Intake query

[Mekk]

I am no mechanic but am interested in learning more about engines so that I may get more out of my bike.

This has been puzzling me a bit lately.

I've noticed that a lot of bike engines have the exhaust port facing the front of the bike and the air intake coming in from the back.

Now, for a simpleton like me, it would make sense to have the air intake at the front as that would be exposed to the most airflow and would surely help with performance...right? I understand there's a vacuum going on with the pistons and I know about the venturi effect... I'm just curious as to why a lot of bikes are designed in this fashion.

Please enlighten.

Cheers.

[scumdog]

My thoughts: the zorst would create a lot of hard-to-get-rid-of heat behind the enjin.

And the intake set up would be sticking waay out on front, specially if it was the olde-worlde carb version.

[Coldrider]

The V twins posed quite a lot of problems for Suzuki & Honda, exhaust pipes clearing the swing arm area, pushing the engine too far forward. That may be why the TL1000 had the shocky underneath, and Honda making the swingarm pivot from the engine.
Interestingly enough Harley D in the 30's made a MB with the generator leaning forward, giving the engine a 3 cylinder look.

[Mekk]

Good points, thanks.

Come to think of it, that heat would be under the seat too.

[Mekk]

Another point...the beemers with flat twin engines also don't have this setup. They'd be able to deal with the heat problem a lot easier having the cylinders out in the open like that.

There must be a more fundamental problem with the design. Perhaps it is just the "too forward" thing... any more thoughts?

[xwhatsit]

I don't think airflow is really a problem, is it? I mean, airboxes are produced for a reason. You want nice still air for carburettion even when the wind is howling past your motorcycle at 160kph. So you build an airbox, which traps the air and makes it still. If you put the air intake in front of the engine, you won't get nice still air.

That said, Kawasaki are famous for their `ram-air' and all of that. I don't know.

You see plenty of V-twins with the rear cylinder `backwards' as you suggest.

I think having it round the normal way is good packaging, as those above have stated. Having long exhaust headers is usually a good idea (at the very least it gives you the added length to get your two or three or four or six headers into one without too many sharp kinks). Yes you would get a hot arse too, lol -- I get one from the carb while at the lights as it is haha. And as has been mentioned you need room for carbs and airbox. To make room for them you'd probably have to stand the cylinders up vertically, or maybe even reverse them so they're sloping backwards. Could be interesting trying to design a frame to cope with that.

[slowpoke]

I used to wonder the same thing, then realised that the engine would have to be positioned further back to cater for the carbs/throttle body/intake runners etc. On a sportsbike especially it would be a major packaging problem with engines needing to be positioned as far forward as possible.
Most of the high performance stuff has got the cylinders canted forward at quite an angle so the carbs etc would almost be underslung. The designers have placed my R1 carbs virtually vertical, giving as straight a shot as possible at the inlet ports and they are enclosed inside an admittedly aftermarket airbox. Turning that around would be a packaging nightmare with severe restrictions on the size/shape of the airbox, and place the stinkin' hot exhaust headers in a patch of virtually still air right under your klacker. It would have all sorts of flow on effects for positioning electrics, insulating fuel tanks etc etc.

[scumdog]

And with carbs out front there would be even more problems with icing-up in cold weather, just another thought.

[merv]

Somehow you've still got to fit aircleaners and really it probably all just doesn't fit and with airbox/ram air now they have gained the benefits without the disbenefits. In the past I'm sure I've seen custom versions of old British stuff that tried the concept.

[pete376403]

Reversed heads used to be considered a hot setup for old 500/650 Triumph twins. This link http://dragsdownunder.tripod.com/articles/peterson.htm is an article about a NZ drag racer whose Triumph had a reversed head, more to accomodate a supercharger than anything else. It was fairly straightforward on the triumphs becuase the bolt pattern was symettrical and with seperate inlet and exhaust cams, either side of the barrel

[scumdog]

Related to the above: my mate had a Honda drag-bike and reversed the head to enable the blower and big-as SU carb to fit.

[Mekk]

Hmm, could you have it by rotating the cylinders so that they're facing forward, like the lower part of an L-twin?

I'm thinking in terms of a parallel twin in this configuration...where the intake ports and carbs (or injection systems) are on top, catching at least some of the air flow...and the exhaust underneath?

Still trying to conserve space, wouldn't that bring more weight to the front?

[Coldrider]

Japanese manufacturers are most unlikely to deviate from convention.
Also in the 60" & early seventies small trailblikes had a rotary carb tucked in the side of the engine case.

[Mekk]

Japanese manufacturers are most unlikely to deviate from convention.

I understand that, I'm trying to think of how they came to those conventions. No doubt they would have thought of every possible configuration by now and chosen the best ones.

[Coldrider]

I don't think the japanese have ever come up with anything new, they copy, modify & perfect, much like the italians, except the italians probably invented.
Even the british got the german motorcycle blueprints from World war reparations, the early BSA (british small arms) are replica DKW's.
One of the first 2 stroke racers was literally stolen from a race track and delivered to the japanese, that reincarnating into the Suzuki 250 rebel.
They are some stories that someone from KBland may have more detail.

[Paul in NZ]

One of the great myths is that the Japanese are not creative engineers. I personally don’t believe that – I’ve always found them pretty adventurous. Sure – at one stage or other nearly every configuration has been tried but to look at something like an RG500 and say Ariel made a Square 4 years earlier is plain daft – yes of course they did…. Moto Guzzi won with L twins yonks before Ducati was even a company but that’s not the point is it. Engineering is about making things work and hopefully making things (in the case of motorcycles) that people want to buy…

Anyway – most manufacturers are pretty open to any wild engineering idea that works but unfortunately consumers tend to like things they understand. In the case of putting the carbs behind the engine, well that’s not always where they have been but development has shown that in most cases, it’s a good place for em because you can whack a great big airbox around them to smooth the airflow into the intakes.

Whats changed is that tyre and chassis / suspension technology has allowed makers to produce engines with much more power without killing the rider in the 1st 5mins. The weight is now carried higher so that under violent acceleration the weight transfer stops the rear wheel from spinning (I think) which makes the whole bike taller etc. On modern sports bikes with inclined engines that’s an issue because the carbs sit on top of the engine, not behind it making even more height.

The other issue is exhausts exiting from the rear would leave us with a short header pipe which may not be a good thing.

Given the trend to full fairings, you could put any old engine config in there I suppose??

[pete376403]

Japanese manufacturers are most unlikely to deviate from convention.
Also in the 60" & early seventies small trailblikes had a rotary carb tucked in the side of the engine case.
Bridgestone and Kawasaki were the main exponents of rotary valves. Bridgestone were persuaded to go out of the motorcycle business by the other Japanese manufacturers in exchange for agreements to buy tyres.
However it wasnt just little bikes. Both Bridgestone, with a 350 and Kawasaki (250 and 350) made some nice rotary valve twins. And I had a Kawaskai F9, 350 rotary valve single trailbike in 1972.
Suzuki *may* have had rotary valve singles early in the piece, but I don't think Yamaha every did, they were more committed to reed valves

[pete376403]

Exhaust ports were probably placed at the front initially to the hottest part of the motor would get the best shot at cooling, bearing in mind all engines used to be air cooled and exhaust valve steel wasn't too flash. Since then it's probably been more a matter of following tradition and, as Paul in NZ says, getting a decent length of exhaust pipe.

[avgas]

I no like hot pipe near my balls!!!
No but seriously i have thought about this (and ran a few designs), and i don't like hot pipes near my balls. Its prob the main reason why im not a vtwin fan.
There were other conclusions like : too much air is bad and motorbike tuning can be done via appropriate air box etc.
But i no like hot pipe near me nuts

[Coldrider]

Yes Suzuki had rotary valves in the Van Van fat wheel beach bikes, my sister had a 50cc and I knew of another RV90. Not sure of the TS50 and TS100 trail bikes, 125cc plus had conventional carbs.
I remember the 350cc kawa trail bikes, had a fancy tail piece/mud guard like a road bike.

[slowpoke]

The other issue is exhausts exiting from the rear would leave us with a short header pipe which may not be a good thing.



Here's another thing that seems to have a few contradictions. We seem to be seeing more bikes like the R6, ER-6N, GSXR600 etc with extremely short exhaust systems. Even drag cars run very short open primary exhaust systems. Yet the conventional wisdom has it that longer exhausts make better power, so what gives? Note the "pigs tail" to lengthen the exhaust of an RC211V below.

[skidMark]

i used to have a yamaha tzr 250 3MA carbys were on the fron zrost on the back that went under the seat and came out the tail.....

carbys ddnt take up much room at all....

airbox then ran up the front and under the tank....

[Mekk]

All good answers, thanks.

So we understand it could be done, but is there one simple answer as to why it isn't?

Space? Cost? Performance?

Or is it just all of them together?

[slowpoke]

i used to have a yamaha tzr 250 3MA carbys were on the fron zrost on the back that went under the seat and came out the tail.....

carbys ddnt take up much room at all....

airbox then ran up the front and under the tank....

Good point, the compact, low hp nature of a small 2 stroke probably opens up a few more packaging options.
On the other hand a modern litre bike has a frikkin' huge airbox, sometimes with CPU controlled internal flaps or bell mouths etc which severely limit your options. This is a much flasher version of the airbox on my R1. My airbox is actually even bigger with the carbs being completely enclosed.