Trying to get my head around this..boost, octane, fuel and porting
05-02-09, 02:29 AM
#26
I've been doing more research and it seems there is no clear answer, mostly lots of people with differing opinions that all make valid points and convincing arguments. I'll try my best to summarize the points I found that add to the conversation at hand while adding some of my own thoughts.
The first thing I would like to mention is that a larger cam, or in our case a ported engine increases and shifts the VE of the engine up in the RPM range as we already established. This also shifts the the peak combustion pressures up in the RPMS, higher rpm's give less flame travel time and less chance of detonation.
An increased overlap (which is a result of porting) reduces peak compression by bleeding off some of the intake charge into the exhaust before ignition. This effect lowers the dynamic compression of the engine.
There is a discussion found here http://web.camaross.com/forums/showthread.php?t=382484 that will make your head spin from all the opinions and technical theory's being tossed around regarding larger cams and their effect on dynamic compression and whether this would allow for an increase in static compression without increasing detonation risks. The thread primarily refers to naturally aspirated engines and wasn't referring to rotaries but the principals would be the same.
Another thread with information regarding boosted applications running high static compression ratios and high overlap cams to prevent detonation is found here http://www.ls1tech.com/forums/advanc...ew-blower.html
here is a quote i took from another site dealing with choosing a cam for motorcycles.
"By using a cam with a longer intake duration, the compression pressure will be similarly reduced because the actual closing of the intake valve will occur with the piston higher in the cylinder. The piston now has less distance to travel to the point of spark plug firing. Less distance to travel on a compression stroke means lower static compression pressure and no pinging. " - (The effect would be similar in a ported rotary)
So after a few hrs of reading various sites and discussions on different types of engines my thoughts and conclusions as far as rotaries are concerned and with as little technical jargon as possible are.....
Porting rotaries increases the overlap which reduces dynamic compression, porting also lends itself for higher RPM operation which reduces the time for detonation to occur because of higher flame speeds at higher rpm. So while there is an increase in actual air going into the engine detonation risk is not increased, and possibly even decreased with a ported engine. Though the increases in combustion pressure due to higher power ouput, may even things out if you're comparing a stock ported vs street ported at the same boost level. But if we make the comparison that was brought up earlier of a stock ported engine running 18 psi to create the same amount of air as a street ported engine at 15 psi, the stock ported engine would have a higher likelihood of detonation due to a higher dynamic compression and it's peak power being at a lower rpm and thus having a slower flame speeds increasing the chance and time for detonation. I personally feel that porting is a safer way to make more power then increasing boost pressure even if intake temps remain the same.
There is a lot of information that I didn't mention due to the post already being very long, and also because it's difficult to explain. In the end it all comes down to many many variables and theory's that will still require trial and error to get right. But there are definitely setups here that are proven to make big power reliably so just pay attention to what makes them unique and hope that by duplicating them it will offer you the same results.
The first thing I would like to mention is that a larger cam, or in our case a ported engine increases and shifts the VE of the engine up in the RPM range as we already established. This also shifts the the peak combustion pressures up in the RPMS, higher rpm's give less flame travel time and less chance of detonation.
An increased overlap (which is a result of porting) reduces peak compression by bleeding off some of the intake charge into the exhaust before ignition. This effect lowers the dynamic compression of the engine.
There is a discussion found here http://web.camaross.com/forums/showthread.php?t=382484 that will make your head spin from all the opinions and technical theory's being tossed around regarding larger cams and their effect on dynamic compression and whether this would allow for an increase in static compression without increasing detonation risks. The thread primarily refers to naturally aspirated engines and wasn't referring to rotaries but the principals would be the same.
Another thread with information regarding boosted applications running high static compression ratios and high overlap cams to prevent detonation is found here http://www.ls1tech.com/forums/advanc...ew-blower.html
here is a quote i took from another site dealing with choosing a cam for motorcycles.
"By using a cam with a longer intake duration, the compression pressure will be similarly reduced because the actual closing of the intake valve will occur with the piston higher in the cylinder. The piston now has less distance to travel to the point of spark plug firing. Less distance to travel on a compression stroke means lower static compression pressure and no pinging. " - (The effect would be similar in a ported rotary)
So after a few hrs of reading various sites and discussions on different types of engines my thoughts and conclusions as far as rotaries are concerned and with as little technical jargon as possible are.....
Porting rotaries increases the overlap which reduces dynamic compression, porting also lends itself for higher RPM operation which reduces the time for detonation to occur because of higher flame speeds at higher rpm. So while there is an increase in actual air going into the engine detonation risk is not increased, and possibly even decreased with a ported engine. Though the increases in combustion pressure due to higher power ouput, may even things out if you're comparing a stock ported vs street ported at the same boost level. But if we make the comparison that was brought up earlier of a stock ported engine running 18 psi to create the same amount of air as a street ported engine at 15 psi, the stock ported engine would have a higher likelihood of detonation due to a higher dynamic compression and it's peak power being at a lower rpm and thus having a slower flame speeds increasing the chance and time for detonation. I personally feel that porting is a safer way to make more power then increasing boost pressure even if intake temps remain the same.
There is a lot of information that I didn't mention due to the post already being very long, and also because it's difficult to explain. In the end it all comes down to many many variables and theory's that will still require trial and error to get right. But there are definitely setups here that are proven to make big power reliably so just pay attention to what makes them unique and hope that by duplicating them it will offer you the same results.
05-05-09, 11:35 PM
#28
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I made this in my spare time to figure out the relation between air, fuel, boost, injector timing, and horsepower.
https://www.rx7club.com/rotary-car-performance-77/horsepower-estimator-tool-reliable-1%25-834745/
I hope it helps.
https://www.rx7club.com/rotary-car-performance-77/horsepower-estimator-tool-reliable-1%25-834745/
I hope it helps.
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