Author Topic: Inlet Manifold Design - 500 Twin  (Read 10463 times)

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Offline Paul K

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Inlet Manifold Design - 500 Twin
« on: October 29, 2016, 10:49:53 am »
A bit of advice needed on inlet manifold design for a 500 cc parallel twin that I'm building for, hopefully, the MCC trials.

It's got a GS500 engine, which has twin carbs, each on a rubber manifold (see photo). I will be using just one carb mounted on a still-to-be-designed manifold.  Ideally, from what I've gleaned in the internet, the manifold should be shaped something like the Meriden Triumphs and similar (see photo).  As I'll be welding this new manifold from steel tubes, can I get away with 90 degree joints at each end (see photo), rather than the curved bends as shown on the Triumph manifold.  I know I can purchase curved pipes and weld those together to create a triumph-type manifold, but the bends I've seen mean the manifold takes up too much room.

The sharp 90 degree joints would promote more turbulence, I'd have thought.  Manifold will be as long as I can get it (within the space available) to gain bottom end torque.

All advice gratefully received.

Offline Jason Potts

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Re: Inlet Manifold Design - 500 Twin
« Reply #1 on: November 05, 2016, 08:59:42 am »
Hi Paul, I got to ask, why do you want to do this?

Offline Paul K

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Re: Inlet Manifold Design - 500 Twin
« Reply #2 on: November 05, 2016, 08:57:22 pm »
Because I've got all the parts to build a 500 cc parallel twin trialer.  The parts were originally a Suzuki GS500 that I converted to dual sport by fitting a DR650 front end and wheels. It's on the Adventure Rider forum.  Having retired and no longer using the bike for commuting, I wondered what I could do with the bike now.  and came up with the idea of a classic trials bike.

I've squeezed the engine into a GS250 tubular steel frame and had a steering stem machined for the DR650 front end to go into the GS250 frame. Rear swinging arm is from a GS450.  So a lot of work in progress and the inlet manifold being one of them.

Offline Tony Bishop

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Re: Inlet Manifold Design - 500 Twin
« Reply #3 on: November 06, 2016, 09:20:18 pm »
Hi Paul,
I am far from any kind of design engineer but the right angle tubing as illustrated for your manifold would appear to me to be rather radical.
If you were attempting to increase power then one of the ways of course is to smooth the intake flow to avoid turbulence. In your case any increased power is not the requirement but I suspect that the right angles would create massive tubulence with a possible subsequent intake imbalance between the cylinders which in turn would cause a lot of head scratching when it comes to tuning the carburettor.
Unfortunately I can't think of an easy answer but how about if you were able to achieve the 90 degrees using two angles ?
I will be very interested to hear how you get on.
Good luck.

Offline Jason Potts

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Re: Inlet Manifold Design - 500 Twin
« Reply #4 on: November 07, 2016, 12:35:58 pm »
Love this idea Paul. Back in the day I used to race Suzuki X7ís in earlystocks and after umpteen blow-ups I was considering going over to the dark side (using a 4-stroke). I was looking at GS400ís as they had spoked wheels and was contemplating fitting GS500 barrels on one. I never did get one but you could fit the standard 500 engine in the 400 frame but by fitting just the barrels to the 400 bottom end you can make a 516cc motor. The GS400 and the GS425 have a longer stroke than the GS450 and GS500.
I didnít realise that the 500 motor would go in the 250 though. The 250 is a single down tube frame (much lighter) and the 400 is twin down tube.
It should make a nice bike when its done and Iíd love to see it but I would definitely stick with 2 carbs. They are pretty bomb proof when running the standard set up.
Regards
Jason

Offline Paul K

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Re: Inlet Manifold Design - 500 Twin
« Reply #5 on: November 08, 2016, 09:19:09 am »
From research I've done on the internet, there appears to be a big crossover of parts between the various Suzuki twin engines. I'd initially thought of using the GS450 frame because it's a duplex frame like the old BSAs, but the exhaust ports on the GS500 face exactly forward, they're not splayed, and as such the header pipes would foul the frame.

I looked at the GS250 frame, which had the single down tube and the GS250 crankcases looked the same size as the GS500, just different mountings. The Eric Cheney frames used a single down tube so it must be ok (ignoring differences in steel type and diameters). Funny thing was, the guy that I bought the GS250 frame from had a project bike with a GS500 engine stuffed into a GS450 frame, but the header pipes were really, really tight with some tight turns to avoid the down tubes.

So the GS500 engine does fit in the GS250 frame, just, with new mountings welded in.  However, to adjust valve clearances will involve removing two mounting bolts and tilting the engine forward to allow the cylinder head cover to be removed.  But as these are checked every 6,000 miles (I think), it won't happen too frequently for a trials bike.  Say once every 5 or 6 years?

As regards twin or single carbs, my belief is that a single carb is better for bottom end torque and tractability.  Andrew Berry is running his 750 Wasp outfit with a single carb and that outfit has no shortage of power. Twin carbs are better for the top end power where the engine would otherwise run out of breath.  Aside from all that, twin carbs won't fit with the GS250 frame, the seat tubes are in the way of the inlet tracts. It'll also make constructing an air box that much simpler to have a single carb.

I'll post some photos when the bike looks in better shape and is accessible.  It's buried under piles of crap at the moment as the garage is a bit chocker block  :)

Offline Jason Potts

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Re: Inlet Manifold Design - 500 Twin
« Reply #6 on: November 08, 2016, 08:26:17 pm »
sounds like a great project. lots of crossover stuff between bandits and GSXR's too. god bless Suzuki for making the amateur builders life easier.  ;D

regards your manifold, 90deg bends are pretty bad for pressure drop. that's without it buggering up your fuel stoichiometry.

I did find this on ebay though which seams a compact solution factory fitted to a CB250 Nighthawk engine.
Must work if Soichiro Honda came up with it.





Offline Paul K

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Re: Inlet Manifold Design - 500 Twin
« Reply #7 on: November 08, 2016, 10:43:50 pm »
Now if only there was a 2-into-1 manifold I could use from the Suzuki range.  By the way, I had to look up what 'fuel stoichiometry' meant.  now that I know, I certainly don't want to bugger it up  :o

Offline Jason Potts

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Re: Inlet Manifold Design - 500 Twin
« Reply #8 on: December 04, 2016, 07:01:14 pm »
How's the GS project going Paul?

Offline Paul K

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Re: Inlet Manifold Design - 500 Twin
« Reply #9 on: December 05, 2016, 08:51:28 am »
Afraid nothing is happening at the moment; not even had a chance to install the modified steering stem and new bearings into the frame to see if they fit ok.  There's too many other distractions/priorities; the garden's been landscaped, Christmas preparations (tree, decorations, present buying) and now I have to prepare the XR400 for the Exeter in January.

I reckon it'll be spring before I get to work on it.  I'd like to use it on the Lands End, but I'm trying to be realistic about completing it.  Early next year, I've got the Beetle to MoT and a couple of local trials so that could impact on completion as well.

I'm going to ask Santa for time for my Christmas present  :)

Offline Jason Potts

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Re: Inlet Manifold Design - 500 Twin
« Reply #10 on: December 06, 2016, 07:12:56 am »
Yeh I know what that's like. I finished my CZ now so just got to put some miles on it. already started work on its eventual replacement (which I promised myself I wouldn't do until id got my first LDT out of the way and learned a few things). cant help myself...  ::)

I got loads of house jobs to do and should really be doing that now but got no enthusiasm for it.  :(

I'm going to ask Santa for some DIY enthusiasm.  ;)

Offline Jason Potts

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Re: Inlet Manifold Design - 500 Twin
« Reply #11 on: January 02, 2017, 02:48:46 pm »
Hey Paul, I saw this on eBay and thought of you.  ;)

http://www.ebay.co.uk/itm/182405651752?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

mind you just had another look at your head picture and I don't think its going to fit..

also this guy is selling a cheap PUCH.

http://www.ebay.co.uk/itm/172457908559?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

not sure if its a split singe but he is selling split single parts on his other listings.  ;)
 


Offline Paul K

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Re: Inlet Manifold Design - 500 Twin
« Reply #12 on: January 02, 2017, 05:19:05 pm »
Regards the inlet manifold, you're correct; it won't fit. There's also the potential problem of using a Y-shaped design. I recall reading somewhere that this design would invariably produce an imbalance between the two combustion chambers, hence the use of the old Triumph shape manifold.

Regards the Puch stuff for sale, I'm pretty sure that would be a 250 cc split single. A good buy for anyone whose got a bike he needs spares for.

The Interstate off-road is the tasty looking one, though, a bit heavy nowadays.

Offline rick howell

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Re: Inlet Manifold Design - 500 Twin
« Reply #13 on: January 02, 2017, 10:27:34 pm »
Hi Paul

this is all way out of my knowledge sphere but...why not get something 3d printed? Accurately measure up what you have and draw it up on CAD (or get someone to do it) and find a 3d printer company with a suitable medium.

Rick

Offline Paul K

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Re: Inlet Manifold Design - 500 Twin
« Reply #14 on: January 04, 2017, 08:04:58 am »
Wow!  There's a thought.  Any idea what the cost for such a component would be?  Presumably, the material would have to be resistant to petrol and high temperatures.