compression+boost?

[Tansey86]

Originally Posted by FreshFRS

im pretty certain that AVCS has a knock sensor internally to help adjust timing.

A positive displacement supercharger will also cause more " sensitivity " to the motor in regards to compression and boost levels because you see max boost the entire power-band, where as for example a turbo or centrifugal style your boost or max boost for that matter is only seen for maybe 3-4k rpms.

[Jesse@JDLAutodesign]

Originally Posted by SkullWorks

also compression helps to spool a turbo,

Are you sure about this? I've had high and low compression motors (with everything else being identical and unchanged) in both my civic and evo. I will agree that the high compression motor is much more responsive (especially out of boost) and based on the logs comes into boost quicker timing wise, but both still made xx psi at a very similar xxxx rpms.

[Tansey86]

Originally Posted by Jesse@JDLAutodesign

Are you sure about this? I've had high and low compression motors (with everything else being identical and unchanged) in both my civic and evo. I will agree that the high compression motor is much more responsive (especially out of boost) and based on the logs comes into boost quicker timing wise, but both still made xx psi at a very similar xxxx rpms.

Well higher compression helps with response/revving so I guess in a way that helps achieve desired psi quicker, maybe not directly by spool time?

[FreshFRS]

Originally Posted by Jesse@JDLAutodesign

Are you sure about this? I've had high and low compression motors (with everything else being identical and unchanged) in both my civic and evo. I will agree that the high compression motor is much more responsive (especially out of boost) and based on the logs comes into boost quicker timing wise, but both still made xx psi at a very similar xxxx rpms.

Thats more the idea, transient response is better with high compression. but it does seem to help spool even if only by a xxx rpms. 300rpm i found with my S2k. went from 11.3:1 stock to 9:1 with a Twin-scroll gt30R pushed boost from 14psi to 23psi for the same power and noticed 300rpm more to spool 14psi.

[Coheed]

Higher compression ratio does not directly impact Volumetric Efficiency of the engine. It helps thermal efficiency, and a higher expansion ratio helps deliver higher pressures on the combustion stroke.

In simple terms, think of a turbo. A turbo compressing air heats it up. Adding an intercooler can increase air density as well as prevent detonation. With the compression ratio of the engine you are doing the same thing, compressing air. Except inside the engine you have no simple way of decreasing the temps. Therefore, the tendency to detonate is higher. Its all just about pressures and temperatures.

Lower compression engine= lower peak pressures and less heat in the cylinders. But with a proper high compression engine you can still use a lot of boost if proper fuels, tuning, and temps are controlled. You just don't want to see detonation, and detonation is more easily prevented by reducing the compression ratio to give a larger margin of error.

[FreshFRS]

8:1 compression N/A motor vs 12.5:1 N/A motor? Which makes more power? Power is determined by how efficiently a motor can convert energy. Therefore a motor with 12.5:1 compression does in fact have a higher volumetric efficiency than a motor with 8:1 compression. the smaller space in the 12.5:1 head allows for more complete fuel burn providing more power per volume of fuel used. That is volumetric efficiency.

[Coheed]

Originally Posted by FreshFRS

8:1 compression N/A motor vs 12.5:1 N/A motor? Which makes more power? Power is determined by how efficiently a motor can convert energy. Therefore a motor with 12.5:1 compression does in fact have a higher volumetric efficiency than a motor with 8:1 compression. the smaller space in the 12.5:1 head allows for more complete fuel burn providing more power per volume of fuel used. That is volumetric efficiency.

You are thinking of thermal efficiency. Volumetric efficiency is the ability of the engine to consume a specific volume of air based on the amount of displacement. Since raising compression ratio doesn't affect how much air the engine consumes, it does not affect volumetric efficiency.

A Rotary engine has great volumetric efficiency, but poor thermal efficiency, because of the low compression ratio in comparison to the otto cycle.

[d1ck]

Originally Posted by Coheed

You are thinking of thermal efficiency. Volumetric efficiency is the ability of the engine to consume a specific volume of air based on the amount of displacement. Since raising compression ratio doesn't affect how much air the engine consumes, it does not affect volumetric efficiency.

A Rotary engine has great volumetric efficiency, but poor thermal efficiency, because of the low compression ratio in comparison to the otto cycle.

To expand on this I will say that the reason that we are seeing high comp engines now is for fuel efficiency. Increasing static compression allows the engine to extract more mechanical energy from the fuel being burned. Note: Given two engines with everything equal except compression ratio they will have identical fuel consumption at a given rpm and MAP. The difference will be that the high compression engine will be producing more torque than the lower compression engine. Because fuel has a finite amount of energy which can be extracted through combustion, a higher compression engine will produce less waste heat (Coolant & Exhaust) and will actually result in an engine which takes slightly longer to spool a turbo. This is because you are getting the same volume of exhaust, but it is slightly cooler. A turbo relies on mass flow rate and heat to spool.

This explains why diesel engines are so much more efficient than gasoline engines. They have compression ratios which can be well over 20:1. Diesel fuel actually contains less energy than gasoline, but due to it's resistance to pre-ignition and detonation we can use higher compression ratios.

[FreshFRS]

oh i get it okay. thanks for the info guys. still doesn't quite explian why my s2k spooled better with high compression than with low compression but i get what you are saying for sure. i think it has to do with how much air is pushed out of the cylinder during the exhaust stroke. 12:1 is going to push more waste air out as compared to 8:1 due to a smaller space at TDC. this could also increase velocity am i right? could you also say it creates more of a vacuum during the intake stroke? therefore allowing the engine to breath in a little more air? im not sure if that is in anyway correct but i can see how it might work like that.


Also diesels burn far cooler than gas engines, its mass flow rate that helps spool not so much temperatures.

[ft_sjo]

8:1 and 12.5:1 means nothing by itself. This is just the static compression ratio and engines, are by far, not very static. This conversation is way off track of actually relevant.

What matters are cylinder pressures. Yes, static CR affects this, but nowhere near as much as the dynamic CR does. The main reason I believe that these tuners are able to simply throw a turbocharger onto such a high SCR engine is because they are controlling the DCR adequately to keep it in one piece. There is no magic in forced induction, sure direct injection helps to some degree, but rarely gives you more than a ratio or so headroom on SCR compared to an equivalent engine.

The bottom line is when the cylinder pressures get too high you get detonation. A high static CR engine is going to have a pretty crappy VE curve to keep it in one piece. No offence to the tuners out there testing at the moment, but most of the bolt-on brigade are producing some very interesting torque curves - some are barely noticeable as forced induction! Some resemble centrifugal superchargers!

The best one in my opinion is the AVO conversion. I'm surprised that it's actually capable of producing such good torque at low engine speeds without det/egt/reliability issues on pump fuel is something to be commended on. I don't know if it's luck or engineering, but I am curious to see some actual data (particularly around final spark angle and the resulting EGT's, both at the exhaust port exit and cat inlet).

Producing a nice wide flat torque band gives you awesome, predictable driveability..

[FreshFRS]

thanks for getting this thread back on track! so how do you calculate dynamic compression? i would love to figure out what ours is would help a ton. if a broad flatish torque curve is best for drivability wouldn't a TVS or twinscrew supercharger be the best in those circumstances? i understand the power limitations, and prefer response to overall power, i get the small turbo idea as well but at a very mild cost in transient response.

[ft_sjo]

First google result: http://cochise.uia.net/pkelley2/DynamicCR.html

I'm not qualified to comment, it's a very complex subject and none of us will have the source data required to calculate it.

[FreshFRS]

Originally Posted by ft_sjo

First google result: http://cochise.uia.net/pkelley2/DynamicCR.html

I'm not qualified to comment, it's a very complex subject and none of us will have the source data required to calculate it.

I agree that its somewhat complex but if i could get all the little pieces of source data i could easily figure out what our dynamic compression ratio is. my guess based on a few points of data (estimates really) i have seen throughout the forum is that our DCR is going to be about 9:1, however variable valve timing throws a monkey wrench in and allows for a big range of dynamic compression in order to control knock (say range from 7.5:1-9:1). this is by all means only a guess so don't quote me on that one.

[ft_sjo]

No, you've got it. The VVT is what makes or breaks this kind of turbo conversion.