Strange boost bumped up with higher EGT's.....

[Katoom]

For those who may have an answer.....


I recently noticed while on a long hot highway camping adventure trip over the weekend that my boost pressure would do something I cant fully explain. For anyone who may not know my driving style, I generally dont pull a trailer with very hot EGT's. For me 1200* is HOT and I prefer to stay under that temperature just because it keeps the engine temperatures happier.....and its just really hot in my opinion.


Nonetheless, grossing about 22k pounds, I was pulling hard up some long grades and because of the immense summer triple digit heat it was hard to keep the EGT's from climbing up to the 1200* range under heavy throttle. Not really wanting to slow my speed down too much I tried a couple times to hold firm on the throttle and by doing so let the EGT's get to 1250*. But what I noticed both times I did this was as soon as the EGT's reached that 1250* mark the boost strangely jumped up 2-3 psi. Not just on the gauge either as I could also hear an audible difference in the turbo.


Does anyone have a logical explanation why this happened?

 

[takedown95]

You may have just reached a point where your drive pressure was able to overcome the boost resistance and drove your charger harder. It's no different than when you put a load on a truck at some point you've built enough drive pressure to push your boost up.

 

[56cummins]

Haven't you heard of "hot unlock". Ounce it gets hot, bam unlock more fuel. Kind of like a nuclear meltdown. :lolly:

 

[Katoom]

You may have just reached a point where your drive pressure was able to overcome the boost resistance and drove your charger harder. It's no different than when you put a load on a truck at some point you've built enough drive pressure to push your boost up.

Interesting point.....

But given boost was only around 30 psi, wouldn't the drive pressure be so close to 1:1 that there would be little resistance to overcome?

And in saying that, why would an increase in drive pressure create a rapid increase of 2-3 psi instead of a steady change?