Exhaust / Intake Ratio

[Apu]

I was thinking about how engine tuners work out what the optimum intake/exhaust ratio is for an N/A engine. Common sense tells me that there’s very little point of having say, a 2” intake and a 3” exhaust…I mean, you can only expel no more that you put in, correct (except under forced induction). Of course, the reverse also applies – a 3” intake is probably not going to work well if you have a 2” exhaust.

Plus the trick to having an exhaust system that works well is velocity. In other words, too big is also not good.

Looking for tech based answers here please, and not “well I reckon this because my butt dyno felt that”.

[hks_kansei]

I always thought there was always more exhaust pushed out than air taken in.

Remember the mix expands during ignition.

[Willstar]

"My butt dyno thinks"...

Keep in mind that hot air leaving the engine is a larger volume than cold air entering.

[Willstar]

***beaten to it!

[NitroDann]

At work but..

theres always more exhaust, Think about it, air goes in, then explodes... What pushes the piston down?

Dann

[Willstar]

Here is a formula for working out the expansion ratio:

PV = nRT (n is the amount of gas and R is a constant)

What this means is that pressure * volume is proportional to temperature.
(The temperature has to be absolute ie. = celcius + 273)

So if you go from 20->40 C, the pressure * volume increases by (40+273)/(20+273) or about 6%. So if you keep the pressure the same the 1000cc will become about 1060cc.

Proudly copied and pasted from
http://www.physicsforums.com/showthread.php?t=267530

Therefore, you really need to know the volume & velocity of air entering as well as the temperature of the gas at the headers to be able to work out the % increase required in the exhaust. Obviously divide it by 4 for the headers, and the whole value for the exhaust (if its 4 into 1).

It seems easier to chuck a ready made one on!

[NitroDann]

Its noway NEAR as simple as willstar implies which is why we still make 5 variations of everything and dyno test. Its not kust temperature its the fact that the fuel explodes and goes from liquid to gas. Its density is also completely different from intale to exhaust.

There is basic rules to follow.

2'' exhaust up to 175rwhp 2.25 up to 220 oddrwhp (dont quote, I have notes for this, not memory).

As for intake the important thing is plenum size and runner length/diametre.

The throttle body can be as big as it wants, however you will lose low rpm throttle modulation with a big TB, and half throttle may flow full power, so half of the throttle travel is wasted.
Again I have notes for this so Cannot quote now.

Enter wazzah:

Dann

[Willstar]

2'' exhaust up to 175rwhp 2.25 up to 220 oddrwhp

It seems easier to chuck a ready made one on!

[lee]

Its noway NEAR as simple as willstar implies

All Willstar was doing is giving some basic, introductory chemistry into why volume increases with temperature. Using this equation (and a couple of others), the ratio between exhaust volume to intake volume can be found within a reasonable tolerance.

The basic rules you provide didn't just come around by accident. An engineer of some description carefully calculated the required intake and exhaust diameters to make the system as efficient as possible within noise and cost constraints, and someone else has calculated them again, this time reducing the importance of a quiet and/or cheap system.

Then again, some haven't calculated it at all, and just whacked a 4" straight pipe going from manifold to rear bumper

[Apu]

I get the gas expansion thing, and that gives you the push, which gives you exit velocity right? And you need that "shot" of gas to exit just in time for the next pulse from the following cylinder - it doesn't all go bang at once!

I'm just trying to figure out if I go bigger throttle body, then how much bigger do I need my exhaust to get.

And Dann, great point about low rpm throttle modulation.

[Willstar]

The 'perfect' exhaust should also use the velocity of the previous 'shot' of gas to help drawer the next shot of gas out of the head.
That's another reason that too big an exhaust path area can have negative effects on horse power.

I agree with what Dann said. Its best to try different exhausts, designed by different engineers that have already gone down the road of formulas and calculations.

A few exhaust shops that i have asked have replied that the basic rule of thumb is to increase the internal diameter by no more than .25" over the stock size. This is only applicable to unmodified cars. So a bigger throttle body may require a bigger increase again.

[NitroDann]

When I get home Ill explain why I dissagree with a lot of what has been written.

But basically the fact that the fuel explodes is of far greater significance than the temperature expansion of the intake air.

Also there are NO people who can calculate everything that goes on inside the engine in regards to combustion and flow, otherwise all engines in F1 would be identical as everyone would just calculate out the perfect combustion chamber/intake/exhaust/cams etc.

People didnt calculate out what exhaust diamtre works best, they tested on a dyno many engine combinations over the years.

No one can calculate it perfectly and every engine whether internally stock or not is exactly the same so its not calculable. If thats a word.

Anyways, you simply choose a plenum size within an accepted norm, then choose runner length (this CAN be calculated) then choose runner diamtre within an accepted norm, choosing whether to go bigger slightly for more top end or smaller for more mid range (which can be almost calculated) then you use valves sizes within the accepted norm for your engine model and the cams you have chosen, and then you build an exhaust the proven right size for the power output, perhaps choosing to step headers up or down for low end or top end.

You can only get so many different sizes of pipe so why calculate it to within 2 percent? You cant anyway. Yo simply dyno it, and choose proven sizes.

Dann

[Gladiator]

I just wish I new WTF you guys are talking about. I guess that's why I pay the professionals.

[lee]

Also there are NO people who can calculate everything that goes on inside the engine in regards to combustion and flow, otherwise all engines in F1 would be identical as everyone would just calculate out the perfect combustion chamber/intake/exhaust/cams etc.

You can't calculate every single aspect of design, otherwise, as you say, every single engine would be exactly the same. There are some limitations in the laws of physics, and some F1 teams chose to push some limits at the expense of other factors. Every design has its advantages and disadvantages. Its why Holden still use a V6 and Ford use an inline 6. Some things like fluid flow and heat soak are very well documented, so engineers use their knowledge in these fields to design what they think will be a system with a balance of minimal restrictions, low noise, and low cost.

People didnt calculate out what exhaust diamtre works best, they tested on a dyno many engine combinations over the years.

People may have just bung a few different pipes of varying diameters to an engine and found the results at the start, but at some point, someone would have found a mathematical correlation between exhaust diameter and power curves. Imagine how much time would be wasted if people had to start from scratch with every single aspect of design in every project ever made!

[quote="NitroDann]You can only get so many different sizes of pipe so why calculate it to within 2 percent? You cant anyway. Yo simply dyno it, and choose proven sizes.[/quote]
You can make a pipe of any size you choose if you custom make it from raw materials, but no one would ever do that, because you'd have to set up your own manufacturing facilities to create the pipe in your chosen diameter. This is why we have standard sizes. Someone would have figured out that an idea exhaust size for a 1.6L mx5 may have been 1.92567" (pulling figures out of my arse), but because there is no standard for that exact size, 2" is close enough. It may not be exactly ideal, but it's close enough that the multitude of other variables would negate any difference anyway.

I'm not disagreeing with you, I'm just saying there's a lot of maths behind the cars we drive

[Willstar]

In other words...
R&D teams don't just plonk 10 random parts on engines, and release the one that works best.