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Hate my 3" thermal exhaust
- Thread starter JOEL12155
- Start date
That's the big question, isn't it?
But let's first lay down some parameters. We're talking about a tuned ST- with stock turbos, etc.
It's not unlikely that ON A DYNO a larger exhaust might produce a bit more peak horsepower. But as I've pointed out (here), in a real-world track environment, you modulate the pedal and shift. In that case, having a higher exhaust velocity (smaller pipe) is beneficial.
Think of it from another point of view- the intake port. You can size an intake port that achieves the best possible port speed (around 100m/s) at say 6500RPM or redline. Or, you can size that port slightly smaller so that the optimal port speed occurs at a lower RPM. You size it smaller because under racing conditions you're not operating at 6500 all the time. So you size it based on the average between the max RPM and lower RPM of your normal operating range- usually a shift point.
Does that help? Oh, and please click the link above if you haven't read it. I don't get paid for views!
But let's first lay down some parameters. We're talking about a tuned ST- with stock turbos, etc.
It's not unlikely that ON A DYNO a larger exhaust might produce a bit more peak horsepower. But as I've pointed out (here), in a real-world track environment, you modulate the pedal and shift. In that case, having a higher exhaust velocity (smaller pipe) is beneficial.
Think of it from another point of view- the intake port. You can size an intake port that achieves the best possible port speed (around 100m/s) at say 6500RPM or redline. Or, you can size that port slightly smaller so that the optimal port speed occurs at a lower RPM. You size it smaller because under racing conditions you're not operating at 6500 all the time. So you size it based on the average between the max RPM and lower RPM of your normal operating range- usually a shift point.
Does that help? Oh, and please click the link above if you haven't read it. I don't get paid for views!
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Taken from your link:
Then why do all those turbo drag cars have such short, fat pipes exiting out the hood/front fender? According to what you said above (and in the previous portion in the post you linked), they are leaving inertial energy on the table, thus not making their car as fast as it could be. True, the pipe does have a sharp-ish change in direction, as it must for packaging reasons as well as those you listed in your post, so there's that going for it. But what about the rest? Where's the cat that helps laminar flow? Where's the smaller diameter pipe that helps with velocity? I mean these things are "straight" pipes off the turbo, often 3" to 4" in diameter. These cars match your scenario above. They are at WOT. They have to shift. Why aren't they running longer, or smaller pipes? Why aren't they wrapped, to keep the heat? Why aren't they designed to increase velocity, so as to help reduce backpressure?
For reference, this is Leroy, a kart with a twin turbo Corvette motor. It's quite fast.
See, here's where I think you have gone astray. You said this:
Why should the turbo RPM drop so much? It's spinning in excess of 140,000 RPM's, with all the mass of the compressor, turbine, and shaft also spinning. In the sub millisecond it takes for our auto trans to shift, the RPM's are not going down all that much. Some, yes. And even if you lose 2000 RPM's on the engine (and thus exhaust gas flow) from the upshift from 6K, that's still 4K RPM's on the engine, more than enough exhaust gas flow for full boost. As soon as the trans re-engages, the turbo is making full boost again almost instantaneously. Want an example? Put a boost gauge in. Or get the live data from ODB2. Do a pull. Keep your foot in it through a number of trans shifts. Do you see the boost value dip at all? On a manual car, where you can hold the trans disengaged (clutch in) between shifts, then you will see the dip. The longer you hold the clutch in. On these trucks? With shifts that last the snap of a finger? Full boost all day long.
Whatever minimal gains you think you will see from optimizing an exhaust flow downstream to help generate velocity, will be far outweighed by the sheer presence of boost. Conversely, whatever small gains you lose by not having an optimized exhaust, as you describe, are easily overshadowed by that boost. If you have to sacrifice a little efficiency to get boost, and get to it sooner, the trade-off is worth it. Which is why these drag cars run the exhausts that they do. Because the trade off is worth it. Your theory is sound. And on a NA motor, that needs as much help as possible to fill to 100% VE, it's worth it. Every little bit helps. On a boosted motor, that can fill 140% VE and more, with a sewer pipe hanging off the turbine? Well, if you want to go faster, you do what you can to let that turbo spool, baby. Everything else is lost in the noise.
Can you force 500 hp through the stock exhaust? Absolutely. As proven by the dyno chart you have referenced. Well, that's a turbo for you, that's forced induction. Emphasis on forced. But just because you can push that power through the stock exhaust, doesn't mean that's the best way to do it.
Why should the turbo RPM drop so much? It's spinning in excess of 140,000 RPM's, with all the mass of the compressor, turbine, and shaft also spinning. In the sub millisecond it takes for our auto trans to shift, the RPM's are not going down all that much. Some, yes. And even if you lose 2000 RPM's on the engine (and thus exhaust gas flow) from the upshift from 6K, that's still 4K RPM's on the engine, more than enough exhaust gas flow for full boost. As soon as the trans re-engages, the turbo is making full boost again almost instantaneously. Want an example? Put a boost gauge in. Or get the live data from ODB2. Do a pull. Keep your foot in it through a number of trans shifts. Do you see the boost value dip at all? On a manual car, where you can hold the trans disengaged (clutch in) between shifts, then you will see the dip. The longer you hold the clutch in. On these trucks? With shifts that last the snap of a finger? Full boost all day long.
Whatever minimal gains you think you will see from optimizing an exhaust flow downstream to help generate velocity, will be far outweighed by the sheer presence of boost. Conversely, whatever small gains you lose by not having an optimized exhaust, as you describe, are easily overshadowed by that boost. If you have to sacrifice a little efficiency to get boost, and get to it sooner, the trade-off is worth it. Which is why these drag cars run the exhausts that they do. Because the trade off is worth it. Your theory is sound. And on a NA motor, that needs as much help as possible to fill to 100% VE, it's worth it. Every little bit helps. On a boosted motor, that can fill 140% VE and more, with a sewer pipe hanging off the turbine? Well, if you want to go faster, you do what you can to let that turbo spool, baby. Everything else is lost in the noise.
Can you force 500 hp through the stock exhaust? Absolutely. As proven by the dyno chart you have referenced. Well, that's a turbo for you, that's forced induction. Emphasis on forced. But just because you can push that power through the stock exhaust, doesn't mean that's the best way to do it.
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What I don’t know is a lot of fancy engineering talk that is going back and forth and over my head. I am not sure if there is a right/wrong or something that has yet to be discovered and could be a new revolution in exhaust/turbo that you guys can collaborate on and become billionaires. Some say it’s penis size while others say it’s “motion in the ocean”, I am sure my wife would say my boat and swimming abilities are subpar.
What I do know is that mcglsr2 seems to have an extent of knowledge that will be beneficial to this forum. This forum is my go to place for everything Explorer ST and I am very appreciative of everyones posts, even some of Firestarters.
Please don’t push him away, Firestarter would have already blocked everyone’s comments while mcglsr2 keeps responding, right/wrong/inconclusive who knows for my stupid brain.
What I do know is that mcglsr2 seems to have an extent of knowledge that will be beneficial to this forum. This forum is my go to place for everything Explorer ST and I am very appreciative of everyones posts, even some of Firestarters.
Please don’t push him away, Firestarter would have already blocked everyone’s comments while mcglsr2 keeps responding, right/wrong/inconclusive who knows for my stupid brain.
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Just for giggles, I hooked up my ODB2 scanner to see live data - to see if the boost drops between shifts. I did a couple pulls on my way to an errand. And…It in fact did…it dropped 0.1 psig between shifts.
Not significant. Lost in the noise. Even if it dropped 1 psi...meh. A teeny blip and then back to full boost (or more specifically, as full as the ECU is set to deliver at that load).
Not significant. Lost in the noise. Even if it dropped 1 psi...meh. A teeny blip and then back to full boost (or more specifically, as full as the ECU is set to deliver at that load).
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Two reasons.
- I haven't replaced the stock downpipes, and I probably never will. So I would never see the benefit of a 3" exhaust (other than it acting like a 2.5" to match the downpipes).
- And, more importantly, the 2.5" was on sale for $1000. The 3" was $1900. I'm not made out of money my man. If I were, then I'd already have an intercooler, 3" downpipes, 3" exhaust and a tune. As it is, this is our family hauler. I have another car that gets what little car money there is. This truck isn't my fun car. Alas. I have to make the hard choices.
The stock exhaust is 2.25", and most likely smaller in places as it is crush-bent and not mandrel-bent. I now have a dual 2.5" with mandrel-bends, high flow resonators/mufflers, that matches my downpipes. What do you mean no gain? And when I get around to putting a tune on it, golden.
I get the vibe you don't like me. I'm not sure why, I didn't do anything to you. I didn't crush your dreams. I even apologized to you for posting dyno charts that turns out you had no interest in seeing. I'm going out of my way to be cordial. So what's up? You don't like what I'm saying? That's okay. I'm sure we can get past that. Is it that I'm questioning TMac? Is that the problem? Is he beyond reproach? Throw me a bone here, how can we smooth this out? And if you aren't interested in doing that, and you want to keep treating me like I'm a dick, you know what? I'm okay with that too.
I get the vibe you don't like me. I'm not sure why, I didn't do anything to you. I didn't crush your dreams. I even apologized to you for posting dyno charts that turns out you had no interest in seeing. I'm going out of my way to be cordial. So what's up? You don't like what I'm saying? That's okay. I'm sure we can get past that. Is it that I'm questioning TMac? Is that the problem? Is he beyond reproach? Throw me a bone here, how can we smooth this out? And if you aren't interested in doing that, and you want to keep treating me like I'm a dick, you know what? I'm okay with that too.
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I have a question.... for you AND @mcglsr2 ... i haven't seen much mention of the cats. With stock downpipes, isn't all of this exhaust discussion a moot point? isn't the stock downpipe and cat going to be EVERYONE's limiting factor, no matter the catback system chosen? so there really would be zero difference between stock/2.5/3.0 on the dyno OR the track until you change downpipes, AND THEN, there would be zero difference between those with 2.5" and 3.0" if they were both running the same aftermarket downpipes?
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