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#341788 - 11/07/10 12:42 am Intake manifold distance and carb performance.  
Joined: Jan 2007
Posts: 250
allflatblack Offline
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allflatblack  Offline
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Jersey City NJ, USA
I have been wondering about this for some time. What difference does carb distance from the head make on carb performance? for instance, my Trident originally has three AMAL 626's set up on a gantry that is then mounted on rubber tubes that mount over stubs that run into the head. I have replaced the Amals with PWK flat slide carbs and have gotten them reasonably dialed in, with the carbs set on the gantry but running individual cables from a coupling. The bike runs great, but I always wonder what it would be like if I had set it up without the gantry. The gantry adds about an inch of manifold piping to the whole setup.
Have seen hot-rods with carb risers in the past, lifting a big four barrel three inches off of the manifold, but in the opposite my A65 has a 929 bolted right to the head. I wonder if this distance has any ill effects or benefits. A benefit that I can think of: more distance for turbulence to increase atomization and fuel/air mixture. Negative?I imagine that the longer distance could effect vacuum leading to weaker response time. I could imagine a snappier throttle.
What do you guys think?

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#341822 - 11/07/10 5:25 am Re: Intake manifold distance and carb performance. [Re: allflatblack]  
Joined: Aug 2001
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BONZO R.I.P. Offline
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BONZO R.I.P.  Offline
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Posts: 3,348
Michigan, USA
This is always a good topic. The general thinking seems to have been longer intake tract=better performance but there is a very real science behind tuning . I am most certain that factory specs were not set for performance , rather built as a compromise between performance and production costs . You might post on the performance board(if you haven't/I didn't check) you should get loads of tech ,those guys map their cams and know all sorts of terms that make my head hurt ....

FWIW-BONZO

#341825 - 11/07/10 5:37 am Re: Intake manifold distance and carb performance. [Re: allflatblack]  
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triton thrasher Online content
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triton thrasher  Online Content
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scotland


Amateur Loctite enthusiast.
#341847 - 11/07/10 10:40 am Re: Intake manifold distance and carb performance. [Re: allflatblack]  
Joined: Sep 2007
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Ob1quixote Online content
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Charleston SC
One of my OT bikes has a long intake. It is a Honda Z50 that has been repowered with a chinese knock-off motor. The long intake is needed because there is an electric starter where the carb would have been on a "regular" motor.

I find it to be a little slow on throttle response. I think this is caused by having a volume of air/fuel mix in the intake that exceeds the displacement of the engine. I can only imagine that this effect is present when extending intakes on larger engines, only to a lesser degree.

Last edited by Ob1quixote; 11/07/10 10:40 am.

When singing "Kung Fu Fighting" is outlawed, only outlaws will sing "Kung Fu Fighting"
#341895 - 11/07/10 3:49 pm Re: Intake manifold distance and carb performance. [Re: allflatblack]  
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DMadigan Offline
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ca, us
In addition to the information in TT's link, C. Taylor (MIT Press) has two books on I.C. engine performance.
Familiar with a clarinet? The engine valve is acting like the reed, the intake length like the clarinet body. The longer the intake, the lower the pitch or resonance (RPM where pressure pulses help fill the cylinder). The larger the diameter of the pipe, the weaker the pressure pulses which is why throwing on big carbs generally hurts low end performance, but small carbs will hurt top end. Which is more important?
Going to EFI this is less of a problem since the pressure wave is not needed to pull in the fuel to the intake stream.

#341983 - 11/07/10 9:48 pm Re: Intake manifold distance and carb performance. [Re: allflatblack]  
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Sisyphus Offline
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There is no such thing as a sympathetic vibration.
#341998 - 11/07/10 10:58 pm Re: Intake manifold distance and carb performance. [Re: allflatblack]  
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johnm Online content
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johnm  Online Content
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New Zealand
As I have said on several occasions Im a big believer in inlet and exhaust tuning for race motorcycles anyway.

The exhaust calculation in mezporting in the link above posted by sisyphus is almost exactly the same as you will get in Blair or AG Bell.

The inlet calculation is the same as Vizard, Panic and Blair all except the 2nd resonance node where Blair gives a different result. I have no idea why.

I have inlets 13 inches long on my 500 twin norton race bike and notice no delays at all.

With respect to the Trident and other street bikes my preference would be first and formost

1. to mount the carbs flexibly with no possibilty of them fouling or touching the frame or other parts. Vibration leading to frothing and starvation is death to any high speed tuning. I would also place a good thermal insulating spacer between the carb and the head. You want to keep the carbs as cool as possible.

2. Once you are sure you have that right then personally my next step on a street bike would be to get good aircleaners fitted to preserve the motor.

And lastly I would tune for lenght. Make the distance just as long as you can make the inlet tract by either a spacer between the carb and head or by adding good shaped bellmouths. But I would not sacrifice 1 and 2 to do this. Generally you will find it is impossible to achieve the calculated lenghts within the room constraints of the frame and motor. From the little I have seen of Tridents this will be especially true because you have very little space.

If you really want to do this properly you should fine tune on a dyno.

John

Last edited by johnm; 11/08/10 1:14 am.
#342247 - 11/09/10 3:11 am Re: Intake manifold distance and carb performance. [Re: allflatblack]  
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allflatblack Offline
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allflatblack  Offline
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Jersey City NJ, USA
Wow! I will look into all of this a bit more over the winter!

Thanks!

#342303 - 11/09/10 3:08 pm Re: Intake manifold distance and carb performance. [Re: allflatblack]  
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BSAJIP Offline
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BSAJIP  Offline
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Ohio
allflatblack,

I have a BSA 441 with an extended intake manifold and a 4+ inch long AMAL stack. Total distance from the intake valve to the stack opening is about 16.5 inches long. It is very responsive, starts easily and exhibits far more punch at lower RPM's than a standard set up. Chrysler pioneered long ram manifolds in the Sixties. The principals of ram tuning apply to m/c's as well. BSAJIP


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