Bambam7

OK- this seems technically appropriate to me- but is the PSI in the chambers at maximum compression for an N/A just the relative atmospheric pressure * compression ratio?

Given that- is compression(PSI value) while under boost in a turbo the intake PSI * compression?
or is it the regular compression ratio (PSI) + intake (boost pressure)?? so- like 95 psi+10 psi? (10 psi of boost)
I Just want to work out some compression figures for my high-comp 90 engine while under boost, and work out fuel requirements....
Thanks
Dan

Icemark

Actually with the 86-92 NA dynamic surge intake its slightly higher than atmospheric because of the port closing post surge, but I couldn't tell you how much.

Bambam7

I didn't take that into consideration- all engines have a slight ram air effect in the intake- due to air velocity in the runners, but it's negligable when figuring into compression I beleive.... it's more an improvement on volumetric effectiecy due to quicker chamber filling.

Bambam7

Hmmm.... I guess there aren't any engineers here.

Evil Aviator

I don't understand the question.

OK, I understand that part. Since the Air/Fuel ratio is a mass ratio, your fuel requirements are based on air mass, not air pressure. It's a whole lot easier to estimate your fuel requirement based on your expected horsepower, but you can use whatever method makes you happy. These links will help.

http://www.maxcooper.com/rx7/how-to/...tem/calcs.html

http://www.rceng.com/technical.htm

http://www.amazon.com/exec/obidos/se...mart%5Fdetail/

The boost gauge reads in "gauge pressure" (go figure, lol) at the intake manifold, which is that amount of pressure above ambient. When not under boost, this will be a vacuum due to the rotors sucking air through the manifold, noted by the boost gauge falling below 0. If you add the gauge pressure to the ambient pressure, you get the "absolute pressure" (psia) which is being fed into the rotors for compression.

BTW, I do not have a degree in engineering.

RETed

What you're asking is what is taught to a 3rd level automotive engineering student - the math is not that bad, but it is tedious.&nbsp I don't feel like playing professor and going through all that crap, since you insist the 20k miles switch reset every 100,000 miles.

Your compression ratio is affected by boost by it's ratio to relative atmostpheric.&nbsp To narrow everything down, just muliply by the Pressure(boost)/Pressure(relative) ratio, and you'll get you answer.

If you're running 14.7psi of boost, this is two times what an NA sees, since relative pressure equals 1 ATM; 1 ATM of boost is 1 ATM over relative atmosphere.



-Ted

Bambam7

So... is regular atmospheric pressure (1 ATM- presumably at sea level) 14.7 PSI?
I just want to figure out my in-chamber pressure at around 10 PSI of boost, so can figure what boost that would equate to on a regular TII with low-comp rotors- just to see what my safe margin of boost is before I really have to worry about pre-ignition on 91 octane.
What I'm asking may seem a little unclear...ok...
Obviously- an engine with higher compression can make more power given the same amount of boost as a low-comp engine- however, detonation is more likely.
So what's the amount of boost the high-comp engine needs in order to make the same power safely as the lower-comp one? (obviously less) That's my question....
I just want to know how much boost my engine can safely hold, and not reduce it's lifetime to a few weeks.
I know a regular TII motor with appropriate fuel increase can run maybe 15 psi perfectly fine.... just want to know what that means my safe limit is.

PS- just wait about the 20K sensor... I've got gremlins doing some digging for me! Muuahahaha!! (*rubbing hands together in evil anticipation*)

Evil Aviator

No, the atmosphere is 14.7 psi (29.92 in Hg) for international standard conditions. The actual atmospheric pressure varies, as shown by the ALTIMETER setting in this link (use plain language):
http://www.flightplanning.navcanada....f&TypeDoc=html

OK, I see... you think that you can figure out your boost threashold by crunching some pressure numbers. Well, it doesn't work like that. First of all, your max boost pressure is a physical limitation of your engine parts. Secondly, detonation is caused by heat, not pressure. There are many other factors which affect the things you are asking, such as humidity, ambient temperature, air/fuel ratio, timing, tuning, engine condition, intercooler efficiency, volumetric efficiency, etc. Sorry, but it is impossible to teach you everything through an internet forum. If you are interested in this field, I highly recommend attending a good university. Otherwise, at least read one of Corky Bell's books and you will have a better understanding of the subject.