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Old 08-22-2008, 08:20 AM   #61
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They did that on Top Gear a while ago during one of their challenges... Jerremy put an oversized turbo in a Mitsubishi (I think) and the car had awful overheating problems.
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Old 08-22-2008, 08:36 AM   #62
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Ok, so now we're going to talk about reversion?

I'd argue that a well designed, and tuned exhaust on a NA engine will be far more benificial than the anti-reversion effects of a baffle (turbo in thise case).

It's better to have no energy wave reflection at all, then to try to control it. Let the pipe diameter control the max torque range for the exhaust, and keep bends minimal. Run a straight through muffler to keep the system open, and not reflecting waves back up the exhaust system.

Just because you have an "open" exhaust system doesn't mean it won't produce low end efficiency. Size the pipes right and the velocity will still be there.
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Old 08-22-2008, 08:39 AM   #63
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Originally Posted by Jay2TheRescue View Post
They did that on Top Gear a while ago during one of their challenges... Jerremy put an oversized turbo in a Mitsubishi (I think) and the car had awful overheating problems.
I remember that, it was a starion.

The data they gave out was very vauge though, and the true reason for the overheating was very unclear. Was it due to a turbo running outside it's efficeny, or under? Was it due to poor exhaust turbine sizing, or simply lack of an intercooler? Turbo sizing is a complicated equation, and just picking any ol' turbo can get you in trouble quick for many different reasons.
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Old 08-22-2008, 09:19 AM   #64
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My apologies if I sound terse here, I'm at work and have to be quick.

Gollum, I think that you'll find that an apples to apples comparison between a turbo specific exhaust system and an NA specific exhaust system will surprise you in terms of backpressure. Yes turbos add some backpressure, but so do normal mufflers. Turbo exhausts are designed to be as low restriction as possible after the turbo since that's necessary for the unit to function properly. In a good turbo exhaust, the turbo itself is the ONLY major restrictions.

Regarding catalytic converters since it came up. Newer generation catalytic converters are much less restrictive than older ones used to be. I read one article that said the increase in backpressure through a good cat is equivalent to less than a 1/4" restriction in the exhaust system.

dkjones, sure turbo engines can still run manifold vacuum. The question is when does the vacuum disappear and either become neutral or switch to a positive pressure situation? Ideally, you want that to happen at the same approximate throttle position you'd find yourself at during cruise. That's the whole point.

In terms of lag... 95% of the DIY and aftermarket turbo systems I've ever seen have had oversize turbochargers, intercoolers, intake piping, or all three. Of course they'll have lag. I'm not going to concern myself with problems associated with design flaws that wouldn't be incorporated into a system that I'd build for myself though. A properly sized turbo system should have little or no turbo lag under any circumstances. Doing it right involves quite a bit of planning and mathematics, but it's not difficult as much as it is labor intensive.
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Old 08-22-2008, 09:44 AM   #65
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My apologies if I sound terse here, I'm at work and have to be quick.

Gollum, I think that you'll find that an apples to apples comparison between a turbo specific exhaust system and an NA specific exhaust system will surprise you in terms of backpressure. Yes turbos add some backpressure, but so do normal mufflers. Turbo exhausts are designed to be as low restriction as possible after the turbo since that's necessary for the unit to function properly. In a good turbo exhaust, the turbo itself is the ONLY major restrictions.

Regarding catalytic converters since it came up. Newer generation catalytic converters are much less restrictive than older ones used to be. I read one article that said the increase in backpressure through a good cat is equivalent to less than a 1/4" restriction in the exhaust system.

dkjones, sure turbo engines can still run manifold vacuum. The question is when does the vacuum disappear and either become neutral or switch to a positive pressure situation? Ideally, you want that to happen at the same approximate throttle position you'd find yourself at during cruise. That's the whole point.

In terms of lag... 95% of the DIY and aftermarket turbo systems I've ever seen have had oversize turbochargers, intercoolers, intake piping, or all three. Of course they'll have lag. I'm not going to concern myself with problems associated with design flaws that wouldn't be incorporated into a system that I'd build for myself though. A properly sized turbo system should have little or no turbo lag under any circumstances. Doing it right involves quite a bit of planning and mathematics, but it's not difficult as much as it is labor intensive.
That was a very good post, and I can agree with all of it. Though here's a question I'll pose -

If a turbo is going to be right at the brink of negative to positive pressure, then will it run out of steam in the higher RPM? The turbo planned for a FE build up would have to have a VERY wide map to work without a significantly large intercooler. Most of the map calculations I'm comming up with to create pressure at cruise RPMS are dangerously close to the surge limit on the turbo. I'd really like to see what you'd recommend for a given setup.

Oh, and mufflers don't create much of any backpressure when they're straight through There's some pretty decent straight through mufflers now days that work well enough for street vehicles not to be too loud and buzzy.

I completely agree about most turbo setups out there. A T3 on a honda is just way too big for most street cars. My 2.8 liter inline 6 does just fine with the stock T3 up to around 270hp, then a T3/T4 50 trim will go from 250 to 400hp easily.

But I have to be going around a 100mph cruise to get my turbo to positive pressure. I know with a modern ball bearing turbo that wouldn't be the case, but how much different would it really be?
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Old 08-22-2008, 09:47 AM   #66
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Quote:
Originally Posted by Gollum View Post
Ok, so now we're going to talk about reversion?

I'd argue that a well designed, and tuned exhaust on a NA engine will be far more benificial than the anti-reversion effects of a baffle (turbo in thise case).

It's better to have no energy wave reflection at all, then to try to control it. Let the pipe diameter control the max torque range for the exhaust, and keep bends minimal. Run a straight through muffler to keep the system open, and not reflecting waves back up the exhaust system.

Just because you have an "open" exhaust system doesn't mean it won't produce low end efficiency. Size the pipes right and the velocity will still be there.
EXACTLY! now if you apply how reversion (or anti reversion) works with the entire system of a turbo it has the ability to make the motor more efficient since it has the ability to be effective over a larger range of flow rates and oscillations in exhaust pressure. because the turbo isnt just a baffle, it is a turbine that carries momentum, it actually INCREASES the fluctuation of high and low pressures as the valves open and close between the valves and turbo. there is a lot more that goes into it, but this gives you a general idea of the possibilities and shows that its more than just a restriction.
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Old 08-22-2008, 09:56 AM   #67
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I know a turbo isn't just restriction, but it's never moving under it's own power. The only instance I could imagine a turbo carrying gasses in a benifictial way would be under spool down.

Now, energy wave reversion... hmm. It would all come down to the manifold design. I'm getting where you're going with this, but I think the cost of building X amount of manifolds to get the turbo just far enough away for cruising RPM wil be difficult. And turbo sizing is extremely complicated, as you've got a very small window here. You don't want the turbo to just give you power when you put your foot into it, you want it to slowly ease on the positive pressure under cruise conditions. Not a simple task. What engine/turbo setup are you considering for a candidate?
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Old 08-22-2008, 12:45 PM   #68
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Quote:
Originally Posted by Hal9000 View Post
But even when a turbo isn't spooled up (like when cruising on the highway for example) it's still idling so to speak. When this is happening, the turbo only produces a small amount of pressure. That won't affect performance substantially, but what it does do is alleviate the need to the engine to suck air into the engine on it's own (like a vacuum pump). The turbo pushes the air in instead. This can make a difference in the VE and FE of the engine.
I don't understand how this could help FE. The energy isn't free; it's not recovered waste energy, the engine has to push the exhaust out to turn the turbo, right?
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Old 08-22-2008, 01:04 PM   #69
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I haven't gone as far as selecting a turbo yet since I'm still on the fence about actually turbocharging my mpg car, but it's an '88 crx with a D15. I'll probably end up with a very small turbocharger. But one of my other cars is a cosworth/mercedes that I'm building for bonneville. It's got a 2.3L 16V engine that I'm pairing up with a hybrid Holset HX 35/40 turbo. This setup should yield roughly 400-450 whp, although with some lag.

I agree with you about the stright through mufflers, most of the newer ones are pretty good. But NA exhausts and even some factory turbo exhausts are usually longer and have more bends than what I would build for a turbocharged engine. In general terms though, even the restriction of exhaust gasses through a turbocharger can be a good thing. A big problem that many garage tuners create for themselves by adding high flow exhausts to NA cars is overscavenging the head. That'll kill your FE and low end torque. Most street engines work best with some moderate level of backpressure unless you're only worried about top speed and high rpms. Zero backpressure is typically a bad thing except for a very few special conditions.

Many older turbos might run out of gas at high rpm, but I'd say that careful selection of even an older turbo could alleviate the problem. The first time I heard about this phenomenon was close to 20 years ago, and turbo design has changed quite a bit in the intervening years. Also, keep in mind, that the boost produced isn't usually linear. It really ramps up steeply in the upper flow ranges in most cases. What I'm talking about is not necessarily having the turbo beginning to spool up and produce real boost at low throttle settings. It's just kind of spinning lazily along and just barely producing pressure.

For turbos with good wide ranges, I'd personally start by looking at the Holset HX series units that have a "power ring" or "map enhancement ring". Basically it's a slit in the intake housing that allows air to be pulled in from the side partway down the inducer. This isn't state of the art technology, but it's new enough that you won't find it on the old t3's and t4's. I can't explain how it works, but it widens the useful gasflow range that the turbo works at substantially. This is basically why the honda guys have been able to get away with running HX 35's off dodge cummins trucks with any level of success. Schwitzer and I think the newer GT series Garrets also carry the same technology. What it will allow you to do is get away with a larger turbo that won't surge at high rpm, but will still spool at a relatively low rpm. Of course, header design, wastegate placement (be sure to go w/ external wastegates since the internal ones usually affect turbo efficiency and increase the rpm that the turbo begins to spool up at by a few hundred rpm) and many other factors can really affect things. I can't think of a way that IC size would affect things except to change the volume of the intake (more volume takes longer to pressurize => lag) and temperature. But those are more performance that FE issues in my opinion.
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Old 08-26-2008, 11:17 AM   #70
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Quote:
Originally Posted by Gollum View Post
I know a turbo isn't just restriction, but it's never moving under it's own power. The only instance I could imagine a turbo carrying gasses in a benifictial way would be under spool down.

Now, energy wave reversion... hmm. It would all come down to the manifold design. I'm getting where you're going with this, but I think the cost of building X amount of manifolds to get the turbo just far enough away for cruising RPM wil be difficult. And turbo sizing is extremely complicated, as you've got a very small window here. You don't want the turbo to just give you power when you put your foot into it, you want it to slowly ease on the positive pressure under cruise conditions. Not a simple task. What engine/turbo setup are you considering for a candidate?
a turbo manifold design is not as complex as an all motor header design. are there large areas for improvement? of course! but even some cars(specifically integras and civics) have seen a small increase in highway fuel economy by adding the turbo kit alone, even with the grossly inefficient log manifolds. most of the time you can figure things out as far as turbo sizing with an online calculator.

My argument isnt that it works every time. My argument is that its very possible and that it isnt rocket science.

cow: a turbo does recover wasted energy: heat. it uses the pressure and heat to spin the turbo because the volume of air going into the motor is much more than the volume coming out. as gollum and i have discussed, the turbo aids in anti-reversion in cruising conditions. there is a vacuum formed in the manifold behind the exhaust valve from both the exhausts velocity AND the turbos momentum. its important to note that the turbo will NOT allow the exhaust gasses back into this vacuum. this not only helps exhaust gasses exit the cylinder, this effect is so strong that it is possible in BOTH NA and turbo setups that this vacuum can actually suck the piston back up the cylinder
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