Thread: Best way to adjust boost pressure

Originally Posted by Blknight.aus

yes but NOT from the overboost spike that happens when you go from full RPM full noise to full idle, something about a lack of fuel to make the heat.

(which you picked up on in your next post.....)

which is why I dont worry about the dump valve on diesels if the boost and fueling is sensible to being with.

I haven’t read all the posts but it seems that we are getting a bit confused.

over spiking boost on a diesel just can not happen, if you are running at full RPM and full fuel then cut back to idle there will be no spike in the boost pressure, there will be a delay in the fuel pressure dropping due to the mass of both turbine wheels but that is all. Unlike a petrol motor which will spike as the throttle body will shut off the airflow.

having a blow off valve on a diesel, waste of time, even a waste gate is really not required but is fitted by manufactures to assist in controlling emissions with no effect on economy and performance.

over boost spiking can and does happen on diesels its just doesnt blow anything apart and because of the nature of diesels not having intake restrictions they arent as big as petrol engines but they do happen usually its only a couple of PSI and you notice it more on diesels with longer runs from the head to the exhuast or restrictive inlet tracts and free flowing post turbo pipes.

and I need to re-edit that post of mine it needs to read

"Full noise Full RPM to Full noise Idle RPM"

(as in when an auto box climbs up a gear and decides to not unlock the TC during the shift or some truckie fumbles the shift and goes up about 5 gears instead of 1)

Originally Posted by clean32

even a waste gate is really not required but is fitted by manufactures to assist in controlling emissions with no effect on economy and performance.

Not true.
Wastegates are needed because turbines can't flow as much gas as the compressors they're bolted to can. To get maximum airflow through a turbo compressor (i.e. maximum performance) you either have to bypass exhuast with a properly set wastegate or use a turbine housing soo large that drivability will be lost.
The large turbine housing is what Ricers do. Good for impressive dyno figures, but useless for driving.

Originally Posted by Dougal

Not true.
Wastegates are needed because turbines can't flow as much gas as the compressors they're bolted to can. To get maximum airflow through a turbo compressor (i.e. maximum performance) you either have to bypass exhuast with a properly set wastegate or use a turbine housing soo large that drivability will be lost.
The large turbine housing is what Ricers do. Good for impressive dyno figures, but useless for driving.

I get your point but would add that as a diesel has a much more limited rev range compared to a ricer thus a limited volumetric range of flow that it is a useless comparison although a good way to make your point.

If we are to look at large displacement low revving diesels, no waste gate is fitted by the manufactures.
If we are to look at high performance or racing diesels, no waste gate is fitted or are deactivated. There is an exception to this where compound turbo setups are used and a waste gate type of setup is used to reduce backpressure to the smaller turbo.
So we only fined waste gates in smaller car type diesels. initially these were fitted to reduce the risk of turbos over speeding etc then we notice that many manufactures dropped the waste gate all together ( Isuzu being an example) or in the more common case where turbos are purchased by an automotive manufacture from a turbo manufacture a turbo designed with a waste gate because that same turbo was also so fitted to petrol motors or commonality of parts ( castings) in this case the waste gate was just there and was of no real benefit.
Lastly we come to waste gates today which are usually electronically activated. These tend to be used to reduce boost at lower HP requirements or at lower fueling to richen up burn thus keeping the greenies happy, a similar result as a EGR.

Any way my post that you quoted was in response to pressure spiking. Something you have posted on in the past indirectly. As well as you have posted on backpressure with posts that seem to contradict your last post? Go figure???

Originally Posted by clean32

I get your point but would add that as a diesel has a much more limited rev range compared to a ricer thus a limited volumetric range of flow that it is a useless comparison although a good way to make your point.

If we are to look at large displacement low revving diesels, no waste gate is fitted by the manufactures.
If we are to look at high performance or racing diesels, no waste gate is fitted or are deactivated. There is an exception to this where compound turbo setups are used and a waste gate type of setup is used to reduce backpressure to the smaller turbo.
So we only fined waste gates in smaller car type diesels. initially these were fitted to reduce the risk of turbos over speeding etc then we notice that many manufactures dropped the waste gate all together ( Isuzu being an example) or in the more common case where turbos are purchased by an automotive manufacture from a turbo manufacture a turbo designed with a waste gate because that same turbo was also so fitted to petrol motors or commonality of parts ( castings) in this case the waste gate was just there and was of no real benefit.
Lastly we come to waste gates today which are usually electronically activated. These tend to be used to reduce boost at lower HP requirements or at lower fueling to richen up burn thus keeping the greenies happy, a similar result as a EGR.

Any way my post that you quoted was in response to pressure spiking. Something you have posted on in the past indirectly. As well as you have posted on backpressure with posts that seem to contradict your last post? Go figure???

No.
Every mobile diesel that I've seen produced in the last 10 years either has a wastegated or variable vane turbo. Even underground mining machinery.
Fixed geometry turbos are now the sole domain of constant speed engines. Mobile engines can't meet emissions or drivability/performance requirements with fixed geometry turbos.

Turbochargers are sized for the application, whether it be a ricer or a diesel the compressor map is still mostly used. A fixed geometry turbo doesn't have the flow range to match.

Isuzu only dropped the wastegate on the 4BD1's from about 88-94. Wastegated turbos were used before and after that. Before 88 they used IHI and garrett wastegated turbos, from 94 they again used garrett wastegated turbos.

You don't need a wastegate to reduce boost at low power. With low demand the engines don't produce much exhaust heat so the turbos don't produce much boost.

Show me this high performance diesel with a fixed geometry turbo.

Hehe, swordfights yet again!

Originally Posted by rovercare

Hehe, swordfights yet again!

You're right there Matt, they're at it again!!!

why does everthing have to get so complicated?? Diesel engines: More fuel = More heat, Petrol engines: More fuel = cooler combustion. Diesel engines: More fuel and boost = more power 'til she melts things (THATS why we use upstream Pyros...) Petrol engines: Why do we waste our time on such inefficient garbage???

Get it? Got it? - GOOD


JC

we have petrol engines to give us a reason to refine petrol.
we need to refine petrol because its very hard to light diesel with a match.

how else are we going to make landrovers resemble jeeps if we didnt set fire to them?