Boost Pressure????

[Fistfull]

How much effect does temperature and humidity have on boost pressure?

The reason I ask, I installed a boost and egt gauge a couple of weeks ago, I have been noting about 18 psi unloaded as I blast up the onramp on to the freeway (havn't had a chance to tow yet). This morning, a little cooler outside (maybe 5 degrees) than it has been, the boost jumped to just above 20 psi before I had to lift due to a suv in my path. This is the same onramp I take everymorning, no other variables I can think of except temp. Is the temperature difference enough to cause this or do I have something sticking?

 

[Dieselnerd]

Shouldn't make any difference. Maybe your wastegate stuck a little bit before opening. When mine was stock the wastegate always opened at 18. 5# which is about right for a 180HP auto transmission truck.

 

[Missouri Mule]

Elkaholic,

Depending on the tempature drop from the previous mornings I think that the cooler air could have effect on boost.

Air(gas) tends to condense when cooler with the air molecules closer together already the turbo compresses them even closer.

Also fuel would be a little thicker(not much) increasing the amount of fuel molecules being injected into cylinders.

How much boost increase I don't know.



Hemi Dart did a post on cooler tempature- more horse power a while ago(last time it started to get cold) . He has one of those pocket dynos or something like that plus gauges.





When I take off before the sun rises (the morning are cooling a little here) my Mule seems to run better than in afternoon when it is 90 plus dergrees in shade but we have more humidty here also.



Try to contact Hemi-Dart see if he has any data on his temps and extra horse power.

 

[cumminsgetme]

Were you in the same gear when you got the 18 # reading as the 20 #. If you were in 3rd or 4th with the 18 # and 5th with 20# that would make sense. 3rd gear usually won't build max boost due to rapping out the rpm's to quick... ... . that's why you dyno your truck in 5th gear... ... . to allow time to build maximum boost.

 

[Fistfull]

Thanks gentlemen, we are headed up the Mogollon (with our little trailer in tow) this AM, have to remember to watch the road instead of the new gauges.

Last month we did 2000 miles up through Utah and Colorado, the pyro didn't show up on the door step till the day before departure, arghhhhhhhhh ... ... .

 

[BushWakr - RIP]

Can I jump in??



I see exactly the same thing when temps cool down as well.

Cooler air is more dense air. The same principle is at work with an inter-cooler. I've seen as much as 3-4psi difference in boost from 75* to 50*. If the weather is real cool/damp its even more noticeable. You have nothing wrong, don't worry.



Regards, Bob

 

[Dieselnerd]

BushWakr, unless I am mistaken, you do not have a wastegate on the turbo in your '93. In that case you would see different pressures at different air densities. '94 and up have wastegates and unless they stick closed or the pressure line going to them is restricted, they always open at the same pressure regardless of the air temperature, altitude or whatever. If the wastegate is operating correctly your maximum pressure will always be the same and on the vintage Dodge we are talking about on this thread, if still stock with an automatic, normal pressure is in the 18-19 pound range.

 

[Fistfull]

Ok with this information and the trip I did this weekend, another question is the 18-19 psi absolute or a differential pressure?

The reason I am wondering, at the lower altitudes WOT in 3rd converter not locked the EGT would get up close to 1300 as we gained altitude the I had to start backing out of the throttle to keep it below 1300, all the while it's making about 19 psi boost.

 

[GSchlegel]

Elkaholic,

It's boost, ie: differential pressure, not absolute pressure like used in aircraft. You get less air (oxygen) with the same boost as your altitude increases. Therefore, higher EGTs.

Hope this answers your question.

Gene

 

[BushWakr - RIP]

RATS!

Dieselnerd, I didn't pick up on that. You are right on. The wastegate should be constant for 'dump pressures'.

Is there a lag when it comes to the reaction timing on the gate opening? Would it hang on long enough if the climb rate was quick enough to go 'over the top' a bit??

Not having a WG i don't see that function.

Hey, I'm learning. Thanks for the correction, I DO appreciate it!



Regards, Bob

 

[Dieselnerd]

BushWakr, there is normally no lag but if it's a bit rusty they can stick sometimes.

 

[gchas]

Hate to beat a dead horse, but must add my two cents worth. The cooler the air is, the more dense it becomes, thus contributing to higher boost pressure (assuming all of the aforementioned factors remain constant). However, the density of air is inversely proportional to humidity, i. e. as moisture is added to air, the density of the air is decreased. Ask any pilot.