Why?

[Bendy]

Why do the exhausts on bikes come out the front of the engine and have to snake their way all the way underneath and then up the side(s) to get to the can at the back? Why don't they just come out the back in the first place?

(I know cock all about engines, it's just something visual that struck me, and I look forward to the answer being devastatingly obvious )

[Demonic69]

Don't they need a certain amount of pipe?
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[tatters]

thats why some people have put the engine in backwards so air is ramed into the carbs and also they can have louder shorter pipes, also think about V4,s
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[cagiva gezzer]

https://www.bikepics.com/cannondale/s440/03/pics.asp

Not ALL bikes.
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"because one stroke isnt enough and four strokes waste two"

[TheShaggyDA]

Might have originally been a problem with carbs icing up - stick them behind the engine, and you have a warmer pocket for them. Out front they are unprotected.

This was the reason that Minis (the real ones, not the BMW "lets ride on the coat-tails of a well known name" compact car) have their engines that way round. It was estimated that the engine lost nearly 10% of it's power by gearing it to run that way round, too.
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[i:6e3bfc7581]But still I fear and still I dare not laugh at the madman...[/i:6e3bfc7581]

[Retro-Man]

In case of an accident, would seem reasonable to me....

It makes sense to me at least to keep the fuel system as close to the centre of the bike as possible keeping it out of harms way.
the by product of this safety feature is that the exhaust has to come out the front (or at least that is easiest way), due to the very nature of engine basic designs.

Cheers
Retro
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[Kickstart]

Hi

It is possible to build the engine reversed. Think the MV Agusta engine was originally designed to be like that, and then there were the reverse cylinder TZR250s.

The problem is partly the length of the exhaust, partly packaging (where the hell do you put all the bits that normally live in that area and which want to be rather cooler than the exhaust) and partly problems with having enough space for the carbs and a nice large air box.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Retro-Man]

so far I think we all have made valid reasons, I would hazard a guess that there is no single reason why, but that all the answers so far are correct.
It simply makes more sense to have the pipes coming out the front due to all the above posts and there are probably even more reasons.
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[Guest]

The hottest part of an engine is round the exhaust valves and header pipes - might as well use all that free cool air for something

[stinkwheel]

Reverse engine MZ250.
https://ratbike.org/photos/y436-1mz.jpg
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[cagiva gezzer]

With a disc valve 2t, there is no reason to have the exhaust on the front. The only benefit would be cooling.

Wern't the reverse cylinder tzr's more prone to seizing???

There must be some benefit, other wise all single 4t's would be running reverse, because if you think about it, it would be like chopping off the front cylinder of a v-twin.

I think its just that it all fits better that way. Exhausts lose less efficiancy, compared to induction, if they have to be routed everywhere. Best to concentrate on getting optimum induction, then solving the exhaust.
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"because one stroke isnt enough and four strokes waste two"

[mchaggis]

On a V-twin there isn't any other way to do it, unless you have a stupidly wide V angle or can put the engine in horizontally. You always want to package the inlet and exhaust as far away from each other as possible to ensure that the exhaust doesn't heat the inlet up too. This means that you tend to end up with the inlet manifold on one side and the exhaust on the other.

In front of the engine there is unfortunately bugger all space really in which to package very much, as the tank and radiator take pretty much all of the available space. Another issue is that normally, you want the exhaust gasses out of the vehicle as fast as you can get them, whilst staying within the environmental controls. This means you end up with them going down and straight out the back.

The obvious answer from my point of view is that packaging 4 hot exhausts through the centre of the bike really doesn't make a lot of sense. There is a lot of convenient space which can have things like electrics, batteries, carburettors, shocks etc etc in the middle of the bike. Most of these things require shielding from the environment, whereas the exhausts don't.

So, in essence, the way the exhausts are just now is the best way to have them as they can be routed away from heat sensitive things, and the space in the centre of the bike can be used better for other more condition critical components.
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[stryker]

Interesting that regarding the exhaust heating the inlet manifold. I say that as the reliant engine on my trike does just that! The inlet manifold is bolted to the top of the exhausts manifold. Water is pumped through the base of the carb as it sits on top of a pretty hot inlet manifold.

Must of been something to do with fitting the engine in a plastic pig, however it runs damn well in this cold weather.
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Now what?

[Kickstart]

Hi

Many cars have heated inlet manifolds. Most also pick up warm air from around the exhaust manifold during winter.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Rory]

It's been touched on already, but I think that by routing the pipes as they are on most bikes, the path of the exahaust gasses is free-er, i.e. fewer twists and turns to slow the gas down.
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[binge]

Well ive just been adjusting the clutch on my mams Suzuki cappacino and I noticed that the exhaustsdo not exit from the front on that. By the looks of it (I didnt have a great look because its so compact) but by the looks of it, the carbs are on the left, and the exhaust exits from the right.

Ben...
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[mchaggis]

[binge]

Right, Just been looking at it, the exhausts do exit from the right, and the inlets are on the left...
Its rear wheel drive too

Ben...
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[mchaggis]

If you can excuse my shoddy Paint drawing and excessively large picture , I presume you mean it's positioned as in the top drawing, whereas in the bottom one, it's positioned as it would be in the bike.

Just out of interest, are there any bikes which have their engine positioned longitudinally? Ah, just thought of ones which will, the shaft driven ones will.

https://www.2tausend1.de/motors/normal/Honda-CX500-1981.jpg
https://home.planet.nl/~motors-20th-century/page_Honda.htm
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I must not be a troll...
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Discovering the delights of Hammerite and a 3/4" brush.

[Smoto Bob]

i always though that having the exhasut out the back of an engin would be good for keeping seat warm

https://homepage.ntlworld.com/karen.curzon/trout/images/3ma/3ma_3.JPG

https://homepage.ntlworld.com/karen.curzon/trout/images/3ma/3ma_2.JPG

One of the cooooolest bikes, ^^ a TZR250 RC (Reverse Cylinder)
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[Guest]

Ah, now we're into GP technology on the road

[loply]

The new Foggy Petronas FP1 has a "reverse engine" but they had an awful lot of difficulty keeping the exhaust cool (and the things around it cool) according to MCN.

So I guess the number one reason is heat...
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[Fortuna]

It's to do with heat.
If the gases coming out the engine touch a cooler surface they contract filling less of a space, therefore exiting quicker.

[iCraig]

[binge]

Is that the triumph 2.5 litre or somthing?

I saw one of them parked out side Laguna Motorcycles but at the time It was a show bike as they hadnt been released to the public...

Ben...
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- Honda XL125S - CBR400 NC29 - VFR400 NC30 - CBR400 NC29 - TL1000R - TeeZeR Pit! - Bandit 12 - YZF450 - LuckyMX125 Pit! - Demon X XLR 140 Pit!
- Stomp KZ 160 Pit! - Demon X D-link 140 Pit! - CRF50F Mini-Pit - MonsterMoto 155cc MiniSM - Honda Chaly 150cc - Yamaha 98' R1
- Honda Chaly 140cc - Thumpstar 190cc 4v MiniSM - Aprilia RS125 Pit-Bike -