Free-flowing Intake and Exhaust for Fuel Economy

[Matt Timion]

I share your questions about the exhaust SVOBoy, and considering the restrictive nature of the VX/HF/CX/HX (all fuel economy cars) intakes and exhaust, I can only assume that Honda did it this way for a reason.

if you want to write a collective letter to Honda, let's do it. IMHO there is no reason to translate it to Japanese, as English is the international langauge now. I have no doubt that the Honda HQ has fluent english speakers available in their engineering department.

I'll even put the letter together and I'll send it off.

[SVOboy]

Me = doesn't like the whole global language thing. Lemme try and get in touch with the guy who used to run jamsat, he prolly knows someone.

[Bunger]

I think I may be able to shed a little light on this subject. First, we have to remember that auto manufactures make compromises when producing a vehicle, sometimes big ones, sometimes small. Material cost, R&D cost, labor cost, customer needs, environmental requirements, sound requirements, etc, will all effect the design and construction of a car. This is why systems like VTEC were invented in the first place, it allows less compromises to take place.

If a car was designed to travel at 55 mph, at 2,000 rpm, on a flat surface, in a straight line and no one cared what it looked like, sounded like or put into our air, I'm sure we would already have the best example of what would yield us the greatest MPG. Unfortunately, few or none of those points are true. So we are stuck with the problem of engineering our own solution from what we know.

Let's start with the intake system. Most air boxes and intakes are designed with a resonator (or 2 or 3) built in, limiting the amount of "growl" that's produced when you step on the throttle aggressively. This probably disrupts the laminar air flow in the intake and causes greater pumping losses, so ditch the resonator(s).

Intake pipe size. This is going to be along the same lines as exhaust pipe size, just imagine your intake manifold is a header (the plenum is the collector), well the intake piping connected to your throttle body is also going to dictate pumping restriction and port velocity into intake manifold itself. I think out of all areas, this one (the intake piping) will have the least greatest effect on mileage.

Now as for the intake manifold itself, the runner length, runner cross section and plenum volume will all greatly effect the volumetric efficiency (VE) of the engine. For instance, as the intake valve opens the air begins to move into the cylinder, and acting like water, air has inertia that will build and have kinetic energy. Now that air inside the runner literally bounces back and forth between the back of the closed valve and the plenum. If the runner is tuned to the correct length and has the right cross section for that RPM (!!!), that pulse of air will be timed to force its way into the cylinder the next time the valve opens. Unfortunately, this is only true for a fairly small RPM range, which is why some engines are designed with 2 stage intake manifolds (i.e. Integra GS-R) This is one of the things that allows an engine to achieve greater that 100% VE without external supercharging.

This is why you see intake manifolds with small long runners on engines like the CRX Hf and Civic VX.

The exhaust manifold (or header) is much the same. Its job is to match up the exhaust pulses, which have a high pressure head, and low pressure tail, so that these 2 ends equal each other out and make for nice even flow. The low pressure "tails" of these pulses can actually suck the exhaust out of another cylinder, this is referred to as exhaust scavenging. Again, exhaust manifolds are only most efficient within a certain RPM band.

Still with me?? As far as exhaust piping goes, you want to try to maintain a similar exhaust velocity as the collector of the header, but again, this will only be true for a certain rpm band as well. Smaller pipe will be a great benefit at lower RPM.

In a perfect world, we would have solenoid actuated valves, no TB or plenum (don't need them with solenoid actuated valves anyways), a material that could change shape instantly to form smaller longer runners at idle and get progressively wider and shorter as RPM increased, same goes for exhaust manifold and pipe. And while we're at it, variable rod / stroke ratio and compression ratio too! =)

Starting from a Honda platform, I would probably take a VX motor, and tighten down its operational RPM band even more. Tune every part of it for 2,000 RPM.

I hope this gives you somewhat of a clearer picture of how everything fits together. This is just the very tip of the iceberg really, we haven't taken into account heat, air density, altitude, etc.

I hope I didn't ramble! =)

[Sludgy]

I had a 6 cylinder Pontiac Tempest in the '70s. I used to drive between Boston and Albany regularly, at about 80 mph. I put a set of Clifford headers on it. It went from about 12 to over 15 mpg. It made me a true believer in headers. Of course, the original Pontiac exhaust manifold was a cast, high-restriction POS, but the experiment proved the theory in my book.

I'm not sure how much help headers are for late model cars, since the newer intake and exhaust manifolds are a lot more free-flowing.

[rh77]

Bunger - awesome info. What I'm stuck with is a '98 Integra LS, Automatic (B18B1). I tend to cruise at 57 mph actual (62 registered on the speedometer) which is around 2300-2500 RPMs if I remember correctly. I do most of my driving on the highway at highway speeds, so around 2000-3000 RPMs. So, should I build a system that's tuned specifically for that range? From stock, what would that entail? I assume a smaller diameter exhaust pipe and a more restrictive intake. From there, would a different intake bolt-on from another Honda engine that's more restrictive? Now your article didn't mention anything about a muffler -- should I go with a free-flow or one with restriction? The exhaust system is literally falling apart, os I need to act soon. I have a leak that is pretty potent at idle (the CAT is fine, so I plan on leaving it). Bottom line is I'd hate to invest in an intake/exhaust system on my fuel-economy experiment car, and have it reduce economy. Thanks again for your expertise...

RH77

[Bunger]

RH77,

Thanks for the kudos, glad you got something from it.

As far as replacing your exhaust with something thats going to be better for mileage, the biggest difference would probably come from a header, which unfortunetly, unless you pay someone (or are able to weld it up yourself) to make you a hand built one, your best bet is to stick with the stock piece.

Now for your exhaust system, you may find a mileage increase from smaller piping, or possibly a smoothly tappering piece right after the manifold collector to act as a venturi, helping to increase velocity without causing too much backpressure. For a muffler, the more free flowing the better, within your tolerance of noise level.

I'm going to be experimenting with some titanium tubing later on, as it seems I can source it for around $1.50 an inch, which isn't too bad considering the weight savings. If I can talk my friend who has an engine dyno into it, I could run some tests to see what changes have the greatest effect. (his setup can meter fuel flow also!)

Sorry I don't have a better answer, I'll keep thinking about it and let you know what I come up with.

[SVOboy]

You're very clever, I have learned something from this but realize now I am more confused.

For a muffler, the more free flowing the better, within your tolerance of noise level.

Why's this?

I'm going to be experimenting with some titanium tubing later on, as it seems I can source it for around $1.50 an inch, which isn't too bad considering the weight savings. If I can talk my friend who has an engine dyno into it, I could run some tests to see what changes have the greatest effect. (his setup can meter fuel flow also!)

Woot!

[Bunger]

Originally Posted by SVOboy

Why's this?

With most mufflers designs, the exhaust velocity is going to go to crap anyways, so at very least, don't create a bottleneck. The good part is, that far back in the system, the exhaust gases have had some time to cool and condense, so flow requirements decrease.

In a perfect world you would probably have a total straight tube that very slowly decreased in diameter to maintain the port velocity with a tip that was designed in such a way that the passing fresh air would create a vacuum, and thus lower pumping losses even more.

[SVOboy]

So I can get a nice sounding muffler for the crx and have it not kill my mileage/possibly help?

Any suggestions?

[Matt Timion]

Originally Posted by SVOboy

So I can get a nice sounding muffler for the crx and have it not kill my mileage/possibly help?

Any suggestions?

define "nice sounding." I've seen too many hondas/crxs in the area drive by...

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WRaAAAAAAAAAAAAAAAAAAAAAAAAAAAAAHW!!!