4 stroke engine characteristics

my brother argues that whilst a free flowing exhaust pipe and or system increases top end at the same time a more restrictive exhaust will increase torque with or without any engine mods.
i think its the stupidest thing ive ever heard. to me its the equivalent of saying that if you block yourr mouth and only breath through your nose when you exercise that you will become more powerfull.
so to settle this argument ive posted this and await feedback.

Don't know for sure and I base this on not a lot of technical knowledge but....

When I had my 426 I put a phaaarkin loud very non restrictive Staintune pipe on it with a removeable baffle. The Staintune guys at the time showed me the Dyno charts from an identical bike with the baffle in and out. Baffle in produced more mid range and baffle out produced more top end.

Riding it you couldn't feel it though....

....Riding it you couldn't feel it though....

Good old Bum Dyno's eh mate? Everyone has one but they aren't real good.

As far as the torque with a restrictive pipe goes, from what I understand it is all to do with gas velocity through the smaller diameter pipe leading to a scavenging effect in the cylinder at lower revs, which then means more fuel and air getting in. The downside is that at higher revs it gets too restricted and you don't get as much fuel and air in.

With a larger diameter pipe the scavenging works well at the higher revs with the gas velocity, but at lower revs it doesn't do much so you don't get as much air and fuel in.

But then I'm no expert either...

explains why a lot of yammy's have that yvvs exhaust valve or whatever it is.

my brother argues that whilst a free flowing exhaust pipe and or system increases top end at the same time a more restrictive exhaust will increase torque with or without any engine mods.
i think its the stupidest thing ive ever heard.

boy
will the arguments rage.....

but its all wrong that statement
top end what...? horsepower or torque?
a more restrictive exhaust...??? what small pipes? short length?
lots of baffles..?? what?

coming back to basics
a free flowing zorst will make no IMPROVEMENT in horsepower unless accompanied
by other things...camshaft head compression changes and others to let the engine
breathe better and better burn the incoming fuel/air mixture

just getting the exhaust gas out quicker with everything else standard
means nothing... what if the inlet side of the engine is small and no more
air/fuel mixture can get in? that means no more grunt

or
another way.....
stick a supercharger on a standard engine (don't change the exhaust)
it WILL make more horsepower

or this.....
the exhaust system on an engine with a TURBO will be more restrictive than standard
...has to be with a bloody great hairdryer in the middle of it,
but it will make more horsepower


my 2c


cheers

Smitty

a free flowing zorst will make no IMPROVEMENT in horsepower unless accompanied
by other things...camshaft head compression changes and others to let the engine
breathe better and better burn the incoming fuel/air mixture

Only if you assume that every other aspect of the engine is perfectly tuned to the existing, more restrictive exhaust. Generally speaking there will be some discrepancy between various ancillary engine systems, and often changing one without touching any of the others will still lead to noticeable improvements.

Take your average carbied Kawasaki sportsbike; these things come from the factory running quite rich jetting and a restrictive exhaust. Adding a free-flowing exhaust to said average Kawasaki sportsbike will see exhaust gases removed from the combustion chamber more efficiently and, in turn, development of greater horsepower. Had the fueling been exactly perfect for the stock exhaust and the current atmospheric conditions then yes, replacing the exhaust with a more free-flowing unit would not increase horsepower (it would actually probably rob a small amount of horsepower). The same could be said for a free-flowing airfilter, higher compression pistons, high-lift cams, etc.

When I had my 426 I put a phaaarkin loud very non restrictive Staintune pipe on it with a removeable baffle. The Staintune guys at the time showed me the Dyno charts from an identical bike with the baffle in and out. Baffle in produced more mid range and baffle out produced more top end.

Not supprised on the dirt those things don't stop spinning the wheel, it is spinning or you are not on the gas!

Had the fueling been exactly perfect for the stock exhaust and the current atmospheric conditions then yes, replacing the exhaust with a more free-flowing unit would not increase horsepower (it would actually probably rob a small amount of horsepower). The same could be said for a free-flowing airfilter, higher compression pistons, high-lift cams, etc.

Dave
I would agree..but how many people get a re-jet when they stick a new can/exhaust on..???

a re-tune/re-jet/timing check or adjustment should be compulsory when you fling a new zorst on


cheers
Smitty

to me its the equivalent of saying that if you block yourr mouth and only breath through your nose when you exercise that you will become more powerfull.

It's nothing like that. Here's something to consider; 4 strokes have an overlap where both the intake and exhaust valves are open. High overlap works well at high revs but results in over scavenging at low revs. Now given that, can you see how an exhaust restriction can help?
Take a look at how two stroke engines with expansion chambers work for an eye opener.

i thought it backward that reducing an engines output
or taking a measure that could potentially reduce the engines output could enhance a factor of performance.

i thought it backward that reducing an engines output
or taking a measure that could potentially reduce the engines output could enhance a factor of performance.

That is the crucial point of error. Just because something might 'look' like it would reduce performance or horsepower, does not necessarily mean it *will*.

Here is a perfect example - When I wanted to set up a set of 40mm Dell'Orto side-draught carbies for my track bike, the local Alfa dyno legend gave me some very interesting advice.......these carbies are nominally rated at 40mm (throat diameter).

His son has a really quick Alfa GTV that also runs DHLA 40mm sidies. It is possible to change the throat diameter with different size inserts. On the street, the young bloke runs 32 or 34mm inserts, as these give him good all-round response on the street.

But when he takes the Alfa out to the track, what size inserts do you think he uses ? 36mm ? 38mm ???

No, the best response, and fastest times, are done with 28mm inserts !! The throttle response is far better, and this gives him better lap times. Its not always about horsepower, but wether you can get it to the ground......

Most people, if given the choice without the knowledge, would always assume in this type of example that 36 or 38mm inserts would give the best result. The lesson ? Bigger is not *always* better. In lots of cases it can be, but sometimes its not........this is why tuning is such an important and often misunderstood thing......and also why, in a nutshell, and for other reasons, your brother is dead right.

that being said Gos, same could apply to turbos on engines, if u go for big turbo it will give u shit response down low which in turn would apply to your 38mm inserts, will give u big open power, where as a small turbo or small inserts would give u small "tight" power

that being said Gos, same could apply to turbos on engines, if u go for big turbo it will give u shit response down low which in turn would apply to your 38mm inserts, will give u big open power, where as a small turbo or small inserts would give u small "tight" power

you've missed the point about 38mm being better because its bigger. On the example with the Alfa, there were *nil* other mods to the engine, which would complement the use of 38mm inserts. For these big inserts to produce better horsepower, a corresponding increase in compression ration, ignition advance, inlet and exhaust valves sizes, cam lift and duration, and engine displacement, would also need to occur. Putting in smaller inserts did not mean the engine can't breathe at high revs.....it would struggle with 20 or 22mm, but 28mm was *perfect* for the track.

Using 38mm inserts without any other changes would make that car basically *undriveable*, as it would never be able to respond to any throttle inputs in order to reach a point where it could make 'big open power'. All the testing the Alfa guys do is on a dyno, so they have a good idea of what works and what doesn't.


Here is the best way of visualising a carby throat. Imagine a normal garden tap. There is a constant pressure being applied to the tap by your local water supplier. You cannot change the pressure of this water supply. In a carbie, the constant pressure is 'atmospheric' pressure, and you cannot change this either ( lets leave forced-induction out of this for the moment..)

OK, going back to the garden tap. If you put a fire hose on the garden tap, what sort of speed does the water come out at ?? Its really slow, its velocity has been reduced due to the size of the hose.......and the pressure has not changed.

If you run the water through a normal garden hose, the speed of the water coming out of the hose is increased substantially. You can still pass more volume of water in a given time, because of the higher velocity of the water passing through...so even though the garden hose is *smaller*, it actually produces a better result.

Its exactly the same with carbie inserts, and carbie sizes in general for that matter. 33mm smooth-bore Mikuni's are a great all-round racing carbie for most big Jap 1-litre air-cooled dinosaurs that are *road* racing. For *drag* racing, where any response under full-throttle is not an issue, then 38mm or 40mm carbs are a better bet.

Its always a matter of compromise. This also applies to exhaust systems. A good 4 into 1 will generally always give you a better top-end response, with a corresponding drop in power at the lower end of the scale. A good 4-2-1 will give better mid-range, without much loss down low, but not quite as much gain up top.......there is no such thing as a *free lunch* .....

yes your rigght gos, i spose its a bit hard to draw a direct comparison between the 2 as they are slightly different, but Generally speaking i spose same theory applies

Carbs are particularly intolerant of low air velocities because they rely in air turbulence to nicely distribute and subsequently evaporate the fuel. Low velocity results in large drops of liquid fuel, which sucks. Direct injection FTW