Backpressure vs Cylinder Pressure

Derek@Vision Diesel · Mar 19, 2012

lincolnlocker said:
is that 1.5:1= boost:backpressure or is it 1:1.5=boost:backpressure?

i just put in a bp gauge and i run 38psi of boost and 58psi of backpressure at wot, is that good or bad?

That is 1:1.5 boost:backpressure i believe.

Charles · Mar 19, 2012

Some people do in fact look to backpressure as if the cast manifold, held to the head with two bolts per cylinder is stronger than the cylinder itself, and that the exhaust system actually held fast while the poor headgasket with 6 fasteners per cylinder and wire backup let go, lol.

The only correlation I could venture would be that higher drive pressure is going to increase exhaust gas reversion, increasing the in-cylinder temperature at SOI, and possibly reduce combustion delay enough to cause a meaningful change to the start of combustion.

Although I don't feel strongly that it would usually produce enough of a change for practical concern in any usual setup, but, I've never sat and tested it either.

As far as ideal.... none? Operating sub 1:1 is the sweet spot. Engines start getting very happy and willing to make power when drive drops below boost.

Extended Power · Mar 20, 2012

Like Charles mentioned in the last sentence: "Engines start getting very happy and willing to make power when drive drops below boost."

That is because the engine is a huge air pump.
If you can forget about the cylinder pressures for a minute...

If the exhaust pressure leaving the engine...and driving the turbo/turbos is 80 psi, and the boost entering the engine is 80 psi...the engine will be able to flow more efficiently, then if the exhaust pressure was at 80 psi, and the boost was at 40 psi. (The engine has to "Work" harder to push the exhaust from the engine...

Camshafts come into play as well...
Valve lift, open and closed duration, as well as valve overlap all play a part in it...

eg: 2 camshafts have the same amount of lift, but one of them has no valve overlap.

The camshaft with no overlap would prolly spool a turbo like crazy, but the engine would prolly only rev to about 2000 rpm.
The same camshaft, but with more overlap would not spool the turbo as easy, but would have more midrange and top end power.

Hope this helps a little bit anyways...

lincolnlocker · Mar 20, 2012

Charles said:
Some people do in fact look to backpressure as if the cast manifold, held to the head with two bolts per cylinder is stronger than the cylinder itself, and that the exhaust system actually held fast while the poor headgasket with 6 fasteners per cylinder and wire backup let go, lol.

The only correlation I could venture would be that higher drive pressure is going to increase exhaust gas reversion, increasing the in-cylinder temperature at SOI, and possibly reduce combustion delay enough to cause a meaningful change to the start of combustion.

Although I don't feel strongly that it would usually produce enough of a change for practical concern in any usual setup, but, I've never sat and tested it either.

As far as ideal.... none? Operating sub 1:1 is the sweet spot. Engines start getting very happy and willing to make power when drive drops below boost.

so is that sub 1:1 all the way threw the rpm range to wot? if so would i be better off actually using the wg on my turbo or is it to pis poor of a design to help with it? if im running 38 psi of boost and 58 psi of back pressure would i be able to gate it enough to lower the bp and still make the power all the way threw the rpm range? or should i just leave it the way it is?

MadDiesel73 · Mar 20, 2012

Hows the gate set now? Shut?

lincolnlocker · Mar 20, 2012

yes shut

MadDiesel73 · Mar 20, 2012

Stock actuator? Tighted down? I would try letting the gate open on spring pressure a little bit and see what happenes

lincolnlocker · Mar 20, 2012

yes. yes. im sure it will drop the bp but will it hinder the performance? like i asked before is it better to have sub 1:1 all the way to wot...

JAP · Mar 21, 2012

If you're talking about a stock turbo, I would cross drill it and tighten the waste gate down a little, then run a back pressure reference to the actuator.

JAP · Mar 21, 2012

Tightening it will allow for low end spool up and the BP will push it open at high back pressure.

Chvyrkr · Mar 21, 2012

lincolnlocker said:
is that 1.5:1= boost:backpressure or is it 1:1.5=boost:backpressure?

i just put in a bp gauge and i run 38psi of boost and 58psi of backpressure at wot, is that good or bad?

What does the gauge say with the engine off?

Gearhead · Mar 21, 2012

Drive pressure mainly adds latent heat for the next combustion cycle all other things things being equal. The lower the drive pressure, the cooler the cylinder temps will be for the most part and thus not add timing effect like the high drive pressure. Of course the lower the drive pressure for a given amount of boost the more air you are actually moving through the engine.

CamTom12 · Mar 21, 2012

Here's another thing too - change the turbine housing to change boost:backpressure.

Wastegates are best used for boost control - you're not going to make more power by wasting exhaust gases around the turbine. You'll keep from overspeeding a turbo or overboosting an engine though.

Homesteader · Mar 21, 2012

Extended Power said:
Like Charles mentioned in the last sentence: "Engines start getting very happy and willing to make power when drive drops below boost."

That is because the engine is a huge air pump.
If you can forget about the cylinder pressures for a minute...

If the exhaust pressure leaving the engine...and driving the turbo/turbos is 80 psi, and the boost entering the engine is 80 psi...the engine will be able to flow more efficiently, then if the exhaust pressure was at 80 psi, and the boost was at 40 psi. (The engine has to "Work" harder to push the exhaust from the engine...

Camshafts come into play as well...
Valve lift, open and closed duration, as well as valve overlap all play a part in it...

eg: 2 camshafts have the same amount of lift, but one of them has no valve overlap.

The camshaft with no overlap would prolly spool a turbo like crazy, but the engine would prolly only rev to about 2000 rpm.
The same camshaft, but with more overlap would not spool the turbo as easy, but would have more midrange and top end power.

Hope this helps a little bit anyways...

So how do we get it close to a 1-1 ratio with out doing the camshaft? Is it possible? Is there some sort of formula?

Charles · Mar 21, 2012

Homesteader said:
So how do we get it close to a 1-1 ratio with out doing the camshaft? Is it possible? Is there some sort of formula?

With a drop in replacement turbo in the stock location and a 88mm charger ahead of it with the stock cam and heads my truck would be better than 1:1 up to ~60lbs of boost, and by ~70lbs I had ~80lbs of drive last I checked these sorts of things.

Exceeding 1:1 is not hard. A good turbo setup will see boost higher than drive without doing anything extravagant.

As for effecting your boost:drive ratio.... it isn't done by altering the wastegate on a given charger. It is done by altering the A/R and turbine size and trim for a given compressor. The cost is that fixed geometry units that exhibit excellent boost:drive ratios up top, will be hard pressed to show acceptable driveability down low, as the A/R will likely be loose, and the turbine large.

Compounds make this problem very controllable however, in that you can run a large, relatively loose turbine section on the first stage compressor, yet have a relatively small turbine section on the second stage compressor to get the show moving that gets bypassed as drive picks up.