Intake/Exhaust Pressure Ratio's - what's the big deal?

[BoostedDrummer]

Doesn't a lower temperature EGT rob horsepower? Lower than normal I mean. I was under the impression that turbos like heat because its violent moving air that helps spin the turbine?

[Force Fed Mopar]

So what about if you had less than a 1:1 ratio? Like, say you have a external wastegate that can flow enough to act like open exhaust once the turbo has reached full spool. Would it drop the pressure or disrupt the flow so much that it would actually slow the turbo back down?

Or put another way, let's say you could get the same exhaust pressure as a supercharged car, as mentioned, which would (seemingly) obviously be lower than 1:1 due to less restrictive exhaust. Would the turbo still spin properly? Superchargers don't care about the ratio as they are crank driven, but I'm sure the turbo does.

[OmniLuvr]

I was under the impression that turbos like heat because its violent moving air that helps spin the turbine?

its not necessarily more violent, it is, but you have to look at heat as energy. if you have hot exhaust running through your manifolds it going to try and "expand" as it moves and eventually cools and looses its extra driving force farther down the exhaust stream as it transfers that energy into the exhaust pipe. now there is still "pressure" there at the end of the tail pipe, but with a little less energy behind it as it cools. so the more you keep your exhaust insulated the more energy you will have present until it starts to cool from distance or from the exhaust pipe itself. which brings up a good point, in which I don't like to wrap my downpipe (as long as its not going to burn something) to allow the exhaust to enter a lower/cooler pressure zone right out of the turbo...

[johnl]

As the exhaust gasses cool, their density increases. Dense gasses have more weight per unit of volume and may impart more energy to the the turbine than faster and hotter but less dense gasses.

[Rrider]

I would think you want it as close to 1:0 as you could get it.. I don't see how the exhaust side being too low could ever be a problem!

[85boostbox]

Nobody that I know has gotten extremely low. I think the lowest I have seen is 1.0/.7.

[OmniLuvr]

As the exhaust gasses cool, their density increases. Dense gasses have more weight per unit of volume

but how far would the exhaust have to travel or how "cool" would the temp have to be? and how would you take advantage of the more dense gasses? would you have to build a header/exhaust system that starts out "larger" in diameter to promote flow, then get smaller as it travels to the turbo to keep velocity up?

the rear mount turbo setups can work well, but they usually utilize a smaller than "usual" turbine/wheel and a larger wastegate and tend to wrap the complete exhaust to help with spool and still have flow, just food for thought...

[ShadowFromHell]

Some great info here for sure, but how would a guy apply this knowledge to a T2 car with the basic mods we all do?

[85boostbox]

Some great info here for sure, but how would a guy apply this knowledge to a T2 car with the basic mods we all do?

Actually most people do without even realizing it. That is why a lot of people misunderstand turbocharging. The biggest way to drop pressure ratio is actually AFTER the turbine. Aka downpipe exhaust. Rob has proved that time and time again and preaches it CONSTANTLY. He gets the most power he can out of the smallest turbine he can with his setup, even with the factory log manfold. And still pertains a 1:1 ratio. I'm using Rob as an example cause he is one of the very few that shares everything and hides nothing. plus his documentation on different stages of his car.

[shadow88]

Ok, so you open up the exhaust with a sewer sized swing valve and down pipe and a nifty exhaust cut out just to cover all the bases.

What would you suggest for head porting, valve size, cam selection, cylinder head design, turbine wheel sizing, turbine housing choices, compressor wheels and housings?

All the usual things people are trying to select to get the correct piece for the most power and ultimately whether they know it or not, ideal pressure ratio.

For example, say I just added an exhaust pressure gauge and I'm double my boost pressure.

I have a huge exhaust already, what else would you change?

[Shadow]

Ok, so you open up the exhaust with a sewer sized swing valve and down pipe and a nifty exhaust cut out just to cover all the bases.

What would you suggest for head porting, valve size, cam selection, cylinder head design, turbine wheel sizing, turbine housing choices, compressor wheels and housings?

All the usual things people are trying to select to get the correct piece for the most power and ultimately whether they know it or not, ideal pressure ratio.

For example, say I just added an exhaust pressure gauge and I'm double my boost pressure.

I have a huge exhaust already, what else would you change?

IF you had your exhaust all done with dump and all that could only happen with the smallest most miss-matched turbo possible. You could Not put any normal turbo configuration on and get that bad of a reading.

So you would need some kind of frankenstein mini, like a 50 trim T04e mated to a mitts turbine housing and wheel running 30psi through a highly efficient set-up.

[contraption22]

It's a careful balance of keeping back pressure low and velocity up before the turbine. After the turbine is much simpler, just make everything big.

[BoostedDrummer]

You'd think there'd be some limit to how "free" the exhaust has to be after the turbine though, right? Too big and you'd run the risk of boost spikes? I seem to remember reading somewhere that having too free flowing of an exhaust will leave the engine feeling gutless on the bottom end.

[1BADVAN]

You'd think there'd be some limit to how "free" the exhaust has to be after the turbine though, right? Too big and you'd run the risk of boost spikes? I seem to remember reading somewhere that having too free flowing of an exhaust will leave the engine feeling gutless on the bottom end.

From what I understand
no not on a turbo car, the bigger/"Free-er" the exhaust is post turbo just will help it spool faster. The issue of boost spikes is due to a wastegate that is insufficient, not because the big exhaust is bad, you could fix boost spikes with a small exhaust but you will also kill power. You probably did read that a free flowing exhaust will cause you to loose torque, but unless you are towing something, that doesn't matter, you want Horsepower. With a turbo car unless you have a huge turbine you will always have some backpressure wanted or not, due to the restriction of the turbo.

[85boostbox]

Boost spikes are usually associated with the sizes of wastegates. Usually not with the exhaust flow. That is why you see a lot of people on here port the waste gate hole for better control.

[85boostbox]

I should add as well that boost spikes can also be attributed to an undersized turbo and running out of breath on a setup.

[Shadow]

You'd think there'd be some limit to how "free" the exhaust has to be after the turbine though, right? Too big and you'd run the risk of boost spikes? I seem to remember reading somewhere that having too free flowing of an exhaust will leave the engine feeling gutless on the bottom end.

Garrett turbos (and most others I would imagine) are designed to run full open, No exhaust! So there is no such thing as too big

What you read is a Myth. Power will Not suffer as your turbo will spool Faster, making More TQ down low, not less.

[shadow88]

Ok, how about I re do the question.

Let's say I have the typical set up, that's a 2.2l turbo 2 with a 2 piece, hybrid 50 trim stage 2 .63 housing and a 3 inch exhaust. I run 25 psi boost. Let's also say the pressure ratio is not 1:1, let's go with 1.5:1 as the revs climb to the upper end.

What can be done to reduce the pressure ratio closer to 1:1 regarding cam duration, lift,advance/ retard, port size, intake design, charge pipes, tb size or anything else you want to mention?

I'm trying to get an idea of things to help reach the "ideal" pressure ratio that isn't a different turbo.

[BoostedDrummer]

You could have the exhaust manifold ported to increase flow is about all I can think of aside from going with either the turbos unleashed log header or a tubular design.

[85boostbox]

Adding any more flow ( can, port work etc etc) is only going to add more pressure. What kind of swingvalve are you running?