Exhaust manifold (vs. headers)

[Rex Chan]

I was going to ask fb this question, but decided FSAE.com was more appropriate:

Most FSAE teams running 4 cylinders use headers/extractors, either a 4-1 or 4-2-1. Is there a really obvious reason that I'm missing why I've almost never seen a team run a manifold (like on a production car)?

There seem to be a lot of benefits in cost/packaging/mass/simplicity/etc. The main drawback is power, but less power = less fuel used. And in FSAE-A, singles (that weigh more than a good Euro 4 cylinder team) have enough power to go very fast.

We plan to do the analysis to see what power loss can be expected, but I seem to remember when I asked for numbers in 2011, it was in the order of 5-10kW loss.

[NickFavazzo]

The power gains from running a nice exhaust system over a bad one are usually from being less restrictive and simply tuning for that. So running a "manifold" may reduce the power but not necessarily the fuel use too.

[Series One]

Efficiency aside, I would have thought difficulty of manufacture would be one reason why cast (I assume you mean cast) manifolds are not very popular. It just isn't worth the time and effort required for a one off car.

[Racer-X]

As far as performance is concerned I would never do it because it would be hard to tune and the runners would be unequal length.

Then there is the fact that the common FSAE bike motors don't come with them stock. So that would mean I would have to cast one. Rather than casting something I would rather make a "shorty" header if space was an issue.

A header could always be designed to perform like a manifold if you wanted...

[Charlie]

You are making a very flawed assumption that more power always equals more fuel used.

Lots of performance gains are from efficiency which will result in a better power-to-fuel used ratio.

I would suspect that a properly tuned exhaust will increase efficiency at a rate that could result in better BSFC, which means you can generate the same power for less fuel.

Making a very efficient package, then tuning for fuel economy, will always give a better result in fuel than restricting power in an inefficient manner.

[Jon Burford]

Rex, I can't see it's worth the effort. I would never deliberately make something worse, it will probably reduce power and not necessarily fuel consumption I should not think, Surely it's overall efficiency you are shooting for?

the manifold you have looks fine to me!!

[Warpspeed]

The vehicle manufacturers are mainly interested in cost, packaging, robustness and ease of assembly.
And a crappy heavy tortuous cast iron manifold is often the result to save a few pennies.

There are definite weight and flow advantages to tube headers, and power too.

Fuel economy on the exhaust side is mainly about exhaust pumping losses.
And as fuel economy is usually much more of an issue at part throttle anyway, you can get by with a horrible exhaust manifold reasonably well.

But if you want to go really fast flat out, that is a very different issue.

[Moreboost]

how is a cast manifold going to be lighter then stainless tube?

[RenM]

Besides Packaging there isn't a single reason why to go for a manifold.

First: For a Prototype its really easy to build your own exhaust headers from steel tubes and it wont cost or weigh a lot.

But of course the main reason is engine tuning:

A low exhaust back pressure is desirable, but its anyways really hard to build an exhaust header that creates a lot of back pressure on its own. (the muffler can however)

Most important are the runner lengths.
The difference between a good exhaust header and a bad one can be massive. Tuning your valve timings and intake and exhaust runner lengths can lead to a huge increase in volumetric efficiency. We saw a difference of 10% at one speed between a badly tuned exhaust runner and a good one.
And don't forget you are also influencing your trapping ratio with the pressure waves at the intake and exhaust valves during valve overlap. A high pressure ratio (high intake pres, low exhaust pressure) can lead to very high overflow losses, where fresh gas is lost to the exhaust including the fuel. This leads to a very bad bsfc and increased fuel consumption!

With a manifold you wont have equal runner lengths for each cylinder, so your cylinder filling will vary a lot, leading to different air/fuel ratios and optimal ignition advance. You can compensate for that if you are adjusting the values for each cylinder at each speed (an overall factor wont be sufficient as the effect changes with the engine speed), but thats going to take a lot of time....

In the end: More power and a better bsfc will always be beneficial for your competition points. If you want to reduce power to reduce fuel consumption go for lean a/f ratio at WOT but dont reduce power by making your gas exchange worse in every respect.

[Rex Chan]

Thanks for all the replies!

Sorry for the confusion - when I was talking about a manifold, I meant the geometry, not making it by casting. I was thinking of 4 very short runners into a log manifold (sort of a reverse of our intake), then one exhaust tube/pipe coming out.

FSAE-A is all about economy (not efficiency, yet), so a manifold is a way to restrict power.

Charlie: I not sure I understand what you're saying about efficiency - doesn't all energy for power come from fuel burned? To be efficient, we'd run at Lambda > 1.00, to ensure complete combustion. We're already running LA=1.00 at WOT for enduro, and hopefully the guys will try to find the lean limit this year. After that, isn't the only to cut fuel used to reduce amount of air entering the cylinder by making the airflow path less efficient?

edit: based on all the above replies, it makes more sense to try to push SAE-A to move to an FSG style efficiency points score for the Fuel event. I quite like the sound and apparent power (only 49kW SAE corrected on the SS engine dyno) of past MUR/Melbourne engines, so would be a shame to give it up (though Monash went to a single in some parts due to this).