Back pressure ?

Rmartin-97 · Mar 20, 2012

Is 45 pounds of boost and 60 psi of back pressure bad? Should I look into a wastegate?

mclean · Mar 20, 2012

is that before or after you subtract the barometric pressure?

Rmartin-97 · Mar 20, 2012

mclean said:
is that before or after you subtract the barometric pressure?

What's the barometric pressure?

outacntrl · Mar 20, 2012

Rmartin-97 said:
What's the barometric pressure?

I think he means atmospheric pressure. It depends on your elevation. I think sea level is 14.7psi. I'm not sure how the H&S reads it. On the DD you have to subtract it from the reading.

mclean · Mar 20, 2012

yeah thats what i meant. turn your key on and see what your bp reading is and subtract that from what you are seeing on your DD. Pretty sure that's how you do it.

Rmartin-97 · Mar 20, 2012

outacntrl said:
I think he means atmospheric pressure. It depends on your elevation. I think sea level is 14.7psi. I'm not sure how the H&S reads it. On the DD you have to subtract it from the reading.

What would I need to do to figure it out.

outacntrl · Mar 20, 2012

Rmartin-97 said:
What would I need to do to figure it out.

What's your elevation? The pressure goes down as elevation goes up. Turn the key on and see what the reading is with the engine off and that should be atmospheric pressure. You need to subtract that from the absolute bp reading you get while it's running.

Rmartin-97 · Mar 20, 2012

Ok thanks guys.

Rmartin-97 · Mar 20, 2012

I dunno what my elevation is I'm in ohio. But on my mini max the exaust back pressure reads 0 with the key on but my barometric gauge on it reads 14 with the key on so doin subtract that from my ebp?

forcefed6.4ford · Mar 20, 2012

Correct. So when you're driving and its showing 24psi. It's actually only 10psi.

Rmartin-97 · Mar 20, 2012

forcefed5.9ford said:
Correct. So when you're driving and its showing 24psi. It's actually only 10psi.

Ok thanks

JD3020 · Mar 20, 2012

If i'm thinking correctly right now, if the back pressure shows 0 with the key on while baro is 14, then its already subtracting 14 for you which means what it shows is what you have. If back pressure showed 14 at idle or engine off, then you would have to subtract 14 to get the BP number.

Rmartin-97 · Mar 20, 2012

JD3020 said:
If i'm thinking correctly right now, if the back pressure shows 0 with the key on while baro is 14, then its already subtracting 14 for you which means what it shows is what you have. If back pressure showed 14 at idle or engine off, then you would have to subtract 14 to get the BP number.

Now I'm confused again.

forcefed6.4ford · Mar 20, 2012

I believe he can monitor both. Absolute and desired. Just be aware of which one your are watching when getting rough with it.

Rmartin-97 · Mar 20, 2012

forcefed5.9ford said:
I believe he can monitor both. Absolute and desired. Just be aware of which one your are watching when getting rough with it.

I didn't se anything about absolute or ddesired. Just have the barometeic reading 14 with key on and ebp readin 0.

JD3020 · Mar 20, 2012

Rmartin-97 said:
Now I'm confused again.

Engine off or at an idle, if the gauge is reading 14, 14.7, 15, or whatever then that means it is showing you the absolute pressure that the sensor is reading. So its atmospheric pressure, plus whatever pressure is being built. So say if it reads 14 engine off, then when its running and shows 60 pounds of back pressure then it is really 60-14, which is 46.

But if at idle or engine off, and it is showing 0psi, then it is already accounting for and subtracting the atmospheric pressure. So if its showing 60psi, that would 60psi of back pressure.

On my OBS the scangauge shows boost and MAP. Boost at idle or engine off is 0, while MAP(manifold absolute pressure) is reading about 14.7. So the boost reading is automatically subtracting 14.7 and showing me what my boost reading is. When i'm building 20psi of boost, the MAP reading is 34.7

outacntrl · Mar 20, 2012

I'm pretty sure ebp is what you want to be watching and if it shows 0 key on engine off then it's already subtracting atmospheric pressure.

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Rmartin-97 · Mar 20, 2012

outacntrl said:
I'm pretty sure ebp is what you want to be watching and if it shows 0 key on engine off then it's already subtracting atmospheric pressure.

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Yea with the key off it reads 0 on ebp so should I look into a wastegate at 45psi boost 60 psi ebp?

forcefed6.4ford · Mar 20, 2012

a 15psi gate spring would get you that referencing boost.

superdutyman97 · Mar 20, 2012

Go down to the fisher building and look at the corner stones. The elevation is etched into one of them. I think it's the front left from in front of dads garage