Supercharger questions

[Uncle Bob]

you honestly think that there will be a 2:1 ratio between the intake pressure at the turbo and the intake pressure at the intake valve??

Now that would be one screwed up engine.

this particular guy has datalogging sensors on EVERYTHING. pressure at the turbo and at the intake manifold. temp sensor before the intercooler and after. Pressure sensor at the exhaust manifold and in the dump pipe. Although I'm not positive where his reading was coming from for the above graph, he stated later that when he saw 10psi at the turbo, he saw 9psi at the intake manifold....which is a sign of a very well designed system. Hardly any pressure drop over the intercooler and throttle body

 

[Gary Evans]

The smaller the turbine A/R the higher the header pressure all other things being equal. Your not going to see header pressures anything like that on a bike or streetable car.
Why don't you measure yours and tell us what you find?

 

[Uncle Bob]

actually, I'm rebuilding the systems as we speak, and I was planning on putting a 1/8" NPT fitting on it just for that purpose. But then, I'm sure whatever findings I post would pull lots of very knowledgable peoples scorn on whats wrong with my bike

But then....nothings stopping others from doing the same thing, no?

BTW, the car in question had a .96 AR on the exhaust.....not exactly "small". Any bigger and the turbo would have a hard time reaching peak boost in initial gear pull, judging by the graph above.

since my turbo uses a .42 AR, I'm sure I'm screwed no matter what eh?



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[Over_Easy]

well, the next friend of yours that has a naturally aspirated bike that you would like to play a gag with, stick a potato up his exhaust can with a tiny hole drilled into the center of the potato, and then challenge him to a race.

Severe restriction of the exhaust costs power.  A very free flowing (large) turbo will still have a 2:1 ratio of exhaust pressure vs intake pressure.  In other words, if you have 15psi on the intake, you will have 30psi on the exhaust.  This pressure reduces efficiency because the engine has to work harder to overcome the pressure in the exhaust, and of course, an SC'd engine will have no notable back pressure in the exhaust, assuming they built it half intelligently, so they will not have this issue.

A undersized turbo can be as high as 4:1 ratio.

Is this why you see turbos with the short exhaust port just out side the fairing?

 

[Gary Evans]

The size of the turbine A/R is only half of the equation. The size of the engine (exhaust gas volume) is the other.
Those exhaust pressure ratios are excessive (for normal applications) and while there may be other reasons a relatively small A/R is the most obvious. A .96 A/R may not be small for that size engine and compressor combination.

 

[Uncle Bob]

no, the reason a dump pipe is used is to reduce back pressure AFTER the turbo. the length, shape, or baffling of the final exit doesn't matter, all that matters is the pressure in the exhaust. The little the better. It is quite possible to have a full exhaust system that is just as good as a short dump pipe, but it requires a large diameter pipe with a very free flowing muffler....most systems offered will have some back pressure vs a dump pipe, and you will lose a little power due to it. It also causes (in an extreme case) for the turbo to spool slower, due to the pressure differential (there's that phrase again...)

 

[Uncle Bob]

The size of the turbine A/R is only half of the equation. The size of the engine (exhaust gas volume) is the other.
Those exhaust pressure ratios are excessive (for normal applications) and while there may be other reasons a relatively small A/R is the most obvious. A .96 A/R may not be small for that size engine and compressor combination.

more involved than that, there's also the size of the turbine, the style of turbine, the trim of the turbine. The weight of the turbine and shaft. Single inlet or tangient housing.

Thats not even touching on what the turbine is connected to.

I'll put it this way, I've seen a lot of dyno sheets on 302's. 900HP is a very respectable number for his boost level. This doesn't happen accidently. The guy is a very knowledgable car builder. I'm going to give him the benefit of the doubt that he didn't randomly pick the turbo in question.

 

[Over_Easy]

no, the reason a dump pipe is used is to reduce back pressure AFTER the turbo.  the length, shape, or baffling of the final exit doesn't matter, all that matters is the pressure in the exhaust.  The little the better.  It is quite possible to have a full exhaust system that is just as good as a short dump pipe, but it requires a large diameter pipe with a very free flowing muffler....most systems offered will have some back pressure vs a dump pipe, and you will lose a little power due to it.  It also causes (in an extreme case) for the turbo to spool slower, due to the pressure differential (there's that phrase again...)

But would you agree that the short "dump pipe" eliminates any concern for back pressure, plus eliminates any extra weight concerns as well?

 

[suzuki4life]

okay...back to the original question

a turbo is more efficient than a supercharger because the added backpressure created invokes less drag than a pulley driven charger requires.

that simple.

Yes a supercharger tends to be more predictable BECAUSE every revolvtion of the crank equals a ratio of the charger thus the charger MUST act in a linear form as the engine will do if both are operating properly.

A turbo however acts and is controlled upon vaccums and pressures created within the engine to control the charger. Lots of variables (ie: leaks, lines, springs, temps etc etc etc etc) which make a turbo a bit more fickle in the end.

 

[suzuki4life]

no, the reason a dump pipe is used is to reduce back pressure AFTER the turbo.  the length, shape, or baffling of the final exit doesn't matter, all that matters is the pressure in the exhaust.  The little the better.  It is quite possible to have a full exhaust system that is just as good as a short dump pipe, but it requires a large diameter pipe with a very free flowing muffler....most systems offered will have some back pressure vs a dump pipe, and you will lose a little power due to it.  It also causes (in an extreme case) for the turbo to spool slower, due to the pressure differential (there's that phrase again...)

But would you agree that the short "dump pipe" eliminates any concern for back pressure, plus eliminates any extra weight concerns as well?

But dump pipes also allow the turbo to spin almost uncontrolably fast in many instances......causing backfires, boost spikes etc. Backpressure on ANY engine is a good idea.

 

[Over_Easy]

no, the reason a dump pipe is used is to reduce back pressure AFTER the turbo.  the length, shape, or baffling of the final exit doesn't matter, all that matters is the pressure in the exhaust.  The little the better.  It is quite possible to have a full exhaust system that is just as good as a short dump pipe, but it requires a large diameter pipe with a very free flowing muffler....most systems offered will have some back pressure vs a dump pipe, and you will lose a little power due to it.  It also causes (in an extreme case) for the turbo to spool slower, due to the pressure differential (there's that phrase again...)

But would you agree that the short "dump pipe" eliminates any concern for back pressure, plus eliminates any extra weight concerns as well?

But dump pipes also allow the turbo to spin almost uncontrolably fast in many instances......causing backfires, boost spikes etc. Backpressure on ANY engine is a good idea.

Just learning a little here, thanks for the info.

Not ready for the turbo, YET.

 

[suzuki4life]

no, the reason a dump pipe is used is to reduce back pressure AFTER the turbo.  the length, shape, or baffling of the final exit doesn't matter, all that matters is the pressure in the exhaust.  The little the better.  It is quite possible to have a full exhaust system that is just as good as a short dump pipe, but it requires a large diameter pipe with a very free flowing muffler....most systems offered will have some back pressure vs a dump pipe, and you will lose a little power due to it.  It also causes (in an extreme case) for the turbo to spool slower, due to the pressure differential (there's that phrase again...)

please draw me a pipe realistically than can perform as well as a 3" long dump pipe in proper sizing. If you have a 3" OD dump 3" long.....and you use a 6" OD full exhaust 40" long(especially with any type of baffling).....at some RPM(ie: hp range or compressor) you will create enough backpressure to lose hp over the dump pipe.

 

[Uncle Bob]

But would you agree that the short "dump pipe" eliminates any concern for back pressure, plus eliminates any extra weight concerns as well?

oh no....the diameter of the dump pipe is just as crucial. Assuming its sized properly, then yes.

 

[suzuki4life]

no, the reason a dump pipe is used is to reduce back pressure AFTER the turbo.  the length, shape, or baffling of the final exit doesn't matter, all that matters is the pressure in the exhaust.  The little the better.  It is quite possible to have a full exhaust system that is just as good as a short dump pipe, but it requires a large diameter pipe with a very free flowing muffler....most systems offered will have some back pressure vs a dump pipe, and you will lose a little power due to it.  It also causes (in an extreme case) for the turbo to spool slower, due to the pressure differential (there's that phrase again...)

But would you agree that the short "dump pipe" eliminates any concern for back pressure, plus eliminates any extra weight concerns as well?

But dump pipes also allow the turbo to spin almost uncontrolably fast in many instances......causing backfires, boost spikes etc. Backpressure on ANY engine is a good idea.

Just learning a little here, thanks for the info.

Not ready for the turbo, YET.

These guys get a bit vague and then carried away if disputed IMHO. Turbo or charger.....entry level...its all in your choices.


If 230hp is what you seek...and the price doesn't bother you....I wouldn't mind a supercharger for something different.


Always remember.....power/speed costs money.....so the more you make...the more you will spend if AND when it breaks.

 

[Uncle Bob]

But dump pipes also allow the turbo to spin almost uncontrolably fast in many instances......causing backfires, boost spikes etc. Backpressure on ANY engine is a good idea.

huh?

Pressure differential is the phrase of the day. The more pressure in the exhaust, the harder the turbo or engine or both have to work in order to overcome it. Tuning issues are another matter. Turbo's do not "spin out of control" because of lack of back pressure, they work more effectively and thats all they do.

back firing, surging, boost spiking, that is all tuning issues.

 

[Gary Evans]

I didn't say he picked the wrong turbo how would I possibly know that. What he needed to achieve his goal may simply require the use of ultra high turbine inlet pressure. What I am saying is that pressures that high are not normal for most applications. Test it and find out for yourself.

 

[Uncle Bob]

please draw me a pipe realistically than can perform as well as a 3" long dump pipe in proper sizing. If you have a 3" OD dump 3" long.....and you use a 6" OD full exhaust 40" long(especially with any type of baffling).....at some RPM(ie: hp range or compressor) you will create enough backpressure to lose hp over the dump pipe.

I never said it was easy, only possible.

 

[Uncle Bob]

I didn't say he picked the wrong turbo how would I possibly know that. What he needed to achieve his goal may simply require the use of ultra high turbine inlet pressure. What I am saying is that pressures that high are not normal for most applications. Test it and find out for yourself.

All I'm saying is thats not what I've seen from my research on the subject, nor the physics involved, as I understand them. This is going in circles.

The end result is a very efficient engine will have a high HP per psi...regardless to what perceived problems one sees in the exhaust manifold pressure.

 

[suzuki4life]

But dump pipes also allow the turbo to spin almost uncontrolably fast in many instances......causing backfires, boost spikes etc. Backpressure on ANY engine is a good idea.

huh?

Pressure differential is the phrase of the day.  The more pressure in the exhaust, the harder the turbo or engine or both have to work in order to overcome it.  Tuning issues are another matter.  Turbo's do not "spin out of control" because of lack of back pressure, they work more effectively and thats all they do.

back firing, surging, boost spiking, that is all tuning issues.

hmmmmm might wanna talk to anyone who produces an entry level kit with an internal gate then about dump pipe directly causing boost spikes because the spool rate tends to cause boost creep. Doesn't tend to happen with full exhausts on the same kits.

 

[suzuki4life]

I didn't say he picked the wrong turbo how would I possibly know that. What he needed to achieve his goal may simply require the use of ultra high turbine inlet pressure. What I am saying is that pressures that high are not normal for most applications. Test it and find out for yourself.

All I'm saying is thats not what I've seen from my research on the subject, nor the physics involved, as I understand them.  This is going in circles.  

The end result is a very efficient engine will have a high HP per psi...regardless to what perceived problems one sees in the exhaust manifold pressure.

Since you are educating us all about turbos.....why not quit your job and contract a company to build them to your specs.........so people don't have to have the problems every other "misfunctioning" kit has?

Like I said...I've owned more than my fair share of turbo bikes.....all have had many of the same characteristics.