Exhaust / Intake Ratio

[Apu]

I think we all know that there's a lot of math that goes into working out the best intake / exhaust set up, and no manufacturer just "plonks it together", but like Dann says exhaust pipe diameters are standard - 2.25", 2.5", etc.

After market manufacturers tend to do 2.5" and 3". Haven't seen a 2.75" exhaust!

I guess what I'm trying to figure out is what is the "typical" intake / exhaust ratio so I know when to go from 2.25" to 2.5". Dann has already partially answered that based on output.

[NitroDann]

I dissagree. Everything about flow cannot be calculated.

Also, RnD Teams dont choose the exhaust.

Dann

[Willstar]

No, they research and develop them!

[noddi85]

Hmm this is going to make my choice of exhaust for my new motor very shitfull to decide on then

42mm quad throttles
Built B6 (could give details but dont think its relevant)
Port matched and polished head
Looking at 9000rpm limit

I was just going to use my 2 1/4 system but now after reading this it might not be right for it.

[NitroDann]

Probably find that 2" is right. Depends on cam profile mostly.

Look, RnD teams dont find the best exhaust for a car and put it on, accountants put the exhaust on. If RnD teams at mazda had done that then Joe at ''Mufflers N Tyres R Us'', wouldnt be able to build a an exhaust for 250 bucks that makes more power and sounds better than stock. think about it.

Anyways you CANNOT calculate everything, if you could every F1 engine would be identical, they would all just calculate the ideal engine and then build it, and they would all calculate the same thing. Maths doesnt lie.

Dann

[wozzah1975]

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.

Unfortunately as Dann stated, the calculations are not simple, and its still very much a black art. Different combinations work on different engines, and different manufacturers have different ways of achieving a similar result. What has changed over alot of years is the methods used to measure air flow in areas of an engine.

I will try and keep it simple explaining it, and I am by no means a complete expert on the subject.

When developing a cylinder head, Inlet port flow/Exhaust port flow on an N/A engine, in general the Exhaust port should flow around 75% of the inlet port. The piping gets bigger after that, because the exhaust gas is still expanding as it comes out of the exhaust port and into the pipes.

Flow ISN'T directly related to size. For instance the throttle body on my XR8 is only 65mm, but it flows more air that alot of the larger expensive off the shelf items flow. Same goes with cylinder head design and exhaust design, bigger doesn't mean it flows more air. There is a bit of a saying about designing cylinder heads, intake and exhaust, "he who flows the most air through the smallest hole wins". For instance, if you have for example a port that has a 45mm hole that flows 200cfm of air, and a 40mm port that flows the same amount (200cfm) of air, the smaller one carrys more air speed, and will almost always give a better power curve on a dyno.

Horsepower and torque are directly related to air flow and air speed, not size. There are given basic rules for calculating starting points on sizes, but they are a guide only, not a rule of thumb.

In relation to what you're talking about (throttle body/exhaust comparison), there is far more that comes into play with that calculation.
- How much air does your original throttle body flow?
- How much air does my engine need before it reaches the limitation of other areas (such as cylinder head, cams etc)

If your throttle body already passes enough air for your engine, then all that will be achieved is the SAME flow, but alot LESS air speed because of the bigger hole. It will be detrimental to the power curve, you will lose alot of low end-mid torque, and gain no more power.

The other issiue is, that quite often big throttle bodies require plenum mods. That can also have a bad effect on the power curve. The design and size of a plenum is very complex on an N/A engine, and VERY easy to upset.

I would do your exhaust mods FIRST. Then if you wish to play around with the throttle body size. You will need to back to back dyno it after each throttle body change, plot a graph and note the differences. You may get a surprise as to what actually works!

I have spent alot of time doing developement work on all sorts of makes and models. One that springs to mind is a BA XR8 Falcon 5 speed I did some work on. STD one of these makes around 195rwkw on the dyno I use. After some good basic mods (exhaust, extractors, intake, piggy back computer) we saw a massive increase in low-mid torque and 230rwkw. The next mod was the throttle body. Magazines were giving rave reviews on the SS inductions stuff at the time, and the owner purchased the big dollar unit with the spacer. We fitted it, and it made no more power, and from 2500-4000 it actually went backwards by about 6rwkw. I rang SS at the time, and they made all the excuses under the sun "the computer needed to relearn", "the other mods we did weren't right" blah blah blah. We left it on for 2 weeks, still no change. The minute we changed it back to the std throttle body it was back, instantly, no computer relearning or anything. Bottom line, no matter what we did to it the throttle body made it worse. After that I got a std throttle body, and filled the large opening with some resin, making it smaller and completely reshaped the entry to the butterfly. We fitted this and instantly made 3rwkw up top, but more importantly 15rwkw @ 3500-4000rpm. This developement was telling us that the std throttle body was already big enough, in fact the opening was already too big as it benefitted from being smaller.

The moral to the story, not all this flash stuff works, and quite often makes things worse.

Cheers
Woz

[wozzah1975]

Hmm this is going to make my choice of exhaust for my new motor very shitfull to decide on then

42mm quad throttles
Built B6 (could give details but dont think its relevant)
Port matched and polished head
Looking at 9000rpm limit

I was just going to use my 2 1/4 system but now after reading this it might not be right for it.

Everything you do to the engine is relavant. What power do you expect it to make? How big are the cams etc? Comp ratio?

Cheers
Woz

[Willstar]

accountants put the exhaust on.

I like your point

[sailaholic]

Dann, point of clarification. Petrol burns (rapidly) rather than exploding.

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[NitroDann]

Point of clarity, an explosion is simply a rate of burning.

When burning hits x feet per second its called an explosion, not sure if petrol hits that, maybe during detonation..?

Dann

[sailaholic]

Why correct me if your not sure? Petrol is classed as a burn.

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[NitroDann]

Are you sure it is when inside a combustion chamber?

I mean Im sure its a burn when you just light a puddle of it on fire.

Dann

[Willstar]

Petrol doesn't burn or explode
It's vapour burns in open atmosphere, and explodes if contained

[Apu]

Thanks for the explanation Woz! Completely get the point about the importance of velocity.

I guess when I've got a few lazy dollars lying around, I'll try it with a spare just for the hell of it to see what can be gained at the top end.

[Matty]

Petrol doesn't explode when contained under normal combustion circumstances, it is still a progressive flame front, albeit much accelerated by the increased pressure and temperature.

Exploding is spontaneous ignition, which is basically what pinging is - the pressure increase caused by the initial combustion causes a shockwave that ignites the rest of the fuel ahead of the normal combustion flame front. That sudden shock wave and instant pressure kick is why pinging destroys engines.