More boost at lower RPM's with smaller exhaust?

[XPC66]

ok then take a hose and then put your thumb over it same CFM at higher psi

And how do you get the higher PSI? The restriction in the downpipe will slow the flow thorugh the trubine, resulting in a slower impellor, resulting a lower boost, resulting in a lower exhaust manifold pressure, resulting in less mas transfer through the turbine, resulting in a slower compressor,.......until it finds a stable state.


The whole thing is moot anyway because one of the first things you do to a stock car is to flick the dump pipe for something less restrictive. The outcomes speak for themselves.

 

[TCIC 300ci superbeast]

ok then i didnt have a turbo that wouldnt build alot of boost and then added a restricter(closing off the EX) plate and then it would build boost



and no the world isnt flat it has some hills before the water falls off the side

 

[Bort62]

ok then i didnt have a turbo that wouldnt build alot of boost and then added a restricter(closing off the EX) plate and then it would build boost



and no the world isnt flat it has some hills before the water falls off the side

Alright dude, here is the thing:

I'm not being rude - I am just stating a concern. Most of the things you have posted on this forum have led me (and I would hope others) to the conclusions that you don't really know anything about the subjects you are giving advice on. Your lack of knowledge is not a problem for me as long as you keep it to yourself.

What is a problem for me is that these forums are a wealth of knowledge for people who are trying to learn about the subjects discussed here. For a person not knowing any better, they may log on here and do a search and dig up one of your posts. They may read what you have written and take it as truth. Not only will they end up likely embarrassed and looking like an idiot when they repeat this information to someone else, but there is a strong chance that they will end up making a poor decision in relation to their car.

That is why I try to rebuke each of you false statements that I find, in the hope that if someone does find themselves in that situation they will at least have a warning that you are wrong.

This forum is amazing because of all the great information here. Unfortunately, every once in a while we get people here who insist on injecting blatantly false information into the mix. Usually they get the hint and pipe down or go away - but you seem to be interested in sticking around.

So I am asking you - please... don't offer any more false advice. If you stick around here and read more than type - you will probably learn a lot. If you think you can help someone - try prefacing your statement with "I think" or some other sort of uncertainty. It's okay to be wrong, but it has the potential to hurt other people when you are so confidently wrong that others believe you.

/soapbox

 

[XPC66]

Ouch that's gotta sting.

I wouldn't be too hard on any members for being sure in their own mind they are right. It's very difficult to undo misconceptions, but with patience..........

 

[TCIC 300ci superbeast]

ok what ever i didnt do it and it didnt work.
i had a pipe 2 1/4" and the turbo wouldnt spool up to the 7psi the stock WG is set at. then i added a restricter plate 1 3/4" hole and then and only then would it spool up to the 7psi need to blow off.
(this was on my old turbo set up)


but what ever it didnt work acording to you.

 

[Thad]

To oversimplify again.
The turbine side is a driven "wheel". Exhuast gas is the driving force. Force = mass x velocity. Ability to work will increase when increasing back pressure which will increase gas velocity as long as mass does not decrease at a rate out of proportion to gas velocity increase.

Who is on second?

 

[XPC66]

ok what ever i didnt do it and it didnt work.
i had a pipe 2 1/4" and the turbo wouldnt spool up to the 7psi the stock WG is set at. then i added a restricter plate 1 3/4" hole and then and only then would it spool up to the 7psi need to blow off.
(this was on my old turbo set up)


but what ever it didnt work acording to you.

So you had an undersized compressor, choked the exhaust enough to cause high exhaust gas reversion and therefore lowered the mass flow to the engine, lowering velocity pressure and raising static (boost gauge) pressure?

 

[SR_Crewchief]

ok what ever i didnt do it and it didnt work.
i had a pipe 2 1/4" and the turbo wouldnt spool up to the 7psi the stock WG is set at. then i added a restricter plate 1 3/4" hole and then and only then would it spool up to the 7psi need to blow off.
(this was on my old turbo set up)


but what ever it didnt work acording to you.

So you had an undersized compressor, choked the exhaust enough to cause high exhaust gas reversion and therefore lowered the mass flow to the engine, lowering velocity pressure and raising static (boost gauge) pressure?

Or to put it another way...

He had too large a turbine and housing for the volume of air to drive it. Then added a restriction that force drive pressure to increase. An excellent example of too large a turbo for the appllication.

Willing to bet if he put a thermocouple into the exhaust manafold before the turbo the EGT's would be a little toasty. That increase EGT then gets a thermal transfer to the compressor side drive intake temps up with a possible end result reduced performance not increased.

 

[XPC66]

yeah but he has put the restrictor after the turbo ( I presume) so the turbine may or may not be OK. You're right about creating a nice collector where the aggragated heated can happily coke the CHRA..

 

[TCIC 300ci superbeast]

yeah but he has put the restrictor after the turbo ( I presume) so the turbine may or may not be OK. You're right about creating a nice collector where the aggragated heated can happily coke the CHRA..

no i put it before the turbo

 

[wallaka]

yeah but he has put the restrictor after the turbo ( I presume) so the turbine may or may not be OK. You're right about creating a nice collector where the aggragated heated can happily coke the CHRA..

no i put it before the turbo

That is a valid solution. I've seen people block one side of a divided housing to use on a smaller engine.

It didn't sound like that when you were arguing though. You need to be clear. Otherwise, you sound like a petulant child even when you might be right.

So, please, think before you post. It'll save a lot of confusion and misinformation.

 

[TCIC 300ci superbeast]

next time ill try to be more clear about it.

 

[XPC66]

Either way the overall result is probably a drop in mass flow, so a drop in power.

 

[wallaka]

Either way the overall result is probably a drop in mass flow, so a drop in power.

Probably, but if the too-big turbo can be had for cheap, a little ingenuity could go a long way. I've usually seen it with 4-cylinder cars using divided-scroll housings designed for diesel that is twice as big (say a Holset HX40 or something). It works out to a .45 A/R or so, down from the .90/.85 that the turbo originally had. Optimal for smaller engines. And, again, optimal for the wallet.

 

[Bort62]

Blocking off half of a twin scroll turbine would give you a lower effective A/R. It's a poor mans way of making the exhaust side of a turbo smaller.

This is NOT the same as putting a smaller pipe or other restriction between the turbo and the engine. That just reduces mass flow and, as a result, would hamper spool-up.

If you want to think of the hose analogy - the turine housing is your nozzle. If you make the nozzle smaller, you will get a higher velocity flow. This will spin your turbine up faster.

However, shrinking the diameter of the hose (the exhaust leading up to the turbo) won't increase the velocity at all, at once you start to get too small will start to restrict it as mass flow is hampered.

Now, there is something to be said for exhaust volume, but that's another situation. It does take a finite time to raise the pressure of the exhaust so that there is significant pressure drop across the turbine wheel. A 2x larger exhaust volume will take approx 2x as long to come up to pressure.

But you would have to dramatically increase/decrease the exhaust diameter/design to really notice the effects of this, IMO.

Even the STS guys w/ the turbo at the end of 7' of 3" exhaust tube see pretty instant spool up w/ a properly sized exhaust housing.

 

[XPC66]

...If you want to think of the hose analogy - the turine housing is your nozzle. If you make the nozzle smaller, you will get a higher velocity flow. This will spin your turbine up faster.

.....

That might be true for hosing off the driveway, but you can't avoid Tp = Vp + Sp. The turbine is there to provide torque to the compressor. Without the required power over time the torque will drop and thus the turbine speed.

By adding a restrictor plate the manifold pressure has increased. For the hell of it let's say the base turbine expansion ratio is 2.0 and the turbo back resistance is 70kPa. This would normally impose a standing pressure in the manifold of:

(70 +101) x 2.0 = 342 kPa aboslute = 241kPa gauge (~35psig)

Seeing as we don't have enough money to increase the A/R, normally you would try to reduce the turboback pressure by putting in the biggest long radiuus exhaut pipe and low drop muffler within budget. So we get the exducer to tail pipe drop down to 20kPa.

Now the standing pressure is

(20 +101) x 2.0 = 242kPa absolute = 141kPa gauge (~20psig)

Thats a total drop of 100Kpa in the manifold for a drop of 50Kpa on the tail pipe, which holds true for the expansion ratio of 2.0. The increase in power will therefore be quite noticeable as the VE of the engine increases.

Now a fixed restrictor plate is installed upstream which imposes a drop between the inducer and the manifold.

What was true for unrestricting the turboback holds for restricting the upstream side. So let's say the plate adds 20kPa

(20 +101)/2 = 60.5kPa absolute = -40kPa gauge drop in pressure on the exducer side of the turbine.

So the turbine is now seeing a reduction of pressure at the inducer of 20kPa and a reduction at the exducer of 40Kpa, for a total reduction of 60kPa. That 60kPa represents a loss of power to drive the turbine, which results in a reduction in turbine speed and therefore a reduction in torque and therefore a reduction in compressor pumping. The VE of the engine has dropped significantly and thus the charge pipe flow is dropping and what would otherwise be velocity pressure is translating into static pressure, making for a great boost gauge reading and BOV psssht, but poor engine response.

 

[Bort62]

XP,
Either you missed my point, or I did not make it clear - but you did a much better job of explaining what I was trying to put in laymans terms.

The end result is the same - physics doesn't lie. Restrictions in the exhaust will not make your car any faster.

 

[XPC66]

I figured as much, but I didn't want someone thinking that a nozzle in the hose would amount to anything than a reduction in motive force. The turbine would spin faster with a jet stream on the blades if there wasn't any compressor hanging off the other end, but with that load it will spin slower as torque conversion takes up the available energy...... like trying to wash a brick off the driveway instead of leaves with the garden house