jbevly

I am curious how much boost I should be seeing driving at 70MPH. I believe I have the 3.73 rear end (not sure) however I know I am at about 1850 RPMS at 70 MPH. The reason I ask this question is that I am currently seeing about 7.5 - 8.5PSI boost. This is near sea level and on a level road with the cruise control on. Also, from time to time the Boost will fluctuate from 7.5 to 12 PSI on the same level road with the cruise on. I will mash the pedal to raise the RPMs and let off till cruise catches, and that seems to keep it from fluctuating until I stop and start again. The fluctuating only happens from time to time, never consistent. Any advice would be great.

SIXSLUG

Well, there's not a 3.73 option for our trucks unless it's aftermarket...probly sittin on 3.54's, and at 70 in 6th I'm pushing about 2150 rpm....

What's your miles per gallon? Pulled any codes?

My boost is only around 6-8 psi flat at that speed

jbevly

It is the factory rear end. I'm guessing you have a manual? Mine is an automatic transmission.

Mike L

hmmmmmmmmmm! at 70mph (~110KPH) I am spinning at 2,000RPM. I have the 354 gearing and a 47RE autotragic. At that speed I am usually pulling 6-8 lbs of boost. Your RPM's seem a bit low and your boost just a nad high. Have you verified your speedo against a GPS? Just in case you are going slower than you think?

Cheers!

Mike

jbevly

Ok, that is the same setup I have. The only thing that may be weighing down the truck is a couple cases of beer, hunting clothes, and some coolers. I am thinking I have a boost leak somewhere. I checked the lower boot from the turbo and it looks to be rubbing against something (ac component?). I'm only getting 18 mpg and I use to get a good 20 MPG hand calculated.

oliver foster

3.73 gearing is not a 2nd gen gearing option from the factory.
Check the inside of your glove box for a sticker that lists the gearing for the truck from the factory.

jbevly

I checked and it has 3.55 rear end. Now, can anyone tell me about how much boost I should be seeing on a level road with no load? Thanks.

johnh

it all depends on how much throttle you are using, 0psi boost to 26psi on a stock configured engine

jbevly

So you are saying on cruise control @ 70 MPH I should see anywhere between 0 and 26 psi? The original question in the post?

johnh

yep!

jbevly

That helps me zero! Thanks!

Mike L

This will be my final input on your query:

At exactly 70.5MPH my truck is spinning at 1,995RPM (+/- 5rpm). My boost gauge, on relatively level hwy, is vacillating between 3 and 4 lbs. Observation tells me to report mostly 3. I did take a video of my Edge and instrument cluster readings and cannot get the MP4 file to view with VLC. Some codec problems I suspect. I was hoping to take some screen shots and post them for you. Oh well,. I did erroneously (post #4) report initially what my boost readings were. Sorry about that.

Nuttin left to say

Mike

Busboy

It might help if you know that cruise control doesn't set the engine speed it sets the desired ground speed and "fluctuates" the engine speed to "attempt" to maintain the set ground speed, this means that boost pressure could be all over the place. Road surface, tire pressure, truck weight like passengers or full fuel, wind speed and direction all play a roll in what the boost pressure would be at a certain road speed.

KATOOM

The cruise control will drastically vary the throttle and not be a good tool to use for any diagnostics..... Even more so when you have a HP timing box.

But to answer what I think you're asking, the boost at highway cruising speeds should be around 5-7 psi, give or take depending on terrain, ambient weather, wind, tire size, speed, etc...

johnh

well said!!! that was my point !