As the title says, I'd like to know what the highest Manifold Absolute Pressure you guys have seen on an N/A car. Have you guys even seen positive pressure on N/A application? Even if the numbers were really low I'd like to know if you've seen positive pressure on an N/A car.

 

How would that be possible?
You could get positive pressure with a turbo but N/A is a vaccume, it wouldn't create a positive pressure. I might be wrong.

 

To be real honest... the reason I'm asking is... As some you read a while ago, I built my-self a RamAir setup. This weekend when I started tuning the PiggyBack with the R4 software I logged a run and I got 0.2lbs positive pressure in multiple spots.

 

I think when RPW was testing their new intake manifold and throttle body they were still far in the negative. I think to reach the 0 pressure you would have to have maybe individual throttle bodies or something. Not really sure. But if you are looking for some information about that RPW will know some more about it.

 

I was wondering if my RamAir setup worked. According to what you're telling me, and please someone else confirm, it looks like my thing is working well.

 

My guess, and it's just a guess, is that the positive pressure came when you were off throttle after heavy acceleration. I think NA cars create more vacuum as they make more power rather than positive pressure. Did your vac numbers frequently go below -70kpa (I think that's like -20ish in/hg)? Without seeing a chart I can't say for sure, but I'll venture a guess anyway.

 

That would be possible. I log another run and pay more attention to where it hits the 0.2lbs.

Can't remember off the top of my head, but according to the vaccum gauge -20in/hg and lower happens at closed throttle high rpm. Like letting go the gas in 3rd at 6000rpm will knock the gauge down to lower than -25in/hg.

Edit: I just re-read your comment...

The more you step on it, the closer you get to absolute pressure(0in/hg-0psi) It mostly depends on throttle position. The bigger the opening, the closer you get to atmospherique pressure.

And to further "argument" the first statement... When I first got the vaccum/pressure gauge, being a new toy in the car I couldn't let my eyes look at anything else. Under heavy acceleration, the needle would often go past the 0 mark and slightly tough the first orange mark. My gauge goes up to 5psi. Each mark on it represent 0.5psi. The very beginning of the first mark could very well be 0.2 psi.


Disclaimer: I might be blaberring and saying things that make absolutely no sens to anyone. Don't flame, just go along and help me understand. I might be right, I might be wrong. I'm only talking about what I see/saw with my own eyes.

 

The highest you should see is 0... which is atmospheric pressure (14.7psi or 1bar from absolute vacuum) relative to atmosphere.. Its POSSIBLE to register a slight increase in pressure from a functional ram-air setup, but very unlikely, a good ram-air setup needs to get air from a high pressure location on the car, then its possible. The odds of reading a positive pressure are very low though..

Engine vacuum is created against the partially closed throttlebody, its because the displacement of the engine is trying to draw more air into the engine than the throttlebody allows.. this is how engine speed is regulated more or less, the the throttle is opened all the way, you've removed the restriction and there is no longer any vacuum because the engines displacement is recieving all the air it needs to fill the cylinders (within reason) In order for a ram-air setup to work and produce a positive pressure, it would have to give the intake manifold/intake ports/cylinders on the engine more air than it can use/draw, creating positive pressure.

On a correctly sized intake and throttlebody, you should never see any vacuum at WOT, I have seen some older engines with very tiny throttlebodies or RESTRICTORS where you will still see a vacuum even at WOT, but its rare on most NA cars nowadays.

 

Ok, then my thinking was way off. Scratch my first post from your thinking.

MalibuJack to the rescuuuue!

 

No prob Hobie. We learn something new everyday.

MalibuJack: So you're telling me my RamAir is most likely working then. When I first thought about it, I was actually looking for a high pressure zone. I first wanted to experiment with taking air from the undercarriage... but then I remembered that was a low pressure zone(good for handling not for ramair), so I decided to use the fog light pocket on the driver side. The rad is located there for a reason. It's there because that's where it can get the most air(making it a very good pressure zone).

 

Yeah, I learn new stuff constantly...the problem being I forget half of it, and I'm too lazy to look up another large portion.

 

sounds like your getting a little pressure. To check for sure use a datalogging tool like digimoto and read the grams of air the MAS is processing. Take ram air off and see if its considerably lower reading. If it doesn't change much then the air isn't there.

The MAS will tell you more exactly whats up.

 

If the air isn't there that would mean the piggyback's reading are wrong?

 

Or that could possibly be a spike occuring from the rapid injestion of air. If your readings are sitting at + pressure for the full pull then look at the mas reading. That will verify your findings.

All i can do is answer your question... if you want to turn this into a piggy problem or are trying to discredit something then fire away. The map sensor used in the piggy is certified to a +/- .01psi. It's a reason the box is so expensive.

 

Nope... Not my intention at all.

That might be it. When I look at the run even though 0.2lbs appears in multiple spots, it's on for very short time. Haven't looked into it yet, but is there a way to post the run file or should I just take a screen shot of it?