, but they won't be equal or equivalent.

Again, I'm not missing the point that higher compression motors are more volumetrically efficient. I've never argued to the contrary. In fact, since the 3rd post in this thread I have stated to keep the stock compression for a number of reasons stated there and throughout this thread.

Again, the point you are missing is that your original statement below seems to be providing misinformation, or rather, no information. It is a statement about what is and not why it is important. Low IATs are better, yes. Turbos make higher IATs, yes. Higher compression motors are more efficient, yes. But this original statement, "higher boost pressure = more heat in the intake charge too"...ok, so what? You say above that, "Both create heat, but they won't be equal or equivalent." Yet you have failed to give any reason to justify this response.

Without anything tangible to go along with that statement, the statement is meritless. It is an unnecessary add on. The OP could ask why that would matter and someone could say, 'cooler IAT's will keep the intake manifold from getting as hot, so I can lean on the manifold without getting burned when servicing my engine', and they would be right.

[wparsons]

You're still missing fundamental things in your argument. Ask the guy tuning the car if IAT matters on a boosted car if you still doubt it has merit.

The skyactiv engine isn't that relevant since it's not boosted and relies heavily on scavenging to empty the the last bit of whatever combusted mixture is in the cylinder still. A turbo engine (assuming a bit of valve overlap) has the pressurized incoming mixture to blow it out.

As for why he was asking about lower c/r, it's probably because the old "rule" was that you needed low compression for boost, not because he's specifically trying to reduce knock at low rpm. Lower c/r gives more margin for error for the tune, but with a good tune you can run higher c/r and boost very safely.

Manufacturers aren't dropping c/r like they used to on boosted engines either, and finding any direct comparison is hard because no one sells an otherwise identical engine that is n/a and boosted. You can find lots of factory boosted engines that have c/r's as high was what used to be considered high for an n/a engine not that long ago. VW/Audi has the 2.0 TFSI at 11.3ish with a small turbo that spools very low in the rpm range. Keep in mind they also have to deal with idiot consumers that'll put 87 octane in an engine that requires 91.

We're also talking about an engine with variable valve timing on both the intake and exhaust cams, by adjusting that in the tune you can easily control the low rpm dynamic compression.

[Irace86.2.0]

Originally Posted by wparsons

You're still missing fundamental things in your argument. Ask the guy tuning the car if IAT matters on a boosted car if you still doubt it has merit.

As for why he was asking about lower c/r, it's probably because the old "rule" was that you needed low compression for boost, not because he's specifically trying to reduce knock at low rpm. Lower c/r gives more margin for error for the tune, but with a good tune you can run higher c/r and boost very safely.

Manufacturers aren't dropping c/r like they used to on boosted engines either, and finding any direct comparison is hard because no one sells an otherwise identical engine that is n/a and boosted. You can find lots of factory boosted engines that have c/r's as high was what used to be considered high for an n/a engine not that long ago. VW/Audi has the 2.0 TFSI at 11.3ish with a small turbo that spools very low in the rpm range. Keep in mind they also have to deal with idiot consumers that'll put 87 octane in an engine that requires 91.

IAT matters. I've said it matters, but not how you have stated it. There is a lot of good conversation here:

https://www.theturboforums.com/threa...-power.346964/

This quote is interteresting:

Auto ignition of fuel is almost completely dependant on temperature and temperature rise from the engine's compression far exceeds the temperature rise from a supercharger.

The old rule is still true. This is one of the reasons why dropping compression can allow for a higher effective compression ratio and absolute more power, especially on pump gas. Modern direct injection and E85 can eliminate a lot of the concerns, so the OP has no reason to gain by dropping compression, unless he is chasing huge numbers and especially wants larger numbers on pump gas, but lower compression has off-boost costs like drops in power and efficiency, so it is best to just stick with stock compression and E85, again, especially if the intent is to not go crazy on power. If I wanted more power than 300whp on pump gas because I had zero access to E85 then I would definitely drop compression.


https://www.sae.org/publications/tec.../2016-01-0703/

A significant issue of a high compression ratio engine for improving fuel economy and low-end torque is prevention of knocking under a low engine speed.

That is from SAE.


Also, what manufactures are doing and what would be best for performance or power don't go hand in hand. Lower compression with high boost motors aren't as fuel efficient. Government regulations for better fuel economy and lower emissions is driving higher compression and direct injection, which can have an advantage of off-boost power and faster turbo spool, but often these engines require 91 octane and don't have the overhead for as much safe boost on pump gas.

In some ways we have a comparison. The FA20D motor has a 12.5:1 compression and the FA20F motor has a 10.6:1 compression. Unless I am mistaken, this power level wouldn't be possible on 93 octane on our cars:

https://www.reddit.com/r/subaru/comm...p_and_347_wtq/

[Irace86.2.0]

To add to this mathematically, Boyle's Law states that pressure is inversely proportional to volume for a fixed temperature, but in all practicality, lowering the volume raises the pressure, as Gay-Lussac's Law states, that pressure and temperature are directly proportional for a fixed volume. What does this mean?

We will see an exponential rise in pressure and temperature as volume decreases...meaning high compression is worse at building heat in the combustion chamber, so this quote of yours...

The temperature increase through compression by the piston isn't going to be the same as by the turbo. Both create heat, but they won't be equal or equivalent.

...is true, except not in the direction you are suggesting (against the high IATs of the turbo). The IAT's from more boost will have much less effect than the heat of higher compression, which again, is why I said that you are really just exchanging one for the other.






https://en.wikipedia.org/wiki/Boyle%27s_law
https://en.wikipedia.org/wiki/Gay-Lussac%27s_law

[wparsons]

The SAE quote is about N/A engines, and has already been addressed by the OEM tuning of the stock engine. Unless you're running at tiny turbo, it won't be an issue. Even worse, the bigger risk of boost down low is bent rods (see high boost from twin screw supercharger issues).

At the end of the day, higher compression ratio is better for thermal and volumetric efficiency, which does help fuel economy, but also power output. Less thermal and volumetric efficiency means it takes more extra boost than the lower c/r to make the same power. That means higher peak cylinder pressures for the same hp output than the high c/r lower boost setup.

[Irace86.2.0]

[falcon_wizard]

For what it’s worth and more from a holistic perspective... the final tuning work was recently completed on my engine. Since I was going to use 91/94 octane (no e85 in my location) The builder/tuner had decided to proceed with 10:1 to maximize long term reliability and detonation protection. I was initially concerned about how this could affect off boost performance, however Final tuning dissipated all my doubts, the car performs amazingly, with power delivery being just as gradual as before, with essentially no lag at all. The car is simply mind boggingly fun to drive, and they we able to bring TQ about 400rpms earlier than before in the low end, as well as achieving a higher tq plateau (up to +60lbft @crank) across the entire mid/top end as well, resulting in lots of extra “area under the curve”. The garrett gt2871 was able to maintain steam to 7500rpm. Note that I was not seeking maximum top end power, but just more usable tq across the entire range for daily driving with the odd hpde. My low boost 91 map now has higher tq across the entire rpm range than my previous stock engine high boost 94 map, with the rest of the setup and unchanged. So for those not going towards higher octane fuel, my feedback on those considering lowering to 10:1 cr would be that very satisfactory results (for my goals) was achieved.