Turbo (Boost) Manifold PSI vs. Exhaust Manifold PSI 6

[Holeshot1]

Should turbo (boost) manifold pressure be equal to exhaust manifold pressure to maximize horsepower and torque?

Application Drag Racing.

Engine V6 231 C.I

RPM Power Curve 2500- 6500

 

[crysta1c1ear]

If we ignore that there is slightly more exhaust gas than air at the intake (due to burnt fuel), aren't you implying a 100% efficient turbo (to generate as much pressure as the pressure that drives it)? I guess that would maximize horsepower, but you'd have trouble finding one.

 

[pmrobert]

I do not intend on being rude or condescending; both previous posters would be well served by reading and learning some basic physics regarding turbochargers. Pressure is only one variable in most of the equations regarding them. Mass flow is really important and various physical attributes of turbos are what help define which is best for your application.

 

[IceStationZebra]

In the simplest sense maximum hp & torque will come from the most boost over the least manifold pressure, assuming an open throttle. B-) But I think that you are making it way too simple to say that boost should equal manifold pressure. There are way too many variables.

I assume that you are trying to hop-up a Buick 231, in which case you will get much better, quicker and cheaper results by buying one of the many turbo kits available instead of trying to design your own. You may also be able to buy parts from someone who is upgrading to the next level. If you truly wish to learn to design your own I concur with pmrobert - get a book or two and get on some of the GN/GNX forums. If you want extreme talk to the tractor pulling folks - though they use alcohol or diesel for fuel.

ISZ

 

[BrianPetersen]

If you do a really good job of matching the compressor and turbine operating points to be as efficient as possible, and you have as little flow restriction upstream of the compressor and downstream of the turbine as possible, then it is quite possible that the intake manifold pressure will be greater than the exhaust manifold pressure. The volume flow rate on the exhaust side is far greater than the intake side because of the higher temperature; this is why this can happen.

But, most of the time, it's going to be operating "off design", and then the reality is that the exhaust manifold pressure will be higher than boost pressure.

The lower you can get the exhaust manifold pressure, the better off you will be. Always.

 

[Fabrico]

"The lower you can get the exhaust manifold pressure, the better off you will be. Always."

For a given turbo output perhaps, but not in general.

 

[Lou Scannon]

chrystalclear, it's actually more complicated than that. It is definitely possible to configure a turbo that will provide higher boost pressure than the exhaust pressure it takes to drive it, albeit over a fairly narrow operating range. This is typical, so I understand, on peaky turbo race engines, in their power band. I have also seen it with my own eyes on heavy duty mobile engines in the peak torque area, as well as on my own turbocharged vehicle in a narrow range.

"The lower you can get the exhaust manifold pressure, the better off you will be. Always."

For a given turbo output perhaps, but not in general.

Fabrico, care to justify why not in general?

 

[Fabrico]

"care to justify why not in general?"

"The lower you can get the exhaust manifold pressure, the better off you will be. Always."

I would say it's the always in that stand alone statement that needs to be justified. From a myriad of possible causes, both internal and external, low exhaust pressure could most easily emanate from doing below potential work. There are likely just as many inefficiencies that would cause low pressure as high. Optimum is the goal.

 

[Lou Scannon]

Agreed, there's too many interactions to justify the original sweeping generalization. I think the intent was, any changes that reduce exhaust manifold pressure while maintaining the same intake manifold conditions are for the better.

 

[turbomotor]

I have done a lot of test work evaluating pumping loop work (both positive and negative work) and would generally agree with Brain Peterson. The lower the exhaust system back pressure relative to a constant intake manifold pressure, the more work one can extract from the pumping loop of a four stroke engine. However, short circuiting of the fresh charge can quickly destruy gains in pumping loop work if the valve events allow a short circuit.

But my expertise is the turbomachinery, so let me comment there. The inherent work balance between the turbocompressor stage and the turboexpander stage is not dirrectly related to pressure ratio or pressure change. It is directly related to enthalpy change (assuming mass flow is nearly identical between the two stages).

So, if you want to maximize the pressure drop across the reciprocator, minimize the compressor inlet temp and maximize the turbine inlet temp. Compressor work is related to enthalpy change so is therefore (assuming constant spec heat) related to delta T. But pressure ratio is related to Delta T divided by T1. Lower the compressor inlet gas temp and you will get much higher compressor pressure for a given shaft work.

Practically speaking, I have precooled the inlet charge with a LN2 cooled heat exchanger and achieved significantly lower turbine work demand for a given compressor pressure rise. I've never done this on a motor sports application (only aerospace), but it may have some real potential if the racing rules allow it.