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Intake manifold distance and carb performance. #341788
11/07/10 12:42 am
11/07/10 12:42 am
Joined: Jan 2007
Posts: 250
Jersey City NJ, USA
allflatblack Offline OP
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allflatblack  Offline OP
BritBike Forum member
Joined: Jan 2007
Posts: 250
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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Re: Intake manifold distance and carb performance. [Re: allflatblack] #341822
11/07/10 5:25 am
11/07/10 5:25 am
Joined: Aug 2001
Posts: 3,348
Michigan, USA
BONZO R.I.P. Offline
In Remembrance
BONZO R.I.P.  Offline
In Remembrance
Joined: Aug 2001
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

Re: Intake manifold distance and carb performance. [Re: allflatblack] #341825
11/07/10 5:37 am
11/07/10 5:37 am
Joined: Aug 2001
Posts: 7,509
scotland
triton thrasher Online content
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triton thrasher  Online Content
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Joined: Aug 2001
Posts: 7,509
scotland


Amateur Loctite enthusiast.
Re: Intake manifold distance and carb performance. [Re: allflatblack] #341847
11/07/10 10:40 am
11/07/10 10:40 am
Joined: Sep 2007
Posts: 1,848
Charleston SC
Ob1quixote Offline
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Ob1quixote  Offline
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Joined: Sep 2007
Posts: 1,848
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"
Re: Intake manifold distance and carb performance. [Re: allflatblack] #341895
11/07/10 3:49 pm
11/07/10 3:49 pm
Joined: Jan 2004
Posts: 3,757
ca, us
D
DMadigan Offline
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DMadigan  Offline
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D
Joined: Jan 2004
Posts: 3,757
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.

Re: Intake manifold distance and carb performance. [Re: allflatblack] #341983
11/07/10 9:48 pm
11/07/10 9:48 pm
Joined: Sep 2007
Posts: 504
Maine
Sisyphus Offline
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Sisyphus  Offline
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Joined: Sep 2007
Posts: 504
Maine


There is no such thing as a sympathetic vibration.
Re: Intake manifold distance and carb performance. [Re: allflatblack] #341998
11/07/10 10:58 pm
11/07/10 10:58 pm
Joined: Apr 2006
Posts: 1,402
New Zealand
J
johnm Offline
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johnm  Offline
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J
Joined: Apr 2006
Posts: 1,402
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.
Re: Intake manifold distance and carb performance. [Re: allflatblack] #342247
11/09/10 3:11 am
11/09/10 3:11 am
Joined: Jan 2007
Posts: 250
Jersey City NJ, USA
allflatblack Offline OP
BritBike Forum member
allflatblack  Offline OP
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Joined: Jan 2007
Posts: 250
Jersey City NJ, USA
Wow! I will look into all of this a bit more over the winter!

Thanks!

Re: Intake manifold distance and carb performance. [Re: allflatblack] #342303
11/09/10 3:08 pm
11/09/10 3:08 pm
Joined: Feb 2008
Posts: 244
Ohio
B
BSAJIP Offline
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BSAJIP  Offline
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B
Joined: Feb 2008
Posts: 244
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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