Exhaust Theory Question

[Cus]

Greetings Humans!

Bear with me, this has been on my mind for a while, and I'm not actually sure how to formulate the question, or even if it is one.... but it is something I'm pondering and don't know the answer to, so brace yourself for potentially damaged reasoning!

My understanding:

On an N/A performance motor you have "tuned length" extractors when you're chasing the final few percent of power - I'm guessing that they're "tuned" to hold the same volume of gas in the pipe from head to joiner in 4-1 headers (or some mathematical ratio for 4-2-1 headers), as the cylinder pushes out in an exhaust stroke. Is this vaguely correct? (not the question, but relevant)

On most turbo setups, there appears to be no thought put into the flow of the gas, just to get it into the turbo as quickly as possible so it can spool and compress. I know that hotter gas takes up a larger volume than cooler gas, so I kind of get the reasoning behind this, but not entirely based on the effect of extractors above.

I think the question is roughly this:

Are people just too excited about installing a turbo and not actually putting thought into the most efficient way of spinning it up and forgoing their few percent of maximum efficiency because they've already picked up 100% - 200% power? Or does moving the turbo that far away from the exhaust ports bring down the efficiency of the turbo by more than the gains in making sure you've got the right gas flow out of the cylinder?

I think that makes sense. Yes. Lock it in Eddie!

[kazama]

Probably more to do with spool time *shrugs dunno* lol

[greenMachine]

On an N/A performance motor you have "tuned length" extractors when you're chasing the final few percent of power - I'm guessing that they're "tuned" to hold the same volume of gas in the pipe from head to joiner in 4-1 headers (or some mathematical ratio for 4-2-1 headers), as the cylinder pushes out in an exhaust stroke. Is this vaguely correct? (not the question, but relevant)

No, I understand it is about the gas velocity and/or pulses, so that they do not compete for space in the exhaust, but rather follow each other out. Thus, if 'tuned' properly one pulse will suck(?) the next along - at least at one point/band in the rev range.

I think.

[speed]

If the question is; would an equal length turbo exhaust manifold flow better than a log style manifold, I suspect the answer would be yes but also suspect that the gains are minimal and not worth while considering the space and plumbing required.
NitroDann would answer this far better than I but I think the gist of it is that turbos are already extremely efficient at extracting the gases from the cylinder head compared to a N/A engine


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[gslender]

Nah, just that tuned length turbo manifolds are expensive and only help with spooling things at the top end (where mostly turbo engine don't need to go) - and if you get the outcome you're chasing with a cheap and reliable log style, then why bother chasing something you don't need. Turbos give you big fat torque from low RPMs through to mid range, then you change gear. If your chasing race car and want consistent spool then a tuned manifold is required in most cases.

[Magpie]

http://iheartstella.com/resources/manuals/tuning/Graham-Bell-Two-Stroke-Performance-Tuning.pdf
Having a thing about 2 stroke road bikes in my youth, this is a very interesting article about exhausts.

[Cus]

Magpie, that's a great document, I read a chapter and a half last night "having a quick flick" and then consumed the exhaust chapter his morning. Knowledge Porn! If y'all could just step outside, I need a moment...

But I'm with the program now, I think...

With extractors the benefit is in the pulling of intake gases into the head as the exhaust gasses are leaving. That's why it's not a *huge* increase in power, but does help worked motors more than stock ones. The term for this is "scavenging" which I thought was something else entirely. But there you go. This is where in/ex cam overlap and such comes into the picture. (ie: both open, so there's a path to pull the intake in while the exhaust is going out the door)

With a (turbo/super)charger none of that matters because you can blow-through as many psi of intake gas as physics will allow you (ie: the rest of the motor) which will always be more than any type of exhaust can pull-through. Cam overlap here I think still helps, but mostly as a pushing the exhaust out, not pulling the intake in. As far as tuned length-ness, as long as they were the same length (in-pattern gas pulses hitting the turbo, making more consistent thrust), the actual length wouldn't matter a huge amount, unless you're wanting time it exactly so as one pulse is getting out of the turbo, the next pulse is going in... but as G said - expensive (or: a lot of stuffing around), not much benefit to normal humans because the turbo is pumping in fresh gas on the other side of the head.

[gslender]

Cam overlap here I think still helps, but mostly as a pushing the exhaust out, not pulling the intake in.

A bit of debate can occur here - namely around what "helps" means. If you have overlap on a turbo then you are essentially sending a fuel/air mixture out the exhaust, so economy can suffer, plus you are sending cold air onto the turbo which can slow it down. Eventually, the exhaust valve shuts and the intake charge is filled with compressed air/fuel but the window of opportunity (think flow) is now restricted by the time between the exhaust valve being shut and the intake valve still being open. With more overlap that window is shorter, and this impacts the airflow - sometimes seen as massive amounts of boost, but HP being about the same as before (as it becomes increasingly hard to force a volume of air to fill the combustion when intake valve is only open (and exhaust closed) for a very short ms of time.

On the other hand, there are benefits to having the turbo push out the exhaust and cool the valves and clear the charge etc, as at higher and higher RPMs, the exhaust header starts to be the limiting factor to HP and overlap is needed to push things along. Normally this approach is taken with bigger and more efficient turbos and injector timing matched to cam/intake valve timing to reduce the issues above. This gets expensive and complex to tune and match up with everything.

G

[Cus]

OK, let's paraphrase all of this again and see if I'm picking up what you've put down.

Basically, any extractors on an N/A car will be an improvement over a stock header, but there's a sweet spot where they'll be of the biggest help, so if you're going for a full-comp engine you want that sweet spot at the RPM range you plan on using the most, but that's in conjunction with all other aspects of the engine being built at the same time. Otherwise it's a case of "they're goodderer than stock" to most people, and that's "goodderer enough".

This still applies to the turbo side of the debate, however the returns are more diminished due to the turbo already doing naturally what you had to plan for with the extractors (ie: moving gases in and out), but once again there's a sweet spot, and if you want maximum torque and/or killerwasps at a particular RPM, you once again have to plan for it way back at the start of the build, and given the power increases from forced induction you can accept a bit of power loss through a log manifold instead of a tuned arrangement because the tuning has less effect, and is chasing a much smaller gain with much larger wads of cash when you can probably offset the loss by just adding an extra PSI or two.

In short to build a high-performance engine of any kind, start with "how much oxygen is in the air today?" and keep doing maths until you've worked out every aspect from the intake to the exhaust, and everything the air, fuel, water and oil touches in between.

*scratches head*

I think my actual issue was not understanding the relationship between exhaust (and intake) flow and piston movement - ie: they're not linked 100% to eachother, there are other things that happen which cause extra flow when aware of and planned for, which allow you to sneak some extra foot-pounds and horses out of the engine.

Up until recently I had no interest in cars, at all. They had wheels, you drove them, and if you were lucky, you remembered to change the oil before it ran out. My first car died of "natural causes". Sometime around this time I found out oil was a thing that needed replacing, or at least an eye kept on it as it leaked out the head constantly. My second car does it too. Car #1 and #2 were both Magnas. I've threatened to set fire to both of them but the second one less often, because I'm starting to understand *why* it "wants to be burnt to the ground", so then I can fix it and don't have to threaten it with fire. WAY less stressful. And now I've got little red, and we get along fine, because he does stuff, and I want to know more about what he does, and how he does it.

Clarification: "Natural Causes" above was the bottom end not having oil (or top for that matter), nothing to do with my threats of death by fireball.

[Okibi]

It's hard to replicate tuned length manifolds give the space available and a cast manifold is lighter and less likely to crack.

This cars is in WA but NitroDann could build something similar for you. If the turbo is mounted nice and low the dump pipe doesn't need to bend so much.

[Cus]

Oh, I'm not building anything (yet), I have a habit of wondering "why?" - and thanks to the internet, if you wonder aloud, you get an answer!

Engine is going to be one of the last things I touch on the car - I'm not going to be a racing driver. I'm not going to be an engine builder. I'm not going to be a tuner. I do want to learn enough about all of these to be able to self-manage going to track days and doing things that would be grounds for loss of licence if done on public roads. Video games *are* cheaper, but also, I think I need an "outside hobby"

I'm an IT person, but everyone knows that "more RAM is better RAM" in a computer. Most people don't really know what it is, or how it does what it does, but they know that much! There's also the clock speed of the RAM, and the load/store latency of the RAM. Having more RAM than you need is not going to make your computer faster. Having too little will slow it down though. It's this kind of zen I'm trying to work out with cars - and thanks to this forum and the people that are willing to answer my questions, all of the parts are starting to make sense in my little brain!

[gslender]

I'm an IT person too, so don't be alarmed if eventually you know enough to be significantly dangerous to the tools and garage!

[speed]

If anything, you IT guys have an advantage over some of us old school types, especially when it comes to aftermarket ecu's and tuning. I'll be sure to ask G many stupid questions once I'm ready to setup my ecu and bolt on my SC.
I wish I didn't have to bother others but sometimes I struggle to catch up and it's been a good 10 years since I've done this, so the tech I know is obsolete.
I'm grateful that I can "lean" on you guys from time to time and I do learn but just not always at the same pace as the younger crowd.


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[Cus]

I have to disagree speed - cars still run on fuel and air, so don't write-off your knowledge just yet!

G - I already know enough to be dangerous! -you don't need to know much for that!

But I must say, the best thing humanity has done for itself was inventing a tool that allows it to communicate in real time, with sound, pictures and video, anywhere on the planet. "Leaning" on eachother is what makes places like this thrive. Someone asks a question, someone else answers it. Sure, sometimes someone shows up and posts a picture of their cat's testicles, but not all of the humans are thinkers!