Big duration cams + turbo
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You really shouldn't say "in most cases" Tony because that simply isn't true. Any car with a T4/T3 hybrid will usually have more exhaust pressure than inlet (if a T2 exhaust side flowed as much as a T4 compressor, it'd be a T4). In all of the turbo cars we've measured, it was at least twice as much. The twin turbo big block mustang a friend had a single Y2k on had a 3:1 ratio! He then swapped to 2 88mm Y2K's. Have you measured your exhaust pressure on the Porsche? I'm not saying its impossible, but if it exists, its almost always on very high dollar road race cars. Drag cars generally have smaller turbines to help spool at the line which of course increases exhaust pressure.
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Originally posted by 400HP930
Damn, too bad I have been too busy then sick to get into this thread till now.
I have seen a lot of good info here with the exception of one incorrect bit of information in one of force fed's post.
Though it depends on the specifics of the engine, turbine and compressor, in most cases the pressure in the exhaust manifold will be very close to if not less than the pressure in the intake manifold.
Damn, too bad I have been too busy then sick to get into this thread till now.
I have seen a lot of good info here with the exception of one incorrect bit of information in one of force fed's post.
Though it depends on the specifics of the engine, turbine and compressor, in most cases the pressure in the exhaust manifold will be very close to if not less than the pressure in the intake manifold.
god you are all sorts of wrong in every single post you make in this forum.
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rest in peace tim.
Advocate for the People's Republic of Awesome
rest in peace tim.
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Revel in your ignorance all you want Matt.
Unless you are going to be pointing out the exceptions like crazy drag and performance setups and/or nearly closed throttle settings this is an easy one to understand and to prove.
Leave it to people that are clueless about fluid dynamics to disagree with me on this one.
If you don't believe me go hook some pressure guages up to your own normal turbo setups and figure it out for yourselves.
Unless you are going to be pointing out the exceptions like crazy drag and performance setups and/or nearly closed throttle settings this is an easy one to understand and to prove.
Leave it to people that are clueless about fluid dynamics to disagree with me on this one.
If you don't believe me go hook some pressure guages up to your own normal turbo setups and figure it out for yourselves.
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1|\| 5h0|27, y0u|2 bl4|\|k37 57473m3|\|7 15 \^/|20|\|g.
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Originally posted by 400HP930
If you don't believe me go hook some pressure guages up to your own normal turbo setups and figure it out for yourselves.
If you don't believe me go hook some pressure guages up to your own normal turbo setups and figure it out for yourselves.
That's what I'm trying to tell you, we have, and in every instance (granted most were hybrids with "small" exhaust's) exhaust pressure is more than compressor.
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Im gonna go with John on this one. Fluid stuff was good back in the day but you can't beat the math. Fluid acts differently under teh same circumstances then air does. IE: Doesn't expand as much under heat, doesn't compress as well, and doesn't dissipate heat at the same rate. All of this can be calculated to a certain extent but just isn't as effective as the computer generated maps done today.
-- PS. My back hurts like hell. Kbye.
-- PS. My back hurts like hell. Kbye.
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Originally posted by Vapor
fluid dynamics are sometimes used for comparison purposes, but are a poor modeling tool for airflow purposes. It might have worked in the 50's and 60's, but today's turbochargers are designed by computers with actual airflow data, not fluid dynamics which are a rough guide and "yesterday's news".
In short, your blanket statement is wrong.
fluid dynamics are sometimes used for comparison purposes, but are a poor modeling tool for airflow purposes. It might have worked in the 50's and 60's, but today's turbochargers are designed by computers with actual airflow data, not fluid dynamics which are a rough guide and "yesterday's news".
In short, your blanket statement is wrong.
What do you think computers are using to calculate and design flows through housings and turbines? Its called computational fluid dynamics for a reason. Before the extreme expense of creating a turbine and testing it is ever invested, 98% of the flow characteristics of a turbine are determined by CFD before it will ever leave the drawing board. The laws of physics are still the same even when we make computers calculate the finite details for us instead of using slide rules.
Force fed is saving himself some embarasment by stating that a way undersized turbine housing can restrict your exhaust flow to get you any crazy pressure ratio you want. Unless someone is looking to get high boost at very low RPMs and screw themselves at the high RPMs doing so is sheer stupidity.
Maybe some of the fucked up hack jobs you have come across are using way undersized turbines, but any properly set up radial turbine should never see more than a 2.5:1 pressure ratio across the turbine. For that reason in any factory setup you will never see a higher pressure ratio than that across the turbine.
If any of you all had bothered learning something more about how centripetal turbines and compressors work than what the other 'tuners' are saying who knows what kind of better numbers you could be squeezing out of your setups.
Just as there is a limit to the pressure an centripetal compressor can generate, there is also a practical limit to how much pressure you would want to run across a centripetal turbine.
So I can save a lot of time of explaning FD to you all, I will just give you a simple chart of typical automotive turbine performance characteristics to prove my point.
As you can see from both the mass flow and efficiency there is a sharp drop in centripetal turbine efficiency and mass flow after the 2:1 pressure ratio is exceeded. Since the power you get from any turbine is based on these two factors, your power output from a turbine tapers off just the same. If you really want to push the envelope then you can go as high as 2.5, but anything after that pressure ratio gains you NOTHING.
Concidering that the maximum pressure ratio you can get out of your centripetal compressor is also around 2.5:1 I hope you can all start to see the basic reason why there is no good reason why you should ever have much higher pressure ratio in the exhaust manifold than the intake.
Anyone who is doing so for any other reason than they can't afford the properly sized turbo for their engine air flows is just doing so because they are ignorant of the facts.
If any of you nay-sayers would have to take even a basic test on the fluid dynamics of centripetal turbines and compressors would get a big fat F.
Last edited by 0HP930; Feb 24, 2003 at 05:30 AM.
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Originally posted by 400HP930
Force fed is saving himself some embarasment by stating that a way undersized turbine housing can restrict your exhaust flow to get you any crazy pressure ratio you want. Unless someone is looking to get high boost at very low RPMs and screw themselves at the high RPMs doing so is sheer stupidity.
Force fed is saving himself some embarasment by stating that a way undersized turbine housing can restrict your exhaust flow to get you any crazy pressure ratio you want. Unless someone is looking to get high boost at very low RPMs and screw themselves at the high RPMs doing so is sheer stupidity.
There are many other examples but these two are the ones I'm most familiar with. Again I ask you Tony, have you ever measured exhaust pressure in your Porsche? This may sound stupid, but physics class is one thing and the real world is another.
