Need crash course on exhaust back pressure...

[RSchwarzli]

So I have the snappy exhaust backpressure gauge now for a few months. From what I gather, boost is always lower than bp. I know I have read before that there is a calculation or at least a recommended number/ratio you want to be in for max efficiency. I have a bad feeling I am not there!



Can someone give me a crash course on this?



Thanks!



Robert

 

[cerberusiam]

Theoretically, optimal ratio is 1:1 across the compressor map. Not gonna see that without a VGT turbo though so using the top of the map as a reference works.



In reality 10% either side of 1:1 across the map is excellent. If boost is much lower than DP you are leaving air on the table and spool capability. If DP goes too high its chopping air flow thru the engine, loosing available power, and wasting energy to push the exhaust thru.



Example: at 30 psi boost DP can be 27-33 psi and you would be hard pressed to measure the difference, all other things being equal.



A stock 3rd gen turbo will make 500 HP or better at 1:2 boost to DP ratio and spool extremely fast across the map. Depending on how you fix the ratio you may or may not loose spool capability and make more power out of the same fueling.



(Hint: BB turbo for best of both worlds)

 

[Greenleaf]

I don't really understand/follow that.



My first question(s); what is "DP" and what is a "BB turbo" ? ball bearing? what would that have to do about boost and back pressure.



I **do** know what a VGT turbo is. After that, I'm at a loss here.



lost in ohio

 

[JLEONARD]

When I did the boat (aka hole in the water, etc) repower using a 6BTA 270 I was concerned with backpressure. A Cummins tech came aboard and had to measure and document exhaust backpressue as part of the warrantee requirements.

Max spec is 4 inches of mercury (Hg) (going from memory) which converts to approx 2 psi.

The highest reading I had was actually about 2 in Hg (1 psi) at about 800-1000 rpm, mainly becuase the raw water which is injected to cool/quiet the exhaust was piling up in the exhaust hose since the run was somewhat flat.



Backpressure went to zero at about 1400 rpm all the way thru WOT which was 2630 rpm pumping over 30 psi of boost (via an HX35M turbo with an antifreeze cooled housing).

Exhaust was via a 5 inch dia hose that was filled approx 30% with water.



I have no friggin' idea how this relates to a hunk of iron in the form of a pickup truck but thought y'all might find it interesting.

 

[cerberusiam]

My first question(s); what is "DP" and what is a "BB turbo" ? ball bearing? what would that have to do about boost and back pressure.

DP is drive pressure on the turbo, or simply back pressure in the exhaust manifold.



BB is ball bearing, as in the Garret and Aurora turbos.



A standard turbo like the Holeset does not have a a bearing to support the shaft, its a bushing that relies on oil pressure to keep the wear surfaces apart. The bearing type turbos spin much easier and don't require as much force to turn them.



The amount of DP needed to produce the same boost is lower, or, more boost is acheivable with the same DP, or, same or less boost but denser air with the same DP. Depends on the compressor and\or turbine size and their configuration.

 

[RSchwarzli]

Theoretically, optimal ratio is 1:1 across the compressor map. Not gonna see that without a VGT turbo though so using the top of the map as a reference works.



In reality 10% either side of 1:1 across the map is excellent. If boost is much lower than DP you are leaving air on the table and spool capability. If DP goes too high its chopping air flow thru the engine, loosing available power, and wasting energy to push the exhaust thru.



Example: at 30 psi boost DP can be 27-33 psi and you would be hard pressed to measure the difference, all other things being equal.



A stock 3rd gen turbo will make 500 HP or better at 1:2 boost to DP ratio and spool extremely fast across the map. Depending on how you fix the ratio you may or may not loose spool capability and make more power out of the same fueling.



(Hint: BB turbo for best of both worlds)

That is great! Thanks!



I run about 31 psi at WOT. DP is about 32-34. Normal driving is about the same as what boost is + or -.



So I guess I am in range.

 

[cerberusiam]

I run about 31 psi at WOT. DP is about 32-34. Normal driving is about the same as what boost is + or -.

Thats what I would shoot for and seems to be the best trade off for performance. What turbo and specs?

 

[RSchwarzli]

What turbo and specs?

Running a 1997 HX35 with 18cm H1C (factory for 1990) housing with 4" exhaust.

 

[AH64ID]

BB turbo's are able to run a larger a/r and and still spool the same as a journal bearing turbo. This allows for lower DP for the same boost.

On my stock setup cruise was 1. 5:1 or higher at times. With my Garrett its 1:1. Accelerating the stock turbo was 1. 5:1 or higher, where unless I am WOT above 2500 rpms the Garrett is 1:1, or very close. At the end of a grade I can be better than 1:1 with the Garrett.

I didn't realize that the Aurora turbo's had BB's.