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Posted By: DavidP Intake spacers? - 07/22/20 5:04 am
The Triumph uses short adapters to mount the carbs to the head, while BSA mounts them directly. Would there be any advantage gained by using those on a BSA?
I'm not using the stock air box, so this is not an issue.
Posted By: triton thrasher Re: Intake spacers? - 07/22/20 8:22 am
Longer intakes can improve performance.

Long rigid intakes between carbs and head can cause bad vibration effects in your carburettors.
Posted By: Andy Higham Re: Intake spacers? - 07/22/20 8:42 am
Originally Posted by triton thrasher
Longer intakes can improve performance.

Long rigid intakes between carbs and head can cause bad vibration effects in your carburettors.

Longer intakes can also adversely affect performance.
There are a couple of ways that inlet port length affects performance
1) Ram pipe effect, the port will have a column of mixture travelling at high speed when the inlet valve is open. when the valve closes this column keeps moving due to its inertia and causes a high pressure area of mixture behind the valve, when the valve opens this mixture is released into the cylinder. Unfortunately at lower RPM this column can stall and the longer the port the harder the engine has to suck to get it moving again.
2) Tuned length, the port length can be tuned the same way as an exhaust pipe to utilise the pressure waves created when the valve opens and closes
3) Friction, the longer a port the more of the mixture is in contact with the port wall. The friction between the port wall and the mixture slows down the charging of the cylinder.
An experiment to try, take a 100M length of say 5mm bore tube cut say 20mm off the end. Blow through the 20mm piece of tube, it's easy. Now blow through the 99.98M length, it will be rather difficult
Posted By: gavin eisler Re: Intake spacers? - 07/22/20 1:46 pm
Beware of moving the carbs back from the head, fuel tap clearance becomes an issue, check this before committing. i rubber mounted mine, used a mikuni flange adapter bolted to the carb flange , fitted a spigot into the inlet port, rubber between, this solves heat issues and vibration, however running the fuel pipes gets difficult and unless supported the carbs soon droop on the rubbers leading to cracks and air leaks, I fixed that by using t140 carb to air box rubbers. The air boxes are a good thing, yeah. less restrictive than two individual pancake filters , and less gritty than no filters at all, keeps out water too, and have a decent volume to boot, IMO they are a performance aid.
Posted By: DMadigan Re: Intake spacers? - 07/22/20 4:53 pm
Depends upon what you are trying to achieve.
To benefit from the acoustic wave the intake length should be:
L (in) - 88000 / RPM
Just adding length to a pipe does increase the boundary layer thickness and, yes, if you put a 100M long intake on it will restrict the flow quite a bit. But as Mark was demonstrating on his 65HP A65 post, port shape also matters. Sometimes you need to add length to fit in a good port shape.
The carbs are ideally rubber mounted on the O-ring in the flange so putting them out on a longer unsupported beam will increase the vibration amplitude.
Dr.Gordon Blair developed a computer program to predict the effects of engine parameters but you probably would not want to pay for the computer runs to see if you can get another HP or two out of these antique motors.
Or you can put the head on a flow bench and test various intake configurations.
Or put the motor on an engine dynamometer and test them that way.
Posted By: kevin Re: Intake spacers? - 07/22/20 5:44 pm
ive experimented just a bit with intake length changes

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^^^this on my not-a-BSA commuter bike. started and ran normally. it's a 5-inch extension that gives a total intake length of 4 inches (intake horn) + 5 inches (tube) + 2 inches (stub) + 2 inches (to valve) = 13 inches. look at the fuel taps. like gavin said, there's clearance issues, which is why i went with 5 inches.

it started and ran normally but i didn't test anything. i just wanted to see whether it would go. a twelve inch piece of tubing can be cut into 3, 4, and 5-inch sections for varying triumph intake tract length from 11, 12, and 13 inches. this is in the range of old panic's intake length predictions from his mathmatical model published in his victory series

i would have preferred to just make longer air horns but these carbs have a weird flange that requires custom mounts on the atmospheric side of the carb.

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Originally Posted by Dick Page
I don't recall the specifics, but I do remember that the results had to be determined experimentally. Too many variables to predict mathematically. Nowadays a computer can probably do it if it is fed enough data.


on the machine i race now, i have the carbs set as close to the head as i can get them, just to try to hold that dimension constant while i mess with exhaust and valve timing. i firmly believe that machines like ours benefit significantly from acoustic tuning of both intake and exhaust systems, but i don't think you can get anywhere quicker with a mathematical prediction over simple cut and test.
Posted By: htown Re: Intake spacers? - 07/22/20 6:12 pm
Chrysler experimented with long tube intake manifolds in the early sixties wedge motors. The longest ones had the carbs mounted over the opposite fenders. Not sure of the physics but they generated more torque at lower rpm. The idea was to get heavy cars off the line quicker. On the more higher rpm motors like the drag race max wedge they went to a short ram.
Posted By: triton thrasher Re: Intake spacers? - 07/22/20 9:24 pm
Originally Posted by Andy Higham
Longer intakes can also adversely affect performance.

Absolutely, or it might do nothing!
Posted By: NickL Re: Intake spacers? - 07/22/20 10:57 pm
Tuning the length is a job for the dyno but Opel and Ford motors i've owned had variable
/split length inlet manifolds.
When racing we found a fair mid range boost from inlets around 12-14 inches. But only
by feel. Most modern motors seemed to mount carbs/inj very close as they rev far higher.
Posted By: DavidP Re: Intake spacers? - 07/23/20 4:00 am
I've seen a few bikes with long intake tubes, including some flat trackers. I suppose they're going for more low-end grunt.
The Triumph adapters, part numbers 71-2811 and 12, are only about an inch long, with a slight bend to make the carbs sit level. Different head, different port geometry, don't know if they would have any effect at all.
Gavin, I've been through all that clearance crap with the Mikunis I had on this bike. Had to use 90 degree fuel taps and the fuel lines still went in odd directions. Those fuel taps are now on my T120 to feed the Mk2's.
Posted By: Danam Re: Intake spacers? - 07/23/20 11:13 pm
I added spacers when cylinder heat was transferring to the carbs
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