4 stroke engine characteristics

[98ninja]

so does a more restrictive exhaust increase torque?
or is that a load of shit?

[Saki]

no more restrictive won't make more torque all it will do is cause the engine to flow less effectively

[Wattie]

so does a more restrictive exhaust increase torque?
or is that a load of shit?

sometimes.

the manufacturer spends thousand on designing an exhaust to gove good all round "useable perfromance"

free flowing stuff gives you more top end. pretty much.

but most gains are made in "set uP" tune what you have got. get the best out of it.

[MiG]

... all it will do is cause the engine to flow less effectively

Which is what you need if you have high power valve timing and want low end power. Sure those priorities are at odds with each other, but that's not the issue up for discussion.
http://www.flowmastermufflers.com/backpressure.html" target="_blank

[Gosling1]

so does a more restrictive exhaust increase torque?
or is that a load of shit?

No, its not a load of shit at all, but its also not the whole story....a more restrictive exhaust (ie like *most* stock exhausts) is designed to give good power characteristics over a wide range of rpm. If you remove the restrictions, you will almost always get a better response (and maybe a couple of hp) at the top-end, but you will lose an equivalent amount of hp ( and more often than not, torque as well), at the lower end of the rpm scale.

In a nutshell, gut your stock ZZR600 mufflers, it will scream from 10grand to 14grand, and be louder, but the low-speed response from 3grand to 7 or 8grand will suck arse. Open the throttle hard, and all you will get is a big loud moan from the exhaust, and not much forward thrust.

Motors are complex things - you can't simplify the argument down to 'more restrictive exhaust = increased torque' - it just isn't that simple.

[javaman]

Stock exhaust is better... that's how mr Kawasaki wants the bike to be .. . case in point I was riding the road between Flowerdale to Yea at an easy pace, and a glance at the speedo revealed it was 200 ... now that never happed with my loud can ! With it, it always seems to be 'hasty'.

[ozten]

Now this is the sort of thread i like theres is heaps of good info on this site

http://mototuneusa.com/thanx.htm" target="_blank

[Daisy]

In a nutshell, gut your stock ZZR600 mufflers, it will scream from 10grand to 14grand, and be louder, but the low-speed response from 3grand to 7 or 8grand will suck arse. Open the throttle hard, and all you will get is a big loud moan from the exhaust, and not much forward thrust.

We have one with a Formula one system and one stock. At 'regulation' speeds the loud one sounds good and feels fast, but to make it work you need to drop a gear or two. Then the stock one will just reel it in like a fish, still in top gear. Of course it *can* get away, but by that stage we're both gonna lose our licences.

[IsleofNinja]

MY TURN LOL!!!

I have a little chuckle to myself whenever the age old exhaust argument comes up

A famous wives tale is that 'the engine needs backpressure' etc

CRAPOLLA!!

FACT 1;There is no such thing as a 'perfect' off the shelf exhaust for a particular engine..All are a compromise to some degree or another.

FACT 2; Small dia header pipes with long individual branches ie stock will provide better scavenging at low to mid rpm providing better general ride ability for 90% of road bike owners. Larger / shorter header branches can suffer from 'reversion' where by the scavenging effect is destroyed and in fact reversed by exhaust pulses having to 'push' the previous one out the pipe at low rpm.However the extra flow at high rpm with large duration cams and high compression engines etc = more HP.

An interesting point is to note the collector merge on an older bike compared to current models..ie Older 4 pots ran a straight 4 into one or 4 into 2 collector/s.
Now it is the norm to run 4 into 2 into 1 to a single or dual cans ...This offers the high rpm flow advantages of tuned length with the scavenging properties of interference type headers.

FACT 3; Louder does not mean faster!! Flow = horspower not noise!!


Nb;My Akro's come with restrictor end caps ..With them in it is legal noise wise and feels a little more torquey out of corners at low revs..Out ,the power comes on harder after around 8000rpm and doesn't crop at 11,000 in higher gears.Mine has the Akro race headers also with no servo operated exhaust valve ....

Clear as mud???

[Gosling1]

......A famous wives tale is that 'the engine needs backpressure' etc....CRAPOLLA!!.....Clear as mud???

well, with an each-way bet - Yes, clear as mud !!

If a 4-stroke engine did not need some type of restriction in the exhaust (which, by definition, creates 'back-pressure' for want of a better term), then all exhausts would simply be 4 straight pipes. No muffler. No anything.....In fact they could exit straight from the head, without any pipes at all .........couldn't they ??

You mentioned that when you fit the 'restrictor' end caps to your Akra's, it feels "a little more torquey out of corners at low revs".... this is a perfect example of 'back-pressure' actually working. If there was no such thing as *back-pressure*, you would not feel any difference at all.

But the actual term 'back-pressure', when applied to 4-stroke motors, is a bit misleading anyway. Its not a 'pressure' effect, more the effect that an exhaust restriction has on gas flow - Wether that exhaust restriction is defined in terms of header length, pipe diameter, etc etc, there are lots of contributory factors. But the term has been around for so long now, its really lost its meaning when 4-strokes are discussed.........but what about smokers ??

Back-pressure is a term that really only applies to 2-stroke engines. In these, each exhaust pulse down the pipe creates a pressure wave that travels at the speed of sound. This wave is then reflected back up the pipe when it reaches the end of the pipe - and if the dimensions of the exhaust pipe are correct, the returning wave actually forces unburnt gases back into the cylinder at the right moment. This 'pressure' wave is what most people are thinking of when they mention the universal term *back-pressure*. Expansion chamber design can vary and improve the response of any 2-stroke motor, its all in the design of the cones, tuned length etc etc.......its a real black art !!

Tuned-length headers for 4-stroke motors that redline at 10,000rpm, regardless of capacity, number of cylinders, engine configuration, air or water cooling etc, all have one common trait - the best tuned length is around 27 inches. This number hardly varies at all between heaps of different bikes. Motors that redline at higher or lower rpm have slightly longer or shorter tuned lengths - but 27 inches is the magic figure . Interference headers are *slightly* different kettle of fish......but only slightly.

[IsleofNinja]

Now THIS is a thread!!!!

Ok 'gos' points taken and noted... I'll accept your definition of backpressure also to a degree, though we almost but not quite grabbed hold of the term 'scavenging'

This ladies and gentlemen is what we seek to optimise in out ultimate goal of 'power' ..

'reversion' as discussed in my previous post is indeed a very , very useful tool in 2 strokers by way of pushing that naughty fuel air mix back up into the combustion chamber to do it's work rather than sneaking out the exhaust pipe to join it's other MRD mates.. This is as 'gos' explained a very precise art to get it right.

This principal has also been tried and proven in 4 stokers in that when megaphone exhausts were discovered and the benifits realised , the reverse cone megaphone was a natural progression which believe it or not has been basis for 4 stroke moto mufflers for decades,particularly Honda.
Which leads me to the afor mentioned baffles in my Aka's.shape being a reverse cone rather than a simple orifice or straight lump of pipe..ie megaphone = power in a very narrow high rpm rev band,adding a reverse cone spreads the power lower through the rev range and adds torque.

In order for scavenging to occur , the movement of each exhaust pulse should serve to 'suck' the following pulse be it from other cylinders or the next exhaust stoke on that cylinder depending on the exhaust configuration. (A vivid example of scavenging at work is the flames from the exhausts of V8 supercars on over run)..

.....Over to you 'gos'

[Neka79]

this is no thread..i dont see monkey porn..or any porn for that matter!!

[IsleofNinja]

INSERT PORN HERE ------------->

[MiG]

Not every thread needs fucking up. The "I wanna 10" thread was begging for it.

[Gosling1]

...... The "I wanna 10" thread was begging for it.....

*Touche* ! it sure was.

Last thing I can add to this is the different effect that the length of the primary headers has on a power curve. Basically, if you are looking for a stonking top-end at all costs, then the best 'collector' system is a 4-1 system , with tuned length header pipes. The tuned length is basically determined by the characteristics of the engine itself......bore / stroke, redline etc.

A 4-1 that uses different length headers is generally referred to as 'interference' headers, and this is where the 'scavenging' effect is most noticeable. Having different length headers will reduce the top-end by a bit, but improve the mid-range response, due to the better scavenging effect of different length headers. Back in the old days, 4-1 interference headers were the only way to get improved mid-range with just bolt-on pipes.

Now, the exhaust systems run 4-2-1, and sometimes 4-2-1-2 ......and all use tuned length header pipes. The addition of a 2-1 section helps a great deal with mid-range response, and you still get the great top-end of a 4-1. The Arrow system I recently helped fit to PC's Z1000 was just a work of art to fit up - and the response and sound is probably the best I have ever heard out of a Z1000.

BTW, this 4-2-1 has been fitted to PC's bike with *NO* PCIII or other mods - and it works *just fine* - I have ridden it a couple of times, there is NO issue with response down low, no flat spots, no hiccups anywhere. This absolutely reinforces my view that a PCIII may not be required on all bikes simply because you have changed the exhaust. Some bikes will need help, but anyone who says they MUST be fitted to all bikes when a (very) free-flowing system has been fitted, is incorrect. - at least in respect of Z1000's !!!