Boost gauge readings (PSI vs. RPM)

[fkovalski]

First off- thanks to all who have replied to my incessant questions... and apologies for posting what seem like mundane inquiries, but I am trying to do my homework as far as providing as best feedback to a repair shop when I get my truck in...

Boost pressure- I am trying to understand the relationship of engine speed (RPM) and truck speed (MPH) ans it relates to boost pressure and how increasing RPM affects boost.

I have an Auto Meter 35 PSI gauge just installed about a week ago. My basic assumptions is that boost is provided by exhaust turning the turbo. RPM is the engine speed that makes the exhaust. Should a constant RPM (e. g. 2000 RPM) produce a constant boost pressure regardless of truck speed (MPH)? I will have different boost readings at a same RPM at different road speeds.

Also- If I am accelerating where RPM and MPH is increasing, should the boost also increase?

When I accelerate hard, it boost will increase to approx 26 PSI but will drop very quick upon lifting of the accelerator pedal. I can cruise at 50-ish MPH at about 2100 rpm and boost will read about 6 pounds. Other times I can cruise at a higher speed at the same RPM and boost will barely register.

Is what I describe makes sense (can you understand what I am trying to describe?) Is what I describe normal?

Thanks.

 

[NGuzik]

Boost pressure has many different things affecting it. Towing vs. empty, 1st gear vs. 6th, flat land vs. steep grades or something like engine upgrades vs. turbo size. . It mostly depends on the load on the motor. So driving around empty to the grocery store you can see high boost pressure (around 20-30ish psi) early in the increase in rpms because the motor becomes loaded quickly and soon become equalized (around 5psi) in each gear. Take the same ride pulling a 40 camper and the motor has a larger load in each gear and uses the turbo longer to make more power, also taking longer to equalize the motor back to a normal engine load. The same idea applies to the transition. It acts as a way to split the motor load down. For ease of understanding in a 6 speed nv5600 6th gear is 1:1 ratio meaning the trans in 6th gear is a direct drive from the motor. It also split the power of the motor down in each gear increasing torque with less load on the motor. This is why first gear is a low speed high torque gear. So to answer your question yes what you describe is very normal. Unloaded driving you should see an increase in boost pressure, but as you reach higher gears boost will become less and harder to see on the gauge. So to say rpms decide your boost psi is hard to recreat each time you drive. Bascly judge your boost pressure on the size of load on the motor. Hope that helps and is clear. . Cheers, nick

*spark notes version- larger load on the motor = more boost pressure. Less load = less boost pressure.

 

[fox]

Boost is related to load and fuel being burnt.
You can have RPM and low boost from no load, or high boost from heavy load.
Spool up ( when you start getting boost) is dependant on load and RPM. 1000 RPM and having the pedal planted will be lower boost than 2000 RPM.

The turbo is more sensitive to HEAT than just RPM.

 

[MarkEkberg]

Rpm is exactly that rpm. Assuming your in any gear it should increase and decrease according to gearing. It gives you the ability to produce boost, but will not on its own.



Speed is exactly that, has nothing to do with boost



Boost is generated by the speed and volume/density of exhaust gasses through the turbo causing it to spin. You need both to produce boost.



When you have no load at 2K rpm, you have exhaust gas speed, but low volume of gasses. If you are at 1K and mash the accelerator you have high volume of gases but no speed of gasses. This is why many diesels smoke for a few seconds till the turbo gets up to speed and gets the fuel/air ratios correct.

 

[JerryNelson]

I have just installed a boost gauge on my truck and when the boost goes past 10 the needle starts to shake. Is this normal or do i have a leak

 

[fkovalski]

I have just installed a boost gauge on my truck and when the boost goes past 10 the needle starts to shake. Is this normal or do i have a leak

I do not know if it is normal or not, but my gage needle does not shake at all... it does respond quite rapidly though when accelerating or decelerating.

 

[AH64ID]

Think of boost as a load meter, the more boost the harder the motor is working. It's work related, so rpms are not as important but still important. Max load at 1500 rpms will make less boost than max lad at 2500 rpms.

You should be able to hit about 32 psi under wide open acceleration.

 

[fkovalski]

I have just installed a boost gauge on my truck and when the boost goes past 10 the needle starts to shake. Is this normal or do i have a leak

Does the boost needle still shake? I previously replied that mine does not do this; that is partially true. Mine is steady at high altitudes (9,000+ feet), but when I go down tot he flatlands 5,500 feet then it starts to shake at around 15+ psi.

Anyone shed some light on this unsteady boost reading at my lower altitudes?

 

[JJPage]

Contact Autometer, they may have a "snubber" kit that will address the unstable needle movement.