25 replies to this topic

[rodomo]

Good question?

Why do drag cars have single pipes coming straight out of the heads?

I thought back pressure was about maintaining some sort of even temperature in crappy old engines that had lo-tech valve materials so the valves wouldn't burn out too quick?

[ls2lxhatch]

This clip from the Dynomation 5 promo video explains why you would tune exhaust pressure.

https://cache.gmh-to...on5_exhaust.flv

[orangeLJ]

I was always told that having no backpressure made the car doughy down low (reduced the bottom end torque)

[FastEHHolden]

An exhaust is a complex thing..its not just a bit of pipe to funnel gas out...its acoustic, its got pressure pulses, its got low pressure areas.

Basically its gets back pressure (I hate that term) as a by product. The exhaust has to be big enough to flow well...but small enough to have a high pressure pulse go down the tube....because behind the high pressure pulse is a low pressure area and that draws thru the fresh air because the inlet and exhaust valve are open (overlap).....its way more complicated than that but you get a basic idea.

[TerrA LX]

So what we are trying to say that an exhaust too big relative to engine output, either by way of diameter or less restrictive (ie mufflers, bends etc) will actually slow down the exhaust cycle in turn hurting torque and final output.

[RIM-010]

So the 3.5" system off of my cousin's VL will be too big for my 173?

Tim

[MRLXSS]

good question, but seems to be a really complicated answer... One of those things that you answer with... "Just cos"

I have always pictured it as a hose... A Garden hose can pump out a fair bit of water quite comfortably and it is a reasonably small size, it doesnt need to produce heaps of pressure (power) to make the water flow at its full potential... Get yourself a bigger hose (Say a fire hose) and you are able to pump out a heap more water, heaps faster than before! (But you need to create a shitload more pressure to push it out, and make full use off the greater hose size) If you still have the same pressure and water flow as before with the garden hose, you aren't going to get any more water coming out, and its just going to trickle out... As they always say... its not the size of the hose that you have, its the way you use it!! HAHA (and can you supply enough energy to it!!)

Is the back pressure an actually air pressure thing? or is it more a sound wave reverberation? If the exhaust pipe is too big, the sound waves reverberate and bounce backwards, sideways etc etc restricting flow there for the larger exhaust size is pointless and just sounds like an untuned piece of crap!

Correct me if i am wrong, this is just how i have always pictured/imagined it

[TerrA LX]

^ sorta, just cause, no really that and hot air is less dense so an over sized exhaust will allow it to cool more before exiting the tail pipe, making it slower.

The aim is to keep the exhaust flowing as fast as possible and as with ur garden hose comparison, a small motor with little overlap will need more and a top fueler making bout 500 a cylinder just needs something to funnel the exhaust away from the car and look pretty.

[FastEHHolden]

Another thing to think about is the fact the exhaust size is fixed...but the gas volume your engine pushes out is not...so your exhaust, no matter what the size, will have a range it works best in...its just working out the compromise that suits your car the best.

[FastEHHolden]

A bloke from Wollongong call up with the Varizorst...however he spells it....he uses an air pressure powered servo to open a butterfly valve to another part of an exhaust system....so the system volume is smallish....when you plant it..it opens up allowing more flow to suit the increased gas flow.

Edited by FastEHHolden, 25 August 2008 - 05:02 PM.

[Struggler]

As a few have said its all about the scavenge effect. A small exhaust will prove efficient on a small engine with small rpm and v/v.

Mechanically supercharged engines don't rely on the exhaust to pull intake charge into the motor so therefore do not really need an exhaust except to direct gas away from the engine. This is why T/Fuel engines use the Zoomie style headers.

Here is a great article on exhaust systems..... http://www.superchev..._exh/index.html

It is interesting to note the author feels that "the exhaust system draws on the intake port as much as 500 percent harder than the piston going down the bore." Interesting stuff.

[FastEHHolden]

[quote Is it true that if its a forced induction engine then you want as big exhaust as possible?

Cheers
[/quote]

Sort of...there is a rule of thumb...and I love rules of thumb...it goes for every psi of restriction you take out of the exhaust...you get 1 psi more boost.


So again you want lots of flow...but the thing to remember about big exhausts on turbo cars is they aren't always on boost....and they can be painful when off boost.....so a smaller exhaust with a higher gas velocity may help a car get onto boost quicker...but ultimately it might have less top end...its all down to compromise again.

[MRLXSS]

When i stepped up from twin 2" to twin 2.25" i noticed my down low torque dropped a bit. But once i got it dyno-tuned i gained more down low than i ever had before! So each exhaust you try would need to be "re-tuned" to get everything right, before you can really judge whats better etc...

[EunUCh]

Very interisting stuff,seems as though exhaust systems will always be a compromise,too many varibles involved,did a bit of looking around and found a book that dealt with tests on exhausts and it seems we are dealing with sound which travels differntly at different temperatures,flow of gas at high velocities,some way out equations are used but basically they used a morrison indicator that measured pressure waves in the exhaust at certain rpm ranges,positive pressure waves at valve overlap are not desireable,if anyone has driven a car with a blocked muffler will testify they dont go real good(too much back pressure similar too a positive pressure wave,just constant) then we have other problems like the length of exhaust pipe which changes the harmonics therefore the "tuning" and time it takes sound to travel from one end to other,then engine rpm changes the whole thing again,what was interisting is that a pipe about 10 in long showed the positive peaks reducing to buggerall very quickly and not as strong,noticed in glossy mag that you can now buy whats called merge collectors,basically they are a venturi where the collector used to be and apparantly they work by keeeping gas speed high and because its high at the merge the "reverse" pressure waves from the rest of the system cant get back to valve and cause havok,everything keeps flowing,bloke up here made a set of merge collectors and header pipes etc. for a 3.8 pajero,3.5 in system,picked up in bottom end power,hes had a bit to do with cars and his idea of backpressure in exhausts should be least as poss.