Compression Increase
11-11-2006, 11:13 PM
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Compression Increase
We've got an RX8 motor to play with for a 914 install and may be putting some new seals in it. It seems like there must be some room to bump compression.
Between the turbos and superchargers there seems to be a pretty safe boost of 5-6 PSI. Despite being intercooled that would result in an equivalent boost of compression of about 30%. If the Otto cycle works the same on a rotary as a piston motor, that suggest there is another 7-9% in efficiency remaining.
Granted boosting compression would take some tuning probably fattening up the mixture with less advance in the timing.
Building compression through adding material to the rotor chambers through wire welding (low carbon) then machining for those wondering how.
Does anyone have any experience with doing this?
Between the turbos and superchargers there seems to be a pretty safe boost of 5-6 PSI. Despite being intercooled that would result in an equivalent boost of compression of about 30%. If the Otto cycle works the same on a rotary as a piston motor, that suggest there is another 7-9% in efficiency remaining.
Granted boosting compression would take some tuning probably fattening up the mixture with less advance in the timing.
Building compression through adding material to the rotor chambers through wire welding (low carbon) then machining for those wondering how.
Does anyone have any experience with doing this?
11-12-2006, 09:55 AM
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Yes, tuning is paramount with FI. In a static equivalent that raises compression to about 13:1. For an NA RX8 that could be leaving about 10 ft lbs of torque on the table across the entre RPM band. Could be anyway. I was hoping to find out if anyone had any practical experience with compression adjustments.
11-12-2006, 10:39 AM
#4
Bummed, but bring on OU!
The Renesis won't see an appreciable increase in power by raising compression. 10:1, 10.5:1, 11:1 etc, it's all the same. Rotarygod will show up and explain it, at some point.
11-12-2006, 06:24 PM
#5
yep rotary is a differant animal when it comes to the compression increasing the hp. the flame front is much differant, and we lose a lot of power via heat as we carry the combustion for a much longer time!
if you want to play with the renesis--and want MAJOR gains--you have to fi. If you want 260-270 hp then that can be done without fi.
olddragger
if you want to play with the renesis--and want MAJOR gains--you have to fi. If you want 260-270 hp then that can be done without fi.
olddragger
11-12-2006, 07:18 PM
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Oldragger,
We're not looking for major gains, in fact if my son would have wanted a turbo, he could have spent less on a complete low mileage RX7 motor. He likes the way the Renesis revs.
One of the builders I knew back in the 80's that built a few of the front running IMSU GTU cars said they ran about 30cc's in the rotor bowls with a total Combustion Chamber Volume of 44 cc's. Thats about 15:1.
We're not looking for major gains, in fact if my son would have wanted a turbo, he could have spent less on a complete low mileage RX7 motor. He likes the way the Renesis revs.
One of the builders I knew back in the 80's that built a few of the front running IMSU GTU cars said they ran about 30cc's in the rotor bowls with a total Combustion Chamber Volume of 44 cc's. Thats about 15:1.
11-13-2006, 07:09 AM
#8
Ok --i see where your thoughts are coming from. with the renesis-engine management is the stumbling block for us rx8 owners. we like to keep things like cruise control, knock sensors and other such fu-fu's. If you are outside of the rx8(which you are) many things can be done. For details contact Paul at MazParts(vendor on this forum), Speed Source, MazSport(again on this forum),Racing Beat etc. DOnt forget your cooling capacity in this swap! There is a better water pump out etc.
If you dont mind going internal--changing to better seals can be a real good thing for higher rpm power. Porting hasnt been proven yet--but I personally dont know of anyone that has attempted that with a differant management system than stock. Now porting WITH a compression increase sure sounds interesting to me.
I am also interested in Mikroniting the Rotor housing.
260-270hp from 1.3L with a useable 10-11K out of a na renasis without DI is amazing.
More things--you may want to look at the mop conversion, ground the individual coils back to the neg post of the battery and a light flywheel and pulleys really help with overall performance. DONT run 5w/20 oil--it will not carry factory oil pressure spec if the ambient temps(and oil temps) are high. 5W/30 is the street oil to use.
Now I shutup.
olddragger
keep us informed about your project
If you dont mind going internal--changing to better seals can be a real good thing for higher rpm power. Porting hasnt been proven yet--but I personally dont know of anyone that has attempted that with a differant management system than stock. Now porting WITH a compression increase sure sounds interesting to me.
I am also interested in Mikroniting the Rotor housing.
260-270hp from 1.3L with a useable 10-11K out of a na renasis without DI is amazing.
More things--you may want to look at the mop conversion, ground the individual coils back to the neg post of the battery and a light flywheel and pulleys really help with overall performance. DONT run 5w/20 oil--it will not carry factory oil pressure spec if the ambient temps(and oil temps) are high. 5W/30 is the street oil to use.
Now I shutup.
olddragger
keep us informed about your project
11-13-2006, 08:36 AM
#11
Racing Beat has done rotor lightening to save on the bearings if taking it up to a higher RPM.
0.2 lbs per rotor
http://www.racingbeat.com/resultset....rtNumber=10008
0.2 lbs per rotor
http://www.racingbeat.com/resultset....rtNumber=10008
11-13-2006, 11:14 AM
#13
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A compression increase isn't going to help. It actually isn't a reason why the Renesis has more power over the 13B. I have a paper somewhere that I'll have to dig up. It's an old SAE paper from Mazda about compression. They found long ago that there was very little power difference between using a 9.0:1 compression ratio or an 11.0:1 ratio. It stayed pretty much equal in that entire range. Above and below that power fell off pretty good. People still have yet to get more power out of a 13B using Renesis rotors in it than they can get with 9.7:1 rotors from the RX-7. They just don't get it.
Mazda used higher compression for one reason, emissions. That may sound strange but a higher compression ratio leaves less room in the chamber after the exhaust has left. They are trying to control how much exhaust gas gets carried back around to the intake side again vs how much is coming in from the outside. The side port configuration does allow unburned hydrocarbons to come back around for emissions reasons but they've used the smaller dish of the higher compression engine to control the amount. It was all calculated. If they lowered compression a little bit, power wouldn't suffer but emissions would go up. Strange but true.
There is also something else to think about. Remember that in a piston engine, we are compressing air in one spot. It doesn't move. We bring it in the engine, leave it there, compress it in place and then send it on it's way. In a rotary we don't do that. We are physically moving the air around the engine. When the engine compresses the air, it not only squishes it, it is also still moving it. Air will actually momentarily flow backwards through the dish in the rotor face during compression. You'd have to see a moving rotor in person to fully understand this. The higher the compression ratio, the smaller the dish which means the smaller the available space for air to flow through. This presents us with a restriction inside the engine and is the reason why power doesn't go up with higher compression after a point.
Now I've heard people say that when we boost an engine we are adding more air to that smaller space so we shouldn't be able to make any more power if the above were true. Remember though that when we add boost, we not only add pressure, we add oxygen. This added oxygen makes a bigger bang which obviously overcomes the forces of internal air restriction. Just simply raising the compression ratio doesn't do this. We aren't adding more oxygen to the mix to make up for it so we have nothing to offset it. This is what nitrous does. It makes a bigger bang not from an increase in airflow but through an increase in oxygen content.
Raising the compression ratio just isn't going to do anything for us. Even the highest rotary race engines only have 10.0:1 compression ratios but some still 9.7:1. I remember talking to Eamon Hurley several years ago and he ran experiments on compression ratio by welding in the rotor faces and then milling them out and rebalancing them to the compression ratio he wanted. He told me back then that 10.0:1 was the highest he could justify going. It's interesting that several years later that holds true in a new engine.
If you really want to get confusing, I can get into how negative timing split can somewhat change the ruels of compression when it comes to non forced induction applications. I'll save that one for another time though.
Mazda used higher compression for one reason, emissions. That may sound strange but a higher compression ratio leaves less room in the chamber after the exhaust has left. They are trying to control how much exhaust gas gets carried back around to the intake side again vs how much is coming in from the outside. The side port configuration does allow unburned hydrocarbons to come back around for emissions reasons but they've used the smaller dish of the higher compression engine to control the amount. It was all calculated. If they lowered compression a little bit, power wouldn't suffer but emissions would go up. Strange but true.
There is also something else to think about. Remember that in a piston engine, we are compressing air in one spot. It doesn't move. We bring it in the engine, leave it there, compress it in place and then send it on it's way. In a rotary we don't do that. We are physically moving the air around the engine. When the engine compresses the air, it not only squishes it, it is also still moving it. Air will actually momentarily flow backwards through the dish in the rotor face during compression. You'd have to see a moving rotor in person to fully understand this. The higher the compression ratio, the smaller the dish which means the smaller the available space for air to flow through. This presents us with a restriction inside the engine and is the reason why power doesn't go up with higher compression after a point.
Now I've heard people say that when we boost an engine we are adding more air to that smaller space so we shouldn't be able to make any more power if the above were true. Remember though that when we add boost, we not only add pressure, we add oxygen. This added oxygen makes a bigger bang which obviously overcomes the forces of internal air restriction. Just simply raising the compression ratio doesn't do this. We aren't adding more oxygen to the mix to make up for it so we have nothing to offset it. This is what nitrous does. It makes a bigger bang not from an increase in airflow but through an increase in oxygen content.
Raising the compression ratio just isn't going to do anything for us. Even the highest rotary race engines only have 10.0:1 compression ratios but some still 9.7:1. I remember talking to Eamon Hurley several years ago and he ran experiments on compression ratio by welding in the rotor faces and then milling them out and rebalancing them to the compression ratio he wanted. He told me back then that 10.0:1 was the highest he could justify going. It's interesting that several years later that holds true in a new engine.
If you really want to get confusing, I can get into how negative timing split can somewhat change the ruels of compression when it comes to non forced induction applications. I'll save that one for another time though.
Last edited by rotarygod; 11-13-2006 at 11:17 AM.
11-19-2006, 08:56 AM
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It isnt how much the mixture is compressed that makes hp gains in turbo cars. Its all about volume efficiency. If you raise the compession ratio you still got the same volume efficiency and therefor no hp gains. However if you use a turbo you raise VE and more air means higher compression. So boost isnt the same as compression.
11-19-2006, 09:47 AM
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Volumetric efficiency, compression, and charge mass are all seperate factors. I was only dealing with one of them.
In most internal combustion engines the higher the compression, the more efficient mechanical power is extracted from the chemical reaction of the charge. That applies to both pistons and turbines.
Typically increasing compression from 8:1 to 11:1 on a piston engine results in a 12-15% increase in power. According to the Otto cycle for a piston engine the gains in efficiency diminish as compression increases to a point where much above 15:1 offers little improvement. GE has published similar slopes on SAE papers regarding turbines and used that higher compression factor to cut fuel consumption by as much as 40% on comparable engines.
On a piston engine the problem with raising compression is with detonation and NOx emissions. On a Turbine the problem is with Turbine blade tempuratures (which GE cleverly added internal cooling vents in the blades).
So the question was based around increasing compression on the rotary, not the charge mass. More importantly we were hoping to find someone who may have some actual experience with changing compression on the Renesis. So far no one on the forum has actually come forward with practical experience, and maybe with good reason.
It's not that big of a deal to us. We'll just end up running stock compression but have chosen to let someone with Renesis experience do some mild porting.
Interestingly I did get a reply back from someone who runs one in a boat and they felt that 14:1 shows definate improvement.
In most internal combustion engines the higher the compression, the more efficient mechanical power is extracted from the chemical reaction of the charge. That applies to both pistons and turbines.
Typically increasing compression from 8:1 to 11:1 on a piston engine results in a 12-15% increase in power. According to the Otto cycle for a piston engine the gains in efficiency diminish as compression increases to a point where much above 15:1 offers little improvement. GE has published similar slopes on SAE papers regarding turbines and used that higher compression factor to cut fuel consumption by as much as 40% on comparable engines.
On a piston engine the problem with raising compression is with detonation and NOx emissions. On a Turbine the problem is with Turbine blade tempuratures (which GE cleverly added internal cooling vents in the blades).
So the question was based around increasing compression on the rotary, not the charge mass. More importantly we were hoping to find someone who may have some actual experience with changing compression on the Renesis. So far no one on the forum has actually come forward with practical experience, and maybe with good reason.
It's not that big of a deal to us. We'll just end up running stock compression but have chosen to let someone with Renesis experience do some mild porting.
Interestingly I did get a reply back from someone who runs one in a boat and they felt that 14:1 shows definate improvement.
04-12-2013, 03:24 PM
#18
Volumetric efficiency, compression, and charge mass are all seperate factors. I was only dealing with one of them.
In most internal combustion engines the higher the compression, the more efficient mechanical power is extracted from the chemical reaction of the charge. That applies to both pistons and turbines.
Typically increasing compression from 8:1 to 11:1 on a piston engine results in a 12-15% increase in power. According to the Otto cycle for a piston engine the gains in efficiency diminish as compression increases to a point where much above 15:1 offers little improvement. GE has published similar slopes on SAE papers regarding turbines and used that higher compression factor to cut fuel consumption by as much as 40% on comparable engines.
On a piston engine the problem with raising compression is with detonation and NOx emissions. On a Turbine the problem is with Turbine blade tempuratures (which GE cleverly added internal cooling vents in the blades).
So the question was based around increasing compression on the rotary, not the charge mass. More importantly we were hoping to find someone who may have some actual experience with changing compression on the Renesis. So far no one on the forum has actually come forward with practical experience, and maybe with good reason.
It's not that big of a deal to us. We'll just end up running stock compression but have chosen to let someone with Renesis experience do some mild porting.
Interestingly I did get a reply back from someone who runs one in a boat and they felt that 14:1 shows definate improvement.
In most internal combustion engines the higher the compression, the more efficient mechanical power is extracted from the chemical reaction of the charge. That applies to both pistons and turbines.
Typically increasing compression from 8:1 to 11:1 on a piston engine results in a 12-15% increase in power. According to the Otto cycle for a piston engine the gains in efficiency diminish as compression increases to a point where much above 15:1 offers little improvement. GE has published similar slopes on SAE papers regarding turbines and used that higher compression factor to cut fuel consumption by as much as 40% on comparable engines.
On a piston engine the problem with raising compression is with detonation and NOx emissions. On a Turbine the problem is with Turbine blade tempuratures (which GE cleverly added internal cooling vents in the blades).
So the question was based around increasing compression on the rotary, not the charge mass. More importantly we were hoping to find someone who may have some actual experience with changing compression on the Renesis. So far no one on the forum has actually come forward with practical experience, and maybe with good reason.
It's not that big of a deal to us. We'll just end up running stock compression but have chosen to let someone with Renesis experience do some mild porting.
Interestingly I did get a reply back from someone who runs one in a boat and they felt that 14:1 shows definate improvement.
What fuel was being used in that boat with the 14:1 compression rotary?
What compression did you finally settle on for your son's 914?
The primary advantage I see with a bump in compression is not making more peak power; but as a bandaid for the low rpm compression leakage we all have.
Over this past week, I was studying the calculated load reported by the PCM during my commute to and from work. At idle, it's reporting 31-33%. Is it any wonder that we're slow off the line and have crummy fuel economy in stop and go traffic?
I'm also curious how the dimensions of our combustion pocket compare to the cavity on Mazda's cavity pistons on the Sky-G engines (width and depth, as length is obviously greater).
07-21-2014, 06:57 PM
#21
Driving my unreliable rx8
There is x amount of room in the engine to hold air. increasing the rotor compression will also reduce the engine displacement volume. Less volume means less oxygen and fuel. That is easy to deal with on a piston engine you can change the stroke and rod length. We can't change the stroke on a rotary. not without changing eccentric shaft and rotor housings also. So you end up loosing oxygen.
Then you have to consider overall chamber pressures. Boost times compression ratio. Lets make this one easy Compare a 9:1, 10:1, and 15:1 ratio at the same Displacement and 6 PSI boost
Absolute=14PSI+6=20PSI
20psi*9:1=180PSI chamber pressure
20psi*10:1=200 PSI
20psi*15:1=300 PSI
The higher the chamber pressure the more likely you'll have a ping. So you have to retard timing and retard power. But lets get them all to the same chamber pressure so we can use the same timing on all of them.
300PSI/10:1=30PSI-14PSI atmosphere=16 PSI boost
300/9:1=33.3PSI-14=19.3 PSI boost
There is about 29LB of air in a renesis stock.
so we have 3 pressure ratios from boost.
20/14=1.42
30/14=2.14
33/14=2.35
before figuring other losses from heat you can multiply that number times your air to get max air. More air means more fuel to burn and more power.
29*1.42=41.18LB air
29*2.14=62.06 LB
29*2.35=68.15Lb
So you can possibly get more percentage of power out of every LB of fuel by raising compression ration, but you can get more overall power by throwing in more air and fuel to burn with lower compression.
Now there's other losses due to turbo efficiency and inter cooler eficiency that reduce the volume of air due to increased temp.
Technically due to these thermal and mechanical losses somewhere around 16PSI of boost at sea level you should double your stock horsepower.
The whole MM saying flow matters not pressure is complete and total bullshit. If that was the case then MAP tuning would not work. Yes more flow = more power, but when you double the air in a space you will double the pressure. I believe it is Boyle's law, but I'll have to double check that. I've got to get to work for now. I'll get back in a bit.
Then you have to consider overall chamber pressures. Boost times compression ratio. Lets make this one easy Compare a 9:1, 10:1, and 15:1 ratio at the same Displacement and 6 PSI boost
Absolute=14PSI+6=20PSI
20psi*9:1=180PSI chamber pressure
20psi*10:1=200 PSI
20psi*15:1=300 PSI
The higher the chamber pressure the more likely you'll have a ping. So you have to retard timing and retard power. But lets get them all to the same chamber pressure so we can use the same timing on all of them.
300PSI/10:1=30PSI-14PSI atmosphere=16 PSI boost
300/9:1=33.3PSI-14=19.3 PSI boost
There is about 29LB of air in a renesis stock.
so we have 3 pressure ratios from boost.
20/14=1.42
30/14=2.14
33/14=2.35
before figuring other losses from heat you can multiply that number times your air to get max air. More air means more fuel to burn and more power.
29*1.42=41.18LB air
29*2.14=62.06 LB
29*2.35=68.15Lb
So you can possibly get more percentage of power out of every LB of fuel by raising compression ration, but you can get more overall power by throwing in more air and fuel to burn with lower compression.
Now there's other losses due to turbo efficiency and inter cooler eficiency that reduce the volume of air due to increased temp.
Technically due to these thermal and mechanical losses somewhere around 16PSI of boost at sea level you should double your stock horsepower.
The whole MM saying flow matters not pressure is complete and total bullshit. If that was the case then MAP tuning would not work. Yes more flow = more power, but when you double the air in a space you will double the pressure. I believe it is Boyle's law, but I'll have to double check that. I've got to get to work for now. I'll get back in a bit.
07-22-2014, 07:17 AM
#25
That would only hold if hot spot control is not likewise controlled. Based on what I have seen of Evans Waterless being used in GT1 racing with compression ratios higher than those being used by Mazda in their latest piston engines, as well as work done by Koenigsegg, I believe there is room for improvement in hot spot control that would remove the need to lower the AFR or retard timing, since both are necessitated by failure to adequately control hot spots.
